JP2012161163A - Dc power transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To operate a system stably even when other power supply or the inverter of a DC power transmission system is connected or not connected with the AC system on the inverter side.SOLUTION: The DC power transmission system comprises a converter 8 which converts AC power into DC power, and an inverter 12 which inverts DC power into AC power. THe converter 8 controls the interchanged power so as to maintain the DC voltage at a constant level. The inverter 12 is provided with a controller 30 for controlling the AC voltage and the frequency. When there is no other power supply 4 nor the inverter of the DC power transmission system in the AC system 2 on the inverter side, the inverter 12 performs output control autonomously so that the AC voltage and frequency of the AC system follow the command values. When there is other power supply 4, or the like, the inverter 12 adjusts the AC voltage and frequency while synchronizing with the AC voltages output therefrom. The inverter 12 includes a limit means of the output current for preventing overcurrent.

Description

  The present invention relates to a DC power transmission system using a self-excited converter as an inverse converter.

  The direct current power transmission system converts alternating current power into direct current power with a forward converter, transmits power in the form of direct current, converts the direct current power into alternating current power with an inverse converter, and supplies power. Once converted into direct current, it is applied to interconnection between systems with different frequencies, interconnection between asynchronous systems even at the same frequency, and power transmission via a long-distance cable. In particular, a self-excited converter is required when there is no other power supply device in the system on the inverse converter side, or in a system with a short-circuit capacity.

  As a control method of a DC power transmission system using a self-excited converter, for example, as described in Patent Document 1, a DC voltage is controlled to be constant by a converter at one end, and an active power is generated by a converter at the other end. A method of supplying power by controlling is known. However, this control method is based on the premise that each converter is connected to an AC system in which other power supply devices exist, and each converter detects the voltage phase of each AC system, and Operation is controlled in synchronization.

  However, assuming power supply to remote islands and remote customers, the aim is to reduce operating costs and CO2 emissions by suppressing these operations as much as possible even if there are no other power generation devices. There is a case. In particular, for direct current power transmission to a non-power supply system, it is necessary to perform reactive power supply from an inverse converter in accordance with an instantaneous load variation and a power factor. It is difficult to instantaneously control the accommodation power according to the fluctuation.

  As a countermeasure for such a case, Patent Document 2 describes that the reverse converter side is detected to be disconnected and the reverse converter is controlled so that the AC voltage becomes constant. .

Japanese Patent No. 2635660 Japanese Patent No. 4287031

  However, when small-capacity power supply devices are dispersedly arranged in the AC system on the reverse converter side, a transmission path for transmitting the disconnection of these power supply devices is necessary, which causes a cost increase. Become. In addition, when the capacity of the power supply device is smaller than the capacity of the inverse converter, the range in which the inverse converter can be stably operated is limited, and thus the power supply apparatus needs to be operated more than necessary. In addition, in the case of using two DC power transmission systems from the viewpoint of ensuring reliability, if two series of inverse converters are controlled so that the voltage becomes constant at the same time, control interference may occur between the two systems. There is.

  The present invention has been proposed to solve the above-described problems of the prior art. That is, an object of the present invention is to provide a DC power transmission system that can realize stable DC power transmission regardless of the presence or absence of a power supply device in an AC system on the inverse converter side.

  In order to achieve the above object, the present invention is characterized by the following configuration in a DC power transmission system including a forward converter that converts AC power into DC power and an inverse converter that converts DC power into AC power. To do.

(1) The forward converter controls the accommodation power so as to keep the DC voltage constant.
(2) Provide a means for controlling the AC voltage and frequency in the inverter.
(3) If there is no other power supply device or DC power system reverse converter in the AC system on the reverse converter side, the inverter will be independent to make the AC voltage and frequency of the AC system follow the command value. Output control.
(4) When there is another power supply device or the like, the inverse converter adjusts the AC voltage and frequency while synchronizing with the output AC voltage.
(5) The inverse converter includes a means for limiting the output current in order to prevent overcurrent.

