JPH0456536B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a frequency control method for an AC system using a power converter used for DC power transmission or the like.

[Technical background of the invention]

A three-phase thyristor bridge shown in FIG. 1 is used as a power conversion device applied to DC power transmission and frequency conversion.

Three-phase thyristor bridge 1 includes thyristor U,
It is composed of Z, V, X, W, and Y, R, S, and T are AC side terminals, and P and N are DC side terminals.

Three-phase thyristor bridge 1 includes thyristor U,
By controlling the phase of Z, V, X, W, Y,
AC voltage can be converted to DC voltage to send power from the AC side to the DC side or from the DC side to the AC side.

Thus, the phase control of each thyristor of the three-phase thyristor bridge 1 is performed in synchronization with the frequency of the alternating current voltage. According to this phase control, the power flow can be changed quickly, so it is sometimes used to improve the frequency of an AC system.

FIG. 2 is a block diagram showing the frequency control mechanism of an AC system for DC power transmission.

201 and 202 are first and second alternating current generators;
03 is an AC bus line.

The AC voltage from the AC bus 203 is transferred to the transformer 204.
is converted into a DC voltage by the three-phase thyristor bridge 205, and the current on the AC side is sent to the DC side. The current converted to direct current, that is, direct current, is sent to three-phase thyristor bridge 208 via DC reactors 206 and 207, and after being converted to alternating current, it passes through transformer 209 to load 210.
is supplied to

The three-phase thyristor bridges 205 and 208 are controlled so that the DC voltage Vd is approximately constant,
The power flow can be changed by changing the DC current Id. 211 introduces AC voltage Vac with a voltage transformer, and with that AC voltage Vac,
The frequency detector 214 detects the frequency F of the AC bus 203.
Detect. Further, the DC power detector 215 detects the DC power Pd based on the DC voltage signal from the DC voltage detector 212 and the DC current signal from the DC current detector 213.

220 is a three-phase thyristor bridge 205, 20
DC current reference I _dp used when controlling the phase of 8
This is an analog control device that calculates 216,2
18 is an adder, and 217 and 219 are arithmetic units that perform PID calculations and the like.

FIG. 3 shows the frequency F of the AC bus 203 in FIG.
Three-phase thyristor bridge 205, 208
FIG. 3 is a power-frequency characteristic diagram illustrating how to reduce the power by controlling the phase.

In FIG. 3, the generator 2 on the AC bus 203
Linear power and frequency characteristics from 01,202
G1 and G2 represent the power and frequency characteristics required by the load, and L1 and L2 represent the power and frequency characteristics required by the load.

Now, in Fig. 3, when the generators 201 and 202 are operated with the characteristic of G1 and the power required by the load 210 is on the characteristic of L1, the specified frequency is
The three-phase thyristor bridges 205 and 208 are phase-controlled so that the supplied power is F _ref and the supplied power P _dp , and operation is performed at point A.

At this time, if the generator 202 suddenly stops generating power, power is supplied only from the generator 201, so the power-frequency characteristic from the generator 201 supplied to the AC bus 203 becomes G2. If the three-phase thyristor bridges 205 and 208 continue to operate with the load characteristic of L1 regardless of the stoppage of the generator 202, they will operate at point B, the frequency will drop to _F2 , and the generator 201 will stop operating. There is a risk that it will not be possible to continue.

In this case, the power characteristics required by the load 210 are
If set to L2, operation will be performed at point C, and the frequency will be F _ref
Therefore, the generator 201 can continue to operate stably. That is, if the power supplied to the load is set to _P2 by the three-phase thyristor bridge 205, the generator 201 can continue to operate stably.

Now, a control method for maintaining the frequency of the AC bus 203 constant as described above will be explained using FIG. 2.

The frequency F of the three-phase AC voltage V _ac introduced from the AC bus 203 via the potential transformer 211 is detected by the frequency detector 214 . The difference between the frequency F and the reference frequency F _ref of the AC bus is calculated by the adder 216 in the analog control device 220, and after being corrected by the calculation device 217, the power reference correction signal ΔP _dp
It is added to adder 218 as .

Moreover, the DC voltage and DC current detected by the DC voltage detector 212 and the DC current detector 213 are as follows:
The DC power detector 215 adds the DC power signal P _d to the adder 218 . The adder 218 obtains a value obtained by subtracting the DC power signal P _d and the DC power reference correction signal ΔP _dp from the power reference P _dp , and this value is passed through the arithmetic unit 219 to the DC current reference I _dp.
get.

The DC current reference I _dp obtained in this way is
This DC current reference increases when the frequency F of the AC bus bar decreases and decreases when the frequency increases.
Three-phase thyristor bridge 205,2 using I _dp
By controlling the phase of AC bus 203
The frequency F of can be kept constant.

