Ring Type Current Transformer Protection Class Sensor

Three Phase Overcurrent Protection.	Unrestricted ground fault protection (Detects ground fault on CT load side)	Combined 3Phase & Unrestricted EarthFault Protection

In some systems it may 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 .

Balanced 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.

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 buildup 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 techniques:

(a) Relays incorporating capacitors to block the dc asymmetrical component. 

(b) Biased relays, where dc asymmetrical currents are compensated by anti-phase coils

(c) Stabilizing resistors in series with current operated relays, or in parallel with voltage-operated relays. Threes limit the spill current (or voltage) to a maximum value below the setting value. 

For series resistors in current operated armature relays. 

Rs = Vkp - VA 

 2Ir/Ir 

Where      Rs = value of stabilizing 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.

Sensitivity

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 magnetizing current of 50%.It is the secondary voltage above which the CT is in magnetic saturation. Differential relays may be set to a required sensitivity but will operate at some higher value depending on the magnetizing 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 magnetizing current and that   of the other CTs in addition to the relay operating current.  Where  Primary Operating Current (P.O.C.) = K(Ir + nle)  K = CT Ratio  Ir = Relay set current  Ie = CT magnetizing current  n = Number of CTs in parallel.  E/Fault Relay, set to trip at 10% Nutral CT Supplies ie.   40A Pri. Fault, will not trip until Relay+ Mag.   Current fault is 56A. Of four CT’s