The present invention exhibits the following effects.
(a) By providing the inverter with means for controlling the AC voltage and frequency, the system can be stabilized regardless of the presence of other power supply devices in the AC system on the inverter side or the inverter in the DC power transmission system. And can drive.
(b) By providing a means for limiting the output current, it is possible to continue the operation of the system while limiting the current even if an accident occurs in the AC system.

1 is a circuit diagram illustrating a DC power transmission system according to a first embodiment. 3 is a graph showing operating characteristics of the inverse converter according to the first embodiment. The circuit diagram which shows the direct-current power transmission system of Example 2, and its control apparatus. The graph explaining the control characteristic of AC-AVR31. FIG. 6 is a circuit diagram illustrating a DC power transmission system and a control device thereof according to a third embodiment. 6 is a graph illustrating control characteristics of the phase / frequency arithmetic circuit 32. The circuit diagram which shows the direct-current power transmission system of Example 4, and its control apparatus. 6 is a graph illustrating control characteristics of the current limiting circuit 33. The DC power transmission system of Example 5, its control apparatus, and the circuit diagram which shows the alternating current system by the side of a receiving power. The time chart explaining the cooperation operation | movement with a direct-current power transmission system and the protection relay in an alternating current system. The time chart explaining the cooperation operation | movement with a direct-current power transmission system and the protection relay in an alternating current system.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the following examples.

  FIG. 1 is a circuit diagram illustrating the overall configuration of the DC power transmission system according to the first embodiment. In FIG. 1, 1 is a power system on the power transmission side, and 2 is a power system on the power reception side. 3 is a power generator included in the power system on the power transmission side, 4 is a power generator included in the power system on the power receiving side, and both are connected to the respective power systems via the circuit breaker 5. 6 is a power transmission end AC bus of the DC power transmission system, 7 is a transformer for forward converter, 8 is a forward converter, and 9 is a DC capacitor. On the other hand, an AC bus 10, an inverse transformer transformer 11, an inverse converter 12, and a DC capacitor 13 are sequentially connected to the power receiving end. The forward converter 8 on the power transmission end side and the reverse converter 12 on the power receiving end are both constituted by self-excited converters, and both are connected via DC lines 14a and 14b.

  Reference numeral 20 denotes a control device for controlling the interchange power from the forward converter to the DC lines 14a and 14b. The control device 20 includes a DC voltage control circuit (hereinafter referred to as DC-AVR) 21 that controls a DC voltage, an AC voltage control circuit (hereinafter referred to as AC-AVR) 22 that controls an AC voltage, and reactive power of the AC / DC converter. A reactive power control circuit (hereinafter referred to as AQR) 23 for controlling the output, a circuit 24 for selecting one of the outputs of AC-AVR 22 and AQR 23, and a forward converter on the power transmission end side from the outputs of DC-AVR 21 and the selection circuit 24 8 includes a converter output control circuit 25 for generating gate pulses of the eight switching elements.

  On the other hand, 30 is a control device for an inverse converter, which is an AC-AVR 31 for controlling an AC voltage, a phase / frequency calculation circuit 32 for detecting the voltage phase of the AC bus 10 and calculating an output frequency, and an output current of the AC / DC converter. A converter output control circuit 34 for generating a gate pulse of the switching element of the reverse converter 12 on the power receiving end side from the output of the AC-AVR 33, the AC-AVR 33 and the phase / frequency arithmetic circuit 32 for limiting so as not to exceed a certain level is included. .

The operation of the forward converter 8 on the power transmission end side is basically the same as that of the prior art, and is briefly described below.
The electric power generated by the generator 3 in the AC system 1 and sent via the power system 1 is converted to DC by the forward converter 8. A control signal calculated so that the voltage across the capacitor 9 matches the command value is sent to the converter output control circuit 25. As a result, the DC voltage can be kept constant regardless of the amount of interchanged power transmitted to the power receiving end via the DC lines 14a and 14b.

  Since the self-excited converter can adjust the reactive power independently of the active power, the forward converter 8 directly controls the reactive power output to the AC system 1 side by the AQR 23 or AC by the action of the AC-AVR 22. The reactive power is controlled so that the voltage of the bus 6 matches the command value. That is, the AC-AVR 22 calculates a signal for making the voltage of the AC bus 6 coincide with the command value, and sends the calculation result to the converter output control circuit 25. In the converter output control circuit 25, a gate pulse is generated using two inputs from the AC-AVR 22 and the DC-AVR 21, and a forward converter is used by means such as PWM (pulse width modulation) control using the gate pulse. 8 controls the on / off state of the switching elements in the circuit 8.

