THREE-PHASE SINGLE-SYSTEM POWER DIRECTION ELEMENT

A. N. Burnashev, N. V. Lisetskii

(Submitted by the Department of Electrical Systems and Networks)

One of the possible ways of improving the protection of parallel lines is, in our opinion, the creation of single-system directional protection based on ferromagnetic current transducers (FCT) and voltage transducers (FVT), responding to all types of faults. Spinelli frequency triplers, connected to the secondary windings of measuring current transformers (CT) and voltage transformers (VT), are used as such transducers. The use of ferromagnetic transducers (FT) makes it possible to simplify the protection circuit, reduce the requirements imposed on the CT, and ensure equal sensitivity of the protection to different types of faults [1].

To create single-system directional protection for parallel lines based on FT, an appropriate power direction element is needed. The circumstance that the voltage at the FT output is, in the general case, substantially nonsinusoidal excludes the possibility of using, for implementation of the power direction element, such principles as the induction principle, the time-pulse principle, etc.

Studies carried out in the laboratory of the Department of Electrical Stations of the Tomsk Polytechnic Institute have shown that one of the acceptable principles is the principle of comparing the absolute values of two electrical quantities \(\dot E_1\) and \(\dot E_2\), formed from voltages taken from the outputs of the FCT and FVT [2, 3]:

\[ \dot E_1 = \dot k_1 \dot U_{(up)} + \dot k_2 \dot U_{(ip)}, \tag{1} \]

\[ \dot E_2 = \dot k_1 \dot U_{(up)} - \dot k_2 \dot U_{(ip)}, \]

where \(\dot k_1, \dot k_2\) are complex constant coefficients selected in accordance with the required angle of maximum sensitivity \((\varphi_{\mathrm{m.ch}})\);

\(\dot U_{(up)}\) is the output voltage of the FVT, which is a nonlinear function of the voltage applied to the relay \((U_p)\);

\(\dot U_{(ip)}\) is the output voltage of the FCT, which is a nonlinear function of the current flowing in the primary windings of the FCT \((I_p)\).

The voltage at the output of the comparison circuit is

\[ U_{\text{out}} = \{\,|\dot E_1| - |\dot E_2|\,\}\frac{1}{k_{\phi}}, \tag{2} \]

where \(k_{\phi}\) is the form factor.

The studies carried out make it possible to conclude that the use of a comparison circuit in combination with MTs makes it possible to obtain a power-direction element that satisfies the basic requirements imposed on devices of this type [4] and, in addition, has certain advantages in comparison with them. Let us name the main ones:

a) simplicity of the relay circuit and, consequently, its higher reliability;

b) smaller overall dimensions, weight, and cost (the MTs used in the circuit are very inexpensive and technologically convenient elements);

c) lower power consumption in comparison with electromechanical relays (the power consumed by the relay is approximately 40 VA; for comparison, we point out that only one RBM-171/1 relay consumes 50 VA, and an RBM-178 consumes 100 VA);

d) a low level of operating currents and voltages in the circuit.

Thus, it may be said that a power relay using MTs combines the advantages provided, first, by the MTs themselves and, second, by the use of semiconductor elements [1, 2, 3, 4]. Figure 1 shows the circuit of a power-direction element using MTs. In the circuit, Tr1 and Tr2 are intermediate transformers with an air gap. Tr1 is connected to the output of the voltage magnetic transducer, and Tr2 to the output of the current magnetic transducer, assembled according to a circuit with circulating currents.

Fig. 1. Circuit of a single-system power-direction element using MTs

The relay uses a circuit for comparing the absolute values of two electrical quantities, with in-phase connection of the rectifier groups (with circulating currents), without buffer resistances on the rectified-current side, as providing a small delay without the use of complex devices for smoothing the rectified currents [2].

As a smoothing device, a filter is used consisting of two parts: a resonant filter—a second-harmonic shunt—and a capacitor connected in parallel with the input of the null indicator. The null indicators are made in the form of two-stage dc amplifiers using germanium triodes, at whose output polarized RP-7 relays are connected (one null indicator is shown in the circuit; in fact there are two of them, since a relay of bidirectional action is considered).

As is known, one of the basic requirements imposed on power-direction elements is that the voltage, do—

sufficient for its operation, must be as small as possible [1]. For the first prototype of the relay this value is 1.5 volts, i.e., somewhat greater than in the best modern designs. However, such voltage sensitivity of the relay is not a limit and can be increased. The relay under consideration operates correctly for any types of phase-to-phase faults and any angles

\[ \varepsilon_p=\widehat{U}_p I_p, \]

corresponding to short circuits on one of the two protected parallel lines. There is a fundamental possibility of constructing a relay that also responds to single-phase short circuits.

The form of the angular characteristics depends on the operating mode of the protected network. In the case of symmetrical modes, the angular characteristic has the form shown in Fig. 2, a. Of the six sectors, each of which corresponds to an angle of \(60^\circ\), two are “operating” sectors, one of which is in the first quadrant and the other in the third quadrant. By selecting the value of the resistance \(R_1\), the relay can be set to operate with the required angle of maximum sensitivity.

Fig. 2. a) angular characteristic of the power-direction element for symmetrical modes; b) the same for asymmetrical modes

Figure 2, b shows the angular characteristic of the relay for an asymmetrical mode (two-phase short circuit). It is similar to the corresponding characteristics of existing relays [4]. Tests have shown that the relay operates correctly for any combination of faulted phases. Figure 3, a shows the characteristic \(U_{\mathrm{cp}}=f(\varphi_p)\), and Fig. 3, b—

Fig. 3. a) dependence \(U_{\mathrm{cp}}=f(\varphi_p)\); b) volt-ampere characteristic

the volt-ampere characteristic of the relay. The operating time of the relay is approximately 15 msec at five times the current.

Conclusions

1. The use of functional converters in combination with a comparison circuit makes it possible to obtain a single-system power-direction relay that responds to all types of faults.

2. In its technical characteristics, the relay meets the basic requirements imposed on power-direction elements.

Literature

1. N. V. Lisetskii, V. I. Khuduguev. Differential protection of generators with functional converters. — Proceedings of the East Siberian Technological Institute. Issue II, vol. IV. Ulan-Ude, 1968.

2. Yu. A. Gavenko. New relay protection in semiconductor devices. State Publishing House of Technical Literature of the Ukrainian SSR, Kiev, 1962.

3. Yu. G. Nazarov. A phase-sensitive circuit as a power-direction element. — Elektrichestvo, 1956, No. 11.

4. A. M. Fedoseev. Fundamentals of Relay Protection. Gosenergoizdat, 1961.