Andreas Uphues

SOGI based grid fault detection for feeding asymmetrical reactive currents to fulfill LVRT requirements

Due to the increasing wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). Additionally the WT has to support the power system stability in LVRT cases by supporting the grid with reactive power. The amount of reactive power feed-in depends on the type of grid fault and the depth of the voltage dip. Therefore this paper shows a reliable grid voltage monitoring consisting on a second order generalized integrator (SOGI) structure. The resulting phase locked loop (PLL) is tolerant against grid faults and the amplitudes and phase angles of the individual phase voltages are detected.

Inverter based voltage sag generator with PR-controller

Due to the increasing wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). Additionally the WT has to support the power system stability in LVRT cases. To evaluate the LVRT capability of grid connected converter, a voltage sag generator (VSG) is required to emulate grid failures. This paper introduces a three phase programmable inverter based VSG, which is equipped with a cascaded control structure consisting of proportional resonant (PR) current controller and PR voltage controller. The described VSG is able to emulate all required voltage sags, propagated through a delta star connected transformer, very precisely. The control structure has been simulated and tested successfully on a 2MW full power testbench.

Test bench for multi-MW grid side wind power converter

This paper deals with an integrated full power test bench for the grid side of a wind power converter. The speciality is the high side voltage and current measurement to use the stray inductance as an additional filter element. The presented test bench is enhanced for fault ride through measurement capabilities with a full power grid simulator.

Frequency adaptive PR-controller for compensation of current harmonics

The use of proportional resonant (PR) current controller in grid side wind power converters instead of the traditional proportional integral (PI) controller has gained a large popularity. Particularly its capability for compensating harmonics in the current waveforms is an essential feature. Due to the replacing of the generator side converter by a simple diode rectifier, harmonics with variable frequencies into dependence of the generator frequency appear in the current waveforms. To reach the IEEE standard for the total harmonic distortion (THD) a frequency adaptive PR-controller for compensating these harmonics is required.

PR-controller in a 2MW grid side windpower converter

To regulate the current in grid connected power converters mostly proportional integral (PI) controller in synchronous reference frame are used. To improve their known drawbacks such as complexity of control structure because of the dq-transformation and the poor behavior concerning low order harmonics compensation, proportional resonant (PR) controller gained a large popularity. This paper describes the differences between using PI-controller and PR-controller in a 2MW grid connected power converter for wind turbines. The control theory of PR filters and implementing them as delta based infinite impulse response (IIR) filters in a fixed-point microcontroller is presented. The PR-controller with harmonic compensation has been tested in a 2MW power converter.

Support of grid voltages with asymmetrical reactive currents in case of grid errors

Due to the increasing wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). Additionally the WT has to support the power system stability in LVRT cases by supporting the grid with reactive power. By feeding symmetrical reactive currents in case of asymmetrical grid errors, as required in many actual grid codes, the phase voltage of the undistorted phase will increase above the upper voltage limit. This paper shows a strategy to feed asymmetrical reactive currents into the distorted grid without increasing the phase voltage in the undistorted phase.

Inverter based test setup for LVRT verification of a full-scale 2 MW wind power converter

With increased wind power penetration, grid codes of system operators require low voltage ride through (LVRT) capability for wind turbines (WT). This paper describes a full power test bench, designed to evaluate the functionality of grid connected converter in nominal operating mode and in case of LVRT. To verify the LVRT capability an inverter based voltage sag generator (VSG) is developed which emulates grid failures.

Comparison of parameter identification approaches with linearised process models based on RLS for induction machines with P > 100 kW

This paper presents a comparison between a continuous time domain approach (CTD) and a discrete time domain approach (DTD) for parameter identification of induction machines P>100 kW fed with a voltage source inverter (VSI). The machine parameters are identified off-line, based on the reference voltage and the measured current at standstill and single-axis excitation by the VSI. The quality of the identified parameters is verified with the comparison of measured and estimated torque for the whole operating range, exclusively the field weakening region.

Crowbar-less ride through of asymmetrical gric faults with DFIG based WECS

Due to the increased renewable power penetration level renewable power plants have to provide low-voltage ride-through (LVRT) capability with simultaneous dynamic voltage support, to ensure the grid stability during grid faults. Concerning doubly fed induction generator (DFIG) based wind energy conversion systems (WECS) large electromotive forces and rotor currents, which may damage the rotor-side converter, or adversely affect the DFIGs controllability are induced into the rotor circuit in case of voltage dips. To handle and limit the rotor currents in case of asymmetrical voltage dips without crowbar triggering, a virtual resistance control approach based on the standard dq-control in the synchronous reference frame is discussed. The theoretical results are compared with those of more demanding virtual inductance control. The LVRT capability is verified with measurement results, recorded during a certification campaign at a 2.1 MW WECS concerning the Indian grid code.

Overview of LVRT-capability pre-evaluation with an inverter based test bench

With increased renewable power penetration level the system operators of power grids require low-voltage ride-through (LVRT) capability of renewable power plants. The LVRT-capability has to be verified during the process of certification with precisely defined short circuit tests on a reactance based test bench. For the development of the fault ride through (FRT) capability the cost intensive reactance based test configuration is replaced by an inverter based voltage sag generator (VSG). This paper deals with an overview of the whole inverter based test configuration including the control structure of the grid emulator and the adjustment of the grid side converters control structure to reach LVRT-capability as well as measurement results for the pre-evaluation.