current transformer for HV Protection (no 164) by hamada1331


									                                         n° 164
                                         for HV protection

Michel Orlhac

Graduated from the Ecole Centrale
de Paris in 1977.
After one year's specialisation at the
university of Stuttgart (Germany), he
entered the overseas projects
department of Stein Heurtey (iron
and steel engineering). In 1980 he
joined Merlin Gerin, becoming part
of the technical section of the High
Voltage Prefabricated Switchgear
Department (P.S.H.T.) where he
completed a study on current
transformers. This Cahier Technique
publishes the results of this study.
At present he is the marketing
manager for France-Transfo, a
subsidiary of the Merlin Gerin

E/CT 164 first issued, march 1995
Cahier Technique Merlin Gerin n° 164 / p.2
current transformers                                                             Current transformers or CTs take up a
                                                                                 lot of space in HV cubicles. Thorough
for HV protection                                                                knowledge of how they work makes it
                                                                                 possible to:
                                                                                 c reduce their dimensions and thus
                                                                                 their cost,
                                                                                 c use standard CTs in a larger number
                                                                                 of configurations.
content                                                                          The purpose of this study is to learn
                                                                                 more about CT operation in association
                                                                                 with protection relays and to lay down a
                                                                                 few rules for sizing them properly.
1. Theoretical review                                                    p. 4    After a brief theoretical review of CT
                                                                                 operation and current protection
                                         Hysteresis - Saturation         p. 5
                                                                                 devices, the behaviour of the CT-
                                         Characterisation of CTs         p. 6    protection relay combination is studied
2. General current protection            Current transformers            p. 7    in two particularly important cases
information                              Functional CTs                  p. 8    in HV:
                                         Protection relays               p. 8    c overcurrent relay supplied by a
                                                                                 heavily saturated CT,
                                         Technological evolution         p. 8
                                                                                 c protection relay connected to two
3. Response of a CT in saturated state   Experiments - Wiring            p. 9    CTs in parallel in duplex cubicles.
                                         Testing with symmetrical        p. 10   This study is completed by
                                         constant currents                       experimental results.
                                         Testing with asymmetrical       p. 11
                                         Conclusions on CTs delivering   p. 12
                                         on an overcurrent relay
4. Parallel cubicle operation                                          p. 14
5. General conclusions                                                 p. 16
Appendix: CT standards                   NF C 42-502 (Norme Française) p. 17
                                         IEC 185                       p. 19

                                                                                  Cahier Technique Merlin Gerin n° 164 / p.3
1. theoretical review

Current transformers consist of a                                                di 2
magnetic circuit in toroid form. The         e 2 = v 2 + R 2 i2 + l 2
                                                                                  dt                                                              i1
primary is made up of n1 turns or simply                dϕ                  dϕ
a single conductor crossing the toroid        e 1 = n1      and e 2 = − n 2
                                                         dt                 dt
(n1 = 1). The secondary is wound in n2       If all the functions described are                                  →
regular turns around this toroid (see                                                                        dI n
                                             sinusoidal of pulsation ω, the following
fig. 1 and 2).                               can be written vectorially:
Ampere's theorem states that the sum         →         →                     →

of the ampere-turns is equal to the          V1 = E1 + (R1 + jl1 ω ) I 1
                                             →         →            →
circulation of the magnetic field vector.    E2 = V2 + R2 I 2
                     →       →               →                 → →                →
n1 i1 + n2 i2 = ∫ H n dI                     E1 = jn1 ω Φ E2 = − jn2 ω Φ
                    Toroid                   →
H = magnetic field                           I1        →       →
→                                                 + I2 = Ie                                                                                      i2
n = tangent unit vector                       n
A transformer is said to be perfect          The wiring diagram in figure 3 and
when                                         equations (1) result in the vectorial                          fig. 1.
    →    →
                                             representation in figure 4.
∫ Toroid n dI = 0
   H                                         The exciting current I e is broken down
                                                          →       →
                                             on the axes Φ and E into:
In the real transformer, this term refers    →         →       →
to the error introduced by the magnetic      I e = Ia + Im                                                                                             i1
circuit and defines the exciting current     c where I a represents the part of this
ie formed at the secondary by:               current lost in the magnetic circuit (iron
n1 i1 + n 2 i 2 = n 2 i e                    losses due to hysteresis and eddy
        n2                                   currents).
If n =      is the winding ratio, the                      →
        n1                                   c and I m is the magnetising current
relationship is written as:                  which transfers power from one winding
 i1                                          to the other by creation of a                                                                  i2
    + i2 = ie                                magnetomotive force which induces the
 n                                                →
                                             flux Φ .                                                       fig. 2.
The transformer can then be
represented (see fig. 3) as having two
parallel elements:
c a perfect transformer of ratio n
delivering a current i1/n at the
secondary,                                                              l1                                                          l2
                                                  i1           R1                                   i1/ n                  R2               i2
c an impedance which consumes a
current ie.
Moreover, each winding, both primary                                                                                ie
and secondary, creates a slight voltage                                            n = n 2/ n 1
drop due to the resistance of the
winding (R1 and R2) and to the leakage
                                                                        e1          n2     n1      e2
inductances ( l 1 and l 2 ). Since, in the    V1                                                                                            V2              Z
case of the CT, the secondary winding                                                                       im             ia
is tight and regular, l 2 need not be                                        perfect transformer
If ϕ is the flux common to both
windings, the following can be written
between the emf e1, e2 and the
                                                                                                                         real transformer
difference in potential v1, v2:
                                 di1         fig. 3: CT schematic diagram.
v 1 = e 1 + R 1 i1 + l 1

