JPH0450667A - Overcurrent detecting circuit of transformer - Google Patents

Abstract

PURPOSE:To detect short-circuit closing quickly and accurately and to make it possible to alleviate the stress due to the short-circuit current as a result by detecting the level and the frequency of the primary current of a transformer. CONSTITUTION:The current flowing through the primary side of a transformer 3 is rectified in a rectifier 5 which is connected through a current detector 2, and the current is detected. In a frequency detecting means, the low level of the current which is rectified in the current detecting means is cut in a comparator 7, and the current is converted into the voltage signal corresponding to the frequency in an F/V converter 8. The signal is compared with the specified preset value in a comparator 9. Thus, the frequency of the primary current of the transformer is detected. At this time, a voltage corresponding to the value twice the voltage without a load input is set in a setter S2. The current which is rectified in the current detecting means is compared with the specified preset value in a comaprator 13 in a level detecting means. Thus, the magnitude level of the primary current of the transformer is detected. At this time, the voltage corresponding to the no-load input level is set in a setter S3.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is applicable to power transmission/distribution transformers, AC arc furnace transformers, etc. that are susceptible to short-circuits, and are designed to prevent overcurrents associated with short circuits when the power is turned on. The present invention relates to a detection circuit for detecting.

[Conventional technology]

Figure 2 is a schematic diagram showing a general example of power supply equipment using a transformer, where 1 is a breaker, 2 is a current detector, 3 is a transformer, 4 is a load, and L is an overcurrent detection relay. (overcurrent relay).

That is, in many cases, an overcurrent relay is connected to the primary side of the transformer 3 via the current detector 2, thereby detecting overcurrent. In other words, an excitation rush current flows through the transformer 3 when the TS side breaker 1 is turned on, and in order to prevent the overcurrent relay from operating due to this current and opening the power supply side breaker 1, the excitation rush current is A method is to set the current relay to a high value with sufficient margin to prevent it from operating, or a method is to combine an overcurrent relay and a time-limited relay to delay the operation of the relay until the excitation rush has attenuated. Furthermore, malfunctions are prevented by short-circuiting the relay coil so that no current flows through the relay.

[Problem to be solved by the invention]

In this way, with conventional methods, the setting value is increased more than necessary to prevent the relay from malfunctioning when the breaker is turned on, and the relay is blinded to short circuits until the excitation rush has attenuated, making it difficult to protect the equipment. There is a problem with not liking it.

Therefore, an object of the present invention is to provide a detection circuit that does not operate with excitation rush current alone and is capable of detecting an excessive current due to a short circuit, thereby eliminating the above-mentioned disadvantages and protecting equipment.

[Means to solve the problem]

A rectifier is connected to the primary winding side of the transformer via a current detector, and the output side of the rectifier is provided with frequency detection means for detecting the frequency of the primary current and level detection means for detecting the level of the primary current. , it is possible to detect an overcurrent due to a short circuit when the power is turned on by using the AND output from both of these means.

[Effect]

To easily and quickly detect an overcurrent accompanied by a short circuit that does not operate with only an excitation rush current.

Here, the mechanism of generating the excitation rush in the single-phase transformer 3A as shown in FIG. 3 will be explained with reference to FIG. 4.

Now, switch IA on the primary side of single-phase transformer 3A in Figure 3
When closing and applying voltage e, let the core magnetic flux be φ and the number of primary turns be n, and the following equation holds between them.

t dφ Therefore, if e −E, sin ωt, then φ = −φ, cos ωt + c (constant), and in the steady state (c = O), e and φ are the fourth
It can be expressed as shown in figure (a). Now, time t=
Assuming that the excitation of the transformer is started at Q, if the residual magnetic flux before that is zero, 1 = 0 and φ = 0, so the magnetic flux φ changes as shown in the same figure (b), and the transformer As a result, the excitation rush current i
flows as shown in the figure. Note that this excitation current i actually gradually attenuates due to losses such as winding resistance and iron loss of the transformer.

On the other hand, if a short circuit is applied, the current at that time is the sum of the short circuit current and the excitation current, and this also gradually attenuates, but when the breaker is short-circuited and when no load is applied, the primary side of the transformer is The waveform of the flowing current is different. In other words, if we rectify these currents and compare their frequencies above the level, as shown in Figure 4 (c), when a short circuit is applied, the frequency is twice as high as when no load is applied, so if this is detected, a short circuit is detected. The principle of this invention is that insertion can be detected.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the invention. In the figure, 5 is a rectifier, 6 is a variable resistor for making the output of the rectifier 5 a constant level voltage signal, 7.9.13
1 is a comparator, 8 is a frequency-voltage (F/V) converter, 10 is a flip-flop, 11 is an amplifier (AMP), 12 is a relay, and SI.

S2. S3 is a setting device for setting comparison voltages for the comparators 7, 9.13, AND is an AND gate, SW is a switch for resetting the flip-flop 10, and other symbols are the same as in FIG. 2. .

That is, this embodiment mainly consists of current detection means for detecting the current flowing through the primary side of the transformer 3, frequency detection means for detecting the frequency of the current, and level detection means for detecting the level of the current. ing.

The current detection means rectifies and detects the current flowing through the primary side of the transformer 3 with a rectifier 5 connected via the radio wave detector 2. The frequency detection means cuts the low level of the current rectified by the current detection means with a comparator 7, converts it into a voltage signal corresponding to the frequency with a known F/V converter 8, and converts the current into a voltage signal corresponding to the frequency with a comparator 9. The frequency of the transformer primary current is detected by comparing it with a predetermined set value. At this time, the setting device S2 is set to a voltage that is twice as high as when no load is applied.

Further, the level detection means detects the magnitude (level) of the transformer primary current by comparing the current rectified by the current detection means with a predetermined setting value using the comparator 13. At this time, a voltage corresponding to the no-load turn-on level is set in the setting device S3. The AND is established when both the frequency and the level are above a certain value, and as a result, the relay 12 is energized via the flip-flop 10° AMP 11, and the breaker 1 is opened by its auxiliary contact.

In this way, it is not necessary to wait for the excitation rush to decay to a certain value in order to detect the short circuit current, so that the detection is carried out quickly.

〔Effect of the invention〕

According to this invention, since the level and frequency of the transformer primary current are detected, it is possible to detect a short circuit more quickly and accurately than before, and as a result, the stress caused by the short circuit current in the transformer is reduced. This has the advantage of being possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a schematic diagram showing a general example of power supply equipment using a transformer, and FIG. 3 is an equivalent circuit diagram showing a single-phase transformer. FIG. 4 is an explanatory diagram for explaining the operation. 1... Breaker, 2... Current detector, 3... Transformer, 4... Load, 5... Rectifier, 6... Variable resistor, 7,913... Comparator , 8...frequency-voltage (
F/V) converter, 10... flip-flop, 11.
...Amplifier (AMP), 12-mN device, AND...AND gate, S1-S3...setting device, SW...switch, L...m electric device for overcurrent detection.

Claims (1)

[Claims] 1) A rectifier is connected to the primary winding side of the transformer via a current detector, and the output side of the rectifier includes frequency detection means for detecting the frequency of the primary current, and level detection means for detecting the level of the primary current. An overcurrent detection circuit for a transformer, characterized in that an overcurrent accompanied by a short circuit when the power is turned on can be detected by the AND output from both of these means.