RU2168824C1 - Method for short-circuit current protection of electrical installation - Google Patents

Abstract

FIELD: protection of power transmission lines against line-to-line shorts. SUBSTANCE: electrical installation current is measured and compared with first and second reference values. Instant disconnection signal is sent when this current exceeds first reference value. When current exceeds second reference value, electrical installation current is stored through first time delay, and present-time current of installation is subtracted from this value to obtain first difference. When electrical installation current rises above second reference value, its phase current is stored through first time delay and present-time phase current is subtracted from this value through second time delay to obtain second difference. If second difference is lower than fourth reference value and first difference, lower than third reference value, disconnection signal is applied. EFFECT: enhanced sensitivity, selectivity, and reduced operating time of protective gear due to eliminating self-start currents of moving load. 4 dwg

Description

 The invention relates to the electric power industry, and in particular to the relay protection technique, and may find application for protecting power lines from interphase short circuits.

 A known method of protecting electrical installations, for example [1], in which the current of the electrical installation is constantly measured, is compared with a reference value, and if after a time delay the current does not decrease less than the reference value, a signal is sent to turn off the electrical switch. However, using this method, it is impossible to distinguish self-starting of a motor load from a short circuit if these currents are commensurate at the first moment.

 The closest in technical essence is the method implemented in the device [2] current protection of the electrical installation against short circuit, which constantly measure the current of the electrical installation, compare it with the first and second reference values, when exceeding the current of the electrical installation of the first reference value, a shutdown signal is sent without delay , when the current of the electrical installation of the second reference value is exceeded, the value of the current of the electrical installation is remembered through the first time delay, the value is subtracted from this value after the second time delay Ia the current value of the electrical current, comparing the resulting difference with a first reference value formed by the third and, if the first difference is less than a third reference value, then a trip signal is supplied.

 However, the following options are possible: a) due to a decrease in the short-circuit current, which is possible in the absence of an automatic excitation controller (ARV) of the generator or with remote short-circuit, the current may decrease during the observation time of the transient; b) due to the operation of an electromechanical ARV generator or an automatic transformer voltage regulator (ARNT), the current may not decrease or decrease slightly during the self-start observation time. In connection with the foregoing, it becomes difficult to distinguish one mode from another for a limited transient time; in such cases, the installation of protection becomes rougher or the duration of the protection increases.

 The objective of the invention is to increase the sensitivity, selectivity and reduce the response time of the protection by detuning from the self-starting currents of the motor load.

 The task is achieved in that the phase of the electrical installation current is additionally constantly measured, when the electrical installation current is exceeded by the second reference value, the value of the electrical installation current phase is stored through the first time delay, the current value of the electrical installation current phase is subtracted from this value, the obtained second difference is compared with the fourth generated reference value, if the second difference is less than the fourth reference value and the first difference is less than the third reference value, t o give a shutdown signal.

 It is known that when starting or self-starting asynchronous electric motors, there is not only a decrease in current (from starting to rated), but also a decrease in the phase of the current. Therefore, the use of complementary parameters will help to reliably recognize short-circuit and distinguish it from self-start.

The invention is illustrated by diagrams, where in FIG. 1 shows the change in current during short-circuit and the computational increment ΔI _K during the time Δ t = t ₂ -t ₁ (t ₁ is the time of establishment of the super-transient short-circuit process, t ₂ is the time less than self-starting of the load);
in FIG. 2 shows the change in the phase of the current during short-circuit and the calculated increment ΔΦ _K for the time Δ t;
in FIG. 3 shows the change in current during self-start and the calculated increment Δ for the time Δ t;
in FIG. 4 shows the change in the phase of the current during self-starting and the calculated increment ΔΦ _K for the time Δ t.

 The method is as follows: measure and calculate the change in current and the change in its phase after the occurrence of a short circuit or self-starting of the load.

When short-circuit occurs, the current increases to I _K and its phase to Φ _K.
If the short circuit is close, then the current value I _t1 exceeds the first reference value I ₁ . In this case, turn off the electrical switch without delay.

If distant short-circuit occurs (located in the reservation zone), then the current value I _t2 does not exceed the first reference value I ₁ , but exceeds the second reference value I ₂ (the second reference value I ₂ somewhat exceeds the rated current of the electrical installation I _N ). In this case, through the first time delay at time t _{1, we} memorize the value of the current I _{t2 of the} electrical installation and its phase Φ _t2 (time delay is necessary so as not to take into account ultra-transient processes). Then, through the second time delay at time t _2, the value of the current of the electrical installation I _t3 (Fig. 1) and its phase Φ _t3 (Fig. 2) are measured. The next step is to find the difference between currents and their angles. Thus, for the time Δt we find the increment of the current and phase:
ΔI _K = I _t2 -I _t3
ΔΦ _K11 = Φ _t2 - Φ _t3,
where I _t2 and I _t3 are the magnitudes of the currents at times t ₂ and t ₁ , Φ _t2 and Φ _t3 are the phases of the currents at times t ₂ and t ₁ . Compare the results. If this increment Δ I _{K is} less than the third reference value and the increment ΔΦ _{K is} less than the fourth reference value (Fig. 1 and Fig. 2), then turn off the switch.

In the start-up or self-start mode (Fig. 3 and Fig. 4) we do the same operations. At the same time, we measure and remember I _t2 , I _t3 and Φ _t2, Φ _t3 . Find the increment of the current Δ I ₃ and its phase ΔΦ ₃ :
ΔI ₃ = I _t2 -I _t3
ΔΦ ₃ = Φ _t2 -Φ _t3
The obtained values are greater, respectively, of the third and fourth reference values, so we do not turn off the switch.

 Thus, the sensitivity and selectivity of the protection according to the proposed method is increased due to the ability to rebuild from the starting currents and self-starting. And the use of information on changing the phase of the current reduces the duration of the protection, since the difference between the start or self-start modes from the short circuit occurs earlier.

Sources of information
1. The theoretical basis for the construction of the logical part of relay protection and automation of power systems / Polyakov V.E., Zhukov SF, Proskurin G.M. et al. Ed. V.E. Polyakova. - M.: Energy, 1979. - p. 240.

 2. USSR patent N 1808160, H 02 H 3/08, Device for current protection of electrical installations from short circuits // M. J. Kletsel, A.G. Koshel, A.N. Metelsky, K.I. Nikitin and V.V. Chelpachenko. Publ. Bull. N 13, 04/07/1993.

Claims (1)

 The method of current protection of an electrical installation against short circuits, which consists in constantly measuring the current of the electrical installation, comparing it with the first and second reference values, when the current of the electrical installation of the first reference value is exceeded, a signal is sent to the deviation without delay, if the current of the electrical installation of the second reference value is exceeded through the first time delay remember the value of the current of the electrical installation, after the second time delay the current value of the current of the electrical installation is subtracted from this value, comp the obtained first difference is obtained with the third reference value generated, characterized in that the phase of the current of the electrical installation is additionally constantly measured, when the electrical current of the second reference value is exceeded, the value of the phase of the electrical current is stored through the first time delay, the current value of the current phase of the electrical current is subtracted from this value compare the obtained second difference with the fourth generated reference value, if the second difference is less than the fourth reference led If the first difference is less than the third reference value, then a trip signal is given.

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