ON THE TYPE OF CURRENT TRANSFORMERS FOR DIFFERENTIAL PROTECTION

I. D. Kutyavin

(Presented by Prof. V. K. Shcherbakov, Doctor of Technical Sciences)

At present, very high demands are placed on the operation of relay protection of electrical systems with respect to its sensitivity, selectivity, and speed of operation. In connection with this, the quality of relay devices is continuously improving, and protection schemes are also being improved. However, in developing modern protection schemes, difficulties often arise because of the unsatisfactory performance of current transformers during transient processes in the primary circuit.

The point is that the operating mode of most types of relay protection is the transient process in the primary circuit of the installation. In this case the protection must accurately determine the nature and location of the fault and carry out the corresponding operation. This is possible when the protection scheme is supplied from measuring transformers which, under these conditions, possess sufficient accuracy or a constancy of errors that can be allowed for in advance.

The current transformers used at present have quite sufficient accuracy only during normal operation of the installation. When short-circuit currents containing an aperiodic component appear in the primary circuit, the errors of these transformers in many cases become unacceptably large owing to strong saturation of the core steel and a sharp decrease in the transformer’s mutual-induction coefficient. This is observed especially when, before the short circuit, the transformer cores have a considerable residual magnetism of positive sign. In addition, the magnitude of the errors of current transformers during short circuits depends on the multiple of the current in this regime with respect to the rated current of the transformer, and the indicated multiple is determined by the power of the system. The greater the power of the system, the greater the short-circuit current and, consequently, the greater the errors of the transformers during short circuits.

In connection with the foregoing, there arises the necessity of developing special transformers for supplying relay protection, possessing different properties depending on the type of schemes in which they are used. In this respect all current transformers intended for relay protection must be divided into the following three categories:

1) current transformers used for earth-fault protection schemes in networks with a small fault current;

2) current transformers used for all types of differential protection;

3) current transformers used for all other protections.

All zero-sequence transformers should be assigned to current transformers of the first category; in most cases, they are used in networks with a small fault current.

in their own way are single-turn, and in the event of ground faults must transform small primary currents (up to several tens of amperes) into the relay circuit with high accuracy; therefore, at present they are made with permalloy cores [1] or with steel cores with additional premagnetization [2, 3].

In recent years, very much attention has been devoted in the Soviet Union to the development of transformers of the first category. In the development of busbar and cable transformers of this category, a great deal has been done by the staff of the Electrical Engineering Institute of the Academy of Sciences of the Ukrainian SSR under the direction of I. M. Sirota [2, 3]. As a result of these works, transformers have been obtained that make it possible to provide highly sensitive protection against ground faults in the most powerful generators.

Current transformers of the second category are used for implementing differential protections based on the balance of secondary currents or electromotive forces. The relay of these protections responds to the geometric difference of secondary currents or to a current proportional to the geometric difference of secondary e.m.f.’s; therefore, the transformers under consideration must be distinguished by a high degree of uniformity in all operating conditions of the installation and must satisfy the following requirements:

1) the transformers must have coincident characteristics representing the dependence of the secondary current on the primary current \(I_2=f(I_1)\);

2) the transformer cores must not retain residual magnetism;

3) the transformers must have a rectilinear characteristic \(I_2=f(I_1)\).

The first two requirements apply to current transformers used for implementing differential protections of generators, two-winding transformers, powerful motors, parallel lines, etc.

A feature of these protections is that, in the absence of damage in their zone of operation, the current transformers of one and the same phase are traversed by the same primary current. Therefore, in order to reduce the unbalance current of these protections during through short circuits, it is necessary that the transformers of each phase have coincident characteristics \(I_2=f(I_1)\). The appearance of residual magnetism in the cores can violate this condition, since the magnitude of the residual induction in different transformers connected in one and the same phase may be different. As a result, the unbalance current during a through short circuit over the first two or three half-periods may be very large and may cause incorrect operation of the protection.

In the differential protection of busbars and three-winding power transformers, the current transformers of one and the same phase, both in normal operating conditions and during through short circuits, are traversed by different primary currents. If the protections under consideration are implemented with the aid of current transformers whose characteristics have arbitrary shape, then under the indicated conditions these transformers, connected in one and the same phase, will operate at different points of their characteristics and therefore will have different errors, which will cause the appearance in the relay of large unbalance currents and may lead to incorrect operation of the protection. It follows from this that the current transformers used for the protections under consideration must satisfy all three of the requirements indicated.

A feature of the current transformers of this group is also that in most cases they operate with a small secondary burden, consisting of the resistance of the wires of the current-circulation circuit.

Coincidence of the characteristics \(I_2=f(I_1)\) can be obtained by selecting current transformers under factory conditions, as is done at present.

The influence of residual magnetism of current transformers on the operation of differential protection can be eliminated by a number of methods, including the following:

1) the use of differential current transformers having two primary windings;

2) increasing the operating induction in the transformer core to 4–5 kG, as a result of which the residual magnetism is very rapidly demagnetized;

3) the use of transformers with AC premagnetization;

4) the use of current transformers with air cores;

5) the use of current transformers with an air gap in the core.

Differential current transformers with two primary windings have long been known in engineering. They can be used only for the differential protection of generators and motors. Similar transformers with concentric current conductors were proposed by the author in 1935 [4] and, in tests, gave such small unbalance currents that with the aid of these transformers it would have been possible to provide protection of generators against earth faults as well. However, to this day these transformers have not been mastered by our industry, despite the fact that their use would greatly simplify and reduce the cost of protecting generators against internal faults.

