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ect160- Harmonics Upstream of Rectifiers in UPS

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ect160- Harmonics Upstream of Rectifiers in UPS Powered By Docstoc
					                                         n° 160
                                         upstream of
                                         rectifiers in UPS

Jean Noël Fiorina

Joined Merlin Gerin in 1968 as a
laboratory technician in the ACS
(Alimentations et Convertisseurs
Statiques) department where he
participated in the performance
setting up procedures for static
converters. In 1977 he obtained his
ENSERG engineering degree
following a 3 year evening course
and rejoined the ACS department.
Starting as development engineer he
was soon afterwards entrusted with
projets. He became later responsible
for design projets in EPS department
(Electricity Power Supplies). He is in
some ways the originator of medium
and high power inverters.
At present he is with the Supplies
Division where, as responsible for
innovations he works on the
preparation on new UPS designs of

E/CT 160 first issued december 1993

cos j1         factor of phase shift =

                                              ∑ Yn2
D%             global distortion rate = 100
Hn %           individual rate of harmonics = 100
l              power factor =
P1             active power of fundamental component
S1             apparent power of fundamental component
n              distortion factor =
                                     cos ϕ1
Y1             effective value of fundamental (current or voltage)
Yn             effective value of harmonic of order n; for current: In (IniN according to standard spec. IEC 146-4)
Zn             impedance value for harmonic n (Un = Zn In)

Cahier Technique Merlin Gerin n° 160 / p.2
harmonics upstream of rectifiers in UPS                                               UPS, like most static converters draw
                                                                                      energy from an A.C. mains network
                                                                                      through rectifiers.
                                                                                      Often, these rectifiers fitted with
                                                                                      thyristors are generators of harmonics.
                                                                                      Merlin Gerin, manufacturers of UPS
                                                                                      equipment are well acquainted with this
                                                                                      problem and consequently have
                                                                                      decided to share their knowledge in this
summary                                                                               «Cahier Technique».
                                                                                      In this treatise, the author highlights
                                                                                      first the need for a standardized co-
1. Harmonics in supply networks        Consequences due to harmonic                   existence between polluting and
                                       currents                               p. 4    polluted equipments.
                                       Need for standardization               p. 4    He then recalls, which harmonic
                                                                                      currents and voltages are produced by
2. Thyristor Graetz bridge rectifier   Harmonic currents generated
                                                                                      conventional (classic) rectifiers
                                       by a Graetz bridge rectifier           p. 6
                                                                                      (thyristor Graetz bridge rectifier) and
                                       Influence of source impedance          p. 7    proceeds to offer various solutions
                                       Current distortion rate                p. 10   designed to minimize harmonics.
3. Minimisation of harmonic            Insertion of inductance at rectifier           Finally, in his conclusion, he alludes to
disturbances                           input                                  p. 11   the appearance in the near future of
                                       Use of double bridge rectifier         p. 12   non-polluting UPS equipment and of
                                       Rectifier circuit with more than               de-polluting converters.
                                       two bridges                            p. 15   Note: harmonic problems occuring
                                       Utilization of a passive filter for            downstream of rectifiers which supply
                                       harmonics                              p. 16   non-linear loads are fully discussed in
4. Conclusions and prospects                                                  p. 18   Cahier Technique No 159. The latter
                                                                                      provides also definitions and
5. Bibliography                                                               p. 20
                                                                                      mathematical formulae relating to
                                                                                      In the present booklet, only the
                                                                                      principal definitions and formulae are
                                                                                      listed in page 2.

                                                                                      Cahier Technique Merlin Gerin n° 160 / p.3
1. harmonics in supply networks

consequences due to                          disturbances produced by some               It should be noted that for a same level
                                             consumers.                                  of current disturbance, the voltage
harmonic currents                            To achieve this, it is necessary to         distortion ratio, at the point of
Harmonic currents generated by certain       define:                                     connection, is dependent on the
equipment, such as static converters,        s first, a maximum distortion rate          network impedance at that point.
discharge lamps, arc furnaces etc..          allowing correct functioning of most
(providing there are many or providing                                                   A solution that is fair, is to authorise
                                             installations (level of compatibility),     disturbance causing power sources
they have of higher power rating             s second, a maximum disturbance rate,
compared with the power of the source)                                                   that are proportional to the power
                                             for each user so that the cumulative        contracted for by each user and for
can adversely affect the operation of        effects of various disturbances thus
other equipment connected to the same                                                    each range of voltage i.e. LV, MV and
                                             generated do allow an operational           HV. Emission levels must be
network.                                     compatibility between all the               considered in domestic and industrial
The effects of these harmonic currents       installations (connected to the same        applications.
are discussed in Cahier Technique            network); all must operate correctly.
No 152 «Harmonic disturbances in             Thus’ if this compatibility is needed to    s  domestic applications
industrial supply networks and their         exist between subscribers, it is also       In the LV range, where the energy
treatment».                                  needed to exist within the installation     distributor is unable to control the
                                             units of individual subscribers.            situation, disturbance levels which
Let’s recall the adverse effects of
                                                                                         have to be observed in equipment units
harmonic currents:                           The end user is therefore burdened          are set in accordance with standards.
s they cause additional heating              with a level of disturbances induced by
especially in line conductors,                                                           As an example, standard specification
                                             equipment units that he has installed       IEC 555-2 referring to «Disturbances
transformers and condensers,                 himself. That is why it is important that
s they induce vibrations and noises in
                                                                                         caused in supply networks by electro-
                                             manufacturers clearly state the             domestic appliances and similar
electromagnetic equipment,                   disturbance levels produced by their
s they can cause interference with
                                                                                         equipment» prescribes limiting
                                             equipment.                                  values of current for each harmonic
communication and «low current»              Standards are therefore needed to set
protection/signalling circuits.                                                          (in appliances drawing an effective
                                             acceptable levels of harmonic               current ≤ 16A - see table in fig. 2).
A distorted voltage can, in addition,        disturbances for the supply networks as
upset the operation of some receivers                                                    s industrial applications
                                             well as for polluters.
such as regulators, static converters                                                    In this sector, there are so far no
                                             Level of operational compatibility          agreed international standards.
(when the crossing through zero of the
                                             Levels of compatibility for Low Voltage     However, a «consensus» appears to
voltage waveform becomes
                                             (LV) public supply networks are defined     emerge on the concept of stages.
                                             in Standard Specification IEC 1000-2-2      s stage 1: automatic acceptance
Thus, one of the factors highlighting the
                                             of May 1990. As the levels retained in      This acceptance is dependent on the
quality standard of electricity supply is
its voltage distortion rate.                 this standard specification are the same    voltage level of network and applies to
                                             as those published in CIGREE                equipment of low power as compared
                                             periodicals (Electra No 77 of July 1991     with the power contracted for
need for standardization                     and No 123 of March 1989), it is            (subscribed). For example, the rule at
As electricity is today regarded as a        probable that levels specified for          «Electricité de France» (EDF) is to
product (in particular in Europe             Medium Voltages (MV) and for High           have a disturbance causing power that
following the directive of 25 July 1985      Voltages (HV) will also correspond to       is inferior or equal to 1 % of minimum
under reference 85/374/CEE), the             these recommendations (see table in         short-circuit power in a normal situation
producer becomes fully liable for            fig. 1).                                    at the point of connection.
damages caused by excess of                  Emission Levels                             This tolerance can be extended if the
harmonics.                                   Limits should be defined for each           total disturbing power is inferior to:
That is why electricity distributors in      subscriber so as to avoid the necessity     - 4 MVA in HV range,
order to be able to guarantee a quality      to carry out systematic controlled          - 500 kVA in MV range,
level acceptable to all consumers, do        checks when the equipment is put into       - 40 kVA in LV range.
set or are compelled to set limits to        service.

