JPH03265424A - Overvoltage suppressor - 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] [Objective of the Invention] (Industrial Application Field) The present invention reduces overvoltages that appear in power systems due to the saturation current of transformers when a transformer is turned on or after a system fault has been removed. The present invention relates to an overvoltage suppression device.

(Conventional technology) When a transformer is turned on, the magnetic flux in the iron core becomes biased and saturated.
It is well known that a so-called excitation inrush current flows. This excitation rush current has a positive/negative asymmetric waveform, such as a part of a sine wave flowing only on one side, and in addition to the DC component and the basic distribution component, this excitation rush current has 2nd, 3rd, 4th, etc. Includes harmonics.

For example, when the AC system resonates with the 3rd harmonic and the impedance of the 3rd harmonic is large and the transformer is turned on, the excitation inrush current of the transformer flows and the 3rd harmonic current flows into the AC system. Inflow A third-order harmonic voltage appears that is determined by the product of the third-order current and the third-order impedance. This harmonic voltage will be superimposed on the fundamental voltage, and if the excitation rush is large or the harmonic impedance of the system is large, an undesirable overvoltage will occur.

Even when a fault occurs in an AC system and the fault is removed, an excitation rush current flows through the transformer, so an undesirable overvoltage still appears after the fault is removed.

For example, assume that a three-phase ground fault occurs in one of two power transmission lines. Here, when an accident occurs, the AC voltage drops to zero during the accident, and the internal magnetic flux of the transformer connected to it does not change. When the power transmission line where the fault occurred is disconnected and the fault is removed, the alternating current voltage returns to its original voltage state because one line remains. At that time, the original voltage is also applied to the transformer, the internal magnetic flux is saturated, and an excitation rush current flows.

Conventionally, when overvoltage occurs due to the excitation current of a transformer, it is necessary to strengthen the AC system by connecting generators in parallel or increasing the number of parallel transmission lines and host transformers.
Methods have been taken to keep the harmonic impedance of the grid low, or to insert filters to shift the resonance point of the grid.

(Problems to be Solved by the Invention) Recently, efforts have been made to improve equipment utilization and efficiency design even in AC systems, and it has become difficult to easily strengthen the systems. In addition, large-capacity transformers are being installed at the end of the system, and overvoltage due to the above-mentioned excitation rush current of the transformer has become a problem.

Conventionally, lightning arresters have been used to suppress excessive overvoltage during lightning surges and coordinate the protection of equipment. However, in recent years, gapless lightning arresters with good performance have been produced, and the use of these to suppress overvoltage due to the excitation rush current of transformers has been considered. The overvoltage here is 1.5 to 1.5 compared to the steady operating voltage IPu.
1.7 PU]/bell, and when this level is set as the operating range, a normal lightning arrester has a large amount of leakage current at all times and cannot be used in terms of energy capacity. Therefore, one possible method is to connect a breaker to the lightning arrester so that the lightning arrester operates only when necessary.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an overvoltage suppression device that suppresses overvoltage in a system caused by magnetizing inrush current of a transformer.

[Structure of the Invention] (Means for Solving the Problem) In order to achieve the above object, the present invention issues a closing command to a breaker to connect a lightning arrester to the grid when overvoltage occurs due to excitation inrush current. It was configured as follows.

(Function) With this configuration, when an overvoltage occurs due to the transformer excitation rush when the transformer is turned on or when an AC fault is removed, the overvoltage is suppressed by the lightning arrester.

(Example) Examples will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of an overvoltage suppression device according to the present invention.

In Figure 1, 1 is a breaker, 2 is a lightning arrester, 3 is an AC bus, 4 is a turn-on command signal for a transformer (not shown), 5 is an undervoltage relay that detects a voltage drop on the AC bus 3, and 6 is a transformer. This is an OR circuit that ORs the circuit breaker closing command and the output of the undervoltage relay and gives a closing command to the breaker 1.

Next, the action will be explained.

First, when a transformer (not shown) is to be turned on, a transformer closing command signal 4 is given, which enters the OR circuit 6, and a closing command is given to the circuit breaker 1 from the OR circuit.
A lightning arrester 2 is connected to an AC bus 3. In this case, an excitation rush current flows when the transformer is turned on, and the harmonic current contained therein flows into the AC system, so harmonic voltages appear as described above, and an overvoltage as shown by the broken line in Figure 6 is generated at the AC bus line. appears in

FIG. 5 shows the voltage-current (V-I) characteristics of the lightning arrester 2, and the voltage is approximately constant in the operating region.

When the lightning arrester 2 is connected, the overvoltage is suppressed by the operating voltage determined by the characteristics of the lightning arrester 2, and the peak overvoltage is cut off as shown by the solid line in FIG.

Next, when a grid fault occurs, the grid voltage drops once due to the fault, and the undervoltage relay 5 is activated accordingly.

