JPH0956073A - Self-excited var compensator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the cost and space of a self-excited var compensator by eliminating the need of a phase-to-phase reactor by connecting the high-voltage sides of multiple transformers for square-wave inverters in series with the high- voltage sides of multiple transformers for PWM inverters. SOLUTION: The high-voltage sides Pa and Pb of transformers Ta and Tb for square-wave inverters are connected in series and square-wave inverters 1a and 1b are respectively connected to the low-voltage sides Qa and Qb of the transformers Ta and Tb. In addition, the high-voltage side Pc of a transformer Tc for a PWM inverter is connected in series to the high-voltage sides Pa and Pb of the transformers Ta and Tb and a PWM inverter 2 is connected to the low-voltage side Qc of the transformer Tc. The voltages outputted from the inverters 1a, 1b, 2 are outputted to a power source Vs side respectively through the transformers Ta, Tb, and Tc. Therefore, the cross current which is generated by the unbalance between the output voltages of the transformers Ta and Tb is not generated and no phase-to-phase reactor is required, because no circulating current is generated between the transformers Ta and Tb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited reactive power compensator suitable for power system stabilization and flicker of a load with large reactive power fluctuations such as in an arc furnace, and as a countermeasure against voltage fluctuations.

[0002]

2. Description of the Related Art In a power system, a variable load such as a large-capacity arc furnace, electric train load, or steel rolling load that causes a system voltage fluctuation due to a reactive power fluctuation fluctuates between the system power supply and the fluctuating load. A reactive power compensator for compensating reactive power due to a load is provided, and an example thereof is a self-excited reactive power compensator (hereinafter referred to as a self-excited SVC).
As shown in FIG. 2, the self-excited SVC includes two square wave inverters (1a) and (1b), one high frequency PWM inverter (2), and a control command unit (not shown). The inverters (1a) (1b) (2) are connected in parallel to a reactive power generation load (not shown) such as an arc furnace by connecting to the grid power supply (Vs) via the grid interconnection transformer (Tm). To be done.
Note that (Xs) is the system impedance.

The transformer (Tm) is a transformer for square wave inverter (Ta) (Tb) and a transformer for PWM inverter (Tc).
The transformer (Ta) (Tb) for the square wave inverter is provided with multiple high voltage side (primary side) (Pa) (Pb) connected in parallel via the interphase reactor (L), and is connected to the reactor (L). It is connected to the system power supply (Vs) via the system impedance (Xs), and square wave inverters (1a) (1b) are connected to each low voltage side (secondary side) (Qa) (Qb). The PWM inverter transformer (Tc) is connected in series to the parallel connection point of the square wave inverter transformers (Ta) (Tb), and the PWM inverter (2) is connected to the low voltage side (Qc). It is connected.

The square wave inverters (1a) and (1b) are connected to the low-voltage side (Qa) (Qb) of the transformers (Ta) (Tb), respectively, and are DC-charged by a converter or the like, as described above. It is a large-capacity low-loss low-speed response inverter that generates a fixed square wave base voltage (Vp) of a fundamental frequency (50, 60Hz) from a DC voltage of a capacitor (not shown). As described above, the PWM inverter (2) is connected to the low-voltage side (Qc) of the transformer (Tc) and compensates the reactive current from the DC voltage of the capacitor (not shown) that is DC-charged by the converter or the like. It is a small-capacity, high-loss, high-speed response inverter that generates a high-frequency variable square-wave voltage component (Vq) to generate a.

At this time, the transformer (Ta) for the square wave inverter
By changing the output voltage phase of (Tb) and making parallel multiple connections, the harmonic components of the compensating currents (Ia) and (Ib) flowing from the transformers (Ta) (Tb) to the power supply side are canceled and reduced.
Even if two square wave inverters (1a) and (1b) or transformers (Ta) and (Tb) have the same capacity, if each output voltage becomes unbalanced due to an error or the like, each transformer (Ta)
A cross current (It) that recirculates between (Tb) occurs, which does not flow to the power supply side and flows into the transformer (Ta) or (Tb), which exceeds the transformer withstand capacity and causes unexpected failures. is there. Therefore, the interphase reactor (L) is placed between the transformer (Ta) and (Tb).
And adjust its impedance value to adjust the cross current (I
t) is controlled so that it easily flows to the power supply side, and the cross current (I) that flows between the transformers (Ta) and (Tb) is controlled by the reactor (L).
suppress t).

Further, in general, in the case where the transformers are serially multiplexed, the sharing of the series voltage applied to each transformer when the system is connected (when the switch is ON) is determined by the exciting impedance ratio of the transformer. On the other hand, on the low voltage side, each inverter (1a) (1b) (2)
Since the voltage required for this is specified in advance, if an error occurs in the above excitation impedance ratios, the voltage on the low-voltage side of the transformer will also become improper and the inverter (1a) (1b) (2) will experience an undesired overvoltage. May be added. Therefore, conventionally, a gap has been provided in the transformer core or an exciting impedance adjusting reactor has been provided on the low voltage side, and the transformer (Ta) (Tb) has a PW.
When the transformer (Tc) for M inverter is connected in series, the excitation impedance ratio is adjusted to an appropriate value to adjust the series voltage sharing. At this time, if the transformers (Ta) (Tb) are connected in parallel multiplex to form a series multiplex configuration with the transformer (Tc), three transformers will have two stages in the series configuration, and thus the series voltage will be shared. Adjustment becomes easy.