The question of increasing the operating induction in current transformers to 4–5 kG in order to eliminate the influence of residual magnetism on the operation of generator differential protection was developed by B. A. Nechaev [5], who, on the basis of his investigations, established that “the magnetization curves of a pair of transformers whose cores are made of steel of the same grade, without special matching as a set, come closer together as the induction increases and coincide at limiting induction. Since current transformers with a small volume of iron maintain limiting induction throughout the entire transient process, the unbalance currents in the relay in this mode should remain small.” This conclusion of B. A. Nechaev must be verified by trial operation of protection using transformers of his design, since transformers with apparent coincidence of the magnetization curves \(B = f(I)\) at high core saturations can produce very large unbalance currents. This is explained by the fact that, under the stated conditions, the magnetization curves become almost horizontal, and then, with an imperceptible mismatch of the curves and with the same induction, the difference in magnetizing currents can be very large.

The question of using current transformers with premagnetization for differential protections is currently being developed in the relay laboratory of the Tomsk Polytechnic Institute.

Current transformers with air cores were first used in 1942 for bus differential protection, which in laboratory tests gave high sensitivity, but in practice this sensitivity proved unsatisfactory because of the insufficient power of the current transformers. Extensive work on the use of current transformers with air cores for differential protection of generators and transformers was carried out by V. M. Izrailyants [6], who in his conclusions indicates that the use of these transformers for the specified protections significantly increases their sensitivity. However, these protections have not yet been verified by operating experience.

A major shortcoming of current transformers with an air core is that they turn out to be bulky and have very small-

have low power and are subject to the influence of extraneous magnetic fields, which makes it difficult to use them for differential protections.

The residual induction in the steel core of a transformer can be reduced to any value by using an air gap of the required magnitude. The first information on the use of transformers of this type in the USA for differential bus protection dates to 1941.

The transformers with an air gap used at that time had a rectangular core with eight air gaps, the total length of which was 17% of the total length of the mean magnetic path of the core; as a result, the equivalent magnetic permeability of this complex magnetic circuit was about six. Operating experience with bus differential protection using the transformers under consideration has not been published.

The idea is that current transformers with an air gap should have a rectilinear characteristic \(I_2 = f(I_1)\) and should not retain appreciable residual magnetism. At the same time, they will have considerably greater capacity than transformers with air cores.

As the author’s investigations have shown, these transformers reduce the residual induction to the operating value with an air gap equal to 0.1–0.2 percent of the total length of the mean magnetic path of the core. If, in this case, the induction in the core at the highest short-circuit currents does not exceed 7–8 kGs, then the characteristic \(I_2 = f(I_1)\) will be rectilinear.

For differential protections that monitor only two currents (protection of generators, two-winding transformers, etc.), the size of the gap in the core must be selected on the basis of eliminating the influence of residual magnetism on the operation of these protections, and it will not exceed 0.1–0.2 percent of the total length of the magnetic path. The induction in the cores of the transformers of these protections under through short circuits need not be limited, since the requirement of rectilinearity of their characteristics is not mandatory here.

Current transformers of this type used for differential protections that monitor more than two currents (protection of buses, three-winding transformers, etc.) must have rectilinear characteristics \(I_2 = f(I_1)\); therefore, their induction at the greatest through short-circuit current must not exceed 7–8 kGs. The air gap for these transformers should be of the order of 3–5% of the length of the mean magnetic path, depending on the maximum value of the primary short-circuit current.

Current transformers of the third category, intended to supply distance protections, maximum-current protections, directional current cutoffs, and some others, must have sufficient accuracy during short circuits in the protected line. The large errors of the presently used transformers at short-circuit currents having a significant aperiodic component lead to failure of the cutoffs to operate and shorten the first zone of distance protections. In this connection, for supplying the protections indicated above it is also desirable to use current transformers with an air gap, which can provide sufficient accuracy under any of the most severe short circuits. It should be noted that special current transformers for these protections have still not been developed, although the need for them is recognized by many.

In conclusion, the following conclusions must be drawn:

1. Differential current transformers and transformers with increased operating induction can be used only for differential-

differential protection of generators, the former being capable of ensuring the highest quality of the indicated protection.

1. Transformers with an air core are hardly likely to become widely used, owing to the inherent shortcomings indicated above.

2. Transformers with an air gap are universal. As the author’s investigations have shown, these transformers, at a high accuracy class, will have sufficient power to supply all types of relay protection. At the same time, they are not subject to the influence of residual magnetism and, at inductions up to 7–8 kg, have a rectilinear characteristic \(I_2 = f(I_1)\). Therefore, these transformers can be successfully used for all types of differential protection, as well as for many line protections.

References

1. Ya. S. Gel′fand et al. Sensitive current protections against ground faults and experience in their operation. Elektricheskie stantsii, 11, 1951.

2. M. I. Sirota. Testing a new protection of generators against ground faults. Elektricheskie stantsii, 6, 1949.

3. M. I. Sirota. Protection of generators against ground faults using zero-sequence bus current transformers. Elektricheskie stantsii, 12, 1950.

4. I. D. Kutyavin. Protection of generators against internal faults. Elektrichestvo, 12, 1936.

5. B. A. Nechaev. On the question of the application of current transformers with a small iron volume for differential protections of generators. Dissertation abstract. Ivanovo, 1952.

6. V. M. Izrael′yan. Air current transformers in schemes of differential protection of generators and transformers. Dissertation abstract. Leningrad, 1952.