JP2008118809A - Method and device for protecting isolated operation of power conversion system for power system operation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for protecting an isolated operation of an inexpensive power conversion system for power system operation with less secular deterioration by using a simple calculating means. <P>SOLUTION: The isolated operation protection device of a power conversion system for power system operation is provided with a synchronizing signal calculating means for calculating a sample value of a synchronizing signal having prescribed amplitude and phase from data obtained by sampling voltage of a connection point of a power conversion circuit converting DC power into AC power and a commercial power system. The device is also provided with a control means controlling output current of the power conversion circuit so that it is matched with a current command value synchronized with the phase of the synchronizing signal. The synchronizing signal calculating means is constituted so that a phase of the current command value changes so that a frequency of output voltage of the power conversion circuit changes to a prescribed value when power of the commercial power system fails. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a protection device for a grid interconnection power conversion system that is linked to a commercial power grid, converts DC power to AC power, and supplies the power to a load or grid on the customer side, The present invention relates to a protection method and apparatus for preventing the system from operating independently during a system power failure.

  According to the grid interconnection regulations JEAC9701-2006, in a power conversion system that converts DC power generated by a solar cell or the like into AC power and supplies the power to a commercial power system or a load connected thereto, In order to prevent an electric shock accident of an operator at the time of a power failure, it has been proposed as one technical requirement to have a function of detecting a system power failure and preventing an independent operation of the system.

  To date, no means has been proposed to completely prevent islanding in any theoretical or actual operational case. Therefore, in the previous regulations, two methods that are recognized as effective, that is, one method each of the active method and the passive method, are adopted, and the combination of these two methods Guidelines are provided to prevent system isolated operation.

  Under such circumstances, there is a system called a frequency shift system as one of the active systems. As a system interconnection power conversion system employing this system, for example, there is a system interconnection inverter device as shown in FIG. .

  In FIG. 5, the converter 101 is connected to the commercial power system 2, converts the DC power of the solar cell 1 into AC power, and supplies the power to the load 3. The output frequency of the conversion unit 101 is controlled to be equal to the oscillation frequency of the voltage controlled oscillation circuit 109.

  The oscillation frequency of the voltage controlled oscillation circuit 109 includes the output of the smoothing circuit 106 that smoothes the output of the phase comparison circuit 105 that compares the voltage phase of the interconnection point 202 and the output voltage phase of the voltage controlled oscillation circuit 109, and the interconnection point 202. Is controlled by an adder circuit 107 that adds the output of the bias generator 108 that fluctuates the output voltage in response to the frequency fluctuation.

  As a result, when the commercial power system 2 fails and the frequency fluctuates, the output frequency of the conversion unit 101 is controlled to change in a direction in which the frequency fluctuation increases.

The control circuit 103 detects the frequency fluctuation, opens the circuit breaker 102, and disconnects the conversion unit 101 from the system (see Patent Document 1).
JP-A-8-275398

  By the way, in the above-described grid-connected inverter device, many hardware such as the phase comparison circuit 105, the smoothing circuit 106, the addition circuit 107, the voltage control oscillation circuit 109, and the bias generation unit 108 are used in order to prevent the independent operation of the inverter. Necessary. Therefore, there is a problem that the circuit configuration becomes complicated and not only the apparatus failure rate increases but also the cost increases.

  In order to solve such problems, the present invention proposes a frequency shift method different from the above using a microcomputer and using software, and operates the power conversion system for grid interconnection independently. It is an object of the present invention to provide a protection method and an apparatus for reliably preventing it.

  Usually, in such a grid-connected power conversion system, a microcomputer is generally used for the control circuit that controls the conversion unit, and this makes it possible to perform simple calculations without adding a circuit. It is possible to provide an isolated operation protection method for a grid interconnection power conversion system and an apparatus thereof that do not increase an apparatus failure rate and cost by realizing a protection method for preventing isolated operation using a means. It becomes possible.