  On the other hand, the operation of the reverse converter 12 at the power receiving end is unique to the present invention, and the operation will be described below. The AC-AVR 31 sends a control signal obtained by calculating the AC voltage of the AC bus 10 at the power receiving end so as to match the command value to the converter output control circuit 34. However, if the control value is such that the command value does not change at a constant value, there are a plurality of DC power transmission systems that perform voltage control with the same configuration, or the generator 4 operates in the power system 2 to perform voltage control. If so, the operation of the inverter 12 may become unstable due to interference with these voltage controls. Therefore, as shown in FIG. 2a, the AC-AVR 31 has a characteristic that the output voltage gradually decreases as the reactive power output from the inverse converter 12 increases.

  By giving such characteristics, for example, when the output voltage of the inverse converter 12 is higher than the output voltage of the generator 4, increasing the reactive power output of the AC / DC converter decreases the output voltage. The driving operation point moves in the direction and settles to a point where balance can be achieved. Further, when the circuit breaker 5 is opened and the generator 4 is disconnected, and there is no other generator in the AC system 2, the operating point is the reactive power required by the load of the AC system 2. Corresponding voltage. That is, stable control characteristics for the AC system 2 can be realized using only the voltage information of the AC bus 10 at its own end.

  Next, the circuit 32 that detects the voltage phase and calculates the output frequency calculates the phase and frequency from the AC voltage of the AC bus 10 using an arithmetic circuit such as a PLL (phase synchronization circuit), for example, and a converter output control circuit 34. The converter output control circuit 34 generates a voltage signal to be output by the converter based on these phase and frequency signals, and generates a gate pulse of the switching element of the inverse converter 12 by PWM control or the like.

  When a general PLL is used in the phase / frequency calculation 32, the detected phase and frequency do not generate errors. In the present invention, the active power output of the AC / DC converter and the phase / frequency calculation 32 are used. The frequency reference shown in FIG. 2b has the characteristics shown in FIG. By giving such characteristics, for example, when the output frequency command of the inverter 12 is higher than the output frequency command of the generator 4, increasing the active power output of the AC / DC converter decreases the output frequency. The driving action point moves in the direction of the movement and settles to a point where it can be balanced.

  Further, when the circuit breaker 5 is opened and the generator 4 is disconnected, and there is no other generator in the AC system 2, the operating point is the active power required by the load of the AC system 2. Corresponding frequency. That is, it is possible to realize stable control characteristics for the AC system 2 using only information calculated from the voltage of the AC bus 10 at its own end.

  The AC-ACR 33 receives the current output from the reverse converter 12 to the AC bus 10, compares it with the upper limit of the output current set inside, and prevents the AC-AVR 31 from exceeding when the upper limit is exceeded. A signal that corrects the output is calculated and output.

That is, according to the conventional technique, the converter must be stopped for protection in the following cases.
(a) The load on the AC system 2 becomes larger than expected and exceeds the output capacity of the inverter 12.
(b) When a short circuit accident occurs in the AC system 2 and there is no current limiting circuit 33, an overcurrent occurs.

  However, in the present invention, the output current of the inverse converter 12 can be suppressed to an operable range by the action of the current limiting circuit 33. Therefore, the inverter 12 can continue operation, and the overload state in the AC system 2 can be eliminated during that time. Further, while the inverter 12 is continuously operated, a short-circuit accident point is detected using a conventionally used protective relay, the accident section is disconnected by a circuit breaker, and the normal operation state is restored. It becomes possible.

  As described above, according to the present embodiment, stable operation is achieved by autonomously controlling the output voltage and frequency regardless of the presence or absence of other power supply devices or an inverter in the DC power transmission system in the AC system 2 on the power receiving side. it can. Even if a short circuit accident or the like occurs in the AC system 2, the operation is continued without stopping the reverse converter 12, and the accident is removed by a protective relay and a breaker in the AC system as before to return to a normal operation state. I can return.