Cahier Technique Merlin Gerin n° 164 / p.4
hysteresis - saturation
Magnetic circuit quality is defined by                       l 1 ω I1                 I1                                «order of creation»
the relationship it imposes between the                                                                                 of values:
induction vector B and the magnetic                              R1 I1                                                      I1, V1    I1, Ie
field vector H.
At a given moment and in a fixed point,              V1      E1
these two vectors are linked by the
                                                                  I 1/ n
relative permeability of the magnetic
material µr such that:                                                                                                       E1
→           →
B = µ o µr H                                                                                       →
A magnetic circuit is thus characterised                                                           →
                                                                                  Im               Φ
by the curve b = f (h) known as the
magnetising curve.                                                                         α    Ia                           Φ
According to the different material                                                                →
types, the curves in figure 5 are                                                                  H
obtained, the results of sinusoidal                           ϕ2
excitation (primary current).                                                                                                E2
In sinusoidal state, b represents voltage
since:                                                       E2

→    Φ→
B=     n
     S                                                                                                                       V2         I2
→            →
E2 = n2 jω Φ                                R2 I2
                                                          l 2 ω I2
→    →                                      N.B.: the real proportions, between the representative vectors of primary and secondary
V ≈ E2                                      values, are not repected.
h represents the exciting current since     fig. 4: vectorial representation of a CT.
            →       →
n2 Ie = ∫ H n dI

assuming that                                  hypotheses:                 magnetising curves                    ie and B as a function of time
                                                                                     b                                          B, ie
H n = H = constant
n 2 Ie = L H                                   perfect
                                                                                               h                                                  t
Perfect transformer
Permeability of the medium is assumed
→                   →    →      I1             linear
H = 0 hence I e = 0 and I 2 =                  transformer
                              n                                                                h                                                  t
This hypothesis approaches the real
situation with CTs since they normally
«work» far below saturation. I2 is then
                                               saturable trans-
the mirror image of I1.                        former without
Linear transformer                             hysteresis                                      h                                                  t
Permeability of the medium is constant
B = Cste x H hence ie and i2 are
sinusoidal functions.                          saturable
Saturable transformer without                  transformer
                                               with hysteresis                                 h                                                  t
Saturation is the sudden variation of µr
from a high value to a low value at the                                                                        exciting current: ie
                                                                                                               induction B
point known as the «saturation bend».
Induction b then increases only slowly      fig. 5: magnetising curves and their incidence on ie.
and ie deforms to form a peak.

                                                                                                       Cahier Technique Merlin Gerin n° 164 / p.5
Saturable transformer with                   c for measurement CTs                             Bear in mind that the less the CT is
hysteresis                                   The module error                                  loaded (the more it is below its accuracy
The magnetising curve is undoubled,                I / n − I2                                  level power Y), the greater its accuracy.
thus indicating the resistance of the        εM = 1                                            Its real accuracy level is therefore
                                                     I1 / n
magnetic circuit to the induction                                                              greater than its rated accuracy level Fp.
                                             The phase error
variations. Curve ie then exhibits a                                                           This point is developed in chapter 3.
characteristic «swing».                      ε ϕ = (I1, I2 ) 10        rd                      Admissible short term current
The magnetising curve of a CT can            An accuracy class X is given (generally           Expressed in kA it is the maximum
easily be observed using an                  0.5 or 1) which expresses limit values            current admissible Ith for one second
oscilloscope. A sinusoidal voltage V2 (t)    of the module error εM and of the phase           (the secondary being short-circuited). It
is applied to the secondary (the primary     shift error εϕ as a function of the load          represents CT thermal overcurrent
is not charged). The current ie (t)          ratio N:                                          withstand.
absorbed then represents the exciting                                                          (standard values are given in the
current and is proportional to the            N = 1 (N var ies from 0.1 to 1. 2)               standards in the appendix).
                                                                                               For times other than 1 second, the heat
magnetic field vector H.                     for N = 1 εM = X (in class 0.5 for I1 = I1n,
                                                                                               conservation law I2 t = cste can be
Integration of voltage V2 represents the     εM = 0.5 %)                                       applied:
flux ϕ2 which is proportional to the         (for value details refer to the standards
                               →                                                               for t < 1 sec. the calculation gives I > Ith,
magnetic induction vector B (see             in the appendix).
                                                                                               thus increasing electrodynamic forces.
fig. 6a).                                    c for protection CTs                              However, the limit guaranteed value is
Integration of a sinusoidal value causes     The composite error εc                            Idyn = 2.5 Ith.
a rotation of π/2 (90°). It is thus                                                   2
                                                      1      1         T      i1 
sufficient on an oscilloscope:               εc =                        i2 − n         dt
c to sweep with ie,                                 I1 / n   T                   
c to apply voltage V2 to the vertical        Protection CTs are characterised by
amplifier.                                                                                                                                V2(t)
                                             3 symbols: Y, P, Fp:
The magnetising curve of the material        Y = error rate (5 or 10),
is thus obtained (see fig. 6b).              P = protection,
                                             Fp = accuracy limit factor which gives                                                       ie(t)
characterisation of CTs                      the limit values of errors εM, εϕ and εc
                                             as a function of the load ratio N.
CTs are characterised in practice by
the following values (according to           For N = Fp
standards NF C 42-502 and IEC 185).
                                             εc = Y
                                             (in class 10P5 for I1 = 5 l1n: εc = 10 %)         a - scales: ie = 0.25 A per square
CT voltage
                                             (for value details, refer to the standards                   V2 = 50 V per square.
This is the operating voltage applied to
                                             in the appendix).
the CT primary. Note that the primary is
at the HV potential level and that one of    For a CT working at a rated induction                                ϕ 2 = ∫ V2 dt (or B)
                                             Bn, a saturation coefficient Ks such that:
the terminals of the secondary (which
must never be opened) is normally                   Bs
earthed.                                     Ks =
Just as for all equipment, a maximum
                                             where Bs is the saturation induction
1 min withstand voltage at standard
                                             characterising the core material.
frequency and a maximum impulse
                                             In practice K s ≈ Fp and they are often                                                 ie (or H)
withstand voltage are defined (refer to
                                             treated as the same in calculations.
the standards in the appendix).
e.g. for a rated voltage of 24 kV, the CT    Accuracy level power
must withstand 50 kV for 1 mn at 50 Hz       Expressed in VA, it indicates the power
and 125 kV impulse voltage.                  that the secondary can supply while
                                             respecting the rated accuracy class Y,
Rated winding ratio                          P, Fp.
Normally takes the form: l1/l2.                                                                                      2 I a = constant
                                             It represents the total consumption of
I2 is very generally 5 A or 1 A (for rated   the secondary circuit (except for CT),            b - scales: ie = 0.25 A per square
values of I1, refer to the standards in      i.e. the power consumed by all the                          ϕ2 = 0.077 V.s per square.
the appendix).                               connected devices as well as the
                                                                                               fig. 6 : oscillographic reading of curves i(t)
Rated accuracy class                         connecting wires.                                 V2(t) and h(b) of a CT, 50/5, 15 VA, 10P20
This depends on whether the CT is used       (for rated values, refer to the standards         where: V2 = 83 V and le = 0.26 A.
for measurement or protection:               in the appendix).

