Differential protection is a unit scheme that compares the current on the primary side of a transformer with that on the secondary side. Where a difference exists (other than that due to the voltage ratio) it is assumed that the transformer has developed a fault and the plant is automatically disconnected by tripping the relevant circuit breakers. The principle of operation is made possible by virtue of the fact that large transformers are very efficient and hence under normal operation power-in equals power-out. Differential protection detects faults on all of the plant and equipment within the protected zone, including inter-turn short circuits.
Principle of Operation
A number of factors have to be taken into account in designing a scheme to meet these objectives. These include:
Each of these is considered further below.
The Matching of CT Ratios
The CTs used for the Protection Scheme will normally be selected from a range of current transformers with standard ratios such as 1600/1, 1000/5, 200/1 etc. This could mean that the currents fed into the relay from the two sides of the power transformer may not balance perfectly. Any imbalance must be compensated for and methods used include the application of biased relays (see below) and/or the use of the interposing CTs (see below).
Current Imbalance Produced by Tap Changing
A transformer equipped with an on-load tap changer (OLTC) will by definition experience a change in voltage ratio as it moves over its tapping range. This in turn changes the ratio of primary to secondary current and produces out-of-balance (or spill) current in the relay. As the transformer taps further from the balance position, so the magnitude of the spill current increases. To make the situation worse, as the load on the transformer increases the magnitude of the spill current increases yet again. And finally through faults could produce spill currents that exceed the setting of the relay. However, none of these conditions is 'in zone' and therefore the protection must remain stable ie. it must not operate. Biased relays provide the solution (see below).
Dealing with Zero Sequence Currents
Earth faults down stream of the transformer may give rise to zero sequence current, depending upon winding connections and earthing arrangements. Since zero sequence current does not pass through a transformer, it will be seen on one side only producing spill current and possible relay operation for an out-of-zone fault. To prevent such occurrence, zero sequence current must be eliminated from the differential scheme. This is achieved by using delta connections on the secondary side of any CTs that are associated with main transformer windings connected in star.
Where CT secondaries are connected in star on one side of a transformer and delta on the other, allowance must be made for the fact that the secondary currents outside the delta will only be 1/√3 of the star equivalent.
Phase Shift Through the Transformer
Having eliminated the problem of zero sequence currents (see above) through faults will still produce positive and negative sequence currents that will be seen by the protection CTs. These currents may experience a phase shift as they pass through the transformer depending upon the transformer vector group. CT secondary connections must compensate to avoid imbalance and a possible mal-operation.
Magnetising Inrush Current
When a transformer is first energised, magnetising inrush has the effect of producing a high magnitude current for a short period of time. This will be seen by the supply side CTs only and could be interpreted as an internal fault. Precautions must therefore be taken to prevent a protection operation. Solutions include building a time delay feature into the relay and the use of harmonic restraint driven, typically, by the high level of second harmonic associated with inrush current.
The use of a bias feature within a differential relay permits low settings and fast operating times even when a transformer is fitted with an on-load tapchanger (see above). The effect of the bias is to progressively increase the amount of spill current required for operation as the magnitude of through current increases. Biased relays are given a specific characteristic by the manufacturer.
The main function of an interposing CT is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main CTs. Interposing CTs are equipped with a wide range of taps that can be selected by the user to achieve the balance required.
As the name suggests, an interposing CT is installed between the secondary winding of the main CT and the relay. They can be used on the primary side or secondary side of the power transformer being protected, or both. Interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.
It should be noted that some of the newer digital relays eliminate the need for interposing CTs by enabling essentials such as phase shift, CT ratios and zero sequence current elimination to be programed directly into the relay.