  FIG. 3 shows an overall configuration including a DC power transmission system and a control device thereof according to Embodiment 2 of the present invention. 3, elements having the same reference numerals as those in FIG. 1 are the same, and description thereof is omitted.

  The difference between the second embodiment and the first embodiment is the configuration of the AC-AVR 31 and the converter output control circuit 34. In other words, in the second embodiment, the voltage detector 15 that detects the voltage of the AC bus 10 and the inverse converter 12 are connected to each other so that the mutual relationship between the main circuit and the control device 30 of the receiving-end AC / DC converter becomes clear. A current detector 16 for detecting a current output to the bus 10 is provided. Further, an effective value detection circuit 35 for detecting the effective value of the AC voltage from the output signal of the voltage detector 15, and a reactive power detection circuit for detecting reactive power using the output signals of the voltage detector 15 and the current detector 16. 36 is provided.

  In the AC-AVR 31 of FIG. 3, 31 a is a comparator that calculates the difference between the reactive power reference Qref and the output of the reactive power detection circuit 36. The result is output to the multiplier 31b, and the product with the slope gain Xs1 is output. An adder 31c adds the voltage command Vref and the output of the multiplier 31b. A comparator 31d calculates the difference between the output signal of the adder 31c and the AC voltage effective value detected by the effective value detection circuit 35.

  31e is an error amplifying circuit, which is composed of, for example, a PI (proportional integration) circuit, generates a correction amount of the control signal of the output voltage so that the error signal which is the output of the comparator 31d becomes small, and adds by the adder 31f Is added to the output signal of the device 31c. Reference numeral 31g denotes an output limiter that restricts the output of the AC-AVR 31 from becoming unnecessarily large.

  The AC-AVR 31 outputs two signals of Vd and Vq to the converter output control circuit 34, but the signal of Vq is actually 0. Next, the converter output control circuit 34 uses the output signal from the AC-AVR 31 and the output of the phase / frequency arithmetic circuit 32 to convert the two-axis signals Vd and Vq from the dq axis to the three-phase circuit. The three-phase signal is converted by 34a, and the three-phase signal is sent to the PWM control circuit 34c. The PWM control circuit 34c uses the signal of the carrier wave generation circuit 34b to generate a gate pulse that controls the on / off state of the switching elements constituting the inverse converter 12.

Next, the operation of this embodiment will be described.
Since the output of the multiplier 31b in the AC-AVR 31 is obtained by multiplying the difference between the reactive power reference Qref and the output of the reactive power detection circuit 36 corresponding to the actual reactive power output by the slope gain Xs1, the reactive power A characteristic having an inclination Xs1 corresponding to a change in output is shown.

  For example, if the voltage fluctuation is to be reduced, the slope gain Xs1 may be reduced. The output of the adder 31c is added to the voltage command Vref, and this characteristic is indicated by the slope Xs1 through (Qref, Vref) as indicated by the solid line in FIG. Thereafter, the difference from the effective value detection circuit 35 is calculated by the comparator 31d so that the effective voltage value of the AC bus 10 matches the output of the adder 31c. The result calculated by the error amplification circuit 31e so that this difference becomes zero is added to the output of the adder 31c and the adder 31f. The signal thus obtained represents the magnitude of the voltage to be output by the inverse converter 12.

The operation of the following converter output control circuit 34 is the same as that of the known one, but only the portion related to the operation of the present invention will be described briefly.
The dq-axis to three-phase conversion circuit 34a uses a two-phase output signal from the AC-AVR 31 that determines the magnitude of the output voltage and an output signal from the phase / frequency arithmetic circuit 32 that determines the phase and frequency. Generate a signal. Further, the PWM control circuit 34c uses the three-phase signal and the signal from the carrier wave generation circuit 34b to generate a gate pulse for controlling the on / off state of the switching elements constituting the inverse converter 12.

  As a result of these series of operations, the inverter 12 appropriately turns on / off the switching element, generates a target AC voltage from the DC voltage maintained by the DC capacitor 13, and passes through the inverter 11 for the inverter. Generated on the AC bus 10.

