JP2791006B2 - Control method of AC / DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an AC power system in which AC / DC conversion equipment is connected to an AC power system for the purpose of DC power transmission and frequency conversion. The present invention relates to a method for controlling an AC / DC converter when the power generation occurs. [Technical Background of the Invention and Its Problems] FIG. 2 is a schematic block diagram of a control device in a conventional DC power transmission system converter. In the converter of the DC transmission system, the DC side of the converters 1A and 1B is connected by the DC transmission line 3 via DC reactors 2A and 2B, respectively, and the AC side of each converter 1A and 1B is a transformer for the converter. It is configured to be connected to respective AC systems 6A, 6B via 4A, 4B and sheath breakers 5A, 5B. The conventional converters 1A and 1B have constant margin angle control circuits 11A and 11B respectively provided with constant current control circuits 13A and 13B.The constant margin angle control circuits 11A and 11B set the minimum margin angle of the converter. The minimum margin angle reference values, which are the outputs of the margin angle setting devices 18A and 18B, and the output of the constant reactive power control circuit 48 required as a converter are added to the margin angle reference values added by the adders 17A and 17B. , And operate so as to follow the margin angles of the converters 1A and 1B. The current reference value output from the constant power control circuit 44 and the DC current detection values obtained by detecting the DC current with the DC current detectors 21A and 21B and converted to a value that can be easily handled as a control circuit are added to the addition circuits 23A and 23.
When the difference is input to B and input to the constant current control circuits 13A and 13B, the DC current flowing through the DC transmission line 3 is controlled so as to follow the current reference value. Only the switch corresponding to the converter that operates as an inverse converter is closed in the switches 24A and 24B, and the current margin output from the current margin setting units 25A and 25B is input to the adding circuits 23A and 23B. The function of the current margin and the control for selecting the output of the constant margin angle control circuits 11A and 11B and the output of the constant current control circuits 13A and 13B, which further advances the control advance angle of the converter, as the output. By the function of the lead angle priority circuits 28A and 28B, if the switch 24B is now closed and the switch 24A is open, the output of the constant current control circuit 13A is provided to the control lead angle priority circuit 28A. Further, the margin advance control circuit 11B is provided to the control advance angle priority circuit 28B.
Is output. (In the following description, the switch 24A is open and the switch 24B is closed for convenience of description.) The outputs of the control lead angle priority circuits 28A and 28B are input to phase control circuits 29A and 29B, respectively, where It is configured to be converted into a pulse signal that determines the ignition timing of 1A and 1B, and to be supplied as a gate pulse signal to the converters 1A and 1B via the pulse amplifier circuits 30A and 30B. Constructing the converter control circuit as described above is a known technique. Now, suppose that an accident has occurred in the AC system 6A, and the voltage of the AC system 6A has dropped significantly. At this time, since the exchanger 1A is a separately-excited converter, commutation cannot be performed. Therefore, the fault of the AC system 6A is detected by, for example, an AC undervoltage relay or the like, and a valve connected to the same arm of the converter 1A is energized.After the fault of the AC system 6A is removed, for example, the AC undervoltage is detected. After the relay is restored, an operation of returning to normal operation can be performed. The operation for energizing the valve connected to the same arm is generally called a bypass pair operation, and is hereinafter referred to as BP pair operation.
Abbreviated as P. The BPP operation consumes only reactive power because the DC voltage of the converter is zero and only DC current is flowing. Therefore, if the sound AC system 6B in which no accident occurs is a strong system, in other words, the ratio of the short-circuit capacity of the AC system 6B to the capacity of the converter, that is, the short-circuit capacity ratio is large, the operation of the converter is not so large. No problem. However, when the short-circuit capacity ratio is small, there is a problem that a large voltage fluctuation is caused in the AC system 6B, which eventually causes a system collapse of the AC system 6B. Next, FIG. 3 shows an example of a circuit configured to prevent the above-described disadvantage. In FIG. 3, the same elements as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 2, the output of the constant power control circuit 44 is directly applied to the adder circuits 23A and 23B. However, in FIG.
Input to 23A and 23B. 51A and 51B are AC voltage detectors for detecting the voltage of the DC system, respectively, and the output signals thereof are input to addition circuits 53A and 53B together with the voltage setting devices 52A and 52B, and the deviation is controlled by the respective constant voltage control. Circuit 54A, 54B
Given to. The amplified output signal is
A is added to the adders 23A and 23B via A and 55B, and is configured to be a current reference value of the constant current control circuits 13A and 13B. In FIG. 2, it is assumed that an accident such as a three-phase ground fault has occurred in the AC system 6A. At this time, as described above, this AC system fault is detected by an undervoltage relay or the like, and the switch 55B is closed and the switch 50 is opened based on the output signal.
