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Introduction to Protection Transformers

Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit In the event of an abnormal or fault condition therein, yet permit other parts of the plant to continue in operation.

Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high overcurrents, requiring the protective current transformer to be linear, and therefore below magnetic saturation. at values up to perhaps 30 times full load current .This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties only. For space and economy reasons, equipment designers should however avoid over specifying protective current transformers ITL technical staff are always prepared to assist in specifying protective CT's but require some or all of the following information


(a) Protected equipment and type of protection.
(b) Maximum fault level for stability.
(c) Sensitivity required.
(d) Type of relay and likely setting.
(e) Pilot wire resistance, or length of run and pilot wire used.
(f) Primary conductor diameter or busbar dimensions.
(g) System voltage level.


IEC Specification

According to 60044-1 protective current transformers are specified as follows:

Rated Output:
The burden including relay and pilot wires
Standard burdens are 2.5,5,75,10, 15 and 30VA

Accuracy Class:
Accuracy classes are defined as 5P or 10P with limits according to the following table extracted from I EC 60044-1

Accuracy Class Current error at rated primary current Phase displacement at rated primary current Composite error at rated accuracy limit primary current
% min centiradians %
5P "1 "60 "1.8 5
10P "3     10



Accuracy Limit Factor

Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of 'Composite
Error' . Composite Error is the deviation from an ideal CT (as in Current Error), but takes account of harmonics in the secondary current caused by
non-linear magnetic conditions through the cycle at higher flux densities.

Standard Accuracy Limit Factors are 5, 10, 15, 20 and 30. The electrical requirements of a protection current transformer can therefore be defined as :


For example: 1600/5, 15VA 5P10
Selection of Accuracy Class & Limit Factor.
Class 5P and 10P protective current transformers are generally used in overcurrent and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such applications are normally working under steady state conditions Three examples of such protection is shown. In some systems, it may be sufficient to simply detect a fault and isolate that circuit. However, in more discriminating schemes, it is necessary to ensure that a phase to phase fault does not operate the earth fault relay.


Phase Fault Stability

Current transformers which are well matched and operating below saturation, will deliver no current to the earth fault relay, since 3-phase currents sum to zero. If however, the transformers are badly matched, a spill current will arise which will trip the relay. Similarly, current transformers
must operate below the saturation region, since, in a 3 phase system, third harmonics in the secondary are additive through the relay thereby creating instability and erroneously tripping the earth fault relay.


Time Grading

Time lags on relays are set in such a way that a fault in a subsection will isolate that section of the distribution only. Accurate time grading can be adversely affected by inaccuracy or saturation in the associated current transformer. The following table is intended to show typical examples of CT applications However, in all cases manufacturers recommendations must be followed.

Protective System CT Secondary VA Class
Pecurrent for phase & earth fault





10P20 or 5P20

10P20 or 5P20

Unrestricted earth fault





10P20 or 5P20
Sensitive earth fault 1A or 5A   Class PX use relay manufacturers formulae
Distance protection 1A or 5A   Class PX use relay manufacturers formulae
Differential protection 1A or 5A   Class PX use relay manufacturers formulae
High impedance differential impedance 1A or 5A   Class PX use relay manufacturers formulae
High speed feeder protection 1A or 5A   Class PX use relay manufacturers formulae
Motor protection 1A or 5A 5 5P10

Balanced Forms of Protection

In balanced systems of protection, electrical power is monitored by the protective CTs at two points in the system as shown. The protected zone is between the two CTs If the power out differs from the power in, then a fault has developed within the protected zone and the protection relay will operate. A 'Through Fault' is one outside the protected zone Should such a fault occur, the relay protecting the protected zone will not trip, since the power out will still equal the power in. Numerous different types of balanced systems exist and advice may often have to be obtained from the relay manufacturer. However, in all cases Sensitivity and Stability must be considered.



Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in zone fault, the current transformer should have a 'Knee Point Voltage' at least twice the setting voltage of the relay.
The 'Knee Point Voltage' (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetising current of 50%. It is the secondary voltage above which the CT is near magnetic saturation.

Differential relays may be set to a required sensitivity but will operate at some higher value depending on the magnetising currents of the CTs, for example:

The diagram shows a restricted earth fault system with the relay fed from 400/5 CTs. The relay may be set at 10%, but it requires more than 40A to operate the relay since the CT in the faulty phase has to deliver its own magnetising current and that of the other CTs in addition to the relay operating current.



That quality whereby a protective system remains inoperative under all conditions other than those for which it is designed to operate, i.e. an in-zone fault Stability is defined as the ratio of the maximum through fault current at which the system is stable to nominal full load current. Good quality current transformers will produce linear output to the defined knee point voltage (Vkp).


Vkp = 2If(Rs+Rp) for stability, where

If = max through fault secondary current at stability limit

Rs = CT secondary winding resistance

Rp = loop lead resisitance from CT to relay

Transient Effects

Balanced protective systems may use time lag or high speed armature relays. Where high speed relays are used, operation of the relay
occurs in the transient region of fault current, which includes the d c asymmetrical component.
The build up of magnetic flux may therefore be high enough to preclude the possibility of
avoiding the saturation region.
The resulting transient instability can fortunately be overcome using some of the following

a) Relays incorporating capacitors to block the dc asymmetrical component.
b) Biased relays, where dc asymmetrical currents are compensated by anti phase coils.



(c) Stabilising resistors in series with current operated relays, or in parallel with voltage operated relays. These limit the spill current
(or voltage) to a maximum value below the
setting value. For series resistors in current
operated armature relays.

Rs = (Vkp/2) - (VA/Ir)


Rs = value of stabilising resistor in ohms

Vkp = CT knee point voltage

VA = relay burden (typically 3VA)

Ir = relay setting current

The value of Rs varies with each fault setting. An adjustable resistor is therefore required for optimum results. Often a fixed resistor suitable for mid-setting will suffice.

Class PX Protection CT's

Class 5P protection current transformers may be adequate for some balanced systems, however more commonly, the designer will specify a special 'Class PX' CT giving the following information.



(a) Turns ratio.
(b) Knee point voltage Vkp.
(c) Maximum exciting current al Vkp.
(d) CT secondary resistance.

Apparatus Protective System Min. Stability Limit x Rated Current
Generator & Synchronous Motors

Differential Earth Fault

Longitudinal Differential




Differential Earth Fault

Longitudinal Differential



Induction Motors / Busbars Feeders

Differential Earth Fault

Longitudinal Differential

1.25x Starting Current, 1x Switchgear short-circuit rating, short circuit rating, 30

Pilot Wire Burden for Class PX CTs

For 'Class X' current transformers, the cross section and length of pilot wires can have a significant effect on the required Vkp and
therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires.
The graph shows the additional burden of pilot leads of various diameters. It should be noted that, by using a 1 amp instrument and CT, the VA burden imposed by the pilot wires is reduced by a factor of 25.

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