  As described above, according to the present embodiment, a command value whose output voltage gently changes with the slope of the slope Xs1 according to the reactive power output of the inverter 12 is generated, and the inverter 12 according to this command value. It is possible to provide a control device for a DC power transmission system that controls the output voltage of the DC power transmission system.

  FIG. 5 shows an overall configuration including a DC power transmission system and a control device thereof according to Embodiment 3 of the present invention. 5, elements having the same reference numerals as those in FIGS. 1 and 3 are the same, and the description thereof is omitted.

  This embodiment is different from the first and second embodiments in the configuration of the phase / frequency arithmetic circuit 32. That is, in the phase / frequency calculation circuit 32 of FIG. 5, 32a is a phase difference detection circuit, and the phase difference between the voltage signal of the AC bus 10 detected by the voltage detector 15 and its own phase signal calculated by the integration circuit 32f. Is calculated. The result is output to the error amplifier circuit 32b, and the operation result is output to the adder 32e.

  On the other hand, 32c is a coefficient multiplier, and the result obtained by multiplying the power command Pref by the coefficient K is output to the adder 32d. 32d adds the angular frequency command ωref and the output signal of the coefficient multiplier 32c and outputs the result to the adder 32e. The adder 32e adds the output of the error amplification circuit 32b. An integration circuit 32f integrates a signal corresponding to the output frequency, which is an output signal of the adder 32e, to calculate a phase signal.

Next, the operation of this embodiment will be described.
Among the phase / frequency calculation circuit 32, the phase detection difference circuit 32a calculates the phase difference between the voltage signal of the AC bus 10 detected by the voltage detector 15 and its own phase synchronization signal calculated by the integration circuit 32f. And output.

  The error amplifying circuit 32b calculates a value corresponding to the frequency adjustment amount so that the phase difference becomes small. That is, when the phase signal calculated by the integration circuit 32f is delayed, the frequency is increased in order to advance the phase, and conversely, when the phase signal is advanced, the phase is delayed. The output signal is output to the adder 32e in the direction of decreasing the frequency.

  Here, the error amplifying circuit 32b does not use an arithmetic circuit including an integration that eliminates the phase difference at all, but outputs a signal proportional to the phase difference in a steady manner, such as adding only a proportional gain or a first-order lag. An arithmetic circuit is used. Thus, it is possible to provide a characteristic in which the phase error and the frequency adjustment amount have a proportional relationship.

  On the other hand, the result obtained by multiplying the power command Pref by the coefficient K by the coefficient multiplier 32c is added to the angular frequency command ωref by the adder 32d. Here, the coefficient K is determined as K = Xt * K1 using the control gain K1 of the error amplifier 32b and the impedance Xt of the transformer for inverse converter 11. As a result, the output of the adder 32d becomes a value larger by Pref * K than the angular frequency command ωref.

  On the other hand, when the inverse converter 12 outputs active power having a magnitude corresponding to Pref, the voltage phase difference Δθ between the output terminal of the inverse converter 12 and the AC bus 10 is equal to the transformer 11 for the inverse converter. Is approximated by Δθ≈Pref * Xt. Here, the output voltage phase of the inverse converter 12 corresponds to the phase of the integration circuit 32f, and the difference from the voltage phase of the AC bus 10 corresponds to the output of the phase difference detection circuit 32a. As a result, the calculation result obtained by multiplying the control gain K1 by the error amplifying circuit 32b has the same magnitude as the output of the coefficient multiplier 32c and the opposite polarity, and the output of the adder 32e becomes a value corresponding to ωref. This action is represented by a control characteristic diagram as shown in FIG.

  Next, the integration circuit 32f integrates the signal of the adder 32e corresponding to the angular frequency to generate a phase synchronization signal. The phase synchronization signal obtained here is output to the converter output control circuit 34, and the following operations are the same as those described in the second embodiment.

  As described above, the output frequency of the frequency command value of the present embodiment changes gently according to the slope associated with the control gain of the error amplifier 32b in accordance with the active power output of the inverse converter 12. As a result, according to the present embodiment, it is possible to provide a control device for a DC power transmission system that controls the output phase and frequency of the inverse converter 12 according to a frequency command value whose output frequency has changed gently.

