– The following article is authored by Jeykishan Kumar K, Energy Efficiency and Renewable Energy Division, Central Power Research Institute, Bengaluru
Solar power plants have been increasing in number each year in India and we also see an increase in the use of renewable energy. The use of renewable energy enables the country to become more independent. Solar power plants mainly consist of solar PV modules, grid-connected inverters and transformers. Owing to an increasing number of power plants, the demand for inverters will be high considering we have a target to achieve 175 GW by the year 2020 and 500 GW by 2025. With one of the leading developing countries, India is pushing onward with a large scale addition of renewable energy by the year 2030. Because of the availability if high renewable energy systems, the stability of the power system needs to be safeguarded. Grid stability is one of the important aspects to consider with regard to energy supply. In order to avoid power outages, it is necessary that power generating plants have control capabilities and protection mechanisms. In the past, these requirements were fulfilled by conventional power plants. In the meanwhile, however, the share of renewable energy sources in the total electricity generation has become so significant that these sources too must contribute to grid stability. Therefore, the transmission system operators have established so-called grid codes with certain critical values and control characteristics that the generating plants have to fulfil. An important part of these requirements is the so-called LVRT capability of generating plants. But what exactly does this term mean? LVRT is a short-form for Low Voltage Ride-Through and it describes the requirement that generating plants must continue to operate through short periods of low-grid voltage that does not disconnect from the grid. Short-term voltage dips may occur, for example, when large loads are connected to the grid or as a result of grid faults like lightning strikes or short-circuits.
In the past, renewable generating plants such as wind turbines were allowed to disconnect from the grid during such a fault and try to reconnect after a certain period of time. Today, because of the significant share of renewables, such a procedure would be fatal. If too many generating plants disconnect at the same time the complete network could break down, a scenario which is also called a ‘blackout’. For this reason, the LVRT requirement has been established which is meant to guarantee that the generating plants stay connected to the grid. Additionally, many grid codes demand that the grid should be supported during voltage drops. Generating plants can support the grid by feeding reactive current into the network and so raise the voltage. Immediately after fault clearance, the active power output must be increased again to the value prior to the occurrence of the fault within a specified period of time. These requirements which at the beginning only applied to wind turbines, now also have to be fulfilled by photo-voltaic systems (PV) and most recently by combined heat and power plants (CHP). Figure 1 shows the result of an LVRT test on a Solar Inverter of 1250 kW tested at 350 kW.
In this diagram, the voltage drops to about 85 per cent of the nominal voltage for a time of 300 ms. The PV inverter recognises the voltage drop and feeds a reactive current of approximately 100 per cent of the nominal voltage into the system for the duration of the fault in order to support the grid. After fault clearance, the active power output is increased to the value prior to the occurrence of the fault within 1 second. Before a generating plant can be connected to the grid, the transmission system operator normally requires a test report or certificate. One of the certification requirements is the measurement of electrical characteristics that includes a test of the LVRT capability. In India, this test can be carried out by a laboratory at the Central Power Research Institute (CPRI), Bengaluru. CPRI is having 540 kVA grid simulator and 750 kW (1000 V & 1800 A) programmable DC source (see Figure 2). During the test, voltage dips are simulated and the behaviour of the inverter is measured and evaluated. The results are documented in a test report which together with other reports forms the basis for certification.
Simulation of LVRT
The simulation of voltage dips requires special technology. Most grid codes and guidelines have specific requirements for the test equipment. According to the international standard for the measurement of power quality characteristics of wind turbines (IEC 61400-21) for example, an inductive voltage divider is recommended which is to be connected ahead of the plant to be tested. For India, CEA Regulations, 2019 for grid-connected equipment. Clause 4(c), Clause B2, Sub Clauses of Central Electricity Authority (Technical Standards for Connectivity to the Grid) (Amendment) Regulations, 2019, Ministry of Power, Notification (New Delhi, dated 6th February 2019) Part III Section 4 provides the requirement for grid-connected equipment for LVRT (see Figure 3).
By using the grid simulator, the voltage dip can be configured. Depending on the respective grid code, different depths of voltage dips have to be simulated, for wind turbines, usually, the dip is less than 5 per cent, 25 per cent, 50 per cent and 75 per cent of the rated voltage are required; for grid-connected inverters usually, the dip is 85 per cent. The duration of the dip is 300 ms only. In some cases, the duration can also be extended to several minutes. German and international guidelines demand the simulation of three-phase as well as two-phase faults. In England, guidelines additionally demand one phase faults against earth. The test system is normally stored in specially equipped standard sea containers and mainly contains the coils and switching devices. Large-size test systems (for generating plants in the multi-megawatt range), often require two or more 40-feet containers. The mobile test system can thus be transported to the respective test site for free-field measurements. PV systems are often tested in the laboratory where the LVRT test system is normally part of the test facility. In cases, however, where manufacturers do not have their own test facility, mobile test containers are used instead.
As an independent measuring institute, CPRI has recently started testing the LVRT capability of grid-connected inverters. Since very recently, CPRI is also able to perform fault ride-through tests with own test systems. These consist of a smaller system for testing generating plants up to 0.5 MW, in grids up to 415 V. The test system is also well equipped for future requirements because it is possible to simulate so-called HVRT tests (overvoltage tests) and FRT test (frequency ride-through from 47.5 Hz to 52 Hz). The first projects with CPRI’s own test setup have already been completed successfully on a solar-based grid-connected inverter of 1250 kW capacity, tested at 350 kW. CPRI is accredited by the NABL (National Accreditation Board of Laboratories), according to ISO/IEC 17025:2017. