JPS6327928B2 - - Google Patents

Description

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

A conventional device of this type is shown in FIG.

In FIG. 1, the outputs of current transformers 2 and 3 provided on both sides of the transformer 1 to be protected are connected to the protective relay 4.
input transformers 5, 6, and 7. 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. Also ₂ bus filter 1
_2.1 The inrush current countermeasure circuit consisting of the bus filter 13 and the comparison circuit 14 has a constant ratio (generally 15%) of the second harmonic (frequency ₂ ) to the fundamental wave (frequency ₁ ) of the differential current _ID . ) It operates in the above cases and supplies the inhibit circuit 15 with an output for locking the output of the comparator circuit 11. This prevents the protective relay from malfunctioning due to inrush current.

However, recent power systems are constructed by connecting power cables and large-capacity capacitors, and in the event of an internal fault in a transformer, the capacitive impedance of these and the reactive impedance of the system or transformer itself may be affected. A resonance phenomenon occurs and 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 in order to eliminate the drawbacks of the conventional ones as described above.
Whether the harmonic component is more than a certain ratio of the fundamental wave component,
Another object of the present invention is to provide a protective relay that detects and protects a transformer from a fault by determining whether the magnitude of the system voltage has fallen below a certain value.

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 the input transformer 17. 1
8 is _{a 2-} pass filter, 19 is _{a 1-} pass filter, and a comparison circuit 20 operates when the ratio of the second harmonic component to the fundamental wave component is above a certain ratio, and the OR circuit 2
2, is input to the inhibit circuit 23,
The signal output from the comparison circuit 14 is locked.

On the other hand, the output of the input transformer 17 is input to the level detection circuit 21, and when the system voltage becomes below a certain value, a detection signal is outputted, and is input to the inhibit circuit via the OR circuit 22, and is then input to the comparison circuit 14.
Locks the signal output from the

Therefore, if the grid voltage contains more than a certain proportion of the second harmonic component, or if the grid voltage falls below a certain value, the operation of the in-rush current countermeasure circuit is locked and the protective relay is enabled to operate. be able to.

Figure 3 shows an example of actually measured waveforms of voltage and current when the transformer is excited from a no-voltage state. This shows that the system 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.

Figure 4 is an explanatory diagram when an internal fault occurs at point F of transformer 1, and the capacitive impedance 24
, and the reactive impedance L of the transformer winding, a oscillating current I flows. Assuming that this current is a combination of the fundamental wave component and the second harmonic component, it is expressed as i=√2Isinωt+√2kIsin(2ωt+α), where k is the content ratio of the second harmonic component.

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, so even if the inrush countermeasure circuit operates due to an internal accident and attempts to lock the output of the protective relay,
As a result, an output from the protective relay can be obtained, so that internal faults in the transformer can be detected and protected.

In addition, if the capacitance of the capacitive impedance 24 is large and, conversely, the reactive impedance L of the transformer is small, the current for the second harmonic will flow, but the voltage value of the voltage transformer 16 itself will become small. There is a possibility that the circuit that compares the second harmonic component and the fundamental component may not operate properly. In this case, if the grid voltage falls below a certain value, the level detection circuit that detects this can prevent the operation of the inrush countermeasure circuit and detect and protect the transformer from internal faults. .

As described above, according to the present invention, if the ratio between the second harmonic component and the fundamental wave component of the grid voltage is a certain ratio or more, or if the magnitude of the grid voltage is below a certain value, the inrush countermeasure is taken. Since the judgment of the circuit is locked and the protective relay can be activated in the event of an internal fault in the transformer, reliable protection can be achieved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional protective relay with inrush countermeasures, Fig. 2 is a circuit diagram of a protective relay of the present invention, Fig. 3 is an actual measurement diagram of current and voltage waveforms when a transformer is excited, and FIG. 4 is a diagram illustrating the generation of harmonics when an accident occurs inside a transformer. In the figure, 1... transformer, 2, 3... current transformer, 4
...protective relay, 5,6,7,17...input transformer,
8, 9, 10... Rectifier circuit, 11, 14, 20... Comparison circuit, 12, 18... ₂ bus filter, 13, 1
9... ₁ pass filter, 15, 23...inhibit circuit, 16...instrument transformer, 21...level detection circuit, 22...OR circuit, 24...capacitive impedance. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A transformer protection relay configured to determine that the transformer is in an energized state and lock the operational output when the ratio of the second harmonic current and the fundamental wave current included in the grid current in the power system is greater than or equal to a predetermined value. When either the second harmonic voltage and the fundamental wave voltage included in the grid voltage are at a certain ratio or higher, or the magnitude of the grid voltage is below a certain voltage value, the transformer is activated. A transformer protection relay is characterized in that it determines that the transformer is not in an energized state and stops locking the operational output.