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Conferences and Forums

August 27, 2012

Considerations on Modeling Inverters for Renewable Resources Within Power System Studies

Grazia Todeschini
Power Systems Senior Consultant, EnerNex

The use of power electronics within residential, commercial and industrial loads has been growing enormously in the last decades. Among the most recent and wide-spread applications are inverters used for interconnection of distributed energy sources to the power grid.

Solar panels produce dc energy, while wind turbines produce energy at variable frequency, depending on the wind speed. Power systems do not tolerate even slight variations from the 60 Hz to operate continuously and reliably. Therefore, power converters are used to interface renewable sources to the power grid and to convert wind and solar power into 60 Hz voltage and current.

Figure 1 shows a conceptual representation of the interconnection of a wind turbine and a solar panel to the grid. Typically, wind power is converted to a dc stage first and then to 60 Hz power, while solar power requires only a stage of conversion, from dc to 60 Hz.

Figure 1

Control overview

The overview of the main blocks included in a power conversion control scheme is shown in Figure 2. These blocks represent a very general structure, applicable to either solar or wind applications. The blocks are showed in a ‘nested’ structure, where the ‘slower blocks’ are external, and the ‘fastest block’ are internal. The slowest part of the controller is responsible to adjust operation of the solar panel or wind turbine to extract the maximum power available from the sun or the wind, respectively. This operation is performed by the maximum point of power tracking (MPPT) regulator for solar panels and by the pitch regulator and speed regulator for wind turbines. The second loop is the power regulator, intended to match the inverter input power (from the wind or the sun) to the output power delivered to the grid. The third block is the current regulator, which controls the converter current output based on the reference signal from the power controller. The more internal block is the switching scheme (typically pulse-width modulation). This last block is responsible to sending the on and off signal to the power switches with a switching frequency of 5-8 kHz. Protection is responsible to trip the converter off-line based on specific system conditions. Signals to the protection block include voltage, current and frequency measured on the grid side and on the dc side.

Figure 2

In addition to power conversion, the control blocks of a power converter stage allow implementing numerous features, such as low-voltage ride through capability, active filter operation, power factor regulation. This complexity offers many possibilities, as well as challenges, when renewable resources are connected to the power system. Power system engineers perform many different studies to determine power system operation, and the impact of renewable resources.

Power system studies can be divided in two main categories: (1) steady-state studies and (2) dynamic studies. For each study within these two main categories, a specific model is developed. It is important to identify the parameters that need to be included in each model. This applies to all system components including the renewable sources.

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