Cahier Technique Merlin Gerin n° 160 / p.4
odd harmonics                                   odd harmonics                                      even harmonics
non multiples of 3                              multiples of 3
harmonic        harmonic                        harmonic         harmonic                          harmonic         harmonic
order n         voltage %                       order n          voltage %                         order n          voltage %
                LV/MV           HV                               LV/MV            HV                                LV/MV           HV

5               6               2               3                5                2                2                2               1.5
7               5               2               9                1.5              1                4                1               1
11              3.5             1.5             15               0.3              0.3              6                0.5             0.5
13              3               1.5             21               0.2              0.2              8                0.5             0.2
17              2               1               > 21             0.2              0.2              10               0.5             0.2
19              1.5             1                                                                  12               0.2             0.2
23              1.5             0.7                                                                > 12             0.2             0.2
25              1.5             0.7
                        12.5            2.5
> 25            0.2 +           0.1 +
                         n               n
Global rate of distortion: 8 % in LV and MV networks - 3 % in HV networks

fig. 1: values indicative of levels (targets) of compatibility for harmonic voltages (in % of nominal voltage at fundamental frequency) in HV power
networks (transports) and MV and LV networks (extracted from paper published in Electra No 123).

s  stage 2: acceptance with reservations           harmonic                                 max. permissible harmonic current
When, in the case of a given user, the             order                                    (in Amperes)
limits stated previously are exceeded,             odd harmonics
the energy producer generally
prescribes a maximum distortion rate at            3                                        2.3
the point of connection. In cases where            5                                        1.14
these levels were likely to be exceeded,           7                                        0.77
the distributor/supplier would reserve             9                                        0.4
the right to ask for complementary                 11                                       0.33
                                                   13                                       0.21
means of compensation to be resorted
to if the distortion rate was exceeded.                                                               15
                                                   15 ≤ n ≤ 39                               0.15 x
s stage 3: acceptance - exceptional and                                                                n
provisional.                                       even harmonics
When the limits stated in stage 2 are
exceeded but without, however, causing             2                                        1.08
the compatibility level to be exceeded -           4                                        0.43
due to the non generation of harmonics             6                                        0.30
by other users -, a provisional                                                                       8
                                                   8 ≤ n ≤ 40                                0.23 x
authorization permit may be granted.                                                                  n
Finally, in an effort to clarify the beha-
viour of harmonic producing equipment,
some standards are now in the process              fig. 2: limits of harmonic components of current in domestic applications (In ≤ 16 A).
of elaboration or modification.

                                                                                                          Cahier Technique Merlin Gerin n° 160 / p.5
2. thyristor Graetz bridge rectifier

UPS equipments consist of an AC/DC
converter (i.e. rectifier), a battery bank
(which can be charged by the rectifier
or with an appropriate current charger)                                charger
and a DC/AC converter (i.e. inverter)                                  rectifier                                 inverter
(see fig. 3).                                            network                                                                    utilization
Generally as the input converter is
expected to provide a charge or to
maintain the charge of the battery at a
constant voltage and to supply the
required power to the inverter, it utilises
usually thyristors arranged in the                                                                 battery
classic form of a Graetz bridge circuit.
There are other types of rectifier circuits
but the three-phase Graetz bridge             fig. 3: circuit diagram of a charger rectifier.
arrangement is the most commonly
used, in particular in high powered UPS
units; hence the following study of
harmonic currents generated by the
three-phase Graetz bridge with a fully                                                                       λ              Id
regulated circuit and of the means of
minimising them.                                                                    T1    T2         T3

harmonic currents
generated by a Graetz                               e1     Zs    e'1    i1

bridge rectifier                                                        i2
                                                    e2           e'2
The rectifier in figure 4 is assumed to
be connected to a high value                                            i3
                                                    e3           e'3
inductance acting as filter to the DC
current Id to ensure that the latter is
perfectly smooth. Initially, the source                                             T4        T5     T6
impedance is considered to be zero.
The line currents I1, I2 and I3 assume
in turn the value (and the shape) of the
DC current Id.
Each thyristor ensures current                                                      Graëtz bridge                           batteries
conduction during 1/3 of a period.
Having assumed a source impedance
equal to zero, the current establishes
itself instantaneously at its value Id as     fig. 4: circuit diagram of charger rectifier.
soon as one thyristor starts conducting.

