EP-3742605-B1 - METHOD IN INVERTER FOR SUPPRESSING COMMON MODE IMPULSE CURRENT GENERATED WHEN SWITCHING ON AN ALTERNATING CURRENT SWITCH AND APPLICATION DEVICE THEREOF

EP3742605B1EP 3742605 B1EP3742605 B1EP 3742605B1EP-3742605-B1

Inventors

YU, Yanfei
NI, Hua
BAI, yang
Cao, Menglin

Dates

Publication Date: 20260506
Application Date: 20200326

Claims (6)

A method for suppressing a common mode impulse current for an inverter topology generated when an alternating current switch is switched on, the inverter topology comprising a direct current port, a direct current bus capacitor, a DC/AC converter, an alternating current filter, an alternating current switch, an alternating current port and a compensation power supply, wherein the direct current port is connected with a direct current power supply, and the alternating current port is connected to a TN type grid, a direct current side of the DC/AC converter is connected with a positive pole and a negative pole of a direct current bus of the inverter topology; wherein one end of the alternating current switch is connected to an alternating current side of the DC/AC converter via the alternating current filter, and the other end of the alternating current switch is connected to the alternating current port, wherein the direct current bus capacitor comprises two capacitors connected in series, wherein one end of a branch formed by the two capacitors is connected to the positive pole of the direct current bus and the other end of the branch is connected to the negative pole of the direct current bus such that the direct current bus capacitor is connected between the positive pole of the direct current bus and the negative pole of the direct current bus, wherein a midpoint of the branch is a midpoint of the direct current bus, and wherein the compensation power supply is connected between the midpoint of the direct current bus and ground, and the method comprises: determining (S101), before the alternating current switch is switched on for the inverter topology, a target value of a common mode voltage between the inverter topology and ground as 0 Volts; controlling (S102) the compensation power supply to change the common mode voltage between the inverter topology and ground to the target value, to ensure that a common mode voltage difference between two ends of the alternating current switch is zero; controlling (S202) the DC/AC converter to operate based on the sampled voltage between the alternating current port and ground, so that a voltage waveform of an alternating current capacitor is identical to a voltage waveform of the alternating current port, wherein the alternating current capacitor is provided in the alternating current filter, and the alternating current filter is configured to connect to the midpoint of the direct current bus through the alternating current capacitor; and controlling (S103) to switch on the alternating current switch.
The method according to claim 1, wherein after the controlling the compensation power supply to change the common mode voltage between the inverter topology and ground to the target value, the method further comprises: controlling (S201) to switch off a switch in series connection with the compensation power supply and arranged between the circuit of the inverter topology and ground.
The method according to claim 1, further comprising: controlling, if the direct current power supply is a P-type photovoltaic module, the compensation power supply to change both a voltage between a positive pole of the direct current port of the inverter topology and ground and a voltage between a negative pole of the direct current port of the inverter topology and ground to positive voltages, in a case that it is unnecessary to switch on the alternating current switch for the inverter topology; and controlling, if the direct current power supply is an N-type photovoltaic module, the compensation power supply to change the voltage between the positive pole of the direct current port of the inverter topology and ground and the voltage between the negative pole of the direct current port of the inverter topology and ground to negative voltages, in a case that it is unnecessary to switch on the alternating current switch for the inverter topology.
An inverter topology, comprising a circuit; and a controller, wherein the circuit comprises a direct current port, a direct current bus capacitor, a DC/AC converter, an alternating current filter, an alternating current switch, an alternating current port and a compensation power supply; a side of the direct current port away from the DC/AC converter is connected with a direct current power supply; a direct current side of the DC/AC converter is connected with a positive pole and a negative pole of a direct current bus of the inverter topology; an alternating current side of the DC/AC converter is connected with one end of the alternating current switch through the alternating current filter; another end of the alternating current switch is connected with the alternating current port; the alternating current port is connected with a TN type grid; the direct current bus capacitor comprises two capacitors connected in series, wherein one end of a branch formed by the two capacitors is connected to a positive pole of the direct current bus and the other end of the branch is connected to a negative pole of the direct current bus such that the direct current bus capacitor is connected between the positive pole of the direct current bus and the negative pole of the direct current bus, wherein a midpoint of the branch is a midpoint of the direct current bus, and the compensation power supply is arranged between the midpoint of the direct current bus and ground, and is configured to output a voltage to the circuit in response to control of the controller; and the controller is configured to: determine, before the alternating current switch is switched on for the inverter topology, a target value of a common mode voltage between the inverter topology and ground as 0 Volts; control the compensation power supply to change the common mode voltage between the inverter and ground to the target value, to ensure that a common mode voltage difference between two ends of the alternating current switch is zero; control the DC/AC converter to operate based on the sampled voltage between the alternating current port and ground, so that a voltage waveform of an alternating current capacitor is identical to a voltage waveform of the alternating current port, wherein the alternating current capacitor is provided in the alternating current filter, and the alternating current filter is configured to connect to the midpoint of the direct current bus of the inverter topology through the alternating current capacitor; and control to switch on the alternating current switch.
The inverter according to claim 4, wherein the circuit further comprises a DC/DC converter arranged between the direct current port and the direct current bus.
A grid-connected power generation system, comprising: a direct current power supply; and the inverter topology according to claim 4 or 5, wherein the direct current power supply is connected with the direct current port of the inverter topology; and the direct current power supply is at least one of a photovoltaic array, a wind power generation system and an energy storage system.

