Power conversion device, grid-connected power conversion system, and ground fault detection method therefor

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 therein. [Background technology]

[0002] In power generation facilities with a PCS (Power Conditioning System) that sells all of its electricity, many 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 earth fault protection, it is common to use an earth leakage breaker on the grid side to cut off the electrical 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 resistor to create a center point to earth, and detect the earth fault by detecting the change in the center point potential caused by the DC side earth fault.

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

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-172616 Summary of the Invention [Problem to be solved by the invention]

[0007] In 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 complex, making it difficult to detect a DC ground fault during grid-connection. Patent Document 1 does not clearly describe a calculation method for when the grid is grounded.

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

[0009] The present invention has been made in view 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 by preventing 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, and by preventing malfunction of systems such as earth leakage circuit breakers. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems and achieve the above-mentioned object, one embodiment of the present invention is a power conversion device of a grid-connected power conversion system which is made up 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 the detection of the 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 cycles for detecting DC ground faults in an order which is 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 multiple DC power sources to a common AC power source, and is characterized by comprising: a first ground fault detection unit that is connected to a first DC power source and detects a DC ground fault in the first DC power source; a second ground fault detection unit that is connected to a second DC power source and detects a DC ground fault in the second DC power source; and a control unit that switches and controls the detection of DC ground faults by the first and second ground fault detection units at different times based on the 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 capable of detecting a DC ground fault and in which a plurality of DC power sources are connected to a common AC power source, the method comprising: assigning timings based on a power supply cycle for detecting a DC ground fault in different orders among the power conversion devices; synchronizing the power supply cycles among the power conversion devices; and switching to DC ground fault detection in the order of the power conversion devices in which the synchronized power supply cycle reaches the assigned timing. [Effects 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 explanation 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; [Figure 2] FIG. 4 is a waveform diagram illustrating power supply phase synchronization according to an embodiment of the present invention. [Figure 3] 3 is a waveform diagram illustrating a period of a power supply period counter according to an embodiment of the present invention. FIG. [Figure 4] 10 is a flowchart illustrating an example of an operation for synchronizing power supply cycles according to an embodiment of the present invention. [Figure 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 INVENTION

[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 some omissions and simplifications have been made as appropriate for clarity of 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, 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 a ground fault 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 downstream of each of the power conversion devices 1, 2.

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

[0018] Similarly, as shown in FIG. 1, the power conversion device 2 is configured by 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 switch 208, etc.

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

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

[0021] Next, the principle by which 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 illustrating power supply phase synchronization according to one embodiment of the present invention, and Figure 3 is a waveform diagram illustrating 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 Figure 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 a direction that reduces the phase difference between these two frequencies, and detects the phase Φ of the system voltage.

[0023] The DC ground 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, creating a ground midpoint. When a DC ground fault occurs, the potential at the midpoint of the DC ground fault detection circuit 105 changes, and a ground fault circuit voltage detector 106 that monitors this potential change detects the DC ground fault. The MCU 104 then stops the operation of the DC / AC converter 103 and performs abnormality processing such as paralleling off the switch 108. Here, the power conversion device 2 has the same functions as the power conversion device 1 described above.

[0024] In each of the power conversion devices 1 and 2, the MCU 104 and 204 have a phase-locked loop (PLL) for detecting the power supply cycle. 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 supply cycle counter that counts the power supply cycle, and is controlled by the MCU 104, 204. A feature of this power supply cycle counter is that it continues counting autonomously at the same cycle as before, even when the power supply is interrupted. Because the power supply cycle counters of each of the power conversion devices 10 and 20 have the same reference cycle (power supply voltage cycle), sharing the same count start time enables synchronization of the counts between the power conversion devices 1 and 2.

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

[0027] In this way, one of the multiple power conversion devices is used as the reference signal generator. Here, it is not necessary for each power conversion device to receive this reference signal simultaneously. 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 independently. Of course, this assumes that there is no discrepancy in the time held by each power conversion device. 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 up, not after it starts up.

[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 synchronized correctly. 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 as shown in the following formula (1). [Year, month, day, hour, minute, second, millisecond, power cycle counter count]

[0029]

number

[0030] Here, the external device reference time is N_K0, and the operator % is the remainder operator. N_MAX is the power supply cycle counter period, which is the period during which the DC ground fault detection circuits of all power conversion devices are enabled in sequence. The power supply cycle counter period (N_MAX) is set to be greater than the sum of the total time during which all power conversion devices detect DC ground faults 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 from the host device. The host device calculates the total of all requested times, t_all. Then, it sets the power supply cycle counter period (N_MAX) so that it satisfies 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 timing of starting and ending ground fault detection is set as follows:

[0035] Start timing: Number of power cycles N_S Phase Φ_S End timing: Number of power cycles N_E Phase Φ_E This timing is assumed to be received from the host device when the power conversion device is connected to the grid, but it is of course also possible to manually set it 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 device starts up (steps S411 and S421).

[0037] In this embodiment, the power supply cycle of power conversion device 1 serves as a reference for synchronization, so after each device is started up, MCU 104 transmits a power supply cycle reference signal to MCU 204 (step S412). When MCU 204 receives the reference signal (step S422), it reads its own power supply 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 supply cycle (step S424), synchronization correction of the power supply cycle is performed based on the reference signal (step S425), and the power supply cycles of MCUs 104 and 204 are synchronized.

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

[0039] First, the power cycle counter of each power electronics device 10, 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 power electronics devices 1 and 2 perform similar operations, the following explanation will focus on power electronics device 1. In Fig. 4, the configuration of power electronics device 2 is shown in parentheses.

[0040] In the power electronics device 1, the current cycle number N_n (n is a natural number) is acquired from its own cycle 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 supply 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 supply cycle, it is determined to enable the DC ground fault detection circuit 105 (Yes route in step S502), and the switch 107 is turned on, switching the DC ground fault detection circuit 105 to be enabled. 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 the DC ground fault is stopped. Subsequently, the gate block stops 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 removal of the DC ground fault (step S509) is started.

[0043] Furthermore, in the above-described processing 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 processing returns to the first step S501, and subsequently, 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 processing 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 synchronously operating power supply cycles 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 one cycle being the power supply cycle counter 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, each DC ground fault detection circuit is autonomously assigned a sequence based on the reference signal, allowing multiple DC ground fault detection circuits to operate autonomously in sequence, preventing interference between the DC ground fault detection circuits. This minimizes the impact of the DC ground fault detection circuits on the power conversion device even when the low-voltage side of the grid is grounded, eliminating malfunctions due to leakage currents and achieving high reliability. In particular, the high-speed sequential detection of the DC ground fault detection circuits is advantageous in reducing costs. For these reasons, a reliable power conversion system for a PCS power generation facility with a total power feed-in system is industrially useful.

[0046] Furthermore, it goes without saying that 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 an embodiment having all of the described configurations. [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 ground 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 ground 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 configured by 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 unit that switches 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 is configured to assign 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, synchronize the power supply cycles between the power conversion devices, and switch the switch unit effectively 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 cycles required for operation of the ground fault detection unit of the power conversion device, and the control unit repeats switching between enabled and disabled of the switch unit for each cycle, which is defined as a total number of count numbers of all the power conversion devices.

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

4. 2. The power conversion device according to claim 1, wherein the control unit controls 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 cycle using 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 the 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 DC ground faults 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 cycles are 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 of which detects 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 synchronizing the power supply cycles of the power conversion devices with each other, and switching to DC ground fault detection in the order in which the synchronized power supply cycles of the power conversion devices reach the assigned timings.

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