Issues in connecting the rooftop systems with the grid
It is important to regulators to specify the guidelines which easily facilitate the connection of grid so that large scale deployment of solar PV systems takes place as roof top systems. Recently Tata Power Company (TPC) installed a 60.48 kWp Rooftop Solar Power Project at Tata Power’s Carnac Receiving Station. The Maharashtra State Electricity Transmission Company Ltd. (MSETCL) has been approached by them to grant connectivity & long term open access for this roof top system. Also to get a clarification on the purchase of electricity by a distribution licensee. Currently the guidelines of MERC allows the transmission open access for Roof Top Solar projects and TPC wanted that this roof top solar power project’s electricity be purchased by their own distribution licensee company (TPC-Distribution) (TPC-D) to meet its Solar based Renewable Purchase Obligations. The installed roof top system has 336 number of panels (28 arrays each containing 12 modules), size of each panel is 1587 mm x 790 mm, area occupied is 549 m2, continuous rating is 60 kVA with all phases equally loaded and output voltage is 400 volts three phase,4 wire, grid tracking. As the open acces from the stete transmission utility has been sought for a capacity of less than 1 MW, it was not allowed by the STU. As per Section 39 (2) (d) (i) of the Electricity Act, 2003 wherein the functions of STU have been specified, the STU is mandated to provide non-discriminatory open access to its transmission system for use by Generating Companies on payment of the transmission charges, and there is no restriction on the eligibility for transmission open access only to a generator having capacity of more than 1 MW. TPC wanted to connect the Rooftop Solar project in the Carnac Receiving station of TPC-T. The nearest interconnection point for the same is 415 V station auxiliary bus of TPC-T. The 415 V auxiliary bus is part of the assets of TPC-T Mumbai Licensed area business.
There is a need to generalise the guidelines for all such small roof top solar power projects which will need grid connectivity in near future. There is also a need to specify the connection interface norms, Metering and Energy Accounting Norms, Issues related to scheduling of such power, and Tariff for such power, etc.
While the TPC Rooftop Solar project can be synchronized with 415 V bus of 415 V side of 22/0.415 kV station transformer, there are issues and concerns of STU in terms of the protections and controls of these roof top systems and issues related to the harmonics. The 415 V DC Voltage generated by Solar Project routed through control panel having DC breaker, inverter, Transformer, A.C. synchronizing breaker and other associated equipments. The Solar control panel is provided with various types of protection.
There are already guidelines specified by MNRE for “Rooftop solar PV and small solar generation program” which sates that Rooftop solar project below 100 kW to be connected to 440 Volt Distribution network and above 100 kW upto 2 MW to be connected to below 33 kV voltage level. The guidelines also states that local distribution utility would sign a PPA with the project at tariff determined by SERC and Distribution Utility would provide certificate of power purchased monthly basis based on the joint meter readings.
The above roof top solar power project is expected to produce only 215 kWh a day and can feed the auxiliary power consumption of the proposed substation only which consumes power at 415 V. However the current guidelines do not allow such small roof top systems to be connected to the STU grid which is generally at 33 kV or 66 kV. The 60.48 kWp Solar Rooftop Power Project of TPC is of small size as compared to the other conventional and non conventional generating plants seeking connectivity to the grid and is the first instance of rooftop small solar power project case in the State which is brought before the Commission since announcement of Jawaharlal Nehru National Solar Mission (JNNSM) at the national level. The Central Electricity Authority is yet to formulate and notify grid connectivity standards and regulations, which shall govern connectivity related aspects for such distributed generation sources. It is important to have a clear guidelines so tha many more roof top systems can be installed in a hassel free manner.