  In order to achieve the above object, the invention according to claim 1 is configured to convert the power of a DC power source into AC power and supply the power to a commercial power system or a load connected to the commercial power system. In the isolated operation protection method for the grid interconnection power conversion system, synchronization having a predetermined amplitude and phase from data obtained by sampling the voltage at the interconnection point between the power conversion circuit for converting DC power to AC power and the commercial power grid By calculating the sample value of the signal and controlling the output current of the power conversion circuit to match the current command value created based on the sample value of the synchronization signal, when the commercial power system fails, The phase of the synchronization signal that is the source of the current command value is changed so that the frequency of the output voltage of the power conversion circuit changes to a predetermined value.

  The invention described in claim 2 is a grid-connected power conversion system configured to convert the power of a DC power source into AC power and supply the power to a commercial power system or a load connected to the commercial power system. A synchronization signal calculation means for calculating a sample value of a synchronization signal having a predetermined amplitude and phase from data obtained by sampling the voltage at the connection point between the power conversion means for converting DC power to AC power and the commercial power system The control means for controlling the output current of the power conversion means so as to match the current command value synchronized with the phase of the synchronization signal is provided, and the output of the power conversion means when the commercial power system fails The phase of the current command value is changed by the synchronization signal calculation means so that the frequency of the voltage changes to a predetermined value.

  According to a third aspect of the present invention, the synchronization signal calculating means according to the second aspect is configured using a state equation and an output equation of a digital control system.

  According to the isolated operation protection method and the isolated operation protection device of the grid interconnection power conversion system of the present invention, when the commercial power system is normal, the current command value is synchronized with the voltage at the interconnection point by the synchronization signal calculation means. is doing. Since the output current of the power conversion circuit is controlled so as to match the current command value, the output frequency is the same as the frequency of the interconnection point (commercial frequency 50 Hz or 60 Hz). On the other hand, when the commercial power system fails, the phase of the current command value is changed by the synchronization signal calculation means so that the output frequency of the power conversion circuit changes to a predetermined value. This frequency change is detected by the frequency relay, and the power conversion circuit can be surely stopped, which is effective.

  In addition, by using the state equation and output equation of the digital control system as the synchronization signal calculation means, the control system and microcomputer software can be easily designed, which is very convenient.

  Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, the same elements as those in the grid interconnection inverter device shown in FIG.

  FIG. 1 is a block diagram showing a grid interconnection power conversion system according to the present invention. The power conversion circuit 11 converts the DC power generated by the solar cell 1 into AC power, and supplies the commercial power system 2 and the load 3 with power. The power conversion circuit 11 and the commercial power system 2 are linked at a linkage point 202, and the voltage at that point is sampled by the voltage detection unit 12 and input to the synchronization signal calculation means 18. Then, the synchronization signal calculation means 18 calculates and outputs a sample value of the synchronization signal Vs (alternating current) having a predetermined amplitude and phase.

  The difference between the voltage command value Vdr set to extract the maximum power from the solar cell 1 and the voltage Vd of the solar cell 1 is amplified by the input voltage control unit 13, and the control signal Vc (direct current) output therefrom is Multiplication unit 14 multiplies synchronizing signal Vs to create current command value Ior (alternating current, frequency is the same as Vs). Then, the difference between the current command value Ior and the output current Io of the power conversion circuit 11 detected by the current detector 4 is amplified by the output current control unit 15, and the output signal is input to the PWM control unit 16 to be PWM controlled. Create a switching control signal. The drive unit 17 operates the power conversion circuit 11 in accordance with the switching control signal. Overall, a current command value Ior that takes the maximum power from the solar cell 1 is created, and the output current Io of the power conversion circuit 11 is controlled so as to match the current command value.

  The frequency detector 19 detects the frequency from the data sampled by the voltage detector 12 and inputs the detected frequency to the frequency relay 20. The frequency relay 20 compares it with a set value to detect a frequency abnormality and inputs it to the abnormality detection unit 21.

