Reverse Power Protection OZTRON eco energy pvt ltd The Technology Source for Green Energy 130 Humayun Kabir Sarani, 10N, Block-A, New Alipore, Kolkata 700 053 Ph: 033 4068 0170 / 2396 6717 E-mail: info@oztron.net
Reverse Power Protection System Description Overview The purpose of Commercial Roof-top solar installations in India is to generate electricity locally and consume locally in the same building. Any excess is exported to the grid. There is no restriction on how much electricity can be exported. If grid is absent a back-up diesel generator is started and the load is shifted to the generator. Usually the diesel generator is sized to support the full load. Electricity generated with diesel is much more expensive. So the solar installation is utilised to reduce the loading on the generator to the extent possible. There are a few constraints: 1. The solar inverters get automatically switched off when grid (or generator) is absent. They cannot operate independently. 2. Generators cannot accept reverse power. That means at no time the PV power can be greater than the load. 3. Generators should not be operated below a certain load, usually about 30% of the rated capacity. Though momentary under loading is permitted. For a large roof-top installation it is possible that the solar power generated could breach the above conditions 2 and 3 while operating with diesel generators and these conditions would have to be prevented. OZTRON Reverse Power Controller (RPC) actively modulates the solar inverters even under rapidly fluctuating loads and varying solar output to ensure that the solar power is always maintained just below the consumption level and tripping is avoided. Design Criteria The design target for a Reverse Power system is to protect DG from reverse Power with the lowest cost, ensuring safety, reliability and long service life. Moreover, the system has to comply with all the relevant electrical, mechanical and environmental standards. Features Smooth control Compatible with popular models of inverters No moving parts or power relays Minimum wiring Quick response, in milliseconds, not minutes Accurate control No start-stop operation for inverters resulting in loss of production No restriction on sizes of inverters Self-resetting
Stability The Control System is designed such that the system remains stable while operating with diesel generators even though there can be fluctuations in the PV output and the load. System Description The Reverse Power Controller System consists of three units: 1. RPC Control Unit 2. Control Junction Box located on the roof close to the inverters 3. Sensing Circuit located in the power house The Sensing circuit measures the current from the diesel generators. This signal is transmitted to the Controller by a current loop. The current outputs for the different PV arrays are monitored by CTs at the Isolator panels and the CT outputs are measured in the Controller. The Controller processes these data and control the Inverters appropriately. The inverter control is accomplished through the RS-485 communication interface or Inverter digital inputs of EPO section. Only the main Controller unit needs a 230V AC supply. There is no damage or loss of data if the power supply runs down and the Controller shuts down. It can be re-booted safely. The sensing circuit and the Junction box do not need any external power. All cabling used between the units are low level (extra low voltage) signals and isolated from the power circuits. These are immune from external interference. However, normal precautions of isolation and shielding should be used. Power Supply Only the main Controller unit needs a 230V AC supply. The Junction Box and the Sensing Unit do not need any external source of power. There is no damage or loss of data if the power supply is lost and the Controller shuts down. It automatically be reboots when the power supply is restored. An internal 24V battery back-up is provided in the Controller so that the communication is maintained in the event of a power outage. All cabling used between the units are for extra low voltage signals and isolated from the power circuits. These are immune from external interference. However, normal precautions of isolation and shielding should be maintained.
Fig1: Block Diagram 2.3 Remote Monitoring The Controller is monitored remotely by Oztron to ensure efficient and reliable operation and firmware updates are installed automatically. An Internet connection to the Controller is required for this purpose. Oztron provides this service free of cost during the warranty period. Subsequently, this will be included in the scope of any future service contract. A LAN based internet or a wireless modem using the Mobile network can be used depending on availability of LAN connection. The advantage of remote monitoring are, Remote communication through customer's LAN connection or independent 3G modem. Remote trouble shooting and maintenance Free software updates Optional Interface with Building Management System
Operation When grid supply is present solar inverters generate the maximum power available. The energy is used on site and if any extra energy is available it is exported to the grid. The RPC control function is disabled under this condition but the RPC is kept in operation. If the grid supply fails, the grid connected inverters go off as per the antiislanding protection requirement. When the diesel generators are switched on the inverters switch on back again, with a delay of a few minutes. The RPC control now comes into operation. It measures the output from the Diesel Generator and solar generator and computes the load. It then automatically adjusts the control relays to set the appropriate operational state and the maximum output power of the inverter array so that minimum load on the diesel generators is maintained at a pre-set value of 20 to 30% of the generator capacity. When the grid is back the diesel generators are switched off. The RPC control is again disabled and the solar array operates at full power. Projects RPC has been successfully installed at following locations 1. WALGA Building, Railway Pde, Perth, January 2014,166kW Roof-top 2. Perth Arena Perth, Western Australia, 2014, 120kW roof-top 3. Calicut Airport, India, February 2016, 750 kwp