DIFFERENTIAL BUSBAR PROTECTION BASED ON FERROMAGNETIC TRANSDUCERS

N. V. LISETSKII, Yu. A. ERSHOV

(Submitted by the Department of Electrical Systems and Networks)

Existing differential protections for busbars have a number of shortcomings, the main ones being: 1) low sensitivity, 2) the possibility of nonselective operation due to large unbalance currents, 3) long operating time.

The proposed protection is based on the principle of restraint by the sum of the currents of the connections, developed by G. T. Grek [1]. The protection contains the following basic elements: 1) sensors—ferromagnetic current transducers, 2) an operating element.

A ferromagnetic current transducer (FCT) converts a three-phase system of currents into a single-phase system of voltages (Fig. 1). Three transformers, whose primary windings are connected in zigzag, constitute a frequency tripler of the Spinelli system [2]. For the protection to operate for single-phase faults, an additional single-phase transformer is added to the circuit, connected in the neutral conductor, which introduces a component proportional to the zero-sequence current.

In three-phase faults on busbars, the FCT output contains mainly third harmonics of voltages [3]. In two-phase and single-phase short circuits, the FCT gives a sum of voltage harmonics, the determining one of which is the fundamental-frequency harmonic [3].

Fig. 1. Circuit of the ferromagnetic current transducer

Equalization of the effective values of the currents through the relay for different types of faults is achieved by connecting a capacitor \(C\) and a linear inductance \(L\) [4].

The operating element contains rectifier bridges VM-1, VM-2, a half-bridge formed by diodes \(D_1\) and \(D_2\), and resistances \(R_1\), \(R_2\), \(R_3\). The operating winding of polarized relay RP-7 is connected in the diagonal of bridge VM-1, while the restraining winding is supplied by the current of bridge VM-2. To simplify the circuit and create a stable characteristic of the operating—

... of the organ, independent of the current distribution among the individual elements; rectifiers of the same type are used in the circuit. In external faults, the restraining winding of the relay is traversed by the sum of the currents of the individual elements. Figure 2 shows a particular case of current distribution in the protection circuits for a fault outside the protected zone.

Fig. 2. Schematic diagram of busbar protection

Let us denote the current flowing through the operating winding by \(I_p\), and the sum of the transformed currents of all connections by \(I_\Sigma\). In accordance with the current directions indicated in Fig. 2, one may write

\[ I_2 = I_1 - I_p; \]

\[ I_\Sigma = I_m + I_1; \]

\[ I_1 R_1 + I_2 R_2 - I_m R_{\mathrm{ц}.m} = 0, \]

where

\[ R_{\mathrm{ц}.m} \]

is the resistance of the restraining-winding circuit, taking into account the resistances \(R_3\) and \(R_m\).

Solving the above equations with respect to the current \(I_m\), we obtain

\[ I_m = \frac{I_\Sigma (R_1 + R_2) - I_p R_2} {R_1 + R_2 + R_{\mathrm{ц}.m}} . \]

This equation shows that the responding organ has a restraining characteristic of the type

\[ I_p = k_m I_m, \]

shown in Fig. 3.

The adjustment of the parameters of the restraining characteristic (the initial operating current and the restraint coefficient) is carried out smoothly by means of the resistances \(R_1\), \(R_2\), \(R_3\).

Figure 4 (curves 1 and 2) shows the changes in the magnetizing forces of the operating and restraining windings as a function of the current multiple for external faults. Curves 3 and 4 of this figure show the changes in the corresponding magnetizing forces for faults within the protection zone. All the indicated dependences were obtained experimentally.

The proposed protection covers the connections of only one busbar system. With a fixed distribution of connections on the second busbar system, a second protection set must be provided. Correct operation of the protection is ensured only with

Fig. 3. Restraining characteristic of the operating element

the prescribed fixing of the connections. If the fixing is violated, the protection must be disconnected. The operating current of the protection is selected

Fig. 4. Characteristics of the operating element for faults within and outside the protection zone

from the condition of preventing operation upon interruption of the current circuit of the FPT of one of the most powerful connections, when the sum of the currents of the remaining connections is equal to this current and the calculated restraining current is equal to zero.

\[ I_{\text{op}} = k_{\text{r}} \frac{I_{\text{load,max}}}{n_t}, \]

where \(k_{\text{r}}\) is the reliability factor,
\(n_t\) is the transformation ratio of the current transformer.

Conclusions

1. The proposed protection provides high selectivity, since, for faults outside the zone, better restraint is obtained than in existing protections. In addition, the restraining magnetomotive force increases almost according to a linear law.

2. The protection has high sensitivity.

3. The circuit is fairly simple.

Literature

1. T. T. Grek, S. Ya. Petrov. Differential busbar protection with restraint. — Elektrichestvo, 1970, No. 10.

2. A. M. Bamdas, V. A. Kulinich, S. V. Shapiro. Static electromagnetic converters of frequency and number of phases. GEI, 1961.

3. N. V. Lisetskii, V. I. Khuduguev, Yu. A. Kulaga. Determination of the harmonic composition of a functional current converter. — Proceedings of the East Siberian Technological Institute. Issue 3, vol. 4, Ulan-Ude, 1968.

4. N. V. Lisetskii, V. I. Khuduguev, Yu. A. Kulaga. A relay of equal sensitivity to various types of faults. — Izvestiya TPI, vol. 179, TSU Publishing House, 1968.