Power converter, energy storage system, and control method

An energy storage system, comprising:

• an energy storage battery;
• and
• a power converter comprising:
• a power conversion circuit, wherein a direct current input end of the power conversion circuit is configured to connect to the energy storage battery through a direct current bus, an alternating current output end of the power conversion circuit is configured to connect to an alternating current power grid through a point of common coupling (PCC);
• a bus capacitor connected in parallel to two ends of the direct current bus, wherein the direct current bus comprises a positive direct current bus and a negative direct current bus, the bus capacitor comprises a positive bus capacitor and a negative bus capacitor that are connected in series, a series connection point of the positive bus capacitor and the negative bus capacitor is a neutral point, the positive bus capacitor is connected in parallel between the positive direct current bus and the neutral point, and the negative bus capacitor is connected in parallel between the negative direct current bus and the neutral point;
• and
• a controller configured to:
• after a difference between a voltage across two ends of the positive bus capacitor and a voltage across two ends of the negative bus capacitor is greater than a specified threshold, control the power converter to inject a second harmonic current into the alternating current power grid, to reduce the difference between the voltage across the two ends of the positive bus capacitor and the voltage across the two ends of the negative bus capacitor, wherein a frequency of the second harmonic current is twice a fundamental frequency of the alternating current power grid, an initial phase angle of the second harmonic current is within a preset interval, the preset interval comprises [−π/4, −π/2], [π/4, π/2], [−π/2, π/2], [φ, π/2], [−π/2, φ−π], and [−π/2, π/2], φ is a power factor of the power converter, and the power factor is a phase difference between an output voltage and an output current of the power converter.