Applications and forms of differential relays

ABB Residual Current Circuit Breaker with rcbo Overload Protection

Differential relays take various forms, depending on the equipment they protect. The definition of such a relay is “one which operates when the vector difference of two or more similar electrical quantities exceeds a predetermined quantity.” We will see later that almost all types of relays, when connected in some way, can work as a differential relay. In other words, it is not so much the construction of relays as the relay is connected in a circuit that makes it a differential relay.

Most RCD applications are of the “current differential” type. The simplest example of such an arrangement is shown in Figure 14. The dotted portion of the circuit in Figure 14 shows the system element which is protected by the differential relay. This system element can be a circuit length, a generator winding, a bus part, etc.

current transformer (CT) is indicated in each connection to the system element. The CTÕ secondaries are interconnected and the coil of an overcurrent relay is connected to the CT secondary circuit.

This relay could be one of the ac types we have considered.

Let us now suppose that the current flows in the primary circuit either towards a load or towards a short-circuit located at X. The conditions are identical to those of Fig. 15. If both current transformers have the same ratio and are properly connected, their secondary currents will only flow between the two cts as indicated by the arrows, and no current will flow through the differential relay.


However, if a short circuit developed somewhere between the two cts, the conditions in Fig. 16 would then exist. If the current flow to the short circuit on both sides as shown, the sum of the secondary currents will flow through the differential relay. It is not necessary for the short circuit current to flow to the fault on both sides for the secondary current to flow through the differential relay. Flowing from one side only, or even current flowing out from one side while the larger current flowing through the other side, will cause a differential current.

In other words, the differential relay current will be proportional to the vector difference between the currents entering and leaving the protected circuit; and, if the differential current exceeds the starting value of the relay, the relay will operate.

It is easy to extend the principle to an element of the system comprising several connections. Consider Fig. 17, for example, in which three connections are involved.

It is enough that, as before, all the cts have the same ratio and they should be connected so that the relay does not receive any current when the total current coming out of the circuit element is vectorially equal to the total current going into it. The circuit element.

The principle can still be applied when a power transformer is involved, but in this case the ratios and connections of the voltage transformers on the opposite sides of the power transformer must be such as to compensate for the variation in amplitude and the phase angle between the power transformer currents on each side. This topic will be covered in detail when we look at the topic of power transformer protection.

The most widely used form of differential relay is the “percentage differential” type. This is essentially the same as that of the overcurrent type current balancing relay described above, but it is connected to a differential circuit.

The differential current necessary for the operation of this relay is a variable quantity, under the effect of the retaining coil. The differential current in the operating coil is proportional to 1 – JE 2, and the equivalent current in the retaining coil is proportional to (JE 1 + I 2 ) / 2 since the operating coil is connected to the center of the retaining coil; in other words, if we let N be the number of turns on the retaining coil, the total ampere-turns are 1 N / 2 + I 2 N / 2, which is the same as if (JE 1+ I 2 ) / 2 had to flow through the entire coil.

The operating characteristic of such a relay Thus, except for the slight effect of the control spring at low currents, the ratio of the differential operating current to the average stress current is a fixed percentage, hence the name of this relay. The term “through current” is often used to refer to I2, which is the part of the total current that flows through the circuit from end to end and the operating characteristics can be plotted using I2 instead of (I1 + I2 ) / 2, to conform to the ASA definition for a percentage differential relay.

The advantage of this relay is that it is less likely to malfunction than a differential connected overcurrent relay when a short circuit occurs outside the protected area.

Current transformers of the types normally used do not transform their primary currents as accurately under transient conditions as during a short period after the occurrence of a short circuit.

This is especially true when the short circuit current is shifted. Under such conditions, assumed identical current transformers may not have identical secondary currents, due to slight differences in magnetic properties or their different amount of residual magnetism, and the difference current may be larger, the greater the current. Short circuit is important. Even if the short circuit current caused by an external fault is not compensated, the CT secondary currents may differ due to differences in CT types or loads, especially in the protection of the power transformer. Since the percentage differential relay has an increasing starting characteristic as the magnitude of the current through increases,

Figure 20 shows the comparison of a simple overcurrent relay with a percentage differential relay under such conditions. An overcurrent relay having the same starting minimum as a differential relay would operate undesirably when the differential current barely exceeds the X value, while the percentage difference relay would not operate.

Percentage differential relays can be applied to system elements having more than two terminals, as in the three-terminal application of Figure 21. Each of the three retaining coils has the same number of turns and each coil has the same number of turns. Produces a restraining torque regardless of which torques are added arithmetically.

The percentage slope characteristic of such a relay will vary depending on the distribution of the currents among the three restraining coils.

Percentage rcds are generally instantaneous or high speed. Time delay is not necessary for selectivity, as the percentage difference characteristic and other additional characteristics to be described later make these relays practically insensitive to the effects of transients when the relays are correctly applied. The settings supplied with certain percentage differential relays will be described in relation to their application.

Several other types of relays with different arrangements could be mentioned. One of them uses a directional relay. Another has an additional constraint obtained from the harmonics and the dc component of the differential current. Another type uses an overvoltage relay instead of an overcurrent relay in the differential circuit. Special current transformers can be used with little or no iron in their magnetic circuit to avoid transformation errors caused by the dc component of the shifted current waves. All of these types are extensions of the fundamental principles which have been described and will be discussed later in relation to their specific applications.

There has been a great deal of activity in the development of the differential relay because this form of relay is by nature the most selective of all conventional types. However, each type of element of the system poses particular problems which have hitherto not made it possible to design a differential relay equipment having universal application.

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