24 May 2016 |

c 0MPRCT-3

ONE
OPTIMISED
NETWORK
EQUIPMENT CT selection for capacitor protection

Introduction These parameters are normally combined in a com-

pact form of specification, for example a current
Medium and high voltage capacitor banks require pro- transformer is described as a 100:5 A, 30 VA 5P10,
tection against faults in the capacitor units and in the 25 kA/1s device when it is to be used in an application
associated equipment such as feeder cables, over- where the highest thermal current rating is 25 kA for 1
head structures and conductors connecting the vari- second, the primary current is 100 A, the secondary
ous parts of the capacitor bank. current is 5 A (the current transformer ratio therefore is
20), the accuracy class is 5P, the accuracy limit factor
Faults inside capacitor units are generally cleared by is 10 and the burden of the secondary circuit is 30 VA.
internal fuses. The operation of these fuses must be
monitored to allow for appropriate alarm and trip in- There is strong interaction between the various rat-
structions to be issued based on voltage stress levels ings that needs to be considered when selecting the
in the capacitor units. The same applies for unfused current transformer rating, as demonstrated in the
units with unbalance current providing an indication following examples:
of the number of shorted elements. Such protection The current transformer is required to remain within
depends on very sensitive detection of relatively small the accuracy class up to the accuracy limit factor, so
currents in the star points of capacitor banks. for example a 100:5 ratio current transformer with ALF
External faults are either phase to ground or phase of 20, burden of 15 VA and accuracy class 10P can
to phase and require more conventional over current produce at most a voltage at the secondary terminals
and earth fault detection. of:

Both of these types of protection require current trans- ALF × PS 20 × 15

formers to provide the necessary detection of current VS,Design ≈ = = 60 V
IS 5
in the primary circuit. Appropriate selection of protec-
tion current transformers is therefore essential for safe The relationship is approximate as the internal resis-
operation of capacitor banks. This selection must be tance of the CT secondary winding may need to be
done to assure correct operation of protection while considered. As another example, a 1 A secondary
minimising cost and avoiding any false operation of current transformer with burden 5 VA and ALF of 20
protection, i.e. ensuring continuity of operation wher- can produce a maximum voltage of
ever possible.
Note that this article discusses only over current pro- ALF × PS 20 × 5
VS,Design ≈ = = 100 V
tection (of which unbalance and earth fault are sub- IS 1
categories). Other protection functions that may be
applied to capacitor banks such as over- and under across the secondary terminals of the current trans-
voltage, or specific protection against harmonic over- former. If the primary current rating was IP = 500 A,
loading are not dealt with here. then the ratio error would be less than 10% at
500 × 20 = 10 kA.

1 Current transformer ratings It is important to remember, especially for unbalance

protection, that protection class specifications do not
make any statements regarding ratio accuracy at cur-
Current transformers are specified in terms of rents lower than nominal current. Ratio errors can be
AS 60044.1–2007, a close variation of IEC 61869- quite large at low current due to the relatively large
1(2007). Due consideration should be given to all the contribution of excitation current.
necessary aspects of the standard, including those of
insulation coordination and testing. The key features
to be specified from a functional perspective are: 2 Selecting primary current

1. Primary current, IP in amperes The nominal primary current IP is selected based on

2. Short time thermal current, Ith in amperes or kilo the expected maximum current that can reasonably be
amperes and duration t in seconds expected to flow through the primary terminals, given
changing network conditions and making provision for
3. For each secondary winding: short periods of overload.
a) Secondary current, IS in amperes For example, if a capacitor bank of 15 Mvar is con-
b) Accuracy limit factor, ALF nected to a 33 kV busbar, and no future upgrades are
c) Accuracy class foreseen for the capacitor bank, the primary current
rating is determined from
d) Secondary burden, PS in volt-amperes
4. Voltage withstand level: maximum continuous,
short time and impulse withstand level. This Maximum power rating
IP = × Safety factor
aspect of the device rating is determined as part Phase voltage
of a substation voltage coordination study and is 15 × 1000
not discussed further here. = √ × 1.5 = 393.4 A
33 3