By the way, conventional control devices used for DC power transmission and the like are often implemented using analog circuits, and the frequency signal F from the frequency detector 214 is often output as an analog voltage value. However, in recent years, a system has emerged in which a control device used for DC power transmission, etc. is constructed using a digital circuit such as a microcomputer.

FIG. 4 is a block diagram showing a schematic configuration of a conventional digital phase control device using the computer of the power conversion device 1 shown in FIG.

401 is a phase detector, 402 is a computer,
403 is gate logic. Phase detector 40
1 is a circuit that takes in AC voltage signals e R , e S , e T from AC side ends _R , _S , and _T of the power converter 1 and extracts a phase signal θ synchronized with the AC side voltage. A phase lock loop (PLL) circuit such as the -34851 is used. computer 402
takes in the frequency signal f of the AC system, the DC power P _d , the DC current I _d , the DC voltage V _d, etc. as feedback amounts, and calculates the phase control signal E _c to be sent to the DC side of the power converter 1. gate logic 403
are the digital phase signal θ and the digital phase control signal E _c
From the thyristors U, Z, V, of the power conversion device 1,
Determine the output time of the ignition pulse TP given to X, W, and Y, and output the ignition pulse TP.

The computer 402 reads the feedback amount at every fixed sampling period and performs calculations. Here, attention is paid to the frequency F as the amount of feedback. Conventionally, an F/V converter that converts frequency into voltage is often used to detect frequency F.

FIG. 5 is a block diagram showing a conventional example in which a frequency signal f is input into a computer 402 using an F/V converter as a frequency detector. 501 is an F/V converter, 502 is an analog-to-digital (A/D) converter in the computer 402, and 503 is a central processing unit (CPU) in the computer 402.
It is. The frequency F from the AC bus is converted into an analog voltage frequency signal f by the F/V converter 501, further converted into a digital signal by the A/D converter 502, and then sent to the CPU 503 as a frequency signal.
Introduced as FCNT and used for calculations.

[Problems with background technology]

When the frequency F is detected as in the conventional example described above, there are the following problems.

(a) When the A/D converter 502 converts an analog quantity into a digital quantity, it generally takes time, which causes the sampling period of the computer 402 to become longer, and the accuracy of calculations by the computer 402 to deteriorate. For example, the 12-bit A/D converter 50 commonly used
The conversion time of 2 is about 25 μsec, and the A/D converter 5 is operated by a microcomputer [Intel 8086 with a 5MHz clock, which is commonly used].
Processing to 02 takes about 10 μs, so A/
The time required for D conversion is about 35 microseconds.

○B Since the computer 402 can only obtain frequency information at each sampling period, significant frequency fluctuations occur only when the computer 402 imports the information, and after that, the computer 402 cannot calculate the calculated value even if it is the reference frequency. Since the frequency is varied significantly, performing frequency control may instead result in disturbance to the AC system.

[Purpose of the invention]

The present invention has been made to solve the problems of the conventional examples, and provides a method for detecting the frequency from the power supply voltage for frequency control by a power converter with high precision and good responsiveness. That is its purpose.

[Summary of the invention]

The present invention uses pulses from a phase detector configured with a PLL circuit that generates pulses synchronized with the frequency of an AC power supply as sampling pulses for a computer in a digital phase control device, and also uses oscillations from an oscillator that performs constant oscillation. Count with a counter,
This is a frequency control method in which the computer reads the counted value from the counter every integer multiple of the sampling pulse to detect the actual frequency, and compares and calculates the actual frequency with a preset standard counted value (reference frequency). .

[Embodiments of the invention]

The present invention will be described below with reference to the embodiments shown in FIGS. 6 and 7. In FIG. 6, the same reference numerals as in FIG. 4 indicate the same or corresponding parts.

FIG. 6 is a block diagram of an embodiment of the frequency control method according to the present invention.

The phase detector 401 receives AC side voltage signals e _R , e _S , e _T
Introducing frequency-synchronized sampling pulses
SPLS is taken out from there as one signal of digital phase signal θ.

The computer 402 generates a sampling pulse
Each time SPLS is received, a control calculation is performed and a phase control signal E _c is derived.

602 is a counter which is initialized by the reset signal RST of the computer 402 and calculates the count value.
Set FCNT to zero and start counting anew.

Reference numeral 603 denotes an oscillator, and a constant oscillation pulse PULS is inputted to the counter 602 as its output.

Therefore, the phase detector 401 outputs the AC voltage signals e _R ,
The sampling pulse SPLS is output by varying the output time interval of the sampling pulse SPLS in response to the frequency variation of e _S and e _T .

After the reset signal RST is output from the computer 402, the counter 602 starts counting the oscillation pulses PLUS from the oscillator 603.