  FIG. 7 shows an overall configuration including a DC power transmission system and a control device thereof according to Embodiment 4 of the present invention. In FIG. 7, elements denoted by the same reference numerals as those in FIGS. 1 and 3 are the same, and the description thereof is omitted.

  This embodiment differs from the first and second embodiments in the configuration of the current limiting circuit 33. That is, an adder 31h is added between the adder 31f of the AC-AVR 31 and the output limiter 31g. Similarly, an adder 31i is added to the output part of the q-axis voltage command, and the output of the current limiting circuit 33 is added. Has been. Further, the output of the current detector 16 is input to the current detection circuit 37 so that the mutual relationship between the main circuit and the current control circuit 33 is clarified, and this output is connected to the current limiting circuit 33.

  The current limiting circuit 33 calculates the difference between the output current limit values Idmax and Iqmax of the d-axis and the q-axis and the output current values Id and Iq coordinate-converted on the dq-axis by the current detection circuit 37 by the comparators 33a and 33b. And output to the error amplifiers 33c and 33d. Further, the output of the error amplifier becomes the output of the current limiting circuit 33 via the output limiters 33e and 33f.

Next, the operation of this embodiment will be described.
The current detection circuit 37 converts the output of the current detector 16, which is a three-phase signal, into coordinates on the dq axis using the output signal of the phase / frequency calculation circuit 32. The output current values Id and Iq obtained as a result are calculated by the comparators 33a and 33b as the difference between the d-axis and q-axis output current limit values Idmax and Iqmax.

  Next, the outputs of the comparators 33a and 33b are composed of, for example, PI (proportional integration) circuits by error amplifiers 33c and 33d, and are calculated so as to obtain converter outputs that reduce the difference in current as an input signal. . Further, the outputs of the error amplifiers 33c and 33d become the output of the current limiting circuit 33 via the output limiters 33e and 33f. However, the output limiter is set only when the currents Id and Iq whose coordinates have been converted by the current detection circuit 37 exceed the current limit values Idmax and Iqmax, and the output becomes valid, and Id and Iq are higher than the current limit values Idmax and Iqmax. If it is small, the output will reach the limit and become zero.

  In this case, when the output current of the inverse converter 12 is smaller than the values corresponding to the current limit values Idmax and Iqmax, the outputs of the output limiters 33e and 33f become zero, so that the AC-AVR 31 is not affected. The operation is as described in Example 2.

  On the other hand, in a situation where the output current is larger than the values corresponding to the current limit values Idmax and Iqmax, a signal such that the difference between Id and Iq and the current limit values Idmax and Iqmax is reduced by the calculation of the error amplifier circuits 33c and 33d. Is output. This signal is added by the adders 31h and 31i of the AC-AVR 31 and acts in the direction of lowering the AC output voltage as shown in FIG. 8, so that the current is limited to values corresponding to Idmax and Iqmax. .

  Therefore, even when a large load exceeding the output capacity of the inverter 12 is input in the AC system 2 or when a short circuit accident occurs in the AC system 2 and an overcurrent flows. As a result, the output current of the inverse converter 12 can be controlled within the operable range by the action of the current limiting circuit 33. Therefore, the inverter 12 can continue operation, and the overload state in the AC system 2 can be eliminated during that time. In the meantime, it is possible to detect a short-circuit fault point by means of an accident detection means such as a protection relay that has been generally used in the past, disconnect the fault section with a circuit breaker, and return to a normal operating state.

  As described above, according to the present embodiment, even if a short circuit accident or the like occurs in the AC system 2, the inverter 12 is continuously operated without being stopped, and the protective relay and breaker in the AC system are maintained as before. Thus, it is possible to provide a control device for a DC power transmission system that can remove the accident and return to a normal operation state.

  FIG. 9 is a configuration diagram including a DC power transmission system, a control device thereof, and a power receiving side AC system showing Embodiment 5 of the present invention. In FIG. 9, elements denoted by the same reference numerals as those in FIG. 7 are the same, and the description thereof is omitted.