Then, the output signal of the constant voltage control circuit 54B changes so as to maintain the voltage of the healthy end AC system 6B constant, and its value becomes the current reference value of the constant current control circuit 13B. The DC current is controlled according to the voltage fluctuation. Therefore, the reactive power of the AC system is controlled by converter 1A. That is, if the healthy-end AC system 6B becomes overvoltage, the constant voltage control circuit 54B increases the direct current, that is, suppresses the overvoltage by increasing the consumption of the reactive power by the converter, and conversely, the healthy end If the voltage of the AC system 6B decreases, the DC current is reduced, that is, the consumption of the reactive power by the converter is reduced, thereby suppressing the voltage drop. This is because the change in the AC voltage is substantially equal to the product of the change in the reactive power and the reactance of the AC system and the reactance, so that the AC voltage can be controlled by controlling the reactive power. If an accident occurs in the AC system 6B in FIG. 2, the switch 55A is closed and the switch 50 is opened, and in this case, voltage control for the sound AC system 6A is operated. The converter works as in the case of the accident according to 6A. However, focusing on the operation of the constant current control circuit on the inverter side in these cases, the current reference value input to the constant current control circuit is controlled by the constant voltage control circuit connected via the adder. The output value is smaller than the output value of the circuit by the current margin signal applied to the adder. This means that if the output signal value of the constant voltage control circuit and the set value of the current margin become the same value, the output of the adder becomes “zero” and the input signal of the constant current control circuit also becomes “zero”.
Therefore, the constant current control circuit cannot function. In such a state, the DC current flowing through the DC transmission line 3 becomes zero or continuous, and the intended purpose of stabilizing the AC system with the control of the converter cannot be achieved. There are inconveniences such as destruction of the thyristor element constituting the converter. [Object of the Invention] Accordingly, an object of the present invention is to eliminate the above-mentioned drawbacks, to minimize the influence on a sound AC system and the AC system on the accident side, and to stabilize the AC / DC converter. The present invention provides a method for controlling an AC / DC converter to maintain a proper operation. [Summary of the Invention] In order to achieve the above object, the present invention provides a constant current control circuit for a converter acting as an inverse converter based on a BPP operation signal of the converter acting as a forward converter. To remove the current margin added to the current reference signal as an input signal. Further, when the current reference output from the constant power control circuit is switched to the current reference output by the constant voltage control circuit of the healthy terminal AC system by the BPP operation, the switching time is delayed by an arbitrary time. This is to prevent the occurrence of AC system overvoltage caused by a sudden change in reactive power. FIG. 1 is a block diagram showing an embodiment of the present invention. Hereinafter, the circuit configuration in FIG. 1 will be described. still,
In FIG. 1, the same elements as those in FIGS. 2 and 8 are denoted by the same reference numerals, and description thereof will be omitted. The circuit 60 is an accident detection circuit such as an AC undervoltage relay. AND circuit 62, timer circuits 63 and 65, inverter circuit 64,
It is composed of switches 56, 59A, 59B, a voltage setting device 58, an adder 57, and a BPP command circuit 66. Next, the operation of this circuit will be described. For example, the converter 1A operates as a forward converter with the output signal of the constant power control circuit 44 as a reference value and further with the output signal of the constant current control circuit 13A prioritized, while the converter 1B
Is the output signal of the constant reactive power control circuit 48 and the margin angle setting device 18
It is assumed that the output of B is in an inverter operating state in which the output added by the adder 17 is used as a margin angle reference value and the output of the constant margin angle control circuit 11B is prioritized. Here, the input signal B of the AND circuit 62 is a signal indicating that the converter 1A is a forward converter, and is a signal that becomes “positive” logic when the converter 1A is a forward converter. In the operating state described above, the AC system 6A
Assuming that an accident has occurred and the accident detection circuit 60 such as an AC undervoltage relay operates due to a drop in AC voltage, the BPP operation circuit 65 instantaneously activates the converter 1A connected to the system at the accident end side. From the forward converter operation to the BPP state. In this case, since the converter 1A that has been BPP-operated cannot control the DC current, the converter 1B changes the output of the constant voltage control circuit 54B from the control of the constant margin angle control circuit 11B to the current reference value. The state shifts to the zero power factor operation state under the control of the constant current control circuit 13B. On the other hand, since the output condition of the AND circuit 62 is satisfied at the same time when the converter 1A is BPP-operated, the constant current control circuit 13B of the converter 1B which is operating as an inverse converter with this output signal is provided. The output signal of the current margin setting device 25B added to the current reference value is removed by opening the switch 59B. By these series of operations, the constant voltage control circuit