The computer 402 performs control calculations at each sampling time and outputs the phase control signal E _c that controls the power converter 1. For example, the computer 402 reads the counter FCNT of the counter 602 every 120 sampling pulses SPLS. outputs a reset signal RST to the counter 602.

Here, when using pulses every 30 degrees in electrical angle as the sampling pulse SPLS (12
The counter FCNT of the counter 602 is an oscillator 60 that oscillates at a constant cycle every 10Hz of the AC voltage signal.
Count value of oscillation pulse PULS from 3
FCNT will be read into computer 402.

In this case, the count value FCNT read by the computer 402 becomes the standard count value FNRM corresponding to the standard frequency of the AC system, but if there is a variation, it becomes a value different from the standard count value FNRM. As can be seen from the above, the count value read by the computer 402 every 120 times of the sampling pulse SPLS
FCNT is a numerical value equivalent to frequency.

Here, the calculation speed of the present invention will be mentioned.

As can be seen from the above description and FIG. 6, the A/D converter 502 in the conventional example is not applied to the digital phase control device 400 of the present invention. Therefore, in the present invention, the counter 602 is only processed by the software of the (micro)computer 402, and the waiting time corresponding to the A/D conversion time is not required.
Pulse from oscillator 603 that oscillates every 10Hz
The time required to count PULS and output the generated count value FCNT is about 10 microseconds.

This is significantly faster than the previously mentioned conventional example, which takes about 35 microseconds.

FIG. 7 is a flowchart outlining the contents of a program executed by the computer 402 of FIG. 6 for each sampling pulse.

The computer 402 outputs the sampling pulse SPLS from the previous 120th time for each sampling pulse.
Check whether the number of times has reached 120, and if it has reached 120, read the count value FCNT of the counter 602,
After outputting the reset signal RST to the counter 602,
Frequency control is performed by correcting the DC current standard value I _dp of the power converter 1 based on the difference between the count value FCNT and the standard count value FNRM which is equivalent to the standard frequency of the AC system.

If the number of sampling pulses does not reach 120, the phase control signal E _c is calculated and output based on the previous DC current standard I _dp , and the program for the current sampling pulse ends.

By performing frequency detection in this way,
The frequency control explained in Fig. 3 is used because frequency detection can be performed in a shorter time than the conventional method that uses an F/V converter and an A/D converter, and because frequency fluctuations are taken as an average value. This can be done quickly and with high precision.

〔Effect of the invention〕

Thus, the present invention has the following effects.

A PLL that generates pulses synchronized with the frequency of the AC power supply, controlling the frequency by a power converter that uses a digital phase control device with a computer.
The pulse signal from the phase detector configured with the circuit is used as the sampling pulse of the computer, and the oscillation from the oscillator that performs constant oscillation is counted by a counter. By reading the count value of , detecting the frequency, and performing frequency control, high-precision frequency control with quick response can be performed.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a power conversion device consisting of a three-phase thyristor bridge, Figure 2 is a block diagram showing the frequency control mechanism of an AC system for DC power transmission, Figure 3 is a power-frequency characteristic diagram of an AC bus, Fig. 4 is a block diagram of a conventional device, Fig. 5 is a block diagram for introducing the frequency signal into a computer, Fig. 6 is a block diagram showing the configuration of an embodiment of the present invention, and Fig. 7 is a flowchart thereof. . 1,205,208...power conversion device, 201,
202... Generator, 203... AC bus, 204,2
09...Transformer, 206, 207...DC reactor, 210...Load, 211...Instrument transformer, 21
2...DC voltage detector, 213...DC current detector,
214... Frequency detector, 215... DC power detector, 216, 218... Adder, 217, 219...
P (proportional) I (integral) D (differential) calculator, 220...
Analog control device, 400... Digital phase control device, 401... Phase detector (PLL circuit), 402...
Computer, 403... Gate logic, 501
...Frequency-voltage converter, 502...Analog-digital converter, 503...Central processing unit (CPU), 6
02... Counter, 603... Oscillator.

Claims (1)

[Claims] 1. In a digital phase control method using a power converter used in a frequency control mechanism of an AC system for DC power transmission, the method includes digital control using a computer to control pulses from a phase detector that generates pulses synchronized with an AC power supply. A sampling pulse that determines the sampling period of the device, interrupts the calculation to the digital control device for each sampling pulse, counts pulses from an oscillator that oscillates a constant pulse with a counter, and calculates an integer multiple of the sampling pulse. Each time, the digital control device reads the count value from the counter, resets the count value of the counter using a reset signal from the digital control device, and sets the count value and the preset value in the digital control device. The frequency fluctuation of the AC power source is captured by the value of the difference from the reference count value calculated, and the DC current reference value is corrected, and the phase control signal of the power converter is derived from this correction calculation and output. A frequency control method using a power conversion device, characterized in that the power conversion device is adjusted so as to keep the frequency of a grid constant.