Cahier Technique Merlin Gerin n° 160 / p.6
Currents supplied by the source have a        The voltage v is such that:
rectangular waveform (see fig. 5).
                                                                d . i1            d . i2
The spectrum is made up of current            v = e1 + L .             = e2 + L .        ,
harmonics:                                                       dt                dt                  Id

        I1                                    hence
In =                                                                       d . i1 d . i2                                                     t
        n                                     2v = e1 + e 2 + L .                +       ,
where n = 6 k ± 1, k taking values 1, 2,                                    dt     dt                  i1
3... (whole numbers/integers) and I1
                                                    d . i1 d . i2            d( i1 + i2 )                                                    t
being the effective value of                  L.          +             = L.
fundamental, i.e. I1 = 0.78 Id.                      dt     dt                    dt
For the first harmonics of current, the                                      d . Id
                                                                        = L.          = 0,
amplitudes vary therefore in function of                                      dt                       i2
I1 :                                                                                                                                         t
s I5   = 20 % of I1,
                                              2v = e1 + e 2 ,
s I7   = 14 % of I1,
                                              or                                                       i3
s I11   = 9 % of I1,
                                                      e1 + e 2                                                                               t
s I13   = 8 % of I1.                          v =              .
The global rate of distortion of this
current is thus 30 %.                         The same phenomenon occurs later
                                              between T2 and T3, then between T3
The global rate of distortion of the          and T1 and also in the negative polarity
voltage is zero in this case, since the                                                           fig. 5: theoretical currents upstream of
                                              of the rectifier between thyristors T4, T5          rectifier with infinite dowstream filter
source impedance has been assumed
                                              and T6.                                             impedance and source impedance = 0.
to be zero (i.e infinite power).

influence of source
impedance                                     a)                                                  b)
Since the source is by nature inductive,               i1           L          T1
its inductance precludes any                   e1                                                 Id
instantaneous variations of current.
The phenomenon of overlap                                                                    Id
When thyristor T2 (see fig. 6a) is gated                                                                                                         t
while thyristor T1 is conducting, current
I2 establishes itself in thyristor T2 while                                                  v              i2
                                                       i2           L          T2
current I1 in Thyristor T1 decreases.          e2
Inductances L oppose sudden sharp
variations of these currents.
                                              e1 , e2: source voltage, phase-neutral
During the time ∆t of commutation                                                                                                                t
                                              v: voltage at + terminal of rectifiers
(see fig. 6a) there is simultaneous
                                              with respect to neutral                                                ∆t
conduction in two thyristors (this            L: line impedance representing
phenomenon is also called «overlap»).         source impedance                                    ∆ t : commutation time
The source is therefore in a state of
interphase short-circuit (phases 1
and 2) limited only by the two                fig. 6: overlap phenomenon.
inductances L.

                                                                                                  Cahier Technique Merlin Gerin n° 160 / p.7
For an angle of lag α of 30° (which
corresponds to a normal working point),
voltages e'1, e'2 and e'3 obtained at the                                                          ω∆t
input to the rectifier and also the line          e'1 , e' , e'           e'1           α              e'2                        e'3
                                                         2    3
current i1 are shown in fig. 7.
The angle of lag α is used for the
regulation of the DC voltage supplied
by the rectifier.
In the case of a rectifier/battery
charger, this output voltage must be
kept constant (see fig. 8) whatever the
variations of the AC voltage or                                                                                                                 ωt
whatever the charging conditions of the
The value of this DC voltage can be
expressed by the approximate relation:
 Ud = 1. 35 . Ueff . cos α − . L . ω . I1
where Ueff: effective value of
«composed resultant voltages»
                    3                        Id
(Ueff = e1 .
if one refers to fig. 6a).                                                                                                                      ωt

Disturbances due to overlap
It is evident that during each half-
period, each of the simple fundamental
voltages is disturbed twice and exhibits:
s a voltage drop when the                    fig. 7: overlap for a thyristor rectifier with an angle of lag α of 30°.
corresponding thyristor is triggered into
s an overvoltage when the current in                              lead storage cells           open load                 nickel-cadmium
this thyristor is turned off.                                     "recombination"              cells                     cells

