A grid independent operation control unit includes a load current estimator to estimate a load current supplied to stand-alone power system in accordance with an output current of the inverter and an output voltage, and a feedback controller configured to PWM control the inverter at a duty ratio feedback calculated to cause the inverter to output an output voltage command value in accordance with the output voltage and the load current. The feedback controller is configured to PWM control the inverter at a duty ratio feedback calculated for output of a normalized output voltage command value obtained by normalizing the output voltage command value with the DC bus voltage in accordance with normalized output voltage obtained by normalizing the output voltage with the DC bus voltage and normalized load current obtained by normalizing the load current with the DC bus voltage.

A system for handling short circuits on an electrical network comprising parallel operated units which are droop controlled for active and reactive power sharing and connected to each other via impedances and protection switches for detecting and handling a short circuit on the electrical network and for disconnecting the faulty part of the electrical network, said units including a DC-source and a grid forming voltage source inverter controlled by a cascaded control structure with an inner voltage control loop and a short circuit control for limiting the output current by changing the output voltage at the output terminals of the inverter as a function of the measured output current and a desired droop voltage provided by a droop controller for voltage and frequency power sharing.

Systems, methods, and computer program products for providing an inertial response by a wind power system to power fluctuations in an electrical grid. The system includes a synthetic inertial response generator configured to generate a power offset in response to fluctuations in grid voltage. The power offset signal is generated by determining a quadrature component the grid voltage using an internal reference voltage having an angular frequency and phase angle that is synchronized to the electrical grid by a control loop. The quadrature component is used to determine a synchronous power level. A control loop error signal is produced by the difference between the synchronous power level and the wind turbine system power output. Changes in the grid frequency produce an error signal that is added to the power set point of wind turbine system output controllers to provide a power system inertial power output response.

An abnormality detection device for a grid interconnection relay to detect an abnormality of the grid interconnection relay upon switching to grid independent operation, and includes an abnormality detector to execute commercial power system voltage determination of determining whether or not there is a commercial power system voltage, and first voltage determination to be executed if it is determined that there is no commercial power system voltage through the commercial power system voltage determination, of causing the power conditioner to chronologically alternately output monitor voltages having different values in a state where a contact of the grid interconnection relay is controlled to open and executing abnormality determination as to the grid interconnection relay according to whether or not each of the monitor voltages is followed by difference between a voltage of the power conditioner and voltage of the commercial power system with respect to corresponding one of the monitor voltages.

A converter system and inverter system are disclosed with individual real and reactive power control for each phase of a poly-phase system. The converter system includes a controller, bidirectional single-phase inverters with AC sides coupled to an AC line filter and DC sides connected in parallel to a link capacitor coupled to DC/DC converters. Each inverter handles a separate AC phase. The controller controls the inverters and DC/DC converters so the current amplitude of each AC phase is independent, and the phase difference of each AC phase is independent. The inverters can be galvanically isolated between the DC and AC sides. The inverters can be non-isolated inverters having line and neutral connectors coupled to an isolated transformer winding, and the output windings of the transformer can be wired in a Wye configuration. The inverters can have local controllers.

A power system stabilizing device includes an arithmetic unit that constructs a system model by collecting various kinds of system information, calculates a transient stability under an assumed accident condition beforehand, and selects, for each assumed accident in advance, a target solar power generator for shutdown and a target power generator to be disconnected to stabilize a power system, and a control unit that shutdowns the shutdown target solar power generator and disconnects the disconnection target power generator from the power system which are selected by the arithmetic unit when a system accident occurs.

A method for three-phase infeed of electrical power from a DC source into a three-phase AC grid by means of an inverter includes measuring phase-specific grid voltages of the three-phase AC grid, and determining a grid frequency from the measured phase-specific grid voltages. The method also includes generating phase-specific voltage reference values from the phase-specifically measured grid voltages and the determined grid frequency, and generating phase-specific target current values using phase-specific predetermined target current amplitude values, the phase-specific voltage reference values and respective grid voltage amplitudes.

The invention concerns a method for coupling to the grid a hydraulic unit having a synchronous generator, a runner, and wicket gates. The method includes a step of increasing the flow of water into the runner from a time t.sub.0 to a time t.sub.1 so that the rotation frequency of the rotor of the synchronous generator is, at time t.sub.1 equal to the frequency of the grid, and closing the circuit breaker at time t.sub.1. A sub-interval from a time t2 to time t1 is defined, with t0<=t2<t3<=t1, wherein a sub-step is executed to apply an adjustment torque to the shaft line via a first actuator that controls the flow of water into the runner and a second actuator coupled to a stator of the synchronous generator.

The invention concerns a method for coupling to the grid a hydraulic unit having a synchronous generator, a runner, and wicket gates. The method includes a step of increasing the flow of water into the runner from a time t.sub.0 to a time t.sub.1 so that the rotation frequency of the rotor of the synchronous generator is, at time t.sub.1 equal to the frequency of the grid, and closing the circuit breaker at time t.sub.1. A sub-interval from a time t2 to time t1 is defined, with t0<=t2<t3<=t1, wherein a sub-step is executed to apply an adjustment torque to the shaft line via a first actuator that controls the flow of water into the runner and a second actuator coupled to a stator of the synchronous generator.

Electrically connecting a first node of a first power grid to a second node of a second power grid includes: determining a phase angle of at least one phase of an AC voltage at the first node in the first power grid; determining a phase angle of at least one phase of an AC voltage at the second node in the second power grid; determining a phase angle metric based on comparing the phase angle of the AC voltage in the first power grid to the phase angle of the AC voltage in the second power grid; comparing the phase angle metric to a phase angle threshold; and if the phase angle metric is equal to or exceeds the phase angle threshold, controlling a dispatchable energy source in the first power grid in a P-Q control mode to adjust the phase angle of at least one phase of the AC voltage at the first node.