Cahier Technique Merlin Gerin n° 164 / p.6
2. general current protection information

Protection devices have many functions
since they have to:
c protect equipment from destruction or
damage as a result of faults (short-
circuit, overload...),
c ensure normal operation of the
installation and its equipment (control,
load shedding...),
c guarantee safety of personnel.

current transformers                          CT with cross primary            Wound type CT with                Wound type CT with
                                              winding (cable)                  wound primary                     wound primary
Since relays cannot be connected              1 secondary - 600/1              winding                           winding
directly onto the MV network, the                                              1 secondary - 200/5               2 secondaries - 200/5 and 100/5
information they receive comes from
                                              fig.7: different types of CTs.
current transformers or CTs (see fig. 7)
and from voltage transformers or VTs.
When primary current is high, the CTs
are of the cross bar type, and when it is     from part of the network to be
low they are of the wound primary type.       monitored (a motor, a transformer, a
CTs have a number of roles to play in         busbar...) to quickly detect and isolate                                                        relay
electrical networks:                          any faults inside that part.                                             I2 = I1/m
c supplying at their secondary a current      Zero sequence protection
exactly mirroring the one flowing in the      This monitors the zero sequence                               I1
HV conductor concerned,                       component Io of the three-phase
c providing galvanic insulation between       current which appears during phase-                fig. 8.
the HV and the measuring and                  earth faults. There are two possible
protection circuits,                          configurations:
c protecting the measuring and                c a toroid transformer encircling the                  I1                                                I'1
protection circuits from damage when a        three phase conductors (if possible).
fault occurs on the HV network.               This configuration (see fig. 10a)                                                    I'1 - I1
Using this current image in the HV            enables detection of small zero
conductor, the relay generates in turn a      sequence currents (1 to 100 A).
tripping order according to the type of       c three CTs achieving in the neutral
protection it provides and the values at      connection of their secondary the sum
                                                                                                 fig. 9.
which it has been preset [threshold(s),       of the three phase currents. This
time delay(s)....].                           configuration (see fig. 10b) is the only
This order is transmitted to one or more      one possible for large and numerous
                                              cables or busbar ducts. It is not                  a)
breaking devices (circuit-breaker,
contactor, switch).                           recommended when the zero sequence
CT configurations vary according to the       current to be detected is 5% less than
type of protection to be provided.            ln (or even 12% for consumer
                                              substations according to standard
Overcurrent protection (see fig. 8)           NF C 13-100 (French Standard)).                                                  Io
This directly uses the «current»
information supplied at the CT secondary
to detect short-circuit or overload           functional CTs                                     b)
currents or calculate the thermal status of   In HV cubicles, the «current
a machine. Note that this configuration       transformer» function takes on a new
type must also contain the protection         dimension as a result of its content and
devices using in addition to VTs:             shape.
c directional overcurrent protection,         Thus:
c power protection (active or reactive).      c a number of CTs can be moulded in                                                                Io
Earth leakage protection (see fig. 9)         the same enclosure: one core for the                                                            relay
This measures the current difference          measurement function, one core for the
between two CTs, one connected                protection function and sometimes even             fig. 10.
downstream and the other upstream             a third core for earth leakage protection,

