While PV project developers are focused to optimize system in terms of optimum yield. Its important to consider whether the energy generation by PV system can be absorbed by the grid, If the grid parameters are not studied properly the profitability and viability of PV projects becomes question mark. It is important to have a stable grid for consistent power evacuation,the grid stability is function of various parameters such has the level of grid substation at which the PV project is connected. A PV system connected at 11KV substation is likely to be less reliable as compared to the same project connected at 33 KV or 132 KV level grid. Many time the grid operator have the feeling that connecting solar PV system will make the grid unstable as the generation from solar is intermittent ;variable in nature. Hence there is a need to carry out a proper load flow study before interconnecting a solar PV system with the grid. It is important to note that PV system are connected at the tail end of the grid and it will rather improve the voltage drop and frequency at the substation level.The below graph shows the improvement of voltage profile along the various feeders with the 25% PV penetration in to grid.(MATLAB/simulink).
The recent experiences of connection of solar plant with the grid have been not so encouraging, as far as our information many projects are facing problem of grid unavailability and there by losing generation .In fact a typical 5MW plant lose about 20 lakh ,if the grid is not available. generally grid substation in India at 33KV level is not designed to accommodate bidirectional flow of electricity and this requires more sophisticated monitoring and communication, control system for integrating PV system at the tail end of the grid to pump electricity back from distribution substation.
The major aspect of monitoring is the variability of the load connected to the grid substation .where the PV system is as well connected it should be noted that on grid substation 33KV at one side there are different loads connected as a demand side feeder. On other side the supply to grid substation comes from 132KV and as well as PV power plant which is variable in nature .At any given point of time the net load at the grid substation need to support is equal to demand minus contribution of supply through the solar PV system. If the demand falls suddenly at grid substation the voltage and frequency will increase and if the same as been not communicate to the PV system .It will trip the PV system which is typically known as grid tripping or over voltage tripping .This can be explained from example that if the voltage level is 33kv at grid sub station ,which is supplying 50 MW load which includes Domestic ,Agriculture, Industrial consumers and suddenly a typical industry connected to grid(20MW)stops its operation .The grid frequency will increase and there by voltage to 36KV and Grid will have a cascading effect in tripping the supply from a solar plant to the grid. In order to safeguard a PV power plant from this type of grid fluctuations there should be provision of grid islanding and many inverters used in solar PV systems have a feature to disconnect it from the grid, so that the grid operators can rectify the problem.
It is important for transmission and distribution utilities to manage voltage drop, as the transmission and distribution lines are in km length and accordingly voltage regulation is accomplished using distributor Capacitor banks and Tap changing transformers .If a feeder of grid substation is connected with PV system ,the voltage regulation becomes important as the grid voltage will dictate the operation of PV plant . IEEE1547 standard allows all grid connected inverters to regulate service voltage and the inverters are capable to produce reactive power needed to regulate the grid voltage. Many inverters are capable to generate and absorb reactive power depending on the requirement .This is the rather positive sign to grid operators that grid connected PV plant may help them to improve grid stability and allow the grid substation to operate close to the rated output voltage .
Dr Sanjay Vashishtha & Rishikesh Muthyal