  This embodiment differs from the first and fourth embodiments in that a timer 38 is added to the control device 30 of the inverse converter 12. This is connected to the output of the current detection circuit 37, and the output is connected to various places related to operation / stop in the control device 30 of the inverse converter 12 although not clearly shown in the drawing. In addition, the AC system 2 includes circuit breakers 2a, 2b, 2c, and 2d disposed at both ends of the power transmission lines 2e and 2f, and protective relays 2g and 2h.

  Here, consider a case where a short circuit accident has occurred in the power transmission line 2f in the AC system 2 due to lightning or the like. When a short-circuit accident occurs, a short-circuit current flows and the nearby AC voltage greatly decreases. Also for the AC bus 10, for example, as shown in FIG. 10, the AC bus voltage greatly decreases at time T 1, and the value obtained by dq-axis coordinate conversion of the current output from the inverse converter 12 by the current detection circuit 37 greatly increases.

  However, the operation is continued in a state where the maximum value of the current is limited by the current limit values Idmax and Iqmax as described in the fourth embodiment. During this time, the protection relays 2g and 2h, which are accident detection means arranged in the AC system 2, detect that a short-circuit accident has occurred at time T2 based on voltage and current information. At the same time, when an opening command is sent to the circuit breakers 2c and 2d including the accident section, the circuit breakers 2c and 2d are opened at time T3 after the operation delay of the opening signal and the operation time of the circuit breaker are required. Thus, since the short-circuit accident section is disconnected, the output current (d-axis current, q-axis current) of the inverter 12 returns to the original state, and the AC bus voltage also returns.

  Assuming that the set value of the termer 38 in the control device 30 of the inverter 12 in FIG. 9 is T0, in this case, the short circuit accident is removed before T0 elapses from the occurrence of the accident, and no large current flows. As a result, the timer 38 is reset at the same time, and its output does not affect the operation of the inverse converter 12.

  That is, even if a short circuit accident or the like occurs in the AC system, the DC power transmission system including the reverse converter 12 and its control device 30 operates while limiting the output current within the settling time of the timer 38. Can continue. Thus, according to the present embodiment, it is possible to provide a control device that can automatically restore the original operation state when the short-circuit accident is removed by the protective relay and the circuit breaker in the AC system. Moreover, the system using this protective relay and circuit breaker does not need to change the conventional method.

  Although the configuration of the present embodiment is the same as that of FIG. 9, the operation of the embodiment 6 of the present invention will be described with reference to FIG. It is the same as that of Example 5 until the short circuit accident occurs in the AC system 2 at time T1 and the voltage of the AC bus 10 decreases.

  The difference is that in Example 5, the protection relay detects an accident at time T2 and removes the accident at time T3. However, the accident is caused by some reason, for example, the failure of the protection relays 2g and 2h and the circuit breakers 2c and 2d. It is assumed that the short-circuit state continues without being removed.

  Such a situation is undesirable for the power system 2 because the voltage drop continues to be unfavorable, and the inverter 12 is also inconvenient because the overcurrent state full of the current limit value continues. Therefore, the timer 38 detects that the overcurrent is not eliminated even when T0 has elapsed from the occurrence of the accident, and issues a stop command to the DC power transmission system. As a result, when there is no other power supply device in the AC system 2, the entire power failure state occurs, and if the short-circuit accident is caused by lightning, the possibility that the short-circuit accident state is eliminated increases. Further, the overcurrent state is also eliminated from the inverse converter 12.

  As described above, according to the present embodiment, when the settling time of the timer 38 is exceeded and the output current of the inverse converter 12 is not eliminated from the limit value, the DC power transmission system itself is stopped by a command from the timer 38. Therefore, it is possible to provide a control device that can eliminate the accident situation.