                                                                                                      Cahier Technique Merlin Gerin n° 164 / p.7
c the enclosure is used to ensure            c replace relays (automation) in the
insulation between two compartments          cubicle,
and plugging-in of the breaking device:      c provide operators with measurement
the CT is then said to be «functional».      of electrical parameters.
An application example is given in the       These units, with their increased
metalclad cubicles for withdrawable          vocation, are:
switchgear (see fig. 11 and 12).             c flexible (protections are chosen
Overall dimensions are thus reduced by       simply by programming),
using one insulating enclosure (the most     c parameterisable (large choice of
appropriate), thus also reducing costs.      settings),
                                             c reliable (they are fitted with self-
                                             monitoring or with watchdog and
the protection relays                        self-test),
The equipment currently available is         c economic (reduced wiring and
based on the three technologies:             implementation time).
electromechanical, analog and digital.
                                             Their digital communication and
The oldest of these is                       powerful algorithms also enable
electromechanical technology: relays         additional functions such as logic
are simple and specialised (current,         discrimination to be performed.              fig. 11: functional CT for HV metalclad
voltage, frequency, ... monitoring) but      This communication capacity means            cubicles (Merlin Gerin).
their accuracy is poor as their settings     that genuine network operation (similar
may be altered over time.                    to technical management of industrial
The last two technologies benefit from       installations) is now possible.
the advantages provided by electronics       Finally, their ability to acquire and
(see fig. 13):                               process the information provided by
c compact dimensions of the device,          sensors allows them to make full use of
c low power required for acquisition of      the performances of the new non-
«current» information (a few fractions of    magnetic sensors.
c response time not dependent on the
current received by the relay,               technological evolution
c reliability increased by lack of           In this current sensor field, sensors with
mechanical parts (no dirt accumulation       wide measuring bands are being
or corrosion, not affected by impacts),      increasingly used instead of current
c low cost since they use mass produced      transformers (1 or 5 A). These sensors
non-specific electronic components.          based on Rogowski's principle (non-
Finally, in the nineteen eighties, digital   magnetic sensors) are currently on the
technology made it possible, thanks to       market and provide distributors with
microprocessor processing power, to          optimised solutions (fewer alternative
produce information processing units         versions and simplified choice) which        fig. 12: installation example of functional
able to:                                     are far more efficient (improved             CTs in a Fluair 200 12 kV HV metalclad
c globally provide the various               response curve linearity) than               cubicle (Merlin Gerin).
protections,                                 traditional transformers.

                                                                                          fig. 13: Vigirack static relays (Merlin Gerin).

Cahier Technique Merlin Gerin n° 164 / p.8
3. response of a CT in saturated state

The emergence of static relays leads to
revision of protection behaviour as a
whole in the case of strong currents: as
the CT saturates beyond a certain
threshold, the first reaction is often to                                                                       i2
avoid this by raising the threshold.
However, this results in both additional                                                resistance
costs (more efficient, larger, more
                                              tested CT                                                              R
space consuming CT) and in the risk of
excessive temperature rise of the
relays.                                                                                                                                measuring
On the contrary, saturation plays a
useful role for the «measurement»
function since primary current image
accuracy is only useful up to the value
of the rated current I1n. Beyond this         standard CT
point, the measurement ceases to be of                                                                i1
any use and saturation must occur for a                                                               u1
low current (2 to 3 I1n) in order to limit
the secondary current and protect the        fig. 14: diagram for checking proper relay operation.
measuring instruments.
It is thus necessary to know the
response of the CT in saturated state to           (A)
ensure the protection device works
properly when the primary current                 500                                                                              1
exceeds rated current strength,
particularly for the high values which
appear if a short-circuit occurs.
In theory, induction in the core reaches
a plateau at the saturation bend, thus                                                                                             2
limiting current strength at the                                                                                                   3
secondary. In actual fact the
experiment performed will show that
current strength at the secondary
slightly increases and that protection
relay operation is quite satisfactory.
experiments - wiring
A current i1 is injected in the CT
primary, and the current supplied by the
secondary in a load Z containing a
relay R and a resistance is analysed
(see fig. 14).
                                                                              10                       100                   500   N = I1/I1n
The currents at the secondary l2 are
given, according to the current supplied     Fig. 15: I2 = f(N) for 1 CT only (15 VA 10P5 100/5).
at the primary (represented by the           Load Z at the secondary:
                 I                           1. relay only,
parameter N = 1 ) for various loads Z        2. Z = rated Z of CT, i.e. 0.6 Ω and cos ϕ = 1,
                I1n                          3. Z = rated Z of CT, i.e. 0.6 Ω and cos ϕ = 0.8.
and various CTs (see fig. 15).