The higher the inductance of line L, the     charging voltage
longer is therefore the duration ∆t of       (high rate)          2.30 < Ucharge <2.50         2.30 < Ucharge < 2.50     1.42 < Ucharge <1.65
these disturbances.                          (in V)
As the line current no longer has a          floating voltage
perfectly rectangular shape, its             (low rate)           2.23 < Ufloating < 2.30      2.18 < Ufloating < 2.25   1.38 < Ufloating < 1.50
harmonic content decreases (strong           (in V)
attenuation of harmonics of high orders).
                                             Sealed batteries (recombination) are generally charged at low rate charge only.
Consequently, the resulting distortion of    Open batteries are charged in two successive voltage steps.
the voltage increases when the line
impedance increases, but this increase       fig. 8: charge of UPS batteries at constant voltage and limited current (according to Gimelec
is not proportional to the impedance         recommendations).
since the harmonic content of current
In addition, the time of commutation         respect to the source impedance for                    The source impedance is represented
decreases when the angle of lag α            different angles of lag α.                             here by the term dXN which corres-
increases which, as a result, brings         The harmonic currents are represented                  ponds to the relative voltage drop on
about - for the same inductance value -      in relative value with respect to their                the DC side. The latter is due to the
an increase in the harmonic content of                                                              effect of total inductance of line.
                                             maximum theoretical value (IniN):
current and the voltage distortion.                                                                 For this rectifier:
                                             InN = Ieff of harmonic of order n
Harmonic content of current                                                                                  1 L .ω . I1
                                                       I1                                            d xN =    .         . 100
The figure 9 extracted from the              IniN =                                                          2     V1
standard specification IEC 146-4,                     n                                             where V1: is the effective value of
shows how harmonic currents vary with        I1 = effective value of fundamental                    simple fundamental voltages.

Cahier Technique Merlin Gerin n° 160 / p.8
      InN                                                                            InN
           . 100                                                                          . 100
      IniN                                                                           IniN
100                                                                            100
                                                             α = 90°
                                                                                                                                         α = 90°
 90                                                               30°          90
 80                                                               20°          80                                                             30°

                                                                  10°                                                                         30°
 70                                                                            70
                                                        0°                                                                                    20°
 60                                                                            60
                                                                                                                    5°              5°
                                                                                                               0°          0°                 10°

100                                                          α = 90°           40
 90                                                                            30
 80                                                                            20
                                                                        d XN                                                                                   d XN

  0       1     2        3    4         5     6     7         8     9            0      1        2         3        4           5         6     7      8   9

a) harmonics of order:            n = 5;          n=7                          b) harmonics of order:                    n = 11;              n = 13

      InN                                                                            InN
           . 100                                                                          . 100
      IniN                                                                           IniN
100                                                                            100

                                     α = 90°
90                                                                             90

80                                                                             80
                                            90°                                                                                          α = 90°
70                                                                             70
60                                                                             60
50                  5°                                                         50                               30°
40                       0°                                                    40                           20°
                         5°                 20°
30                                                                             30                    5°
20                                                                             20
                                   0°                                                                 5°
                                            10°                                                            10°
10                                                                             10
                                                                        d XN                                                                                   d XN

  0       1     2        3    4         5     6     7         8     9            0      1        2         3        4           5         6     7      8   9

c) harmonics of order:            n = 17;         n = 19                       d) harmonics of order:                    n = 23;              n = 25

fig. 9: amplitude variation of harmonic currents with respect to source impedance for various angles of lag α in a three-phase Graetz bridge

                                                                                                           Cahier Technique Merlin Gerin n° 160 / p.9
In a balanced three-phase circuit, dXN               Power factor of rectifier                                   As a first approximation, since the
represents half the relative voltage drop            s as the current drawn by the rectifier is                  overlap angle is small compared with
of the line.                                         highly distorted, the RMS current has                       the angle of lag, a phase shift equal to
By calling U’cc this relative voltage drop           therefore a value superior to that of the                   α can be retained, hence:
that can be likened to a short-circuit               fundamental. The effective value of
                                                                                                                 cos ϕ1 = cos α .
voltage, it is possible to write:                    current can be calculated by applying
                                                     the basic formula:
d XN =      . U' cc                                                               ∞
          2                                          Ieff =          I   2
                                                                             + ∑ (I     2
                                                                                                +I   2
                                                                                                             )   direct current (downstream)
                                                                         1              6k +1        6k −1
                                                                                 k =1
current distortion rate                              with a theoretical value of current
Assuming a source impedance equal to                 (source being of infinite power) equal to:
zero and a perfectly filtered DC current,                                  I1                                                                             t
                                                     I   6k ±1   =
the effective value of each current                                      6k ±1
harmonic can be expressed as:
                                                     hence                                                       line current (upstream)
In =                                                                       1
                                                                                  1
     n                                               Ieff =          1+        +      + ......                       i1
                                                                           5    7
In this instance, the harmonic content is
independent of α (∆t = 0).                           i.e.
The global rate of theoretical distortion            Ieff = 1.05 . I1
is given by the expression:                          s in addition, the phase shift between
                                                     the current and the voltage has a
                                                     minimum value equal to α, to which
                      ∑ (I   2
                             6k +1   +I   2
                                          6 k − 1)
                  k =1                               must be added approximately half the                        fig. 10: factual currents upstream and
D % = 100 .                                          overlap angle ω ∆t.                                         downstream of rectifier.

that is = 30 %.
Note: in practice, for calculation                                   voltage distortion
purposes, the line current does not                                  rate D (%)
strictly assume the theoretical shape                                                                                          α = 50°
taken as a basis for the calculations,                                                                                             40°
since perfect smoothing of DC current                                     30
cannot be achieved (see fig. 10).                                                                                                  30°
As a result, the harmonic content of                                                                                               20°
current is slightly modified; in particular                                                                                        10°
it is observed that harmonics of order                                                                                              0°
6 k - 1 are increased whereas those of
order 6 k + 1 are decreased.
Voltage distortion rate
The figure 11 shows the variation of                                      10
the voltage distortion rate at the
rectifier input with respect to the total
source impedance referred to the
short-circuit voltage U’cc and the angle
of lag α set for thyristor control. It is                                    0             5             10         15           20         U'cc
                                                                                                         (5)                    (10)        (d XN)
clearly seen that this distortion rate
increases very rapidly and that it is                fig. 11: variation of voltage distortion rate with respect to source impedance for different values
difficult as originally anticipated to               of lag angle α .
remain below a value of 5 %.

