JPH0487523A - Overload detector for ac filter - Google Patents

Abstract

PURPOSE:To judge an overload state by providing a filter passing frequency components to be absorbed in an AC filter, means for sampling the output signal therefrom and means for determining the effective value of sampled data, and then comparing thus determined effective value with a predetermined value. CONSTITUTION:A current transformer CT is connected with the earth wire of an AC filter circuit 7 and the overload state of the AC filter is detected based on an output signal from the current transformer CT. A filter 2 passes such frequency components of the output current from an auxiliary CT as to be absorbed by the AC filter 7. A sample & hold circuit 3 samples the output signals from the filter 2 at a predetermined time interval. A processing circuit 5 determines the effective value and when it exceeds a preset value, a judgment is made that an overload state has occurred and a detection signal is outputted. According to the constitution, heating of the AC filter is determined through operation and steep temperature rise is detected quickly, resulting in protection of the AC filter.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to an AC filter overload detection device used in a relay or the like that protects AC filter equipment from overload.

('b) Prior Art Conventionally, for example, in an AC/DC converter station for DC power transmission, AC filter equipment has been provided to reduce harmonic currents superimposed on the system. Such an AC filter is
Normally, a single tuned filter for each order for low-order harmonics below the 13th order is combined with a bypass filter for high-order harmonics exceeding the 13th order.When designing an AC filter, economic efficiency and filter efficiency are considered. Reactive power in the fundamental wave,
Taking into account the resonance between the filter and the system when the tuning shifts due to frequency changes in the AC system, etc., the allocation of capacitors to each shunt and the sharpness (Q) are determined to maximize the filtering effect. .

Generally, the capacity of an AC filter is designed in advance according to the content of harmonic current generated on the AC system side in AC/DC conversion equipment, but harmonic currents may be superimposed on the system through other paths. In some cases, unpredictable harmonic current components may flow into the AC filter, and the AC filter may become overloaded. Therefore, in the past, a temperature sensor was attached directly to the reactor, which generates the most heat among the AC filters, to detect the overheating state of the reactor.

(C) Problems to be Solved by the Invention However, the method using the temperature sensor cannot detect a sudden temperature rise. Conventionally, a current transformer is installed in each shunt of an AC filter, and overcurrent in each shunt is detected from the output of the current transformer. It was not possible to determine the heat loss of the AC filter, and therefore it was not possible to determine overload.

SUMMARY OF THE INVENTION An object of the present invention is to provide an AC filter overload detection device that accurately calculates the heat loss of an AC filter and detects an overload state.

(Means for solving the d1 problem) The heat generation of the accelerator and resistor is a particular problem when an AC filter is overloaded. Therefore, in order to provide overload protection, the harmonic component expressed by the following equation must be Ideally, a system that monitors heat loss, including heat loss.

ΣIn2Rn (n≧l)...(1) where n is the harmonic order, ln is the harmonic component of the current, R
n is the equivalent resistance for each harmonic.

However, the heat loss due to the fundamental wave current (n-1) exists as a base component with little fluctuation, and it may be considered that the heat rise is determined by the harmonic components. Therefore, it is sufficient to detect fluctuations in heat loss due to harmonic components.

The AC filter overload detection device of the present invention is a device connected to a current transformer provided in the ground wire of an AC filter that absorbs harmonic current of a predetermined order or higher harmonic, and A filter that passes the frequency component of the harmonic current of the order to be absorbed among the inflowing current, a quantization means that samples the output signal of this filter at regular time intervals and A/D converts it, and It consists of: an effective value calculation means for calculating the effective value of each sampling data; and a means for determining an overload state by comparing the calculated effective value with a predetermined value.

te1 action The first example of the configuration of the AC filter overload detection device of the present invention is as follows.
As shown in the figure. In FIG. 1, 7 is an AC filter circuit consisting of a capacitor CO, a reactor Lo, and a resistor Ro, and is connected between the system and ground. Further, a current transformer CT is connected to the ground wire of the AC filter circuit 7, and the AC filter overload detection device 6 detects the overload state of the AC filter from the output signal of the current transformer CT. In this example, the AC filter overload detection device 6 includes an auxiliary CT that further transforms the output of the current transformer CT, a filter 2, a sample hold circuit 3, an A/D conversion circuit 4, and an arithmetic processing circuit 5.
The filter 2 passes the frequency component of the harmonic current of the order absorbed by the AC filter circuit 7 out of the voltage conversion signal of the output current of the auxiliary CT. The sample hold circuit 3 and the A/D conversion circuit 4 correspond to "quantization means" and sample the output signal of the filter 2 at regular time intervals and convert it into digital data. The arithmetic processing circuit 5 corresponds to "effective value calculation means" and "overload state determination means", and calculates the effective value by calculating the root mean square of each sampled data in one cycle of the harmonic current flowing through the AC filter circuit 7. The value is determined, and if the effective value exceeds a predetermined set value, it is regarded as an overload condition and a detection signal is output. Note that the effective value calculation of the harmonic current does not necessarily have to be performed by calculating the square root of the square mean of each sampling data, but can be calculated as the square value of the effective value by performing the root mean square calculation of each sampling data in one cycle of the harmonic current. If the square value of the effective value exceeds a predetermined set value, it may be assumed that an overload condition exists and a detection signal may be output.