                                                                                                     Cahier Technique Merlin Gerin n° 164 / p.9
testing with symmetrical                        Nevertheless, the rms current I2                  v both relays trip from their threshold ß
                                                continues to increase as is shown in              right up to Nmax.
constant currents
                                                line 2 in figure 15.                              Testing at reduced load
Testing at resistive rated load
                                                As I2 increases, the power supplied at            The secondary load only comprises the
The test was carried out using a CT with
                                                the secondary P2 = Z l2 and the power
                                                                       2                          relay and the connecting wires.
low performance: 10P5, 50/5 with a
                                                delivered at each relay Pr = R l2 also
                                                                                2                 Compared with the rated load of 15 VA,
rated load Z of 15 VA (at 5 A) made up
                                                increase. This accounts for the                   this represents a load of roughly 9%.
of an overcurrent relay and a resistance.
                                                tripping of both relay types as from
Two relays were used:                                                                             c results
                                                threshold ß to which they were set right
v a Vigirack static relay,                                                                        Curves i2 (t) (see fig. 16c) and i2 (N)
                                                up to Nmax.
v an electromechanical relay.                                                                     (see line 1 in figure 15) show that the
                                                Testing at rated partly inductive load            saturation bend is far higher than at
As both these relays have a low
                                                This test resembles the previous one.             rated load.
internal resistance, a resistance was
                                                However, a choke is placed in the                 This bend follows the law:
added to reach roughly 0.6 Ω, i.e.
                                                                                                  K s (P2 + R 2 I2 ) = constant
                                                secondary circuit to represent the case                           2
15 VA at 5 A (connecting wiring
                                                of an electromechanical relay
included). Because the inductance of                                                              with P2 = Z l 2 is the total power
                                                connected by itself to the secondary                            2
the electromechanical relay was low                                                               supplied at the secondary (consumed
                                                which would consume the rated power
(15 µH, i.e. cos ϕ = 0.95 for the relay                                                           by the relay and the connecting wires).
                                                of the CT. In practice, these relays
only), the load can be considered to be                                                           R2 = internal resistance of the CT
                                                never fall below cos = 0.8.
purely resistive in both cases.                                                                   secondary winding,
                                                In this test, the current I1 explored the
The test consisted in making current l1                                                           Ks = saturation coefficient (real or rated).
                                                range I1n = 50 A to I1 max = 16,400 A,
vary in the range I1n = 50 A                                                                      Thus, in practice, when a CT delivers into
at I1max = 54 kA                                                              N max
                                                i.e. N max = 328 and η =            = 65.6        a load less than its rated accuracy level
                                                                               Fp                 power (in VA), saturation occurs at a far
I.e. N max =          = 1,080 and               c results                                         higher overcurrent level than the rated
                                                Current i2 (t) assumes the curve given            saturation coefficient Ks.
     N max 1,080
η=        =      = 216                          in figure 16b. The presence of a choke            This phenomenon must be taken into
      Fp     5                                  spreads out the peak, hence the lower                                         .
                                                                                                  consideration and calculated for each
(the latter value indicates the level of        value I2 (see line 3 in figure 15).               application since it may generate
saturation to which the CT was                  With respect to testing at pure resistive         overcurrents in the secondary which
subjected).                                     load:                                             are incompatible with the thermal and
c results                                       v I2 is multiplied by a factor of 0.65,           dynamic withstands of the relays
The current i2 (t) collected at the             v the total power supplied at the                 connected to the CT secondary (for
secondary takes the form of a peak              secondary is multipled by a factor                calculation, refer to the conclusions
above: N = 10 (see fig. 16a).                   of 0.4,                                           given below).

                                           v2                                                                            v2

fig. 16 a - CT 15 VA 10P5 50/5                  fig. 16 b - CT 15 VA 10P5 50/5                    fig. 16 c - CT 15 VA 10P5 50/5
testing at purely resistive rated load          testing at rated load with cos ϕ = 0.8            testing at reduced rated load
I1 = 16,400 A                                   I1 = 16,400 A                                     relays + connecting wires - I1 = 14,200 A
scale: i2 = 100 A/square; v2 = 100 V/square.    scale: i2 = 25 A/square; v2 = 50 V/square.        scale: i2 = 100 A/square; v2 = 5 V/square.

Cahier Technique Merlin Gerin n° 164 / p.10
testing with asymmetrical
The test was performed using an
asymmetrical current, i.e. the sum of a            scales:
symmetrical sinusoidal current and a               v1
DC component with the following
characteristics:                                   i1 500 A/mm

     ≈ 2.3
                                                   i2 10 A/mm
These values are slightly less than
those in standard NF C 64-100 for
                                                                           no saturation
      = 205 = 1.8 2                                b)
i.e. 20% of asymmetry at 70 ms.
The secondary load is identical to that
of the main test at resistive rated load           v1
comprising an electromechanical or
static relay.
                                                   i1 1,000 A/mm
c results
Both relays correctly respond in a
few ms and in the same manner as in
symmetrical testing throughout the                 i2 10 A/mm
range explored (up to Î1 = 140 kA peak
with Irms = 54 kA).
c the first peak seen at the secondary
by the relays is enough to make them
trip, if its energy is sufficient: this is the
case for Irms greater than 2 kA but
below this value (see fig. 17a) the third
peak is required;                                  scales:
c the CT does not saturate during the
first negative peak of the primary
current for Î1 = 4 k ;
c the response delivered by the CT on
the first negative peak of the primary
(or even secondary) is normally shorter
than the responses in steady state
(which is reached as from the sixth

c the above points show that for higher
peak factors (case of off-load                     i1 2,000 A/mm
energising of transformers with an
                                                   i2 20 A/mm
      = 3.7 ), there is a risk of the
response at the secondary disappearing
during the first peaks. If, in addition, the
time constant of the primary current DC
component is high (t = 80 ms in the
case quoted), this disappearance                 fig. 17: CT secondary responses on an asymmetrical primary current for:
continues until the primary current                       a) lrms. ≈ 1.4 kA,
crosses the 0 axis. This phenomenon is                    b) lrms. ≈ for 14 kA and Î1 ≈ 32 kA,
shown on the curves in figure 17                          c) lrms. ≈ 54 kA and Î1 ≈ 140 kA.
(tripping time moves to 68 ms).