Cahier Technique Merlin Gerin n° 160 / p.10
s allowing that the effective value of       since:                                                               1
voltage approaches very closely that of                                                         that is: λ =         . cos α
the fundamental (which is true when                   P         3 . U1 . I1 . cos ϕ1
the distortion rate is low), the power       λ =        =
                                                      S           3 . Ueff . Ieff               Note: for more details refer to standard
factor λ can be expressed with good                                                             IEC 146-4 § 424.
approximation as:                            (U1 and Ueff. representing line to line
λ = 0.95 . cos α                             voltages)

3. minimisation of harmonic disturbances

Curves in figure 11 clearly show that        It is, of course, possible to combine              Theoretical calculation of distortion
the voltage distortion rate at the           these methods so as to optimize the                rate
rectifier input grows rapidly in impor-      results.                                           For each harmonic of order n, there is a
tance even when the source                                                                      voltage component V’n at the point B,
impedance is very low. It is therefore                                                          such that:
necessary to reduce this rate of             insertion of inductance at
                                                                                                V' n = n . (Ls + LF ) . ω . In
distortion so as to allow the use of         rectifier input
rectifiers of non-negligible power com-                                                         where ω: pulsation of fundamental.
                                             The circuit diagram corresponding to
pared with the contracted for power.                                                            Voltage Vn measured at point A is:
                                             one phase is shown in figure 12.
Since harmonic currents are
responsible for the voltage distortion
                                             The insertion of inductance LF reduces             Vn = n . Ls . ω .In
                                             the distortion rate of current. The                               Ls .
when they flow across the source
                                             voltage distortion rate at point A                 Vn = V' n .
impedance, reduction in their amplitude                                                                     Ls + LF
will bring about an improvement to the       decreases.
voltage waveform.                            Its value can be calculated from the               By applying this reasoning for each
                                             value obtaining at point B.                        harmonic and calculating the total
To achieve this, three classic methods       Inductances Ls and LF form a divider               distortion, it becomes evident that, if the
are utilized:                                for harmonic voltages.                             voltage distortion rate measured at
s the insertion of an additional
inductance in the rectifier input in order
to attenuate the amplitude of
harmonics (especially those of higher                                       Ls         A        LF         B
orders),                                                    e
s the use of several rectifiers fed by
voltages appropriately phase shifted.
It is possible, with this method to
eliminate - by combining currents - the
most troublesome harmonics (that is                                                    D                   D'
harmonics of the lowest orders for they
have the highest amplitudes).                      LF: filtering inductance of rectifier
s the retention of a single Graetz                 Ls: total inductance of source (generator + cabling)
bridge rectifier to which is added a               e: source of perfect voltage
passive filter designed to eliminate the           D, D': voltage distortion rates
most troublesome harmonics and to
reduce the amplitude of other                fig. 12: harmonic separation (decoupling) through use of additional inductance.

                                                                                                Cahier Technique Merlin Gerin n° 160 / p.11
point B is D’, the voltage distortion rate    Taking into account, the load rates of             use of double bridge
at A is:                                      transformer and of the rectifier:
              Ls                                                                                 (see fig. 13)
D = D' .                                      Uccs becomes: 4 % x
                                                                             = 1.7 %
            Ls + LF                                                      630                     This principle consists in utilizing a
                                                                                                 transformer with two secondary
Remembering that:                                                                                windings which supply voltages with a
                                              UccL becomes: 12 % x            = 9.1%             phase displacement of 30° between
       Ls . ω . In                                                        350
Uccs =                                                                                           them; each of those secondaries
           Vn                                                                                    supplies a Graetz bridge rectifier which
       LF . ω . In                            If, on average, the thyristors operate
                                                                                                 provides a six-phase rectification.
UccF =                                        with a lagging phase angle α of 20°, it
           Vn                                                                                    The rectifiers must supply identical DC
                                              is then possible to determine the
where Vn = effective value of the                                                                currents to ensure that the AC currents
                                              distortion rate from figure 11:
simple fundamental voltage.                                                                      drawn from the transformer secon-
As an example, if Ls is such that             D = 18. 8 %                                        daries have the same value.
Uccs = 2 % and LF such that                   (α = 20°; Ucc = 10. 8 %)                           Under those conditions, there occurs a
UccF = 6 %, then their sum                                                                       recombining process between the
                                              hence a distortion rate across the                 harmonic currents generated by each
Uccs + UccF = 8 %.                            transformer terminals amounting to:                one of the rectifiers in the primary
For an angle of lag α equal to 30°, the                                                          winding of transformer and calculations
curve in figure 11 gives a distortion rate    D = 18. 8 % x
                                                                 1.7 %
                                                                       = 2.9 %                   show that harmonics of order 6 k ± 1
of 19 %. The distortion rate at point A is                      10.9 %                           (k being an odd number) are eliminated.
D = 19 % x       = 4.75 %.
Without the inductance LF in the circuit,                                                                                      I .R 1
                                                                                                                        + R1
the distortion rate would have been
that referred to Ls alone, that is
Uccs = 2 %, value which gives on the                                                            i11
curve in figure 11
D = 10 %.
The insertion of inductance LF has
made it possible, in this case, to reduce
the voltage distortion rate by a factor
greater than 2 compared with the rate                 J1
                                                                                                                        - R1
level in other utilizations.
Application                                                                                                             + R2
                                                                                                                               I .R 2
An UPS unit, rated at 300 kVA, supplies               J3
a load of 250 kVA with a cos ϕ = 0.8; its
efficiency is 0.92 and the power factor                                                         i12
of its rectifier is λ = 0.82.
The apparent power drawn by the
rectifier is therefore:                                                                         i32

 250 x 0. 8
              = 265 kVA .
0. 92 x 0. 82
                                                                                                                        - R2
The rectifier is fed from a transformer
rated at 630 kVA; Uccs = 4 % and is
related to an inductance corresponding
to a UccF value of 12 % and calculated        fig. 13: basic diagram of a rectifier with two phase staggered bridges.
for a rectifier power rating of 350 kVA.

Cahier Technique Merlin Gerin n° 160 / p.12
                                                                                                     This is, in particular the case of 5 th and
                                                                                                     7 th harmonics whose theoretical
                                I 11                                                                 amplitudes are the most important.
                                           30°                                                       11 th and 13 th harmonics are retained
                                                                                                     but the 17 th and 19 th harmonics are
                                                                                                     The remaining harmonics are therefore
                                                                                                     of order 12 k ± 1 (k being a whole
                                I 21
                                                                                                     The figure 14 indicates the current
secondary                                                                                            drawn by the transformer primary and
                                                                                                     resulting from currents supplied by the
                                I 31
                                                                                                     two secondaries.
                                                                                                     The line current has a shape which is
                                                                                                     much closer to a sinusoidal waveform
                                                                                                     than that of the current obtained with a
                                                                                                     single rectifier. The two rectifiers can be
                                                                                                     connected in series or in parallel
                                                                                                     (see fig. 15).
                                I 12                                                                 When the two circuits are put in parallel
                                                                                                     and considering that the instantaneous
                                                                                                 t   voltages delivered by each one of the
                                                                                                     two rectifiers are not equal (since they
                                                                                                     are displaced from each other by 30°), it
                                I 22                                                                 is necessary to add an inductance with
                                                                                                     a centre tap in order to maintain a
                                                                                                 t   continuous flow in each rectifier.

                                I 32
                                                                                                     a)                          L1
                                                                                                                    + R1                  +R
                                                                                                                    - R1

                                                                                                                    + R2

                                                                                                                    - R2                  -R

                       I12 − I 22                                                                t
              I' 1 =                                                                                 b)
                            3                                                                                       + R1

                                                                                                                    - R1               L2

                                                                                                                    + R2

                                                                                                 t                  - R2                         -R
primary       J1 = I11 + I' 1
                                                                                                     L1 , L2 : inductances of DC current filtering
                                                                                                     λ: separation (decoupling) inductance with
                                                                                                     centre tap point

fig. 14: shape of currents drawn by rectifier and resultant in primary of transformer with two       fig. 15: connection in series (a) or in parallel
secondaries.                                                                                         (b) of two rectifiers.

                                                                                                     Cahier Technique Merlin Gerin n° 160 / p.13
In the absence of this inductance,                   The theoretical rate of distortion is                          obtained with a two-rectifier circuit to
conduction would be ensured at each                  thus:                                                          that obtained with a single rectifier is:
instant only by that rectifier that delivers
the highest voltage.                                               ∞                                                 1
                                                                                                                       ≈ 0.7.
There are several variants (patented by                           ∑ (I           2
                                                                                       ) + (I
                                                                                 12 k+ 1
                                                                                                12 k−1
                                                                                                        )            2
Merlin Gerin) of the circuit diagram                              k =1
                                                     D% =                                                   . 100   For a higher source impedance, the
shown in figure 13 (see fig. 16) which                                                I1
lead to the same result as regards level                                                                            gain is more substantial since higher
of harmonics.                                        that is D ≈ 15 % which represents half                         order harmonics decrease rapidly as
                                                     the value obtained with a single rectifier                     the source impedance increases.
Distortion rate of current
                                                     (see start of paragraph 2).                                    However, the gain remains rather
Assuming zero impedance upstream of
rectifier and a perfectly smoothed DC                Distortion rate of voltage                                     «modest» and in practice a ratio of 0.5
current, the effective value of each                 The voltage distortion rate is dependent                       in favour of the double bridge circuit is
current harmonic is of the following                 on the source impedance.                                       to be retained.
               I1                                    For a very low source impedance, (sum                          Example:
form: In =
               n                                     of impedances upstream of rectifier(s)),                       s for an angle of lag α of 30°, the ratio
where n = 12 k ± 1                                   the ratio between the distortion rates                         between the two distortion rates

                                                                             +                                                                                  +



a) circuit connection with power transformer                                               b) circuit connection with autotransformer

                    A          a                                         A                 a                                             A        a

                                                                                            α                                                      α


                                                         c                                                  B               c                                   B
     C                              B

                                                              C                                     b                             C                       b
simple star                                             double star                                                         polygonal
c) various connection circuits for autotransformer

fig. 16: circuit connections to obtain a phase shift of 30° and various connection methods for autotransformer.

Cahier Technique Merlin Gerin n° 160 / p.14
amounts to 0.66 with U’cc = 8 %
and 0.55 with U'cc = 16 %;
s for angle α = 0, the ratios are 0.53                                                                                R1
and 0.37 respectively.
This ratio between the distortion rates                                                                                           -
takes no account of the inductance of
the phase shifting system.
                                                                              α1                                      R2
rectifier circuit with more                                                             α2
than two bridges
(see fig. 17)
The basic idea here is to increase the
number of transformer secondaries with                                                                                R3
respective phase displacements                                                                α3
depending on the number of secondaries                                                                                            -
retained for the purpose of eliminating
other harmonics of current.
Three rectifier circuit arrangement
For this form of arrangement, the phase
displacement must be such that:
s α 1 = 0°,                                                                   αn
s α 2 = 20°,
s α 3 = 40°.
In this case, the only harmonics remaining
are those of order 6 k ± 1 (where k =                                                                                             -
multiple of 3) that is 18 k ± 1.
The first harmonics of current are           fig. 17: example using n rectifiers.
therefore 17 th and 19 th followed by
35 th and 37 th harmonics.
Four rectifier circuit arrangement                                       α1=0
In this case, the phase displacement
are as follows:
s α 1 = 0°,
s α 2 = 15°,                                                             α2
s α 3 = 30°,
s α 1 = 45°.
The only harmonics remaining are
then those of order 24 k ± 1.
The first harmonics are therefore the
23 rd and 25 th following by 47 th and
49 th.
These arrangements are of interest in
so far as they make it possible to
obtain relatively low distortion rates of                                αn
current and voltage.
They have the disadvantage of being
complex and expensive.
Consequently, their utilization is
reserved for equipment of high power
                                             fig. 18: principle of phase shifting.
For instance, aluminium electrolysis
process, which utilizes DC current           Special case of circuit connection              between them and the currents drawn
supplied from power sources of               called «phase shifting»                         by each rectifier have identical
several MW, requires circuit                 (see fig. 18)                                   amplitudes.
arrangements consisting of up to             When several UPS units are operated             It is then possible to supply the
72 phases!                                   in parallel, they share the load current        rectifiers from auto-transformers which

