JPS6070918A - Overcurrent relay - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field This invention relates to an overcurrent relay for disconnecting a transformer from a power system in the event of a short-circuit accident or the like of a load connected to the transformer.

(bl Conventional technology and its disadvantages) At the receiving end of a high-voltage power system, a transformer is connected via a circuit breaker, and an overcurrent relay is installed between the circuit breaker and the transformer to prevent overloads and short circuits. Figure 1 shows a one-phase current transformer CT installed between a circuit breaker and a transformer, and a conventional overcurrent relay OCR connected to this current transformer CT. It is a block diagram showing the overcurrent relay OCR.The overcurrent relay OCR includes a time-limited element detection circuit 1 that detects an overcurrent caused by an overload, and an instantaneous element detection circuit 2 that detects a transient overcurrent caused by a sudden short circuit accident. have.

The time limit element detection circuit 1 is connected to a trip relay X via an inverse time limit circuit 3 for setting a trip time according to the magnitude of the detected overcurrent. Therefore, when the time-limiting element detection circuit 1 detects an overcurrent due to an overload via the current transformer CT, a trip signal is supplied to the trip relay X according to the magnitude of the overcurrent, and a circuit breaker (not shown) is activated. Open. Additionally, if a short circuit occurs in a load connected to a transformer (not shown), the operating time of the instantaneous element will be several tens of seconds.
Since the time is set to be very short, m seconds, the instantaneous element detection circuit 2 operates to open the circuit breaker before the time-limited element detection circuit 1 operates.

However, when the circuit breaker is turned on and the transformer is connected to the power grid, the magnetic saturation of the transformer's core causes a transient excitation inrush current that is several times the rated current for several cycles to several seconds. This excitation inrush current is detected by the instantaneous element detection circuit 2.
may detect an overcurrent & cause the relay OCR to malfunction. In particular, as transformers have become smaller in recent years and iron core materials with high magnetic permeability have been used to improve magnetic characteristics during steady state, the excitation inrush current has increased, leading to an increase in malfunctions of overcurrent relays (OCR).

Therefore, in order to prevent the overcurrent relay OCR from malfunctioning, it is sufficient to increase the setting value of the operating current of the instantaneous element detection circuit 2. It's not appropriate. Another method is to detect the second harmonic component of the magnetizing inrush current and suppress the overcurrent relay OCR, but since the short circuit current also contains the second harmonic component, the magnetizing inrush current and short circuit current cannot be completely determined. For this reason, the conventional overcurrent relay OC
R had the disadvantage that it could not effectively prevent malfunctions when the circuit breaker was closed.

(C1 Purpose of the Invention This invention was made in view of the above-mentioned circumstances, and its purpose is to provide an overcurrent relay that can prevent malfunctions caused by magnetizing inrush current of a transformer when a circuit breaker is closed. In summary, an inverse time-limiting circuit that provides an inverse time-limiting characteristic that makes the operating value of the instantaneous element detection circuit a larger value over the entire operating range due to the magnetizing inrush current of the transformer connected in series with the circuit breaker, It is characterized by being connected to an element detection circuit.

(El Embodiment FIG. 2 is a block diagram of an overcurrent relay according to an embodiment of the present invention connected to a current transformer CT, and FIG. 3 is a diagram showing operating time characteristics of the same overcurrent relay.