                                                                                                   Cahier Technique Merlin Gerin n° 164 / p.11
conclusions on CTs                                   current value, even if the CT is strongly
delivering on an
                                                     Thus, the CT saturation coefficient Ks
overcurrent relay                                    must be calculated not according to the
The above tests show that for both                   short-circuit current lcc but according to
electromagnetic and static relays,                   the maximum setting threshold of the
tripping is obtained whatever the                    associated relay (see fig. 18 and 19).

                                                                                         choice of Ks                     Icc
                                                       I1n     ITIn            I1r = β I1n              Ith = Icc θ               ∞

                                                               I2n                                        I2max θ        I2 max
                                                                      Ir min                 Ir max                   Ithr

             of the         I1n           = rated current
             network        I1r           = β I1n setting current
                            Icc           = short-circuit current
                            θ             = maximum short-circuit time

             of the relay   Ir min to Ir max = setting range
                            Ithr             = admissible short term current (1s)

             of the CT      ITIn          = rated primary rating
                            I2n           = rated secondary current
                            I2 max        = CT response to Icc

fig. 18: characteristics to be considered for defining a CT.

Cahier Technique Merlin Gerin n° 164 / p.12
1. the saturation threshold Ks
must correspond to the maximum
setting value of the relay.
                                                  2. the CT must thermally withstand
3. this CT must                                   the current Icc for a time θ at least
electrodynamically                                equal to the breaking time of the
withstand the peak                                short-circuit by the circuit-breaker.
value 2.5 Icc.

4. the secondary circuit must
thermally withstand the maximum
secondary rms current I2 max
created by Icc at a primary for the
time θ.                                                5. the relay setting range
                                                       (Ir mini, Ir maxi) must be large
                                                       enough to cover the CT
                                                       response at the setting
                                                       current of network B I1n.

fig. 19: general rules for sizing a CT.

                                          Cahier Technique Merlin Gerin n° 164 / p.13
4. parallel cubicle operation

Power supplies with double busbars                  encountered in this system is highly                      provide insulation between
are frequently used in HV network                   complex lockings:                                         compartments and to plug in the
configurations.                                     c cubicles connected in duplex (see                       breaking device. This arrangement
                                                    fig. 20). Using standard elements, this                   makes it necessary to connect the
There are currently two solutions for                                                                         relays (which are not backed up) on
most cubicles:                                      solution can advantageously replace
                                                                                                              each CT secondary. This has resulted
c the double busbar cubicle: the circuit-           the double busbar, as it is more
                                                                                                              in the study below concerning
breaker may be connected to either                  reliable.
                                                                                                              operation of two identical CTs
busbar without discontinuity of service.            As on the new cubicle generations, the                    connected in parallel on the same
One of the drawbacks often                          CTs are standard elements used to                         load.

                               1455                                    1300                                         1455



fig. 20: connection of 2 cubicles in duplex relay

Wiring diagram                                                                                      I1
Connection of two cubicles in duplex,               standard CT
as shown in figure 18, results in the
diagram in figure 21 for protection.
                                                                                                     resistance                 relay
One of the CTs (said to be «live») is               tested «live» CT                                                        R
supplied at the primary by the
                                                                                 shunt         i2        ir       shunt          Ir
HV network; its secondary supplies a
current i2 broken down into a current im
on the secondary of the other CT (said                                                                                                  recorder
to be «dead») and a current ir on the                                                                               Im
rated load of 15 VA made up of an
electromagnetic or static relay and a
pure resistance.                                                                    I2              tested «dead» CT
The tests were performed on two
identical CTs of the same series (15 VA             fig. 21: wiring diagram for study of a parallel-connected CT.
50/5 10P5 as in the above paragraphs).

Cahier Technique Merlin Gerin n° 164 / p.14
These are given in the curves of                                                                                                         scales:
figures 22 (currents as a function of
time) and figure 23 (root mean square                                                                                                    im 9.4 A/mm
currents and tripping times). The
following observations are made:
c both relays quickly respond from their                                                                                                 ir 9.55 A/mm
tripping threshold ß to η = 72,
c the static relay trips in a constant time
T ≈ 20 ms, whereas the                                                                                                                   i2 19 A/mm
electromagnetic relay reacts as a
function of I2 (T ≈ 80 ms at the tripping
                                                                                                                                         i1 955 A/mm
threshold to T ≈ a few ms at η = 72);
c the secondary current I2 continues to
increase but two separate zones
                                                                                                                                         Vigirack relay
v before η = 10, Ir ≈ I2 and lm << lr
the secondary current flows entirely into
the relays since the «dead» CT acts as        fig. 22: currents at the secondary of 2 parallel-connected CTs. I1 = 12,500 A.
an infinite impedance,
v after η = 10 Im ¡ I2, which means
that the secondary current mostly                I(A)
flows into the «dead» CT but,                    T(ms)
                                                                                                              I2 (total secondary current)
however, Ir continues to grow, thus                                                                           Im (current flowing in the «dead» CT)
causing the relays to trip (do not forget          100
that ir is dephased by       with respect                                                                     Ir (current flowing in the relay)
to im);
c the current lr flowing in the relays                                                                        T: static relay tripping
during testing with two CTs is lower
than for testing with one CT (at                    10
N = 300, roughly - 40%).                                                                                      T: electromagnetic relay tripping

The connection of two CTs in parallel
presents no problem:
c for low currents: hardly any current               1
                                                         1                10                  100            N = I1/I1n
flows into the «dead» CT,
c for high currents: sufficient current       fig. 23: rms currents and tripping times of the relays connected to 2 CTs in parallel (10P5 50/5 15 VA).
flows into the relay to trip it.

