Improvements in Line Differential Relays

Authors: Roberto Cimadevilla and Javier Garcia-Villalobos, ZIV, Spain

Line Differential Relay Based on Instantaneous Values Review of the operating principle of a line differential protection: A line differential protection system is based on several IEDs, each of them located at one end of the protected line. To simplify the explanation, the two-ended line of Figure 1 is considered. Each of the IEDs measures the local current and receives, via the communication channel, the currents measured by the remote IED. The local
IED calculates the differential and restraint currents using the following formulas:

The local IED samples the current at several time instants, as it is shown in Figure 2. The current can be measured as instantaneous values or phasors. The remote IED must sample the current at the same time instants. Any synchronization error in the time instants will generate a false differential current.

Advantages of using instantaneous values instead of phasors: Line differential relays based on instantaneous values present several advantages over the ones using phasors. Some are detailed below.

Calculation of the harmonics in the differential current: This calculation allows the protection of groups line plus transformer or even transformers alone. The first application is required when, because of economic reasons, there is no breaker between the line and the power transformer and both elements must be protected by the same differential relay. The second application is needed when the distance between the CTs in the first and the second windings is very long. This requires very long wires, creating an excessive burden in one of the CTs, limiting the use of a transformer differential protection.

It is important to note that harmonic blocking or restraint units require the use of the harmonics present in the differential current. If the units are based on the harmonics present in the first winding currents, the operation may not be correct for inrush or overexcitation conditions that occur with the transformer loaded, as the harmonic content in the first winding currents may be much lower than the one included in the differential current. Inrush with the transformer loaded can occur when clearing an external fault or during a sympathetic inrush. If just the phasors at the fundamental frequency are interchanged between both line differential IEDs the harmonic information of the remote end currents is lost.

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