Optimised Network Equipment Pty Ltd 41/ 2 Benson Street Toowong QLD 4066
ABN 56 151 739 374 PO Box 1951 Toowong QLD 4066
www.onegrid.com.au info@onegrid.com.au
The use of a safety factor is common in all compo- The bank is located in an air-insulated switchyard
nent selection, and in this case is deemed necessary some 80 m away from the control room. Calcula-
to make provision for short term overloads that are tions based on the assumption that the conductor will
allowed according to capacitor standards, and addi- be 4 mm2 , and some allowance for deviations in the
tional provision for harmonic distortion that can appear cable route results in secondary circuit resistance of
continuously in the capacitor bank. RS = 0.9 Ω.
The preferred range of standard primary currents de- Two sets of current transformers are required: three
fined in the standard is 10,15, 20, 30, 50 and 75 and identical single phase devices for each phase, on the
their decimal multiples, and one should pick a stan- line side of the capacitor bank, and a single device
dard value as close as possible to these. In this case, located in the star point of the capacitor bank. The
neither 500 A nor 300 A would be deemed close to the single line and three phase schematic indicate the
calculated primary current, and the standard primary locations of the current transformers.
current of 400 A should be selected. A B C

OC/EF CTs
3 Selection of rated burden 1 1

The output rating in VA of the current transformer = 2

must be selected so that it exceeds the product of 2

rated secondary current IS (either 1 A or 5 A) and the

impedance of the current circuit, including the current
UB CT
transformer, cabling and protective device. 3
3
Modern protection relays have very low internal
impedance and hence the burden is determined
largely by the cable between the current transformer 4.1 Line side CTs
and the protection relay. The resistance of this con-
ductor is The purpose of the line side CTs is to detect over
current and earth faults in the feeder between the
ρl location of the CT and the capacitor bank, including
RS = the capacitor bank, i.e. at any of the fault locations (1,
A
2 and 3) indicated in the schematic, including line-line
where ρ is the resistivity of the conductor (17.1 nΩ· m faults.
for copper), l is the circuit length in meters, and A is For a secondary current rating of 1 A, the burden
the area of the conductor in square meters. is therefore PS = IS2 × RS = 0.9 VA, and for a 5 A
Since the burden of the cable is determined by I 2 R secondary the burden is 22.5 VA.
the same cable will have 25 times the burden if used The nominal current of the bank is 131 A, and using
with a 5 A secondary current rating compared to a 1 A a safety factor of 1.5 this implies a primary nominal
rating. Therefore 1 A secondary ratings are normally current IP = 200 A. Given the secondary burden based
used when the current transformer is located in a on conductor resistance only, it appears that a CT with
switchyard. rated burden of 2.5 VA would be more than sufficient.
The actual circuit resistance is often not known at Note that secondary current of 5 A would require a
the time of selecting current transformers. Assuming rated burden of 30 VA.
that the CT will not be over-burdened (i.e. the rated Consider a typical CT described as 200:1, 2.5 VA,
burden will exceed the actual circuit and internal bur- 10P10. Such a CT can produce a maximum voltage
den) the total circuit resistance can be inferred from of
RS = PS /IS , for example a CT with burden 5 VA and
secondary current rating 5 A has total circuit resis- ALF × PS 10 × 2.5
tance of 1 Ω. VS,Design ≈ = = 25 V
IS 1
Standard current transformer burdens are 2.5, 5, 10,
15 and 30 VA. Current transformer costs generally across the CT secondary terminals.
increase with increasing burden.
A fault current of 31.5 kA implies that the CT will at-
tempt to produce 157.5 A through the CT secondary
4 Complete example circuit, and given the burden of 0.9 Ω, a voltage of
157.5 × 0.9 = 141.75 V across the terminals. This
clearly exceeds the capability of the current trans-
A 66 kV, 15 Mvar capacitor bank connected as an un-
former and hence any protection relay connected to
grounded double star requires line side and sensitive
the CT will not correctly detect a fault.
unbalance protection. Calculation has shown that the
bank should trip before the unbalance current reaches There are two rating decisions available to correct
0.8 A. The fault level at the busbar is 31.5 kA. the situation: the rated burden and the accuracy limit