                                                                                                      Cahier Technique Merlin Gerin n° 164 / p.15
5. general conclusions

The conclusions in chapters 4 and 5           Nevertheless, CT saturation, as shown
show that:                                    in this experiment, should not be
c the relays operate correctly in both        considered a handicap:
cases studied:                                c when a CT «supplies» one or more
v high CT saturation,                         measuring instruments, saturation, by
v parallel-connection of two CTs;
                                              limiting rms current at the secondary,
c static relays give the most reliable
                                              protects the devices which, moreover,
response (constant operating time for
                                              do not generally need to be very
all currents greater than the setting
threshold).                                   accurate above l1n.
Moreover, static relays generally have        c when a CT «supplies» a protection
a very small acquisition time, thus           device, operation is ensured even if
meaning operation is more reliable            saturation occurs. The idea of sizing
when the CT is strongly saturated and         a CT according to the highest current it
supplies a very short current impulse.        may have to withstand at the primary
Do not forget, however, that the              must therefore be rejected. Moreover,
transient phenomena considered were           this oversizing is risky for the relay and
limited to the asymmetrical current less      cabling which could be seriously
than:                                         damaged.
  Î = 2.5

Cahier Technique Merlin Gerin n° 164 / p.16
appendix: CT standards

NF C 42-502 (French Standard)                                                                            Normal rated current values
Rated insulation levels                                                                                  c at the primary (in A): 10 - 12.5 - 15 -
The insulation levels recommended by the standard are given in table II A presented                      20 - 25 - 30 - 40 - 50 - 60 - 75
in figure 24.                                                                                            and their decimal multiples or
                                                                                                         Preferential values are given in bold.
highest voltage for                  withstand voltage                                                   c at the secondary (in A): 1 - 5
equipment (kV)                       1 minute at standard frequency        to impulse voltage            Accuracy class
                                     (rms value) (kV)                      (peak value) (kV)
                                                                                                         c measurement CTs
0.6                                  3
                                                                                                         The normal accuracy classes are:
1.2                                  6
                                                                                                         0.1 - 0.2 - 0.5 - 1 - 3 - 5.
2.4                                  11
                                                                                                         The rated frequency operating range is
3.6                                  16                                    45
                                                                                                         96% to 102% of rated frequency.
7.2                                  22                                    60
                                                                                                         For transformers of accuracy classes
12                                   28                                    75
                                                                                                         0.1 - 0.2 - 0.5 and 1, the current error
17.5                                 38                                    95                            and phase shift in the rated frequency
23                                   45                                    95                            range must not exceed the values in
24                                   50                                    125                           table III (see fig. 25) when the
36                                   70                                    170                           secondary load is between 25% and
52                                   95                                    250                           100% of accuracy load.
72.5                                 140                                   325                           For transformers of accuracy classes 3
                                                                                                         and 5, the current error in the rated
fig. 24: insulation levels (table II A).                                                                 frequency range must not exceed the
                                                                                                         values in table IV (see fig. 26) when the
                                                                                                         secondary load is between 50% and
accuracy         current error                   phase shift, ±
class            (ratio error)                   for current values
                                                                                                         100% of accuracy load.
                 as a percentage, ±,             given as a percentage                                   In all cases, the load used must be
                 for current values              of rated current                                        inductive with a power factor of 0.8,
                 given as a percentage                                                                   unless the corrresponding power is less
                 of rated current                minutes                    centiradians
                                                                                                         than 5 VA, in which case its power factor
         % I1n 10        20      100 120         10    20        100 120    10     20      100    120    is the unit. On no account must the load
0.1              0.25 0.20       0.1       0.1   10    8         5    5     0.30   0.24    0.15   0.15   be less than 1 VA.
0.2              0.5     0.35    0.2       0.2   20    15        10   10    0.60   0.45    0.3    0.3
0.5              1.0     0.75    0.5       0.5   60    45        30   30    1.8    1.35    0.9    0.9
1                2.0     1.5     1.0       1.0   120   90        60   60    3.6    2.7     1.8    1.8

Note: after agreement between manufacturer and user, guarantees can be provided for
accuracy and phase shift, between 120% and 200% of In n.
fig. 25: error limits (table III).

accuracy         current error
class            (ratio error) as a percentage, ±, for current values given
                 as a percentage of rated current
         % I1n 50                                          120
3                3                                         3
5                5                                         5

There is no phase shift limit for classes 3 and 5.
fig. 26: error limits (table IV).