The effective value calculation means performs the effective value calculation as follows. First, the square value of the harmonic current component flowing through the AC filter circuit 7 that appears as the output signal of the filter 2 can be determined by the following equation.

Here, ω=2πf, f is the harmonic frequency, 1(1) is the current, i(m) is the current sampling data, and m is 2I! This is the number of sampling data when divided into equal parts.

Therefore, the amount of heat generated by the harmonic current of the AC filter circuit 7 is determined by the following equation.

Here, R is the equivalent resistance of the AC filter circuit 7.

(f) Embodiment FIG. 3 shows a circuit diagram of an AC filter facility according to an embodiment of the present invention, and FIG. 4 shows the configuration of an AC filter overload detection relay.

In Fig. 3, a circuit consisting of a capacitor C1, a reactor L1, and a resistor R1 is a fifth-order harmonic AC filter, a capacitor C2, reactors L21, L2, and a resistor R.
The circuit consisting of 2 is a 7th harmonic AC filter, and both AC filters are connected to the bus bar via a circuit breaker CBI. The circuit consisting of capacitor C3, reactor L3, and resistor R3 consists of 1) AC filter for harmonics, capacitor C4, reactor L41. The circuit consisting of L4 and resistor R4 is an AC filter for 13th harmonic, and further includes capacitor C5.
, resistor R5, and reactor L5 is a high-order harmonic AC filter, and these AC filters are connected to the bus bar via circuit breaker CB2. Furthermore, current transformers indicated by CTI to CT5 are provided in the ground wire of each AC filter.

The fourth AC filter overload detection relay is a device connected to each of the current transformers CTI to CT5 shown in FIG. 3 to detect the overload state of each AC filter. In Fig. 4, CTll-CT15 is the current transformer CTI~C shown in Fig. 3.
These are auxiliary current transformers respectively connected to the outputs of T5. 2
1 to 25 are band pass filters and bypass filters that allow only the tuning frequency components of each AC filter to pass among the output signals of the auxiliary current transformers CTII to CT15. The multiplexer 26 selects the output signal of each filter, the sample hold circuit 27 samples and holds the output signal, and the A/D converter circuit 28 converts the sampling signal into digital data and stores it in a predetermined area of the RAM 31 by DMA transfer. Store. CPU29
By executing a program written in advance in the ROM 30, overload detection control of the AC filter is performed. The RAM 31 is provided with an area for storing sampling data and a working area for other effective value calculations. The operation panel 33 is a panel that includes an operation section for setting a set price for determining an overload state and a display section for displaying the detection result of the overload state, and is controlled by the interface circuit 32. The printer 35 records the load status of each AC filter, and is controlled by the interface circuit 34. When an overload condition is detected, the contact output circuit 36 performs operations such as tripping the circuit breaker based on its output.

Since the current flowing through each AC filter provided with the current transformers CTI to CT4 shown in FIG. 3 is dominated by the fundamental wave component and the single harmonic current of each order, the filter 21 shown in FIG. 24 may be any filter that blocks at least the fundamental wave component. On the other hand, in addition to the fundamental wave component, the AC filter equipped with the current transformer CT5 shown in FIG.
Next, four high-order harmonic currents of 36±1st order flow in,
In this high-order harmonic AC filter, heat generation from the resistor R5 connected in parallel to the reactor L5 rather than the reactor L5 poses a problem. Therefore current transformer CT5
The shunt ratio of the signal detected by the resistor R5 must be considered. The frequency characteristic of this shunt ratio has bypass filter characteristics, and the bypass filter 25 shown in FIG. 4 acts as a circuit for correcting the frequency characteristic of the shunt ratio.

In addition, the sample hold circuit 27 shown in FIG. The CPU 29 performs the calculation shown in equation (3) or equation (5) on one cycle of sampling data of the harmonic signal stored in the buffer area, and determines the overload of the AC filter by comparing it with the set value. Determine the condition.

(g1 Effects of the Invention According to this invention, since the heat generation of the AC filter can be determined by calculation, it is possible to quickly detect a sudden temperature rise and protect the AC filter. Since the heat generation is calculated by calculating the effective value of the harmonic current flowing through each AC filter,
For example, compared to the calculation method using Fourier integral and the method using matrix calculation, there is no error in principle, the calculation is easy, the time required for calculation is relatively short, and it is possible to use a common calculation method for each branch. It has the advantage of being possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a configuration example of the present invention, and FIG. 2 is a diagram for explaining its operation. FIG. 3 is a circuit diagram of an AC filter facility to which an AC filter overload relay according to an embodiment of the present invention is applied, and FIG. 4 is a configuration diagram of an AC filter overload relay according to an embodiment of the present invention.

Claims (1)

[Claims] (1) A device connected to a current transformer installed in the ground wire of an AC filter that absorbs harmonic current of a predetermined order or higher harmonic, which absorbs the current flowing into the AC filter. a filter that passes frequency components of harmonic current of orders of 1 to 3; quantization means that samples the output signal of this filter at fixed time intervals and A/D converts it; and quantization means that calculates the effective value of each of the above sampled data in one cycle. An AC filter overload detection device comprising: effective value calculation means; and means for determining an overload state by comparing the calculated effective value with a predetermined value.