Single-System Differential Protection of Power Two-Winding Transformers

N. V. Lisetskii, Yu. A. Kulaga

(Presented by the scientific seminar of the Department of Electric Power Stations)

Existing types of differential protection for power transformers, like many other varieties of protection, can be significantly simplified and their characteristics improved by introducing ferromagnetic transducers (FT) into the circuits [1], [2].

Possible variants of single-system differential-protection circuits for a two-winding transformer having the winding-connection group Y/Δ-11 are shown in Fig. 1. As can be seen from this figure, the use of an FT makes it possible to implement protection in two variants: according to a circulating-current scheme (Fig. 1, a) and according to a voltage-balance scheme (Fig. 1, b). Power transformers with this winding-connection group have unequal current distribution in the incoming conductors—

Fig. 1

— in cases of two-phase short circuits on the high- and low-voltage sides. With such faults in the protection zone, the voltage at the output of the FP will differ from zero and is capable of causing relay operation, if the primary winding of the FP is made double, with different numbers of turns, connected in “zigzag.”

The primary windings of single-phase intermediate transformers are made with taps for balancing the currents in the protection arms and for changing the operating current of the FP. The FP, together with the intermediate transformers, can be installed directly at the current transformers, which practically short-circuits the latter and leads to a significant saving in connecting wires. Installation of the protection is thereby simplified.

To detune the differential protection from inrushes of magnetizing current and from unbalance currents during external faults, the effect of saturation of ferromagnetic materials is widely used. The FP also operates with saturated cores. However, when the FP is connected directly to current transformers, the current in the relay during magnetizing-current inrushes assumes the greatest values, depending on the type of differential scheme. In a scheme with circulating currents it reaches 2–2.5 times its rated value. The large currents in the relay in the presence of saturation are explained by the nature of the change in the internal resistance of the FP, which in this differential scheme constitutes a considerable share of the resistance of the entire circuit.

When the FP is connected to current transformers through saturating intermediate transformers, detuning from energizing currents is obtained at lower protection operating currents.

The primary winding of the FP is a variable inductive load for the intermediate transformers; therefore their operating regime during magnetizing-current inrushes differs somewhat from the analogous operating regime in ordinary differential-protection schemes. Fig. 2 shows an oscillogram of the change in the secondary current of the intermediate transformer when a maximum unipolar inrush flows through the primary winding. Part of the aperiodic component of the energizing current in the first period passes through the primary winding of the FP as

Fig. 2

due to a decrease in its inductive resistance. The magnitude of the first peak of the secondary current is additionally increased if the polarity of the residual induction in the core of the intermediate transformer does not correspond to the instantaneous value of the forced magnetic flux. However, with a proper choice of the parameters of the intermediate transformer, the maximum value of the first peak of the secondary current does not exceed 1–1.5 times the amplitude value of the rated current, which ensures reliable blocking of the protection operation.

When implementing single-system differential protection according to the voltage-balance scheme (Fig. 1, б), the secondary winding of IT-2 is connected in series with the relay. In view of this, the change in the internal resistance of the IT from the side of connection has only a slight effect on the total resistance of the entire secondary circuit. The current in the relay in this case does not exceed 1.5 times the rated current.

The connection of the IT in this scheme through intermediate saturating transformers leads to an inadmissibly low multiple of the current in the relay during internal faults. The scheme under consideration (without intermediate saturating transformers) can be successfully replaced by differential cutoff. For its selective operation, a relay pickup setting equal to 2–2.5 times the rated current is required.

A model of the proposed single-system differential protection was tested when switching on a transformer of 2.5 kVA capacity at a voltage of 220 V. The unipolar peaks of the magnetizing current in this case, as shown by the oscillograms, reached 7–8 times the amplitude value of the rated current. The number of turns of the primary winding of the intermediate saturating transformers corresponded to the rated pickup current of the protection. An electromagnetic relay of type ET-520 was used as the actuating element.

When switching on the transformer from the star side, in order to obtain the largest magnetizing-current inrushes, the winding connected in delta was opened. This was done in order to avoid the “helping” effect of the delta [3]. When a transformer with an isolated neutral was switched on, periodic inrushes of magnetizing current occurred, the maximum values of which reached 1.5 times the amplitude value of the rated current. The oscillograms shown in Fig. 3 show the change in the current in the relay when the primary windings of the intermediate saturating transformers were traversed by unipolar switching currents (Fig. 3a) and when a periodic inrush was present in one of the phases (Fig. 3b). As can be seen from the oscillograms, in the first period the maximum value of the current in the relay does not exceed 0.9–1 of the amplitude value of the pickup current. But since the current curve in the first period contains the second harmonic, its rms value is 0.6–0.7 of the rms value of the relay pickup current. In the second and subsequent periods, a sharp decrease in the current in the relay is observed because of the increase in the internal inductive resistance of the IT. This leads to additional automatic blocking of the protection after the first period.

An electromagnetic relay, when a current equal to the pickup current flows through its winding, does not have time to operate within one period. Taking this into account, for desensitizing single-system differential protection against magnetizing-current inrushes, it is possible to recommend a pickup-current setting equal to (0.7–1) of the rated current.

The presence of saturating elements leads to a slight slowing of single-system differential protection when an aperiodic component appears in the internal short-circuit current. This slowing is inherent in all protections in which, for blocking, use is made of-

Figure 3a

Figure 3b

the saturation principle or braking from the extracted aperiodic component of the fault current is used.

Tests of a model single-system differential protection under internal faults showed that, with a minimum sensitivity coefficient equal to 2, the relay operating time for a three-phase short circuit does not exceed 1.5–2 periods (0.03–0.04 s), and for a two-phase short circuit, 2–3 periods (0.04–0.06 s). The aperiodic component decay constant in this case reached 0.1 s. The relative speed of operation of the proposed differential-protection circuit in comparison with protections implemented with RNT or DZT relays is explained by the increased current multiple in the relay. With a primary current equal to twice the protection operating current, the current in the relay is equal to

\[ \begin{array}{ll} \text{for a 3-phase s.c.} & I^{(3)}_{p}=1.5\,I_{p.\mathrm{op.}},\ \text{and}\\ \text{for a 2-phase s.c.} & I^{(2)}_{p}=1.95\,I_{p.\mathrm{op.}} \end{array} \]

The change in the current in the relay at triple frequency in the three-phase fault mode additionally reduces the operating time of the protection.

Conclusions

1. FPs can be used to construct differential protection of power transformers both on current circulation and on voltage balance.

2. The combination of intermediate saturating transformers with an FP ensures reliable restraint against magnetizing-current inrushes.

3. Owing to the increased current multiple in the relay, the proposed protection has a relative speed advantage in comparison with protection implemented with RNT and DZT.

4. A single-system protection according to the voltage-balance scheme can be used as a simple and more sensitive replacement for differential cut-off.

5. In the protection schemes, a single actuating element is used.

References

1. N. V. Lisetskii, Yu. A. Kulaga, V. I. Khuduguev. Determination of the harmonic composition of a functional current converter. Izv. TPI, vol. 191.

2. N. V. Lisetskii, Yu. A. Kulaga, V. I. Khuduguev. Equal-sensitivity relay for various faults. Izv. TPI, vol. 179.

3. A. D. Drozdov. Electrical Circuits with Ferromagnetic Cores in Relay Protection. GEI, 1965.