Even when the output of the 54B reaches a minimum value at which the converter can stably operate, for example, due to a decrease in system voltage, the output signal of the constant current control circuit 13B is affected by the current margin (the constant voltage described above). If the output value of the control circuit is n (p,
In u), when the current margin setting value is also n (pu) and the current margin operates, the output of the adder 23B becomes n−
n = 0 (p, u)), and as a result, it is possible to prevent problems such as intermittent occurrence of a DC current. Further, after the time set by the timer circuit 63, an operation is performed to shift the control from the control by the constant power control circuit 44 to the control by the constant voltage circuit 54B. According to this operation,
Since the control transition is performed after the constant voltage control circuit 54B has already responded, the fluctuation of the current reference value caused by the transition can be minimized, and the fluctuation of the reactive power consumed by the converter is also minimized. Therefore, voltage fluctuation in the AC system 6B can be prevented. On the other hand, when the AC system 6A recovers from an accident, the timer circuit 66 operates and the voltage setting unit
The arbitrary value set in 58 is used as the current reference
Since it is configured to be added to the adder 57 via 56, when restarting the converter related to the AC system or the reclosing of the breaker, the constant current control circuit of each converter provides an arbitrary current reference. Can be input for an arbitrary period, and the occurrence of overvoltage due to reclosing can be prevented by increasing the reactive power consumption of the converter. In the example shown in FIG. 1, only the converter control circuit at one end constituting the converter is described, but it goes without saying that the same control can be performed in the event of an AC system fault on either side. Also, in this example, an example in which the current margin is separated has been described.
Conversely, it is also possible to separately add a signal having a polarity opposite to the current margin, or to invert the polarity of the current margin. Further, when the current margin is set to -α (pu), the lower limiter of the output of the constant voltage control circuit is set to −kα (p, u) [where k> α] under the BPP operation conditions of the forward converter. The same effect can be expected. Also, in the embodiment of the present invention, the case where an AC system accident has occurred has been described as an example. Needless to say, the present invention can be applied to such cases. [Effects of the Invention] As described above, according to the present invention, when an accident occurs in the AC system, the DC current of the converter does not become zero, and the AC system voltage is controlled by controlling the converter. Can be stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a control block diagram of an AC / DC converter used in a DC transmission system, and FIG. FIG. 4 is a block diagram of protection control of FIG. 1A, 1B …… Converter, 2A, 2B …… DC reactor 3 …… DC transmission line, 4A, 4B …… Transformer transformers 5A, 5B …… Shiya breaker, 6A, 6B …… AC system 11A, 11B ... constant margin angle control circuits 13A, 13B ... constant current control circuits 17A, 17B, 23A, 23B, 42, 46 ... addition circuits 18A, 18B ... minimum margin angle setting devices 24A, 24B ... switch 25A, 25B Current margin setting units 21A and 21B DC current detectors 22A and 22B Current-voltage conversion circuits 28A and 28B Control lead angle priority circuits 29A and 29B Phase control circuits 30A and 30B Pulse increase Width circuit, 41: Power setting unit 43: Power detector, 45: Reactive power setting unit 47: Reactive power detector, 44: Constant power control circuit 48: Constant reactive power control circuit 50, 55A, 55B switches 51A, 51B AC voltage detection circuits 52A, 52B voltage setting units 54A, 54B constant voltage control circuits 53A, 53B addition circuit 60 accident detection circuit 62 AND circuit 63 66 Timer circuit 64 Inverter circuit 65 BP P operation circuit 56, 59A, 59B Switch 58 Voltage setting circuit 57 Addition circuit

Claims (1)

(57) [Claims] A constant current control system that controls a direct current flowing between the forward converter and the inverse converter so as to follow the output of the constant power control circuit; and a margin angle of the forward converter and the inverter is a margin angle reference. A constant margin angle control system for controlling so as to follow the value, and a control method of the AC / DC converter for controlling the forward converter and the inverse converter, wherein an accident occurs in the AC system on the side of the forward converter. In this case, a constant current control value of the constant current control system of the inverter is set by the output of the constant voltage control circuit that maintains the AC voltage of the inverter constant, and the constant current of the inverter is constant. A method for controlling an AC / DC converter, comprising performing an operation of removing a current margin signal applied to a control system.