Harmonic analysis of a photovoltaic power plant


Background

The scope of work concerned a solar power plant located in North America. The objective was to analyze the harmonics produced by the photovoltaic units in order to verify the compliance with the IEEE 519 standard and measured at the point of common coupling (PCC).

The power plant is composed of 12 Inverters producing 1.2MW each and a substation connecting inverters to the upstream network.

Realization

Powersys used for this study the software EMTP-RV and its capabilities to simulate accurately frequency dependent equipment.

  • Models

For the harmonic analysis, the most important part of the system is the modelling of the inverters, which are producing harmonics by converting the DC current from photovoltaic cells to AC voltage. Two models are developed. First, the inverters are represented by the equivalent impedance of their outputs filters for the resonance-frequencies investigation. For the harmonic analysis, the inverters are represented by an equivalent harmonic current source using the harmonics level given by the manufacturer.

 

photovoltaic pannel electric plan

 

The upstream network at the PCC is modelled as a Thevenin source which is valid over the frequency range of interest. Cables of the collector system are modelled as classic PI models and the saturation of transformers is not represented.

  • Simulations

Different configurations of the system are studied according to the internal circuits (and the number of inverters) which are connected.

Two types of simulations are performed :

               - A frequency scan to measure the system impedance and identify possible resonance frequencies.

               - Time domain analysis to simulate the current generated by the power plant and calculate the different harmonic distortion levels (IHD and THD).

Then, the harmonics levels are compared to the limits given by the IEEE 519 standard.

 

PPC curent ITHD curve

Results

The study reveals the photovoltaic power plant is in compliance with the standard :

  • Current and voltage IHD, voltage THD and current TDD are below the IEEE 519 limits,
  • For all configurations of the system, a resonance is identified beyond the range of harmonics of the IEEE 519 standard.
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