JPS6327927B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a protective relay with inrush current countermeasures that prevents the relay from operating due to the excitation inrush current of a transformer.

A conventional device of this type is shown in FIG. In FIG. 1, the outputs of current transformers 2, 3 located on both sides of the transformer 1 to be protected are fed to input transformers 5, 6, 7 of a protective relay 4. A ratio differential circuit section consisting of input transformers 5, 6, 7, rectifier circuits 8, 9, 10 and comparator circuit 11 calculates a differential current I for the sum of output currents I ₁ and I ₂ of current transformers 2 and 3. When the ratio of _D is above a set value, the output is supplied to the inhibit circuit 15. In addition, an in-rush current countermeasure circuit consisting of _{a 2-} pass filter 12, _{a 1-} pass filter 13, and a comparator circuit 14 has a differential current I _D
It operates when the ratio of the second harmonic (frequency ₂ ) to the fundamental wave (frequency ₁ ) is a certain ratio (generally 15%) or more, and the inhibit circuit 15 has a function for locking the output of the comparator circuit 11. Provides output. This prevents the protective relay from malfunctioning due to inrush current.

However, modern power systems are constructed by connecting power cables and large-capacity capacitors, and in the event of an internal fault in a transformer, the relationship between these capacitive impedances and the reactive impedance of the system or the transformer itself may be affected. A resonance phenomenon occurs in
Harmonic current flows.

As the capacity increases, the frequency of the harmonic current approaches the fundamental wave, and recently, second harmonics have started to be generated.

Therefore, even in the event of an internal fault in the transformer, the protective relay is locked by the inrush current countermeasure circuit, causing the problem that no operational output is produced.

This invention was made to eliminate the drawbacks of the conventional system as described above, and it can prevent transformer accidents by determining whether the second harmonic component of the grid voltage is greater than a certain ratio of the fundamental wave component. The purpose is to provide a protective relay that can be detected and protected.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2, reference numeral 16 denotes a potential transformer, which introduces the grid voltage and supplies it to an input transformer 17. 18 is _{a 2-} pass filter, 19 is _{a 1-} pass filter, and the comparator circuit 20 operates when the ratio of the second harmonic component to the fundamental wave component is a certain ratio or more;
Locks the signal output from the

Therefore, when the system voltage contains a certain proportion or more of the second harmonic component, the operation of the inrush current countermeasure circuit can be locked and the protective relay can be operated.

Figure 3 shows an example of actually measured waveforms of voltage and current when a transformer is excited from a no-voltage state. This shows that the voltage contains almost no second harmonic component. Therefore, in this case, no output is output from the comparator circuit 20 on the grid voltage side, and the output of the protective relay 4 is blocked by the function of the inrush current countermeasure circuit.

FIG. 4 is an explanatory diagram when an internal accident occurs at point F of the transformer 1, and the capacitive impedance 22 and
Reactive impedance L of transformer winding
A resonant current I flows. Assuming that this current is a combination of the fundamental wave component and the second harmonic component, and where k is the content ratio of the second harmonic component, it is expressed as i=√2Isinωt+√2kIsin(2ωt+α).

The voltage of the instrument transformer 16 is v=Ldi/dt =√2ωLIcosωt +√2k2ωLIcos(2ωt+α) becomes.

Therefore, the ratio between the fundamental wave component and the second harmonic component of the system voltage is m=2k, which is larger than the ratio k of the system current.

This 2k is detected by the comparator circuit 20 and the signal of the inrush countermeasure circuit is blocked.
Even if the inrush countermeasure circuit operates due to an internal fault and attempts to lock the output of the protective relay, it will be blocked and the output from the protective relay will be obtained, making it possible to detect and protect the transformer from internal faults. become.

As described above, according to the present invention, if the ratio of the second harmonic component to the fundamental wave component of the grid voltage is equal to or higher than a certain ratio, the determination of the infrastructural countermeasure circuit is locked, and an internal fault of the transformer is detected. Since the protective relay can be activated in response, reliable protection can be achieved.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional transformer protection relay with inrush countermeasures, Figure 2 is a circuit diagram of a transformer protection relay of the present invention, and Figure 3 shows current and voltage waveforms when the transformer is excited. The actual measurement diagram, FIG. 4, is a diagram illustrating the generation of harmonics in the event of an internal fault in the transformer. 1... Transformer, 2, 3... Current transformer, 4... Protective relay, 5, 6, 7, 17... Input transformer, 8, 9, 1
0... Rectifier circuit, 11, 14, 20... Comparison circuit, 1
2, 18... ₂ pass filter, 13, 19... ₁ pass filter, 15, 21... inhibit circuit, 16
...Instrument transformer, 22...Capacitive impedance.
In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A transformer configured to determine that the transformer is in an energized state and lock the operating output when the ratio of the second harmonic current included in the grid current in the power system to the fundamental current is equal to or higher than a predetermined value. In the protective relay, when the second harmonic voltage and the fundamental wave voltage included in the system voltage exceed a certain ratio, it is determined that the transformer is not in the excitation state, and locking of the operational output is stopped. Characteristic transformer protection relay.