JPS64908B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a constant current control method in an AC to DC converter such as a DC power transmission system.

In DC power transmission, it is common to use constant current control on the forward converter side (REC) and constant voltage control or constant margin angle control on the inverse converter side (INV). FIG. 1 shows a block diagram of such a constant current control system.

In Figure 1, 1 is the main transformer for the converter, 2 is a forward conversion device using a thyristor valve, 3 and 4 are DC reactors (DCL) that smooth the DC current, and 5
6 is a reverse converter using a thyristor valve that converts direct current into alternating current, and 6 is a main transformer for the reverse converter.

Next, the operation of the constant current control system will be explained. The direct current flowing through the direct current system is detected by a direct current transformer (DCCT) 7 and converted by a signal converter 8 into a value Idc corresponding to the direct current. 9 is Idc and current setting value
This is a comparison amplifier that adds Idp with the polarity shown.
10 is a constant current control loop that makes the deviation of the comparison amplifier 9 zero, and 11 is an automatic pulse phase shifter (APPS) for controlling the firing angle of the thyristor valve from the control signal 10.

Figure 2 shows DC current detection performed by CT71 installed on the DC winding side of the transformer.
Components with the same reference numerals as in the figures have the same functions. 12
is a filter that smoothes the current waveform of CT71. Although only the forward converter side has been described above, the inverse converter is also provided with a constant current control system, and the structure and operation of the control system are the same as those of the forward converter, so in the following discussion, Let us consider the operation of a forward converter.

Now, the following problem occurs in the constant current control device as shown in FIGS. 1 and 2. That is, the first
In the control system shown in the figure, the time constant of the constant current control loop 10 is small, so although the response is fast, ripples in the DC current may occur.Furthermore, even if a failure such as an arm short circuit occurs in the converter, Since no current flows through the DCCT 7, the fault current cannot be suppressed by current control. On the other hand, in the constant current control system shown in Figure 2, the current waveform on the DC winding side of the transformer is connected to the normal 120° wide rectangular wave current in parallel to the valve, as shown in Figure 3. Here, ripples occur due to the charging/discharging currents of a resistor (not shown) and an capacitor, so a smooth filter 12 as shown in FIG. 2 is required. Therefore,
Although arm short-circuit accidents can be detected, due to the delay in control by the filter, there is a drawback that suppression of fault current is delayed not only in arm short-circuits but also in DC transmission line faults.

Generally, the magnitude of the fault current is determined by the DC reactor.
It differs depending on whether it is inside the DCL, that is, the converter side, or outside, that is, the power line side. Accident inside the DCL,
For example, in the case of an arm short circuit or a DC bus fault, an excessive fault current flows, so high-speed constant current control is required. On the other hand, in an accident outside the DCL, such as a DC transmission line accident, although excessive current does not flow as in an accident inside the DCL, fast current control is required. On the other hand, it is desirable to respond slowly and reliably to fluctuations in DC current due to disturbances in the AC system.