2
factor. A conversation with CT suppliers will assist in 0.8 A in the primary circuit the primary current rating
selecting the optimum solution from a cost perspective: IP should not exceed 5 A. Should a ratio of 5:5 be
in this case it is possible to maintain the ALF of 10 and selected, then the maximum secondary current that
increase the burden to 15 VA. Then VS,Design becomes should ever flow before a trip instruction is given is
150 V and the CT will allow correct detection of the 0.8 A. To improve resolution the 1 A input of a protec-
fault. Alternatively, the ALF can be increased to 30 and tion relay is often selected despite being connected to
the burden increased to 5 VA for the same outcome. a 5 A CT secondary, since no more than 1 A should
ever occur in this circuit.

4.2 Unbalance CT
4.3 High voltage example
The purpose of the unbalance CTs is to detect very
small changes in the capacitance of the bank. As A typical EHV capacitor bank with a large number of
it is connected in the star point of the bank, it only units in series in each phase and the bank connected
detects the current that will flow as a result of unequal in a grounded H configuration is shown below. Line
capacitance in any of the six branches of the capacitor side and neutral side CTs may be used to provide
bank. over current and earth fault protection as well as some
form of differential protection.
If the capacitor bank star point is not earthed, then no
A B C
fault in the bank (three-phase, phase-phase, phase-
phase-ground, or phase to ground) will cause current
OC/EF CTs
to flow in the unbalance CT. If the star point is earthed, 1 1
then phase-ground or phase-phase-ground faults will
cause fault current to flow in the unbalance CT. In the
2 = 2
case of an ungrounded bank, if there is simultaneously
a fault to ground in the star point and a phase to
ground fault in the capacitor bank, then fault current 3 UB CTs
will flow though the unbalance CT.
As this CT is not intended to provide detection of over 3
current and earth faults, it is only necessary to ensure Neutral CTs

that the CT can withstand such fault currents, and not

to be able to detect such currents accurately. The
maximum fault current through the unbalance CT can
be determined from the fault level at the busbar and
taking into consideration the exact location and type The unbalance CTs are generally required to regis-
of fault, and the effect of the capacitor and reactor ter very low unbalance current, with trip levels in the
impedance. range of 200 mA. CTs with good accuracy in the lower
current ranges are therefore required. In the case of a
In this example, a phase-phase-ground fault in the typical 330 kV 160 Mvar capacitor bank with damping
connections between the damping reactors and ca- reactors of 11 mH and a network fault level of 50 kA,
pacitor bank (location 2 in the figure above) will result a typical line side CT specification will be 500:1 A,
in a fault current of approximately 12 kA. Since the 10P20 5 VA, for the same physical layout as earlier.
bank star point is not grounded, the unbalance current
It should be noted that fault rating and voltage with-
transformer will not detect any current.
stand should be related to the actual fault currents and
If the bank star point was grounded, the same fault voltage stresses that may occur at different locations
between damping reactor and capacitor current will in the bank in order to obtain an optimised solution.
result in a current of less than 200 A in the unbalance
protection CT. It is clear that this CT does not require
the same fault rating as the line side devices. 5 Concluding notes
The selection of the unbalance CT ratings is some-
The use of Intelligent Electronic Devices (IEDs) with
what different from line side over current protection:
integrated metering results in conflicting requirements
fault rating is determined by the maximum fault cur-
of the current transformer. For the protection applica-
rent that the CT could in fact experience, and may
tion, it should not saturate and be accurate for high
be different from the line side ratings. The burden is
current values. For the purpose of metering it should
selected to ensure that the the CT is capable of driv-
be accurate for currents less than the nominal cur-
ing the relatively small unbalance current through the
rent and saturate for high currents to protect metering
secondary circuit. To ensure reliable, accurate detec-
devices. IED burden is now much lower than before
tion of relatively low current the use of measurement
and A/D converters saturate at a certain current value.
class 1 or better CTs is recommended.
The classical arguments for selecting current trans-
The ratio selection is the most important aspect, and formers have been modified and the use of dual rated
should be selected as low as possible to ensure op- metering and protection class current transformers is
timum resolution at low current. For a trip current of now common.