                                                                                             Cahier Technique Merlin Gerin n° 160 / p.15
produce the required phase shifts
according to the number or rectifiers         connection              number of rectifiers harmonics
(instead of utilizing circuit arrangements    type                    in service
with transformers).                                                                        H5   H7   H11                    H13      H17 H19      H23 H25
The auto-transformers utilized can be         2 rectifiers            2                      0          0        1          1        0     0      1            1
of the same type as those shown in                                    1                      1          1        1          1        1     1      1            1
figure 16.
                                              3 rectifiers            3                      0          0        0          0        1     1      0            0
The polygonal circuit arrangement is
mostly utilized for economic reasons.                                 2                      1/2        1/2      1/2        1/2      1     1      1/2          1/2
The principal disadvantage of this            4 rectifiers            4                      0          0        0          0        0     0      1            1
system is due to the fact that harmonic
                                                                      3                      1/3        1/3      1/3        1/3      1/3   1/3    1            1
rates increase when one of the UPS
units is shut down.                           fig. 19: variation of harmonic content of current in principal connection systems.
Table in figure 19 gives the harmonic
content in principal circuit connections
in which all the rectifiers - except one -           IH          Ls                    L'F                    I' H              LF
are operational.

                                                                                                              I" H
utilization of a passive filter
for harmonics
The filter is tuned to a particular
Its effectiveness is highest at this
frequency, but several filters are                                                               Cp
needed to strongly attenuate several
The introduction of passive filters is
always critical because of risk of            fig. 20: basic circuit of passive filter for harmonics.
(Refer on this subject to Cahier              filter and this harmonic no longer                                     I Hn         Zsn                     I' H n
Technique No 152: «Les perturbations          affects other users.
harmoniques dans les réseaux                  s as regards 7 th harmonic, because of                                                                  I" H n
industrielles, et leur traitement»).          its proximity to the tuned frequency, the
Filter utilized by Merlin Gerin for           parallel impedance is still low and
                                              consequently a large proportion of this                                                      Zpn
UPS units of high power rating
The figure 20 shows the equivalent            harmonic is also eliminated.
basic circuit for one phase.                  s finally, as regards harmonics of
                                              higher orders, the parallel impedance of
The parallel arm of the filter consists of
                                              the filter is very close to that of its                       fig. 21: equivalent circuit diagram of filter for
a circuit tuned to the 5 th harmonic
                                              inductance Lp: the filter thus functions                      harmonics.
which is the most important. The series
                                              as a current divider.
arm of the filter comprises an induc-
tance whose function is to achieve            For harmonics of higher orders:
                                                                                                            the gain following the insertion of induc-
separation of the parallel arm from the                          Lp                                         tance L’F alone, is at least 3 whatever
source.                                       IHn = I' Hn .                                                 the value of the source impedance.
                                                            Lp + Ls + L' F
Calling Zpn and Zsn the impedances of                                                                       The figures 22 and 23 illustrate the
parallel and series arms of the filter        if Lp is chosen so that                                       shape of line currents with and without
tuned to harmonic of order n and                                                                            the presence of a filter, as well as the
                                              Lp ≈ Ls + LF then                                             spectra of these currents for a rectifier
assuming that the current generated by
the rectifier for this order is I’Hn, then                1                                                 comprising an input inductance and a
                                              IHn =         . I' Hn                                         filter inductance such that:
the current supplied by the source is:                    2
                                                                                                             L' F = LF with UccF = 10 %
                  Zpn                         Global distortion rate of voltage
IHn = I' Hn .                                                                                               The rectifier is supplied from a source
                Zpn + Z sn                    Detailed calculations of the voltage
                                                                                                            such that Uccs = 2 %.
                                              distortion rate obtained at the source
(see fig. 21).                                                                                              For a current harmonic of order n, the
                                              output, are beyond the terms of
                                                                                                            voltage VHn developed across the
s as regards 5 th harmonic, the parallel      reference for this technical booklet.
                                                                                                            source impedance is:
impedance is equal to zero.                   Let’s however consider an example:
All the current of 5 th harmonic flows        s if L’F = LF with UccF = 12 % and                                                               I Hn
                                                                                                            VHn % = Uccs % . n .
thus through the parallel arm of the          s if Lp corresponds to Uccp = 15 %,                                                                I1

