A system and method for controlling a grid-connected inverter to provide negative sequence current during unbalanced grid operating conditions. The system uses a combination of feedforward and feedback controls to compute voltage signals which are used to control the inverter switches. The system includes both positive and negative sequence current controllers with voltage feedforward terms. The measured grid voltage is directly fed forward to the positive sequence control through a predictive algorithm, so that the instantaneous voltage information is kept, reducing the influence of grid voltage harmonics on the quality of the output current. The predictive voltages include positive, negative and harmonic component information of the grid voltage signals.

A three phase inverter can include: a first converter and a second converter connected to an input source in parallel, respectively; a first single phase inverter connected to the first converter through a first inverter first input node and a first inverter second input node and providing a first inverter first output node and a first inverter second output node; a second single phase inverter connected to the second converter through a second inverter first input node and a second inverter second input node and providing a second inverter first output node and a second inverter second output node; and a common output node connected to the first inverter first output node and the second inverter first output node.

A device and method for generating single/split or three phase AC voltages from a DC source with 1 to 2 times gain in output voltage without using any DC/DC boost or an output transformer. An isolation/multiplexer/mixer circuit successively charges multiple power modules, allowing each power module to generate output voltage(s) with desired magnitudes and phases, and allows independent outputs of each power converter modules to be reconnected to obtain up to two times the conventional possible output voltage. An isolation block eliminates the common mode noise problem. The gain in output voltage and isolation between the output converters eliminates the need of the front end DC/DC converter or an output transformer for most of the DC voltage sources, which improves cost, power density, efficiency and reliability of the inverter.

A device and method for generating single/split or three phase AC voltages from a DC source with 1 to 2 times gain in output voltage without using any DC/DC boost or an output transformer. An isolation/multiplexer/mixer circuit successively charges multiple power modules, allowing each power module to generate output voltage(s) with desired magnitudes and phases, and allows independent outputs of each power converter modules to be reconnected to obtain up to two times the conventional possible output voltage. An isolation block eliminates the common mode noise problem. The gain in output voltage and isolation between the output converters eliminates the need of the front end DC/DC converter or an output transformer for most of the DC voltage sources, which improves cost, power density, efficiency and reliability of the inverter.

[OBJECT] To provide a radio-frequency power source capable of outputting radio-frequency power having a desired waveform changing at high speed. [SOLUTION] A radio-frequency power source 1 includes two DC-RF converting circuits 4A, 4B and an RF combining circuit 5 for combining the outputs from both DC-RF converting circuits 4A, 4B. The DC-RF converting circuits 4A, 4B amplify radio-frequency voltages v.sub.a, v.sub.b inputted from a radio-frequency signal generating circuit 8, and output radio-frequency voltages v.sub.PA, v.sub.PB. The RF combining circuit 5 outputs radio-frequency voltage v.sub.PX at a ratio corresponding to the phase difference between the radio-frequency voltages v.sub.PA and v.sub.PB. A controlling circuit 9 switches the phase difference between 1 and 2. As a result, the power P.sub.X outputted from the RF combining circuit 5 becomes pulsed radio-frequency power having a high level period and a low level period. Since the switching of the phase difference can be performed at high speed, it is possible to output pulsed radio-frequency power with a high switching frequency between the first level and the second level.

A single-phase Energy Utilization Tracker (EUT) inverter that comprises two DC/AC conversion modules. At any time, the two modules combined can sequentially extract and convert most the power provided by a DC energy source into two AC power (voltage) trains. The first AC power (voltage) train conforms to the power grid convention; while the second AC power train has a 90 degree phase difference to the specific power line pair. In according to the principle described herein, this single-phase EUT inverter further comprising a phase adjuster to adjust the phase of the second AC power (voltage) train by 90 degrees to become synchronous with the first AC power train; both AC power trains being then suitable to deliver into the same power line.

In an electric power conversion device which performs power conversion between multiphase AC and DC, a first converter cell of a first arm for each phase of a power converter includes: a capacitor; a Leg A having upper and lower arms having switching elements; and a Leg B having upper and lower arms one of which has a switching element and the other of which has only a diode, and a second converter cell of a second arm includes: a capacitor; and a Leg Aa having upper and lower arms having switching elements. A control device has a steady mode and a protection mode. When short-circuit between DC terminals of the power converter is detected, the control device switches from the steady mode to the protection mode, turns off all the switching elements of the first converter cell of the first arm, and controls the second converter cell of the second arm so as to perform reactive power compensation operation.

In a multilevel converter, three circuit breakers are respectively connected between three arms and three reactors. One circuit breaker is a DC circuit breaker configured to interrupt direct current when a short circuit accident occurs between two DC power transmission lines. Each of the two circuit breakers is an AC circuit breaker configured to interrupt alternating current when the short circuit accident occurs. When the short circuit accident occurs, the two AC circuit breakers are brought into the non-conductive state and then the DC circuit breaker is brought into the non-conductive state, thereby interrupting the short circuit current.

In a multilevel converter, three circuit breakers are respectively connected between three arms and three reactors. One circuit breaker is a DC circuit breaker configured to interrupt direct current when a short circuit accident occurs between two DC power transmission lines. Each of the two circuit breakers is an AC circuit breaker configured to interrupt alternating current when the short circuit accident occurs. When the short circuit accident occurs, the two AC circuit breakers are brought into the non-conductive state and then the DC circuit breaker is brought into the non-conductive state, thereby interrupting the short circuit current.

A power converting method for high frequency inverter and low frequency inverter connecting in parallel, which is for converting a direct current power into an alternating current power, includes the following steps. A low frequency inverting module which electrically connected to the direct current power is provided. A high frequency inverting module which is electrically connected to the low frequency inverting module in parallel is provided. A high frequency switching duty ratio of the high frequency inverting module is adjusted to output a second current according to a first current produced by the low frequency inverting module. The second current is for compensating ripples of the first current.