The present invention was made in order to eliminate such drawbacks of the conventional constant current control system, and eliminates blind spots in current detection, and enables current control without current ripple or transient overshoot during steady state. However, in the event of an accident, constant current control is performed by changing the speed of suppression of the fault current depending on the nature of the accident.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. FIG. 4 shows only the forward converter side of the constant current control system according to the present invention. 72 in Figure 4
is DCCT installed on the return side, 13 is DCCT7, 7
81 and 82 are signal conversion circuits that convert the DC current into Idc ₁ or Idc ₂ ; 14 to 16 are the detected values Idc ₁ ,
A comparator that adds Idc ₂ , current setting value Idp, and constants K ₁ and K ₂ to be described later with the polarity shown, 17
18 is a constant current control loop (ACR) with a relatively large time constant that makes the deviation of comparator 14 zero, and 18 is a constant current control loop (MACR) with a small time constant that makes the deviation of comparator 15 zero. , 19 is the comparator 16
The constant current control loop (HACR) 20 has a time constant smaller than the constant current control loop 18 so as to make the deviation of
11 is a minimum value selection circuit that selects the minimum value among the 19 outputs, and 11 is a pulse phase shifter APPS. K ₁ and K ₂ are bias values added to the comparator
K ₁ < K ₂ and K ₂ is set to be larger than the ripple of the transformer DC winding current i (usually about 20 to 30% of the DC current) as shown in Figure 3. shall be. If K ₁ and K ₂ are set like this,
Since the current detection value Idc ₁ and the set value are always equal, the constant current control loop 17 has no output, and the biases K ₁ and K ₂
Therefore, the comparators 15 and 16 have a certain finite value, so the constant current control loop MACR18 and the constant current control loop HACR19 are saturated to +, so the constant current control loop ACR17 is selected because its output is the minimum. On the other hand, when an accident occurs in the DC transmission line and the detection current Idc ₁ of the DC current transformers 7, 72 increases and approaches the set value Idp + K ₁ , the HACR 19 is still saturated on the + side, but the MACR 18 is in the operating region. ACR17 is input to the minimum value selection circuit 20,
The smaller MACR18 is selected.
If Idc ₁ is almost the same as Idp + K ₁ , MACR18
Since the output of is smaller than the output of ACR17,
The output of MACR18 is selected. However,
Since MACR 18 has a smaller time constant than ACR 17, MACR 18 quickly performs constant current control to suppress fault current. In addition, an arm short circuit occurs and the detection value Idc ₂ of DCCT71 becomes the set value Idp + K ₂
When it becomes approximately the same as , the output of the constant current control loop 19 becomes the minimum, so the HACR 19 is selected and more rapid constant current control is performed. Also, at this time
If K ₂ is set as described above, there is no need to provide a filter, so there is no delay in control caused by providing a filter. In other words, the triple loop constant current control system allows constant current control to be quickly performed depending on the nature of the accident. In FIG. 4, the DC current transformer 72 and the maximum value detection circuit 13 may be omitted; they are provided only to duplicate the DC current transformer and have no other meaning. It is from.

As described above, the present invention provides rapid control using the constant current control loop 19 having the smallest time constant in response to converter failures such as arm short circuits and DC bus failures, which require the most rapid current control. I can,
Failures in the DC transmission line outside the DC reactor 3 can be controlled relatively quickly by the constant current control loop 18 with the second smallest time constant, and DC current fluctuations due to disturbances in the AC system can be controlled relatively quickly. For,
The constant current control loop 17 with the largest time constant allows for smooth control. In other words, the converter 2 can be controlled at the most appropriate speed for each failure.

Furthermore, in the present invention, by setting the constant K ₂ added to the comparator 16 to be larger than the ripple of the current on the secondary winding side of the transformer 1, high-speed control can be achieved without using a filter.

As described above, according to the present invention, since the time constant of the constant current control system is configured to be switched depending on the magnitude of the fault current, there is an advantage that rapid constant current control is possible depending on the nature of the fault. .

[Brief explanation of drawings]

Fig. 1 and Fig. 2 are diagrams showing a conventional constant current control system, Fig. 3 is a diagram showing a waveform of a transformer DC winding current, and Fig. 4 is a diagram showing a constant current control system according to the present invention. . In the figure, 2 is an AC/DC converter, 7, 71, 7
2 is a current transformer, 13 is a maximum value selection circuit, 14, 1
5 and 16 are comparators, 17, 18 and 19 are constant current control loops, and 20 is a minimum value selection circuit.

Claims (1)

[Claims] 1 A first means for detecting a current on the AC side of an AC/DC converter equipped with a constant current control system, a second means for detecting a current on the DC side of the AC/DC converter, which always detects the AC/DC current based on the current setting value. a first constant current control system with the largest time constant for controlling the ignition phase of the converter;
If the set value is larger than the set value of the first constant current control system and the time constant is smaller than that of the first constant current control system, and the output from the second means exceeds the predetermined value, the a second constant current control system for controlling the ignition phase of the converter, the setting value of which is larger than the setting values of the first constant current control system and the second constant current control system, and the time constant of which is greater than the setting values of the first constant current control system and the second constant current control system; a third constant current control system that is smaller than the second constant current control system and controls the firing phase of the AC/DC converter when the output of the first means exceeds a predetermined value;
It is characterized by comprising a minimum value selection circuit which selects the minimum value among the outputs of the third constant current control system and outputs it to the pulse phase shifter for controlling the firing angle of the AC/DC converter. A control device for an AC/DC converter.