Cahier Technique Merlin Gerin n° 160 / p.16
             ∞                                 I1                                                                                 current spectrum
D% =         ∑ VHn2                                                                                                               order      IHn / I1
           n = 2                                                                                                                  H5         33 %
hence                                                                                                                             H7         2.7 %
                                                                                                                                  H11        7.3 %
                          ∞               2                                                                               t       H13        1.6 %
                                 IHn 
D % = Uccs % . n .        ∑      I1 
                                                                                                                                  H17        2.6 %
                        n = 2                                                                                                     H19        1.1 %
                                                                                                                                  H23        1.5 %
By taking the values listed in figure 22,
                                                                                                                                  H25        1.3 %
the distortion rates at the source output
are 4 % without filter, and 1 % with filter                                                                                       global rate
respectively.                                                                                                                     of distortion
                                                                                                                                  of current ≈ 35 %
For comparison purposes, it is to be
noted that in the case of a two-bridge        fig. 22: line current of rectifier without harmonic filter.
rectifier having the same input
inductance, harmonics of orders 5, 7,          I1                                                                                 current spectrum
17 and 19 are eliminated, which results                                                                                           order      IHn / I1
in a distortion rate at the source output                                                                                         H5         2.1 %
equal to 1.9 %.                                                                                                                   H7         1.4 %
The harmonic filter is in this case,                                                                                              H11        3.6 %
practically twice as effective compared                                                                                       t   H13        0.7 %
with the use of a two-bridge circuit                                                                                              H17        1%
arrangement.                                                                                                                      H19        0.7 %
Furthermore, this is a less costly                                                                                                H23        0.6 %
solution which can be resorted to after                                                                                           H25        0.5 %
the equipment has been put in service.                                                                                            global rate
                                                                                                                                  of distortion
Additional characteristic of
                                                                                                                                  of current < 5 %
harmonic filter
The presence of the parallel arm of the       fig. 23: line current of rectifier with harmonic filter.
filter tuned to 5 th harmonic causes the
appearance of a capacitive current at
fundamental frequency. This capacitive
current improves the power factor cos ϕ
of the rectifier.

                                                                                                         Cahier Technique Merlin Gerin n° 160 / p.17
4. conclusions and prospects

Thyristor rectifiers of classic types         possible to bring these disturbances      intended to deal with a particular
utilized in UPS equipments are sources        down to an acceptable level.              polluting load or with the whole of the
of harmonic disturbances and                  These solutions are nowadays perfectly    installation; this principle can be
adversely affect the power factor of the      «mastered» and widely applied.            compared to the one adopted for
installation.                                 The figure 24 gives a synthesis of        effecting «acoustic de-pollution»
These pollutions are acceptable as long       advantages and disadvantages for          (i.e. emission of «sounds» in phase
as the power rating of an UPS                 various solutions.                        opposition to the sounds to be
equipment is low compared with the                                                      neutralised).
                                              In the not too distant future, the
short-circuit power rating of the             multiplication of polluting equipments,   By utilizing a different regulation
network.                                      the changes in standards and the          strategy, the same converters can also
When the voltage distortion rate              requirements of energy distributors       achieve self compensation of the power
exceeds acceptable values (in the             should lead to the use of «clean»         factor cos ϕ of the installation.
order of a few %), corrective measures        rectifiers (this has already been         In order to make these devices, which
must then be taken.                           achieved in single phase equipment        are technically feasible, available to
The simplest solution and the most            thanks to the technique of sinusoidal     industry, it is necessary to ensure that
common consists in inserting a series         sampling).                                their production costs are acceptable
inductance which achieves harmonic            Furthermore, a converter utilising the    compared with those of classic
decoupling.                                   technique of PWM (pulse width             solutions.
When this measure is found to be              modulation) can, by making use of         The principles of such converters and
insufficient, the use of phase staggered      appropriately adapted regulation          of their possibilities will be developed in
rectifiers or passive filters makes it        control, behave as an active filter       a future Cahier Technique.

Cahier Technique Merlin Gerin n° 160 / p.18
circuit type                            diagram    observations

a) no reducing interface                           s acceptable if power required is low
                                                   compared with short-circuit power
                                                   of network

b) series inductance                               s   simple, reliable

                                                   s   can be used in most cases

                                                   s inductance can be added after equipment
                                                   has been put in service

                                                   s   economic

c) double bridge and transformer                   s complicated (requires balancing of voltages,
with two secondaries                               of Icc's, of currents in rectifiers)

                                                   s   to be considered at design start

                                                   s   expensive

d) double bridge with                              sthe solution for parallel connection of
autotransformer                                    UPS units in active redundancy

                                                   s compared with circuit C
                                                   s same effectiveness and drawbacks
                                                   s smaller losses
                                                   s more economic

e) passive filter                                  s   simple, reliable

                                                   s   best persorming

                                                   s can be inserted after equipment has been put
                                                   in service

                                                   s   more economic than solution with two rectifiers

fig. 24: comparison of anti-harmonics solutions.

                                                          Cahier Technique Merlin Gerin n° 160 / p.19
5. bibliography

Standard specifications                       Merlin Gerin Cahier Technique           Other publications
s IEC 146-4: semi-conductor                                                           s Directiveof 25 July 1985 under
converters part 4: Method for specifying      s Les  perturbations harmoniques dans   reference 83/374/CEE.
performances of and test procedures           les réseaux industrielles, et leur
                                                                                      s Electra     No 77 - July 1991.
with UPS.                                     traitement : Cahier Technique No 152
                                              by P. ROCCIA and N. QUILLON.            s Electra     No 123 - March 1989.
s IEC 552-2: Disturbances caused in
supply networks by electro-domestic           s Inverters and Harmonics (case
equipment and similar equipment.              studies of non-linear loads): Cahier
                                              Technique No 159 by J.N. FIORINA.
s IEC 1000-2-2: Electromagnetic
compatibility (CEM) part 2:
Section 2: Levels of compatibility.

                                                                                      Réal. : Illustration Technique Lyon -
Cahier Technique Merlin Gerin n° 160 / p.20                                           DTE - 12/93 - 2500 - Imp. : Léostic Seyssinet-Pariset