Research Focus - Power Quality

  • Control of Grid-Connected and Stand-Alone Inverters

    The field of distributed generation of electrical power is growing rapidly. Electrical power is supplied to the grid by injecting current. It is very important, that the injected current does not reduce the power quality of the grid – on the contrary, the current shall support or even improve the power quality of the grid. In particular, the injected current has to result in a defined reactive power, without feeding any distortion power into the grid.

    Various projects at ISEA deal with control of grid-connected inverters. In the field of photovoltaics, inverters with a power range from some kVA (string inverters) up to some hundred kVA (central inverters) are considered and developed at ISEA. The behaviour of passive and active loads, like PV inverters and wind turbines for example, can be analyzed under various grid conditions using a grid simulator. Besides simulating almost ideal grid conditions, a grid simulator is able to simulate disturbances like voltage dips, unbalanced voltages or harmonics. For further information on this topic, please have a look at the section "High Power Grid Simulator".

  • Active Filter

    A Shunt Hybrid Power Filter for industrial applications has been developed with excellent harmonic filter characteristics at low costs. The SHPF consists of a passive part created by a transformer and a capacitor bank and an active part which comprises a three-phase voltage source IGBT inverter. The combination of passive and active parts results in a filter with outstanding characteristics. Compared to traditional Shunt Active Power Filters (SAPF), SHPFs have better characteristics. Furthermore, the active part can be dimensioned smaller, due to reduced current and voltage requirements of the IGBTs as well as reduced losses. As a result also the initial costs decreased.

  • Power Factor Correction (PFC)

    In various projects, the ISEA has developed Power Factor Correction stages for different applications and power levels. High efficiency and low component size has been the main focus. Special solutions for requirements like a continuous neutral conductor have been successfully developed.

  • High Power Grid Simulator

    Operating conditions in electrical distribution systems are often disturbed, due to various reasons, e.g. voltage dips, short interruptions, voltage fluctuation, unbalanced voltages, variation of power frequency, and harmonics. Consumer loads have to be able to operate properly under these conditions up to a specified level. Furthermore, distributed power sources like wind turbines and photovoltaic systems are required to react in a specified way to certain disturbances; depending upon the nature of the disturbance they shall either substitute the grid or shut themselves down. These reactions and the tolerable limits for disturbances are specified in various standards, like IEC 61000, EN 50160, IEEE 1547, and IEC 61400.

    Based upon the specifications for test generators or grid simulators respectively, that are given by these standards, a closed-loop control for a 1 MVA multilevel inverter has been designed at ISEA. A combination of state-feedback control and dynamic output-feedback results in a stable, exact, and robust control, which is capable of handling unbalanced output variables without any problems. The grid simulator will be used for testing linear and nonlinear, active and passive loads. In the field of distributed generation, tests like the "low voltage ride-through" test for wind turbines and islanding test for photovoltaic systems can be performed. The following figure shows an example of a three-phase voltage drop and subsequent unbalanced voltages.

  • Compensation of pulsating power consumption and reactive power

    In order to guarantee a high power quality of the grid it is important to compensate reactive power loads and harmonics. A one phase load causes a pulsating power consumption which must be compensated by a large energy storage device in the DC-link. Dependent on the application the size of these expensive energy storage components can be reduced. It is possible to extent the load to a resonant circuit and add an additional network to extent this single phase load to a three-phase load. By special modulation algorithms the power consumption can be smoothened and it can be realized that only constant power is drawn from DC-Link for sinusoidal current and voltage waveforms at the output. To attain the best compensation an optimal converter topology dependent on the application must be chosen. As a current research topic matrix converters are investigated. This type of converter has no energy storage device in the DC-Link.
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