Description

FIELD The present disclosure relates to the technical field of automatic control, particularly to a method for suppressing a common mode impulse current for an inverter generated when an alternating current switch is switched on, and a device for applying the method. BACKGROUND In a new energy grid-connected power generation system such as a photovoltaic power generation system, a wind power generation system, and an energy storage system, transmission and conversion of energy is performed by an inverter. As shown in Figure 1, in the inverter, a mechanical alternating current switch such as a relay or a contactor is arranged between an alternating current filter and an interface for a power grid. When the alternating current switch is switched on, the inverter is connected to the power grid, thereby performing the conversion and transmission of energy. In order to avoid an impulse current resulting from a voltage difference between two ends of the alternating current switch when the alternating current switch is switched on, before the alternating current switch is switched on, the inverter may generate a voltage with an identical frequency, phase and amplitude to those of a voltage of the power grid in an alternating current capacitor of the inverter (namely a differential mode capacitor of the alternating current filter), via a DC/AC converter of the inverter. However, with the above solution, only a differential mode impulse current can be suppressed. If there is a common mode voltage difference between two ends of the alternating current switch before the alternating current switch is switched on, the common mode impulse current remains when the alternating current switch is switched on. In an inverter topology with an AC side capacitor connected to a DC side midpoint, for example, in a three-phase T-type three-level inverter with an AC side capacitor connected to a midpoint of a direct current bus as shown in Figure 1, a common mode impulse current generated when the alternating current switch is switched on sequentially passes through an alternating current capacitor, a direct current bus capacitor, an equivalent direct current capacitor-to-ground (for example, a Y capacitor ) and/or a parasitic capacitor (which is uniformly expressed by C1 in Figure 1) and ground, then the common mode impulse current passes through a power grid, a Residual Current Device (RCD) and an alternating current switch, and returns to the alternating current side, as shown by a dotted line. Since a corresponding current loop does not pass through an alternating current inductor (namely an inductor in the alternating current filter), the impulse current can be very large. Besides, as an equivalent capacitance-to-ground of a power generation system (for example, a Y capacitor) and a parasitic capacitance increases, the common mode impulse current increases. The common mode impulse current may trigger an external leakage current protector (namely the RCD in Figure 1) to open, resulting in that the inverter is disconnected from the power grid; or may generate a transient interference on a control system and driving waveforms of the inverter, resulting in an operation malfunction of the inverter. WO2015/074964A1 discloses a transformerless inverter connected on the input side to a generator and on the output side to a polyphase AC grid. The inverter comprises a polyphaser inverter bridge, a switch between a center point of the input-side DC link and a connection for a PEN conductor of a polyphase AC gird. When first operating conditions are present, electric power is fed into the AC grid from the DC link when the switch with the inverter bridge is closed. During a transition from the first operating conditions to second operating conditions which differ from the first operating conditions, the switch is opened, while during a transition from the second operating conditions to the first operating conditions, the switch is closed. WO2017/162892A1 discloses a method for connecting a DC-AC converter device comprising a first connection on the DC side, formed by two terminals and a second connection on the AC side for connection to an electrical grid by means of at least one connection switch. The method comprises a connection step in which the connection switch is closed, and a step prior to the connection step, consisting in measuring at least one reference voltage between a point on the DC side of the device and the earth and determining if this reference voltage is inside or outside the pre-established safe range; the connection step being performed only when it is inside said range. DE10 2011 055220A1 discloses a process of connecting an AC output of a transformerless inverter of a solar power plant to an internal AC power grid at an input side of a galvanic isolation, while an offset voltage for shifting a potential center point of a photovoltaic generator connected to the inverter is applied. The process includes: synchronizing