UPS system having a function of parallel operation

A UPS system comprising:

• a plurality of UPS units connected in parallel, with the UPS units provided with identical or different capacities, with each of the UPS units including a DC/AC inverter and a control circuit, with the control circuit provided in the DC/AC inverter and used to control the DC/AC inverter;
• with each of the control circuit of the DC/AC inverter including a voltage feedback control circuit, a current feedforward control circuit, an adder and a PWM circuit;
• wherein outputs of the voltage feedback control circuit and the current feedforward control circuit are sent to the adder to obtain a modulation signal which is sent to the PWM circuit to obtain drive signals for controlling each of the DC/AC inverters of the UPS units;
• and
• wherein the current feedforward control circuit operates the DC/AC inverter as a virtual fundamental resistor and a virtual harmonic resistor connected serially to an output terminal of the DC/AC inverter.

A UPS system comprising:

• a plurality of UPS units connected in parallel, with the plurality of UPS units provided with different capacities, with each of the plurality of UPS units including a DC/AC inverter and a control circuit, with the control circuit provided in the DC/AC inverter and used to control the DC/AC inverter;
• wherein each of the control circuits controls the corresponding UPS unit depending upon a capacity ratio of the corresponding UPS unit, wherein a ratio of output currents of the plurality of UPS units approaches a capacity ratio of the plurality of UPS units;
• wherein each of the control circuits includes a current feedforward control circuit;
• and
• wherein the current feedforward control circuit operates the DC/AC inverter as a virtual fundamental resistor and a virtual harmonic resistor connected serially to an output terminal of the DC/AC inverter.