According to the above configuration, when the load current is detected and the reactive power generation due to the load fluctuation is detected, the compensating reactive power (Q) is generated in each inverter (1a) (1b) (2),
It is supplied to the system bus through the transformer (Tm) to cancel the reactive power due to load fluctuation. The reactive power (Q) is
Determined by Q = {Vs · (Vp + Vq) / Xs}, the voltage component (Vq) by the PWM inverter (2) is variably controlled and set appropriately.

[0008]

A problem to be solved by the invention is that when two transformers (Ta) (Tb) for a square wave inverter are connected in parallel and multiple in a self-excited var compensator, an interphase reactor (L) is generated. This is because the reactor (L) is on the high-voltage side, and its insulation equipment is also required. Therefore, the overall size of the transformer (Tm) becomes large and the cost also increases. Therefore, an object of the present invention is to provide a self-excited SVC in which the interphase reactor (L) is omitted to reduce the cost and save the space.

[0009]

SUMMARY OF THE INVENTION The present invention is a transformer for a plurality of square wave inverters in which a large capacity low speed response square wave inverter for generating a base voltage equal to a system voltage is connected to a low voltage side, and for canceling a reactive current. A PWM inverter transformer in which a small-capacity, high-speed-response PWM inverter that generates a voltage component for generating a compensation current is connected to the low-voltage side, and the added value of the output voltage of each inverter is connected to the system impedance via the system impedance. When the reactive power for compensation is generated by controlling the amplitude in the same phase as the voltage, the high-voltage side of a plurality of transformers for square wave inverters and the transformers for PWM inverters are connected in series.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a self-excited var compensator according to the present invention will be described below with reference to FIG. The same parts as those shown in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. The difference is that in the configuration of the transformer (Tn), each high-voltage side (Pa) (Pb) of a plurality of transformers (Ta) (Tb) for a square wave inverter is connected in series and connected to each low-voltage side (Qa).
Square wave inverters (1a) and (1b) are connected to (Qb). Moreover, the transformer for the square wave inverter (T
a) The high-voltage side (Pa) (Pb) of (Tb) is further connected in series with the high-voltage side (Pc) of the PWM inverter transformer (Tc), and the PWM inverter (2) is connected to the low-voltage side (Qc).

According to the above configuration, the square wave inverter (1
Voltages output from a) (1b) and PWM inverter (2) are transformers (Ta) (Tb) and (Tc) that are serially multiplexed.
Is output to the power supply side. Then, since no return current occurs between the transformers (Ta) (Tb), the transformers (Ta)
The generation of the cross current (It) due to the imbalance of the output voltages of (Tb) is eliminated, and the interphase reactor (L) is unnecessary. Further, as in the conventional case, harmonics are reduced by changing the phase of the output voltage of the transformer (Ta) (Tb).

[0012]

EXAMPLES Further specific examples of the present invention will be shown below. First, by connecting three transformers (Ta) (Tb) (Tc) in series multiplex, the excitation impedance ratio is adjusted so that each series voltage distribution during grid interconnection (switch ON) is adjusted appropriately. There is a need to. Therefore, in the square wave inverters (1a) and (1b) having the same performance, a gap is provided in the transformer core to adjust the exciting impedance ratio. Also, two square wave inverter transformers (Ta) (T
An exciting impedance adjusting reactor is provided between b) and the PWM inverter transformer (Tc) to adjust the exciting impedance ratio.

[0013]

According to the present invention, the high voltage sides of a plurality of transformers for a square wave inverter in a self-excited var compensator are connected in series and the square wave inverter is connected to each low voltage side. Also, the insulating equipment is not required, the number of parts is reduced, and low cost and space saving can be realized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a self-excited reactive power compensator according to the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional self-excited reactive power compensator.

[Explanation of symbols]

 1a, 1b Square wave inverter 2 PWM inverter Tn Transformer Ta, Tb Square wave inverter transformer Tc PWM inverter transformer Vs System power supply Xs System impedance

Claims (1)

[Claims] 1. A plurality of square wave inverter transformers in which a large-capacity low-speed response square wave inverter that generates a base voltage equal to a system voltage is connected to a low voltage side, and a voltage for generating a compensating current for canceling reactive current. PWM that connects a low-capacity, high-speed PWM inverter that generates a component to the low-voltage side
It is equipped with an inverter transformer, and when the reactive power for compensation is generated by controlling the amplitude of the added value of the output voltage of each inverter in the same phase as the system voltage via the system impedance, Transformer and PW
A self-excited reactive power compensator, characterized in that the high-voltage side of a transformer for an M inverter is connected in multiple series.