Insulation resistance detection circuit and method

A circuit, comprising:

• a controller;
• an inverter circuit, comprising a positive input terminal, a negative input terminal, and N alternating current output terminals;
• a first direct current voltage source;
• a second direct current voltage source;
• a filter circuit, comprising N input terminals and N output terminals;
• and
• a switch circuit, comprising N input terminals, N output terminals, M relays, and at least one impedor, wherein each relay of the M relays comprises N relay switches, an input terminal of an ith relay switch of a first relay is used as an ith input terminal of the switch circuit, and an output terminal of an ith relay switch of an Mth relay is used as an ith output terminal of the switch circuit, and wherein in response to the quantity M being greater than 1, ith relay switches of the relays are sequentially connected in series;
• wherein N is an integer greater than 1, M is an integer greater than or equal to 1, and i is an integer greater than or equal to 1 and less than or equal to N;
• wherein a positive electrode of the first direct current voltage source is connected to the positive input terminal of the inverter circuit, a negative electrode of the first direct current voltage source is connected to a positive electrode of the second direct current voltage source, and a negative electrode of the second direct current voltage source is connected to the negative input terminal of the inverter circuit;
• wherein an ith alternating current output terminal of the inverter circuit is connected to an ith input terminal of the filter circuit, an ith output terminal of the filter circuit is connected to the ith input terminal of the switch circuit, and the ith output terminal of the switch circuit is connected to an ith terminal of an alternating current power grid;
• wherein at least one relay switch in the switch circuit is connected to the at least one impedor in parallel;
• and
• wherein the controller is configured to:
• perform (i) opening a first relay and closing second relays, or (ii) switching an on/off status of at least one of the second relays;
• and
• determine, based on a first measurement value between the positive input terminal of the inverter circuit and an earthing point, whether the positive input terminal of the inverter circuit is short-circuited to protective earthing or has low resistance to protective earthing;
• and
• determine, based on a second measurement value between the negative input terminal of the inverter circuit and the earthing point, whether the negative input terminal of the inverter circuit is short-circuited to protective earthing or has low resistance to protective earthing, wherein the first relay is a relay in the M relays that is connected to the at least one impedor in parallel, and the second relays are relays other than the first relay in the M relays.