Photovoltaic (PV) systems have variable power generation because of variable parameters (day/night, angle of the panel, clouds, etc.); therefore, it is more challenging to manage distributed voltage, increasing wear and tear in electromechanical utility equipment, and configure protection system with larger distributed generation PV (DGPV) systems. Some of the major issues and their impacts of integrating DGPV are discussed next.
The voltage level of different nodes of distribution system rises locally beyond the acceptable range due to integration of more and more distributed generation PV (DGPV) systems, wind, micro-turbine, and so on. The value of voltage rise depends on feeder characteristics, voltage rating, overhead or underground lines, size of the wire, etc.; the location of solar PV; and load pattern. Advanced inverters can be used to mitigate and eliminate undesirable voltage by shifting the phase angle of sinusoidal current output to absorb or inject reactive power. Advanced inverters can provide capabilities that can benefit the power system, external controllability, real power curtailment in response to excess generation, voltage and frequency ride-through, and so on.
Reverse Power Flow
Conventional power flows are unidirectional, but the higher deployment of DGPV can reverse the power flow, which can be supplied to loads of neighbouring feeders or injected back into transmission lines. Tap-changing transformers are used for voltage regulations that use recirculation schemes to avoid arcing and other issues when changing voltage ratios under load and these can limit reverse power rating for tap-changing transformers. Some older units in system may ignore the reverse power flow and may sense it as an overload in the system and cause voltage droop and the regulators will increase the voltages, which is exact opposite behaviour required to mitigate the DGPV voltage rise. The bigger challenge about reverse power flow is distribution–transmission interface. This interface may require the approval of many parties and it also needs research on market exchanges and guidelines to support bulk level of penetration of DGPV. This is often known as frequency-watt control because an increase in grid frequency is the first measurable change of excess generation compared to load.
During mid-day, the PV generations reach the peak and the demands are low. It can create a situation of overgeneration by variable generators (VGs). Without intercession, generators and motors connected to the network would increase rotational speed, which can cause damage. To prevent overgeneration, system operators may curtail DGs output by either inverters or disconnecting plants. This requires system operators to control generation resources, which is not easy with large power plants and abnormal for smaller systems (distributed or rooftop PVs). The curtailment of the output of VGs will reduce economic and environmental benefits. As curtailment increases, at a point, the additional installments of PVs are not worth the cost. Other technical challenges of increased/overgeneration of PV or any other DGs that limit the resilience of system include contractual and institutional restrictions on plant operation, including long-term ‘must take’ contracts, self-scheduling, and combined heat and power plants.
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January - June 2019 (Combined Issue)