  The effective value calculation unit 22 calculates the effective value of the connection point voltage from the data sampled by the voltage detection unit 12 and inputs the calculated value to the voltage relay 23. The voltage relay 23 compares it with the set value to detect an abnormality in the system voltage and inputs it to the abnormality detection unit 21.

  Operation signals of various protection relays (not shown because they are not related to the present invention) such as the frequency relay 20 and the voltage relay 23 are input to the abnormality detection unit 21. The abnormality detection unit 21 gives a stop command to the drive unit 17 when the protection relay operates, and stops the power conversion circuit 11.

  By the way, in FIG. 1, all the parts enclosed by the broken line are processed by the microcomputer program.

  As is apparent from the above description, the output frequency of the power conversion circuit 11 is equal to the frequency of the synchronization signal Vs, which is defined by the synchronization signal calculation means 18. As the synchronization signal calculation means 18, a method comprising a state equation and an output equation of a digital control system is simple in terms of design and explanation, and here, a method of calculating with these equations will be described.

  The synchronization signal Vs is calculated by the following two equations.

here,
k: an index representing time t _k = kΔt (Δt: sampling interval),
u (k): voltage sampling data of the interconnection point 202 at time t _k ,
x (k): state vector at time t _k (column vector having n state variables as components),
y (k): sample value of the synchronization signal Vs at time t _k ,
A: n-order square matrix, B: n × 1 matrix, C: 1 × n matrix.

  Each element of the matrix A, B, C has a phase characteristic of the pulse transfer function derived from [Equation 1] and [Equation 2], the phase is 0 degree at the commercial frequency, and the rate of change of the phase in the vicinity thereof is positive. The phase is determined so that the phase at a frequency lower than a predetermined value from the commercial frequency is positive and the phase at a frequency higher than the predetermined value is negative.

  For example, matrixes A, B, and C when the commercial frequency is 60 Hz and the sampling interval is 80 μs, and examples of the state equation and the output equation are shown in [Formula 3] to [Formula 5] below. Further, the frequency characteristics of the pulse transfer function of this example are as shown in FIG. 2 (FIG. 2A: gain characteristics, FIG. 2B: phase characteristics).

  From the above, when the commercial power system 2 is normal, the phase of the output current of the power conversion circuit 11 with respect to the connection point voltage is 0 degrees, and is in phase with the connection point voltage. When the commercial power system 2 loses power and the frequency changes, the phase of the output current of the power conversion circuit 11 changes accordingly. That is, the phase is delayed when the frequency is lower than the commercial frequency, and the phase is advanced when the frequency is higher.

  Hereinafter, the operation of this embodiment will be described. The output frequency of the power conversion circuit 11 always fluctuates slightly due to, for example, fluctuations in circuit element values due to temperature changes. When the commercial power system 2 is normal, the output current of the power conversion circuit 11 is controlled so as to be in phase with the connection point voltage, so that the output frequency is always controlled in a direction that matches the frequency of the connection point. . Therefore, in this case, the frequency relay 20 does not work and the power conversion system does not stop.

  Next, the operation when the commercial power system 2 fails will be described. In order to simplify the description, the load 3 is assumed to be an RLC load as shown in FIG. 3, and a load in which an isolated operation in which the resonance frequency of the LC coincides with the commercial frequency (60 Hz in this embodiment) is likely to occur. And An example of the phase characteristic φz of the impedance Z of the load is shown in FIG. 4 (FIG. 4 also shows the phase characteristic φs of the pulse transfer function of the synchronization signal calculation means).

  Now, it is assumed that the frequency of the interconnection point 202 is 60 Hz, and the output power of the power conversion circuit 11 and the power consumption of the load match. In such a case, since power is not supplied from the commercial power system 2 to the load 3, the power conversion system continues to operate even when the commercial power system 2 is disconnected (single operation).

  Although the output frequency of the power conversion circuit 11 always fluctuates slightly, if this fluctuation does not increase, the frequency relay 20 does not operate, and the power conversion system continues to operate independently. However, according to the present invention, the control works in the direction in which the frequency fluctuation increases, the frequency relay 20 operates, and the power conversion system stops.

  When the commercial power system 2 is disconnected, the phase characteristic of the interconnection point voltage Vo with respect to the output current Io is the phase characteristic φz of the load impedance Z (Vo = Z × Io). The phase characteristic of the synchronization signal Vs with respect to the interconnection point voltage Vo is the phase characteristic φs of the pulse transfer function of the synchronization signal calculation means 18.

  The phase of the current command value Ior is the same as the phase of the synchronization signal Vs (Ior = Vc × Vs). Since the output current Io is controlled to coincide with the current command value Ior, the phase characteristic φi of the output current Io with respect to the interconnection point voltage Vo is the same as the phase characteristic φs of the pulse transfer function.

  Now, assuming that the output frequency fo of the power conversion circuit 11 fluctuates and becomes higher by Δf than the current commercial frequency fs, the phase of the output current Io increases with positive feedback. That is, when the output frequency is increased by Δf, the phase of the interconnection point voltage Vo is delayed by Δφz due to the phase characteristic φz of the load impedance. On the other hand, the phase of the output current advances by Δφs due to the phase φs of the pulse transfer function. Eventually, the phase of the output current Io advances by Δφi = Δφs−Δφz. Furthermore, since the phase of Δφs is delayed and delayed by Δφz with respect to this phase, the phase of the output current Io increases with positive feedback.

  As is apparent from the relationship between frequency modulation and phase modulation, generally, there is a relationship of 2πf (t) = dφ (t) / dt between the instantaneous frequency f (t) and the instantaneous phase φ (t). . Here, when considering the variation Δfo (t) of the output frequency fo from the commercial frequency fs and the variation Δφi (t) of the instantaneous phase of the output current (subtracting 2πfs × t from the instantaneous phase of the output current) A similar relational expression 2πΔfo (t) = d {Δφi (t)} / dt holds.

  As is apparent from the characteristics shown in FIG. 4, since Δφi> 0 when the output frequency fo is in the range of fs <fo <f2, the change in the phase of the output current Io increases in the positive direction. Therefore, the variation Δfo of the output frequency of the power conversion circuit 11 also increases in the positive direction.

  When the output frequency fo becomes higher than f2, since Δφi <0, the phase change of the output current Io increases in the negative direction. Therefore, the variation Δfo of the output frequency of the power conversion circuit 11 also increases in the negative direction.

  When the output frequency fo is equal to f2, Δφz = Δφs and Δφi = 0, so neither the phase nor the frequency changes.

  From the above, when the output frequency fo of the power conversion circuit 11 fluctuates and becomes slightly higher than the commercial frequency fs, it converges to the frequency f2.

  Next, the case where the output frequency fo of the power conversion circuit 11 fluctuates and becomes lower by Δf than the commercial frequency fs which is the current value will be described. In this case as well, as apparent from the characteristics shown in FIG. 4, since the output frequency fo is Δφi <0 in the range of f1 <fo <fs, the phase change of the output current Io is apparent. Increases in the negative direction. Therefore, the variation Δfo of the output frequency of the power conversion circuit 11 also increases in the negative direction.

  When the output frequency fo becomes lower than f1, since Δφi> 0, the phase change of the output current Io increases in the positive direction. Therefore, the variation Δfo of the output frequency of the power conversion circuit 11 also increases in the positive direction.

  When the output frequency fo is equal to f1, Δφz = Δφs and Δφi = 0, so that neither the phase nor the frequency changes.

  From the above, when the output frequency fo of the power conversion circuit 11 fluctuates and becomes slightly lower than the commercial frequency fs, it converges to the frequency f1.

  After all, according to the present invention, in the case of an isolated operation, if the frequency fluctuates even a little, the output frequency fo of the power conversion circuit 11 becomes f1 or f2. [Equation 1] and [Equation 2] of the synchronization signal calculation means 18 are set in consideration of the load impedance so that the frequency f1 or f2 is a frequency that can be detected by the frequency relay 20. Therefore, in the present invention, isolated operation can be detected by the frequency relay.

  Here, in order to simplify the explanation, an RLC load in which an isolated operation is easily generated such that the output power of the power conversion circuit 11 matches the power consumption of the load 3 and no power is supplied from the commercial power system 2 is considered. It was. However, in many cases, power is supplied from the commercial power grid to the load, so if the grid fails, power is not supplied from the grid, and not only the frequency but also the load voltage exceeds the specified range. Become. Therefore, by combining the frequency relay 20 and the voltage relay 23, it is possible to detect the isolated operation more reliably.

  As is apparent from the above description, according to the present invention, it is not necessary to add a circuit for protection of isolated operation to the control circuit of the grid interconnection power conversion system, and the microcomputer used therefor It is possible to use the software to realize the function for protection from isolated operation. For this reason, it is possible to reliably detect isolated operation of a power conversion system for grid interconnection without causing an increase in cost and an increase in failure rate due to the addition of circuits, fluctuations in circuit elements, fluctuations due to temperature, and deterioration in performance due to aging. Can be prevented.

  In addition, since it can be configured from the state equation and output equation of the digital control system, it is very convenient because it can be freely designed without restrictions on design due to circuit element values, compared to the case where it is realized with a circuit. It is.

  According to the present invention, it is possible to provide an isolated operation protection method and an isolated operation protection device for a grid interconnection power conversion system that are inexpensive and have little deterioration over time, using simple calculation means.

It is a block diagram which shows the power conversion system for grid connection of this invention. It is an example of the frequency characteristic of the pulse transfer function of the synchronous signal calculation means which comprises the islanding operation protection apparatus of the said grid connection power conversion system. It is an example of load. It is operation | movement explanatory drawing of the said independent driving protection apparatus. It is a block diagram which shows the conventional grid connection inverter apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solar cell 2 Commercial power system 3 Load 4 Current detector 5, 6 Circuit breaker 11 Power conversion circuit 12 Voltage detection part 13 Input voltage control part 14 Multiplication part 15 Output current control part 16 PWM control part 17 Drive part 18 Synchronization signal calculation Means 19 Frequency detector 20 Frequency relay 21 Abnormality detector 22 Effective value calculator 23 Voltage relay 201 Input point 202 Interconnection point Vd Solar cell voltage Vdr Voltage command value Vc Input voltage controller output Vs Synchronization signal Io Power conversion circuit Output current Ior Current command value

Claims (3)

  In the isolated operation protection method for a grid-connected power conversion system configured to convert the power of a DC power source into AC power and supply the power to a commercial power system or a load connected to the commercial power system, A sample value of a synchronization signal having a predetermined amplitude and phase is calculated from data obtained by sampling a voltage at a connection point between a power conversion circuit that converts electric power into AC power and a commercial power system, and the sample value of the synchronization signal is By controlling the output current of the power conversion circuit so as to match the current command value created in the above, the frequency of the output voltage of the power conversion circuit changes to a predetermined value when the commercial power system fails. An isolated operation protection method for a grid interconnection power conversion system, wherein the phase of the synchronization signal that is a source of a current command value is changed.   In a grid-connected power conversion system configured to convert DC power into AC power and supply the power to a commercial power system or a load connected to the commercial power system, DC power is converted to AC power. Synchronization signal calculation means for calculating a sample value of a synchronization signal having a predetermined amplitude and phase from data obtained by sampling a voltage at a connection point between the power conversion means for conversion and the commercial power system, and synchronized with the phase of the synchronization signal Control means for controlling the output current of the power conversion means so as to match the current command value is provided, and when the commercial power system fails, the frequency of the output voltage of the power conversion means changes to a predetermined value. Thus, the isolated operation protection device for a grid interconnection power conversion system, wherein the phase of the current command value is changed by the synchronization signal calculation means. 3. The isolated operation protection device for a grid interconnection power conversion system according to claim 2, wherein the synchronization signal calculation means comprises a state equation and an output equation of a digital control system.

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