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 =
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 =
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
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
> 25 0.2 + 0.1 +
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
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
generated by a Graetz e1 Zs e'1 i1
bridge rectifier i2
The rectifier in figure 4 is assumed to
be connected to a high value i3
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
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,
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.
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
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»
(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
. 100 . 100
α = 90°
α = 90°
90 30° 90
80 20° 80 30°
0° 0° 10°
100 α = 90° 40
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
. 100 . 100
α = 90°
90° α = 90°
50 5° 50 30°
40 0° 40 20°
30 30 5°
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
) direct current (downstream)
1 6k +1 6k −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
n Ieff = 1+ + + ...... i1
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
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
= 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
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
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
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
The figure 14 indicates the current
secondary drawn by the transformer primary and
resulting from currents supplied by the
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.
+ R1 +R
- R2 -R
I12 − I 22 t
I' 1 = b)
3 + R1
- R1 L2
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
There are several variants (patented by ∑ (I 2
) + (I
12 k+ 1
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
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.
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
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
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
utilization of a passive filter
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 %
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
Additional characteristic of
of current < 5 %
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
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
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
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
s more economic than solution with two rectifiers
fig. 24: comparison of anti-harmonics solutions.
Cahier Technique Merlin Gerin n° 160 / p.19
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