DESCRIPTION OF SYMBOLS 1 ... AC system on the power transmission side, 2 ... AC system on the power reception side, 2a, 2b, 2c, 2d ... Circuit breaker, 2e, 2f ... Transmission line, 2g, 2h ... Protection relay, 3, 4 ... Generator, 5 ... Breaker, 6, 10 ... AC bus, 7, 11 ... Transformer for converter, 8 ... Forward converter, 12 ... Reverse converter, 9, 13 ... DC capacitor, 14a, 14b ... DC line, 15 ... Voltage detector , 16 ... current detector, 20 ... control device for power transmission / reverse converter, 21 ... DC voltage control circuit, 22 ... AC voltage control circuit, 23 ... reactive power control circuit, 24 ... selection circuit, 25 ... converter output control Circuit: 30 ... Control device for receiving-end AC / DC converter, 31 ... AC voltage control circuit, 32 ... Phase / frequency arithmetic circuit, 33 ... Current limiting circuit, 34 ... Converter output control circuit, 35 ... Voltage effective value detection circuit, 36 ... reactive power detection circuit, 37 ... current detection circuit, 38 Timer, 31a, 31d ... Comparator, 31b ... Multiplier, 31c, 31f, 31h, 31i ... Adder, 31e ... Error amplification circuit, 31g ... Output limiter, 32a ... Phase difference detection circuit, 32b ... Error amplification circuit, 32c ... coefficient multiplier, 32d, 32e ... adder, 32f ... integrator circuit, 33a, 33b ... comparator, 33c, 33d ... error amplifier, 33e, 33f ... output limiter, 34a ... dq axis → three-phase converter circuit, 34b ... Carrier wave generation circuit, 34c... PWM control circuit

Claims (5)

In a DC power transmission system including a forward converter that converts AC power to DC power and an inverse converter that converts DC power to AC power,
The forward converter has a control device that controls accommodation power so as to maintain a DC voltage constant;
The reverse converter causes the AC voltage and frequency of the AC system to follow the command value when there is no reverse converter side AC system, regardless of the presence or absence of other power supply devices or DC converter reverse converters. In the case where there is another power supply device or the like, it has a control device that adjusts the AC voltage and frequency while synchronizing with the output AC voltage.
The control device provided in the reverse converter includes a means for limiting an output current for preventing an overcurrent to the reverse converter side AC system. The control device of the inverse converter,
A circuit for detecting the reactive power output of the inverse converter;
A circuit for calculating a difference between the command value of the reactive power output and the detected reactive power output;
A circuit for multiplying the difference by a slope gain;
By adding the output of this circuit to the AC voltage command value, a command value whose output voltage changes gently according to the reactive power output of the inverse converter is generated, and the command value is compared with the value detected by the AC voltage. And a circuit for calculating the error,
A circuit for adding the calculation result and the command value;
By these circuits, a command value of an internal voltage to be output by the inverse converter is generated, and coordinate conversion and PWM control are performed according to the command value to control the output voltage of the inverse converter. The DC power transmission system according to claim 1. The control device of the inverse converter,
A circuit for detecting the instantaneous value of the AC voltage of the AC system on the reverse converter side;
A circuit for detecting a phase difference between the instantaneous value of the alternating voltage and the synchronization signal calculated by itself;
A circuit for amplifying the phase difference with a proportional gain, and
The value obtained by multiplying the active power command value by the coefficient and the frequency command value is used as a new frequency command value, and the conversion device actually adds the frequency command value and the output value of the error amplified circuit. As a frequency to be output, a circuit that generates a synchronization signal that is a command value of an output voltage phase by integrating the frequency is provided.
The DC power transmission system according to claim 1 or 2, wherein the output frequency of the inverse converter is controlled by performing coordinate conversion and PWM control using a synchronization signal generated by these circuits. The control device of the inverse converter includes a circuit that detects an AC output current of the inverse converter, and a circuit that performs coordinate conversion of the output current into a d-axis and a q-axis,
The means for limiting the output current includes a circuit for comparing the upper limit value of the output current on the coordinate axes of the d axis and the q axis, a circuit for amplifying an error exceeding the upper limit value, and the error amplified value for the error amplified value. 4. The AC output voltage is reduced when an overcurrent occurs by adding the AC voltage to the output of the circuit that controls the AC voltage. The direct current power transmission system described in 1. The control device on the reverse converter side includes an accident detection means for detecting that an accident has occurred in the AC system on the reverse converter side, and even if a certain time elapses when the accident detection means detects an accident. The direct-current power transmission system according to claim 4, further comprising a timer for stopping the reverse converter when not recovering from the accident state.

Images