In FIG. 2, this overcurrent relay OCR is the conventional overcurrent relay OCR shown in FIG. 1 provided with an instantaneous element inverse time limit circuit 4. The inverse time limit circuit 4 has almost the same configuration as the inverse time limit circuit 3, which is a known time limit element, but the time constant is set small in order to improve responsiveness. By connecting the inverse time limit circuit 4 to the subsequent stage of the instantaneous element detection circuit 2,
The trip signal supplied from the instantaneous element detection circuit 2 to the trip relay X has an inverse timing characteristic as shown by the solid line in FIG. Therefore, the instantaneous element detection circuit 2
The operating value of is larger than the magnetizing inrush current of the transformer over the entire operating range. In other words, in a range where the overcurrent value is large, it has a limited time characteristic that supplies a trip signal in several tens of milliseconds, which is the same as the conventional overcurrent relay shown by the dashed line, but even within the protection range of the instantaneous element detection circuit 2. In the range where the overcurrent value is small, it has an inverse time limit property. Due to this inverse timing property, the instantaneous element operating characteristic curve does not intersect with the transformer's magnetizing inrush current characteristic curve shown by the two-dot chain line, unlike the conventional instantaneous element operating characteristic curve shown by the one-dot chain line. The magnetizing inrush current of the transformer attenuates over time as shown in FIG. When the circuit breaker is turned on and connected to the power supply, a current of peak value A, which is more than ten times the rated current, flows through the transformer for the first 1 to 2112, and then gradually decreases to 2.
It settles down to steady value B after ~3 seconds. Therefore, if the instantaneous element operating characteristic curve intersects the attenuating magnetizing inrush current characteristic curve as in conventional overcurrent relays, a malfunction will occur, but in the overcurrent relay OCR of this invention, the instantaneous element operating characteristic curve has an inverse time limit characteristic. Since the excitation inrush current does not intersect with the excitation inrush current characteristic curve, the excitation inrush current will not cause malfunction. In use, the setting value at which the instantaneous element operating characteristic is a limited time operation of several tens of milliseconds is set smaller than the current value at the time of a short circuit on the secondary side, which is defined by the % impedance of the transformer connected in series with the circuit breaker. Approximate values are 20 to 25 of the transformer's rated current, assuming the transformer's % impedance is 4%.
It's double. Therefore, from the short circuit current defined by the transformer impedance to the abnormal short circuit current caused by the resonance between the transformer impedance and the connected load, the circuit breaker has a limited time characteristic of several tens of milliseconds. Open. For a fault current smaller than the current at the time of a short circuit on the secondary side defined by the % impedance of the transformer, as shown in FIG. 3, an inverse time-limiting characteristic occurs and the circuit breaker is opened. In addition, in order to have an appropriate inverse time characteristic, the slope of the inverse time characteristic part of the instantaneous element operation characteristic curve is adjusted so that the slope of the inverse time limit characteristic part is approximately parallel to the excitation inrush current characteristic curve as shown in FIG. It is desirable to set constants, etc.

Effects of ffl Design As described above, according to the present invention, the inverse time characteristic makes the operating value of the instantaneous element detection circuit larger than the magnetizing inrush current of the transformer connected in series with the circuit breaker over the entire operating range. Since the inverse time-limiting circuit that gives the following is connected to the instantaneous element detection circuit, there is an advantage that malfunctions caused by the excitation inrush current of the transformer when the circuit breaker is closed can be completely prevented.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional overcurrent relay, Fig. 2 is a block diagram of an overcurrent relay according to an embodiment of the present invention, and Fig. 3 is a block diagram of a conventional overcurrent relay.
The figure shows the operating characteristics of the same overcurrent relay. CT → Current detector, 0CR-Overcurrent voltage detector, X-Trip relay. Applicant Tateishi Electric Co., Ltd. Agent Patent Attorney Hisao Komori

Claims (1)

[Claims] +11 The time-limited element detection circuit or the instantaneous element detection circuit detects, via a current transformer, that the current in the electric path exceeds the set value of the time-limited element detection circuit or the set value of the instantaneous element detection circuit. Then, in the overcurrent relay that trips the circuit breaker by tripping the trip relay, the operating value of the instantaneous element detection circuit is used to excite the transformer connected in series with the circuit breaker over the entire operating range. An overcurrent relay characterized in that an inverse time limit circuit that provides an inverse time limit characteristic that increases the inrush current to a larger value is connected to the instantaneous element detection circuit. (2) The overcurrent relay according to claim 1, wherein the inverse time limit circuit is set to a time constant whose inverse time characteristic is substantially parallel to the excitation inrush current characteristic curve.