When the undervoltage relay 5 operates, the circuit breaker 1 is closed, so that the same effect as when the transformer is closed is brought about, as shown in FIG. 6, against the overvoltage generated by the transformer excitation rush after the fault has been removed.

According to the above embodiment, the breaker is normally opened to prevent voltage from being applied to the arrester, thereby preventing excess energy withstand capacity due to leakage current of the arrester, and preventing it from occurring only when the excitation rush current of the transformer flows. The effect of suppressing overvoltage peaks to the operating range of the lightning arrester can be obtained.

FIG. 2 is a block diagram of another embodiment according to the present invention.
The same parts as those in the figures are given the same reference numerals and the explanation will be omitted.

In this embodiment, when an overvoltage occurs due to the excitation inrush current of the transformer, the lightning arrester is intended to be turned on.

In FIG. 2, 7 is a voltage transformer PT, and 8 is an overvoltage relay.

FIG. 3 is a configuration diagram of the overvoltage relay 8, where 81 is a level detector [D, 82 is an off delay, 183 is an on delay, 184 is NOT, 85 is an on delay, and 186 is 0.
R287 is a flip-flop.

Next, the action will be explained.

When harmonic currents included in the excitation rush of the transformer flow into the AC system, a transient overvoltage as shown by the broken line in FIG. 6 appears on the AC bus 3, similar to the embodiment described above. This voltage is PT7
Level detector LD21 in overvoltage relay 8 via
given to. The level detector [D81 outputs an output when the set value ID is exceeded, as shown in FIG. By having a timer longer than one cycle of the fundamental wave, the off-delay 82 has a timer longer than one cycle of the fundamental wave, as shown in FIG.
The output of 2 continuously detects voltage peaks. onday 1
In No. 83, the output is output when the output of the off-delay 82 continues for a preset time. That is, it is detected that the peak overvoltage has continued for a preset period of time, and "1" is applied to the set terminal S of the flip-flop 87. Then, "1" is set to the output terminal Q of the flip-flop 87, which turns on the circuit breaker 1 and connects the lightning arrester 2 to the grid. As shown in FIG. 5, the voltage is approximately constant in the operating region. When the lightning arrester 2 is connected, the overvoltage is suppressed by the operating voltage determined by the characteristics of the lightning arrester 2, and the peak overvoltage of the system voltage is cut as shown by the solid line in FIG. 6.

When the overvoltage of the system becomes small and the output of the LD shown in FIG. 3 ceases, the output of the off-delay 82 also becomes zero, and the output of the on-delay 83 also becomes zero. Then onday-83
The zero output of is inverted by NOT 84 and sent to OR circuit 8.
G, reset the flip-flop 87) Give "1" to one terminal R, and set the output Q of the flip-flop 87 to "1".
Set I2 to 0'' and open breaker 1.

Lightning arresters have a thermal energy withstand capacity, and if this capacity is exceeded, the arrester will be destroyed, so if the overvoltage continues for a long period of time that exceeds the thermal energy capacity, it is necessary to take measures to prevent destruction.

On-delay 83 output comes out, flip-frog 87
When the circuit breaker 1 is turned on via the circuit breaker 1, the other on-delay 85 detects whether the output of the circuit breaker 83 continues for a preset time and outputs an output "1". The output of the on-delay 85 passes through the OR circuit #I86 and gives "1" to the reset terminal R of the flip-flop 87, setting the output Q of the flip-flop 87 to "0" and opening the circuit breaker 1.

According to this embodiment, as in the previous embodiment, it is possible to suppress the overvoltage peak to the operating range of the lightning arrester only when an overvoltage occurs. Furthermore, the effect of preventing the surge arrester from exceeding its energy capacity after operation can be obtained.

Effects of the Invention] As explained above, according to the present invention, the surge arrester is connected only when an overvoltage occurs, so that the overvoltage can be reduced without causing the surge arrester to exceed its energy withstand capacity. A suppressor can be provided.

[Brief explanation of drawings]

Fig. 1 is a diagram showing one embodiment of the overvoltage suppression device according to the present invention, Fig. 2 is a block diagram of another embodiment, Fig. 3 is a diagram showing the internal configuration of an overvoltage relay, and Fig. 4 is the overvoltage detection principle. FIG. 5 shows the voltage-current characteristics of the arrester, and FIG. 6 shows the voltage waveform when the arrester operates. 1... Breaker 2... Lightning arrester 3... AC bus 4... Transformer closing command signal 5... Undervoltage relay 6... OR circuit 7...
Voltage transformer PT 8... Overvoltage relay 81...
・Level detector [D 82...Off delay 84...N0T 86...OR circuit 83...On delay 85...On delay 87...Flip-flop

Claims (1)

[Claims] In an overvoltage suppression device that suppresses overvoltage in an AC system caused by the magnetizing inrush current of a transformer, when an overvoltage occurs due to the magnetizing inrush current, a closing command is issued to the surge arrester to connect the surge arrester to the grid. An overvoltage suppression device featuring: