Power conversion device, system interconnection power conversion system, and method for detecting earth fault thereof

Abstract

To provide a power conversion device, a system interconnection power conversion system, and a method for detecting earth fault thereof, for obtaining high reliability of not causing malfunction of systems such as earth leakage circuit breakers without interference between earth fault detection circuits of the power conversion devices interconnected to each system even when a system low voltage side is grounded.SOLUTION: It is controlled so that timing based on a power supply cycle for detecting DC ground faults is assigned in different orders between power conversion devices 1 and 2, each of MCUs 104 and 204 synchronizes the power supply cycle between the power conversion devices 1 and 2, and DC ground faults are detected in order from the power conversion devices whose synchronized power supply cycles reach the timing for detecting earth fault.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power conversion device, a grid-connected power conversion system, and a method for detecting a ground fault in the same. [Background technology]

[0002] In power generation facilities with a power conditioning system (PCS) that sells all electricity, many of them are equipped with a contact prevention plate for interconnection and use an ungrounded system that does not require grounding on the low voltage side. In power generation facilities that sell surplus electricity, the low voltage side for interconnection is often grounded.

[0003] For maintenance management, stable and reliable earth fault detection is essential for all low voltage sides of interconnection transformers. For AC side earth fault protection, it is common to use an earth leakage breaker on the grid side to cut off the circuit.

[0004] Regarding DC side earth fault protection, a commonly used method is to insert two high resistance resistors between the positive and negative sides of the DC circuit of the grid-connected power conversion device, and ground the midpoint through the resistors to create a center point between the grounds. Earth faults are then detected by the change in center point potential caused by an earth fault on the DC side.

[0005] Patent Document 1 discloses a technology for ensuring system stability by detecting a ground fault using an inverter that links multiple DC power sources to a common AC power source. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-172616 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the case of a grid-connected power conversion system in which multiple DC power sources are connected to a common AC power source, the DC ground fault detection circuits provided in each power conversion device interfere with each other during grid-connected operation. In particular, when the grid low-voltage side of the interconnection transformer is grounded, the route through which the ground fault current flows becomes complicated, making it difficult to detect a DC ground fault during grid-connection. Patent Document 1 does not clearly state a calculation method for when the grid is grounded.

[0008] Furthermore, if the number of interconnection power conversion devices in the interconnection transformer described above increases, the current passing through the DC ground fault detection circuit increases even if no DC ground fault occurs, making the earth leakage breaker on the AC side more likely to malfunction.

[0009] The present invention has been made in consideration of the above background, and aims to provide a power conversion device, a grid-connected power conversion system, and a ground fault detection method thereof that are capable of achieving high reliability without causing malfunction of systems such as earth leakage circuit breakers, without interference between the ground fault detection circuits of the power conversion devices connected to the grid even when the low-voltage side of the grid is grounded. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems and achieve the above-mentioned objects, one embodiment of the present invention is a power conversion device of a grid-connected power conversion system which is composed of a plurality of power conversion devices and which connects a plurality of DC power sources to a common AC power source, and which comprises: a ground fault detection unit which detects a DC ground fault; a switch unit which switches detection of a DC ground fault between enabled and disabled; and a control unit which has a power supply cycle and controls the switching of the switch unit, wherein the control unit assigns timings based on the power supply cycle for detecting a DC ground fault in an order different from that of the other power conversion devices, synchronizes the power supply cycles between the power conversion devices, and switches the switch unit to enabled when the synchronized power supply cycle reaches the assigned timing.

[0011] Another embodiment of the present invention is a grid-connected power conversion system that connects a plurality of DC power sources to a common AC power source, and is characterized in comprising a first ground fault detection unit connected to a first DC power source and detecting a DC ground fault in the first DC power source, a second ground fault detection unit connected to a second DC power source and detecting a DC ground fault in the second DC power source, and a control unit that switches and controls detection of DC ground faults by the first and second ground fault detection units at different timings based on a power supply period.

[0012] Another embodiment of the present invention is a method for detecting a ground fault in a grid-connected power conversion system which is composed of a plurality of power conversion devices each for detecting a DC ground fault and which connects a plurality of DC power sources to a common AC power source, the method comprising the steps of: assigning timings based on a power supply cycle for detecting a DC ground fault in different orders between the power conversion devices; synchronizing the power supply cycles between the power conversion devices; and switching to detection of a DC ground fault in the order of the power conversion devices in which the synchronized power supply cycle reaches the assigned timing. Effect of the Invention

[0013] According to the present invention, even if the low-voltage side of the system is grounded, there is no interference between the ground fault detection circuits of the power conversion devices connected to each system, and it is possible to achieve high reliability that does not cause malfunctions of systems such as earth leakage circuit breakers. [Brief description of the drawings]

[0014] [Figure 1] 1 is a circuit diagram showing an example of the configuration of a ground fault detection device according to an embodiment of the present invention; [Diagram 2] FIG. 4 is a waveform diagram illustrating power supply phase synchronization according to an embodiment of the present invention. [Diagram 3] 4 is a waveform diagram illustrating a period of a power supply period counter according to an embodiment of the present invention. FIG. [Figure 4] 4 is a flowchart illustrating an example of an operation for synchronizing power supply periods according to an embodiment of the present invention. [Diagram 5] 5 is a flowchart illustrating an example of an operation of detecting a ground fault according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the following description and drawings are merely examples for explaining the present invention, and appropriate omissions and simplifications have been made in order to clarify the explanation. The present invention can also be implemented in various other forms. Furthermore, unless otherwise specified, each component may be singular or plural.

[0016] First, the grid-connected power conversion system of this embodiment will be described with reference to Fig. 1. The grid-connected power conversion system of this embodiment is configured such that, for example, as shown in Fig. 1, a plurality of DC power sources 51, 52 are connected to a common AC power source, and DC to AC power conversion is performed by grid-connected power conversion devices (hereinafter referred to as power conversion devices) 1 and 2. Also, an earth leakage breaker 3 that performs a cutoff operation when an electric leakage is detected, and a step-up transformer 4 that interconnects the outputs of the power conversion devices 1, 2 to the AC power source are connected to the rear stage of each of the power conversion devices 1, 2.

[0017] As shown in FIG. 1, the power conversion device 1 is composed of an AC voltage detector 101, a DC / DC converter 102, a DC / AC converter 103, an MCU (Micro Controller Unit) 104 which is a control unit, a DC ground fault detection circuit 105 which is a ground fault detection unit, a ground fault circuit voltage detector 106, a switch 107 which is a switch unit, a switch 108, etc.

[0018] Similarly, as shown in FIG. 1, the power conversion device 2 is composed of an AC voltage detector 201, a DC / DC converter 202, a DC / AC converter 203, an MCU 204 which is a control unit, a DC ground fault detection circuit 205 which is a ground fault detection unit, a ground fault circuit voltage detector 206, a switch 207 which is a switch unit, a switch 208, etc.

[0019] Since the power conversion devices 1 and 2 have the same configuration, each part will be described using the power conversion device 1. The AC voltage detector 101 detects the AC voltage to be output to the AC power source. The DC / DC converter 102 performs DC / DC conversion on the output of the DC power source 51 to control the step-up / step-down, and the DC / AC converter 103 converts the DC / DC converted DC voltage into an AC voltage.

[0020] The MCU 104 is composed of circuits such as a CPU, a DSP, a memory, and an I / O, and controls operations related to power conversion and ground fault detection. A DC ground fault detection circuit 105 detects a DC ground fault on the DC power supply 51 side. A ground fault circuit voltage detector 106 detects a DC voltage from the output of the DC ground fault detection circuit 105. A switch 107 switches between enabled and disabled states of the DC ground fault detection circuit 105 under the control of the MCU 104, and a contactor 108 switches between open and closed states under the control of the MCU 104 depending on whether a ground fault is detected.

[0021] Next, the principle by which the DC ground fault detection circuits 105 and 205 detect ground faults in a preset order will be described with reference to Figures 2 and 3. Figure 2 is a waveform diagram for explaining power supply phase synchronization according to one embodiment of the present invention, and Figure 3 is a waveform diagram for explaining the period of a power supply period counter according to one embodiment of the present invention.

[0022] The MCU 104 has a phase-locked loop (PLL) for detecting the power supply cycle. As shown in Fig. 2, the MCU 104 compares the phase of its own clock frequency with the phase of a reference frequency (system AC voltage frequency) based on the power supply cycle, adjusts the phase of its own clock frequency in the direction that reduces the phase difference between these two frequencies, and detects the phase Φ of the system voltage.

[0023] The DC earth fault detection circuit 105 inserts a high resistor between the positive and negative sides of the DC circuit, and the midpoint is grounded through the resistor to create a center point between the grounds. When a DC earth fault occurs, the potential at the midpoint of the DC earth fault detection circuit 105 changes, and a ground circuit voltage detector 106 that monitors this potential change detects the DC earth fault, and the MCU 104 stops the operation of the DC / AC converter 103 and performs abnormality processing such as disconnecting the switch 108. Here, the power conversion device 2 has the same functions as the above-mentioned power conversion device 1.

[0024] In each of the power conversion devices 1 and 2, the MCU 104 and 204 has a phase-locked loop (PLL) for detecting the power supply period. In the power conversion devices 1 and 2, the phase of each clock frequency is compared with the phase of a reference frequency (power supply voltage frequency), and a process is performed to adjust the phase on the power conversion device side in a direction to reduce the phase difference between these two frequencies.

[0025] As shown in Fig. 3, each of the power conversion devices 1 and 2 has a power cycle counter that counts the power cycle, and is controlled by the MCU 104, 204. A feature of this power cycle counter is that even when the power supply is cut off, it continues counting autonomously with the same cycle. Since the power cycle counters of each of the power conversion devices 10 and 20 have the same reference cycle (power supply voltage cycle), the counting can be synchronized between the power conversion devices 1 and 2 when the counting start time is shared.

[0026] The above-mentioned start time may be received as a reference signal from outside, but here it is assumed that the MCU 104 of the power conversion device 1 generates a base reference signal. In this case, the reference signal generated by the MCU 104 is supplied to all of the interconnected power conversion devices, and is supplied to the MCU 205 in the configuration example of FIG.

[0027] In this way, one of the multiple power conversion devices is used as the reference signal generating device. Here, it is not necessary for each power conversion device to receive this reference signal at the same time. It is sufficient to receive the current power count number N_K. After receiving the reference signal, each power conversion device counts the power cycle. Of course, this is premised on there being no time lag between the power conversion devices, and if there is a delay in the power cycle, then correction is performed before (in advance of) the grid-connected power conversion system 1 starts operating, not after.

[0028] In each of the power conversion devices 1 and 2, if the delay of the signal compared to the reference signal is greater than the power supply period, there is a possibility that the power conversion devices 1 and 2 may not be properly synchronized. In this case, the power conversion device 2 also receives information such as the time and date of the power conversion device 1, which is the external reference signal generator, and uses its own time information to calculate the starting count number N_K according to the following formula (1). [Year / Month / Day / Hour / Minutes / Seconds / Milliseconds / Power Supply Cycle Counters]

[0029]

number

[0030] Here, the external device reference time is N_K0, and the operator % is a remainder operator. N_MAX is the power cycle counter period, which is the period during which the DC ground fault detection circuits of all the power conversion devices are enabled in sequence. The power cycle counter period (N_MAX) is set to be greater than the sum of the total time during which all the power conversion devices detect a DC ground fault and the abnormality processing time.

[0031] Each power conversion device requests its own detection operation time and abnormality processing time when a ground fault is detected to the upper device. The upper device calculates the total of all requested times, t_all. Then, the power cycle counter period (N_MAX) is set to satisfy the following equation (2).

[0032]

number

[0033] This power supply cycle counter period (N_MAX) is received from a higher-level device (not shown) when the power conversion device is connected to the grid. Of course, there is no problem in manually setting it in each connected power conversion device.

[0034] The timing of ground fault detection for each power conversion device must be set in advance. For example, the start and end timings of ground fault detection are set as follows:

[0035] Start timing: Power cycle count number N_S Phase Φ_S End timing: Power cycle count number N_E Phase Φ_E This timing is assumed to be received from a higher-level device when the power conversion device is connected to the grid, but it is of course also possible to manually set this timing in each power conversion device.

[0036] Next, the operation will be described with reference to Fig. 3 and Fig. 4. First, synchronization of the power supply cycle will be described with reference to Fig. 3. Fig. 3 is a waveform diagram illustrating the cycle of a power supply cycle counter according to one embodiment of the present invention. First, when power is applied to each of the power conversion devices 1 and 2, the entire devices are started up (steps S411, S421).

[0037] In this embodiment, the power cycle of the power conversion device 1 is used as a reference for synchronization, so after each device is started up, MCU 104 transmits a reference signal for the power cycle to MCU 204 (step S412). When MCU 204 receives the reference signal (step S422), it reads out its own power cycle and checks whether there is a delay in the cycle. Here, if the delay in the cycle is equal to or greater than the power cycle (step S424), a synchronization correction is made to the power cycle based on the reference signal (step S425), and the power cycles between MCUs 104 and 204 are synchronized.

[0038] Next, DC earth fault detection of this embodiment will be described with reference to Fig. 4. Fig. 4 is a flowchart for explaining the DC earth fault detection operation according to this embodiment. In this embodiment, as shown in Fig. 1, there are two power conversion devices in the grid-connected power conversion system, and the order of DC earth fault detection is the power conversion devices 10, 20. The following description is executed by each of the MCUs 104, 204.

[0039] First, the power cycle counter of each of the power conversion devices 10 and 20 starts as shown in the flowchart of Fig. 4 after setting the cycle (N_MAX) of the power cycle counter, the start count N_K, and the detection timing. Since the power conversion devices 1 and 2 perform the same operation, the following explanation will be given of the power conversion device 1. The configuration of the power conversion device 2 is shown in parentheses in Fig. 4.

[0040] In the power electronics device 1, the current period number N_n (n is a natural number) is acquired from its own period counter (step S501). Here, n=1 for the power electronics device 1, and n=2 for the power electronics device 2.

[0041] In the power conversion device 1, the cycle number N_n is confirmed based on the time based on the already synchronized power cycle, and it is determined whether it is time to detect a ground fault (step S502). If it is determined that it is time to detect a ground fault for cycle number N_1 in the count of the power cycle, it is determined that the DC ground fault detection circuit 105 is enabled (Yes route in step S502), and the switch 107 is turned on, switching the DC ground fault detection circuit 105 to an enabled state. If it is not time to detect a DC ground fault, the switch 107 is maintained in the off state.

[0042] If a DC ground fault is detected while the DC ground fault detection circuit 105 is operating effectively (Yes route in step S504), the switch 107 is turned off (step S506) and detection of a DC ground fault is stopped. Then, the gate block stops the operation of the power conversion device 1 itself (step S507) and cuts off the conductor connected to the grid (parallel-off process: step S508), and further, abnormality processing such as removing the DC ground fault (step S509) is started.

[0043] Furthermore, in the above-mentioned process flow, if a DC ground fault is not detected in step S504 (No route), the switch 107 is turned off (step S505), and DC ground fault detection within the time given to the DC ground fault detection circuit 105 is terminated. Then, the process returns to the first step S501, and then the MCU 204 of the power conversion device 2 checks its own cycle number N_2 and determines that it is time to detect a DC ground fault (step S502). Thereafter, the process of FIG. 5 is executed in the same manner as that of the MCU 104.

[0044] In this way, in each of the power conversion devices 1, 2, when the count of the power cycle in synchronous operation reaches the timing assigned to its own cycle number, it is determined that it is the timing (turn) to perform ground fault detection, and the switches 107, 207 are switched under the control of the MCUs 104, 204. This switching control is repeatedly performed with the power cycle counter cycle being one cycle, so that the DC ground fault detection circuits 105, 205 can perform ground fault detection in turn for each cycle without interfering with each other.

[0045] As described above, according to this embodiment, since each DC ground fault detection circuit is autonomously assigned an order based on the reference signal, the multiple DC ground fault detection circuits can be operated autonomously in order, and therefore the DC ground fault detection circuits do not interfere with each other. As a result, even if the low-voltage side of the system is grounded, the influence of the DC ground fault detection circuit of the power conversion device is minimized, and no malfunction due to leakage or the like occurs, making it possible to obtain high reliability. In particular, there is an advantage in that the cost is reduced because the DC ground fault detection circuits are operated in order at high speed. For these reasons, a reliable power conversion system in a power generation facility of a PCS for selling all power is industrially useful.

[0046] Furthermore, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the above-described embodiment has been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to the embodiment having all of the configurations described. [Explanation of symbols]

[0047] 1 Power conversion device 2. Power conversion device 3. Earth leakage circuit breaker 51 DC power supply 52 DC power supply 101 AC voltage detector 102 DC / DC converter 103 DC / AC converter 104 MCU 105 DC earth fault detection circuit 106 Earth fault circuit voltage detector 107 Switch 108 Switch 201 AC voltage detector 202 DC / DC converter 203 DC / AC converter 204 MCU 205 DC earth fault detection circuit 206 Earth fault circuit voltage detector 207 Switch 208 Switch

Claims

1. A power conversion device of a grid-connected power conversion system that is composed of a plurality of power conversion devices and connects a plurality of DC power sources to a common AC power source, A ground fault detection unit that detects a DC ground fault; a switch section for switching between enabling and disabling detection of a DC ground fault; A control unit having a power supply cycle and controlling switching of the switch unit; Equipped with the control unit assigns timings based on a power supply cycle for detecting a DC ground fault in an order different from that of the other power conversion devices, synchronizes the power supply cycles between the power conversion devices, and switches the switch unit to an active state when the synchronized power supply cycle reaches the assigned timing.

2. 2. The power conversion device according to claim 1, wherein each of the power conversion devices is assigned at least a count number of power supply periods required for operation of the ground fault detection unit of the power conversion device itself, and the control unit repeats switching between enabled and disabled of the switch unit for each cycle, the total number being the sum of the count numbers of all the power conversion devices being one cycle.

3. 2. The power conversion device according to claim 1, wherein the control unit synchronously corrects its own power supply period using the power supply period of the other power conversion device as a reference.

4. 2. The power conversion device according to claim 1, wherein the control unit controls a parallel-off of the grid-connected power conversion system when the ground fault detection unit detects a DC ground fault.

5. 2. The power conversion device according to claim 1, wherein the control unit controls the power conversion device to stop operation when the ground fault detection unit detects a DC ground fault.

6. 2. The power conversion device according to claim 1, wherein said control unit detects the power supply period by a phase-locked loop.

7. A grid-connected power conversion system that connects a plurality of DC power sources to a common AC power source, a first ground fault detection unit connected to a first DC power supply and configured to detect a DC ground fault in the first DC power supply; a second ground fault detection unit connected to the second DC power supply and configured to detect a DC ground fault in the second DC power supply; a control unit that switches and controls detection of a DC ground fault by the first and second ground fault detection units at different timings based on a power supply cycle; A grid-connected power conversion system comprising:

8. 8. The grid-interconnected power conversion system according to claim 7, wherein the control units are arranged in a one-to-one relationship with the first ground fault detection units and the second ground fault detection units, and the power supply cycle is synchronized between the control units to detect a DC ground fault.

9. A method for detecting a ground fault in a grid-connected power conversion system that is configured with a plurality of power conversion devices each for detecting a DC ground fault and that connects a plurality of DC power sources to a common AC power source, comprising: a power conversion device that detects a DC ground fault by switching on detection of a DC ground fault in the order of the power conversion devices in which the synchronized power supply period reaches the assigned timing, the power conversion device being switched on in the order of the power conversion devices in which the synchronized power supply period reaches the assigned timing.

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