                                                                                                         Cahier Technique Merlin Gerin n° 164 / p.17
c Protection CTs
The normal accuracy limit factor values
are: 5 - 10 - 15 - 20 - 30 - 40.
The rated frequency operating range is        accuracy              ratio error                  phase shift                composite error
90% to 110% of rated frequency.               class                 for currents                 for rated current          for accuracy
                                                                    between In                                              limit current
The normal accuracy classes are 5P                                  and 2 In                                                (as a %)
and 10P.                                                            (as a %)                     minutes     centiradians
For accuracy level power and in the           5P                    ±1                           ± 60        ± 1.8          5
rated frequency range, the current            10P                   ±3                                                      10
error, phase shift and composite error
must not exceed the values in table V
(see fig. 27).
                                              fig. 27: error limits (table V) .
To determine the current error and
phase shift, the load must be inductive
and equal to the accuracy load with a         c the admissible times for the
power factor of 0.8, unless the               admissible short term current are set
corresponding power is less than              from the cold state. However, at the
10 VA; in this case the load could be         user's request, the manufacturer is
resistive (unit power factor). To             obliged to indicate, for a given type of
determine the composite error, the load       device, the admissible short term
power factor may be between 0.8               current based on a state corresponding
(inductive circuit) and the unit, the value   to operation, the heating current and
being set by mutual agreement                 maximum ambient temperature.
between manufacturer and user.
                                              However, in the latter case, verification
Accuracy level power                          of admissible short term current cannot
The normal accuracy level power               be made mandatory as an acceptance
values are: 2.5 - 5.0 - 10 - 15 - 30 - 75 -   test.
100 VA.                                       Admissible current peak value (Idyn).
Admissible peak current and short             The admissible current peak value is
term current                                  2.5 Ith. However, another value can be
Admissible peak current and short term        accepted provided it is stated on the
current (Ith). The short term current (Ith)   identification plate.
must be specified for each transformer.
Their preferential values are given in
paragraph 10.1 (see fig. 28).
                                              highest           Ith
c for the highest network voltage less
                                              network           (kA)
than or equal to 36 kV, the admissible        voltage
short term current value is                   (kV)
constructively linked to rated current        3.6               10     16         25   40
value. It is thus frequently expressed as     7.2               8      12.5       16   25   40
a multiple of rated current, for which the    12                8      12.5       16   25   40
preferential values are: 40 - 80 - 100 -
                                              17.5              8      12.5       16   25   40
200 and 300.
                                              23                8      12.5       16   25   40
c if no admissible values as a function       24                8      12.5       16   25   40
of time are given, it is accepted that the
                                              36                8      12.5       16   25   40
transformer can withstand for a time t,
                                              72.5              20     25
expressed in seconds, a current with a
root mean square value given by the           100               20
formula:                                      245               20     31.5
                                              420               40
          I th
I' th =
                                              fig. 28: preferential values of Ith
where t2 > t1 bearing in mind that Ith is     (paragraph 10.1).
given for t1 (= 1s).

Cahier Technique Merlin Gerin n° 164 / p.18
IEC 185
This is the reference standard. The
NF C 42-502 (Norme Française) differs
only slightly from it. The differences are        highest                        rated lightning impulse                  rated short term
as follows:                                       voltage                        withstand voltage                        withstand voltage
                                                  for equipment Um               (peak value)                             at standard frequency
Rated insulation levels                           (rms value)                    network power                            (rms value)
The IEC standard gives two tables:                                               ≤ 500 kVA       > 500 kVA
c the same table as the NF C standard             kV                             kV               kV                      kV
for European countries,                           4.40                           60                 75                    19
c another table as per USA practice
with slightly more stringent values: refer        13.20                          95                 110                   34
to table II B (see fig. 29).
Normal rated current values
                                                  26.4                           150                                      50
Same preferential values at the primary.
At the secondary possibility of a I2n = 2 A.      36.5                           200                                      70

Accuracy class
c measurement CT
Current errors in module and phase are            fig. 29: rated insulation voltages set for the U.S.A (table II B).
the same in class 3 and 5. For classes
0.1 - 0.2 - 0.5 and 1, the errors are the
same, except for the 10% of I1n column
which is replaced by 5% of I1n with the
errors listed in table IV A in figure 30.         accuracy        error εM                          error εϕ
Moreover, the IEC standard defines two            class           for I1 = 5 % of I1n               for I1 = 5 % of I1n
additional classes, 0.2 S and 0.5 S for                                                             minutes                    centiradians
CTs with special applications                     0.1             0.4                               15                         0.45
(connection with special electrical energy        0.2             0.75                              30                         0.9
meters). In this table, the module and
phase errors are given for I2n = 5 A only.        0.5             1.5                               90                         2.7
c protection CT                                   1               3                                 180                        5.4
The IEC gives the same limit errors.
The only difference is that the accuracy          accuracy        error εM for                      error εϕ for values as a %
limit factor, Fp = 40, does not exist.            class           values as a % of                  of rated current I1n
Accuracy level power                                              rated current
The IEC only gives the same normal                                I1n                               minutes                    centiradians
values up to 30 VA. Beyond this point,                    % l1n 1        5     20       100 120 1         5    20 100 120 1           5   20      100 120
power can be chosen to meet needs.                0.2S            0.75 0.35 0.2         0.2   0.2   30 15      10 10      10   0.9 0.45 0.3       0.3   0.3
Peak current and short term current               0.5S            1.5    0.75 0.5       0.5   0.5   90 45      30 30      30   2.7 1.35 0.9       0.9   0.9
Unlike the NF C standard, the IEC
standard does not define preferential
values of Ith for each network voltage.
However, application of the law i2 t = Cste       fig. 30: accuracy class (table IV A).
to define the Ith is limited to: 0.5 < t < 5 s.

                                                                                                              Cahier Technique Merlin Gerin n° 164 / p.19
                                              Réal.: Sodipe - Valence - Photo.: IPV
                                              Edition: DTE - Grenoble
Cahier Technique Merlin Gerin n° 164 / p.20   03-95 - 2500 - Printing.: Clerc

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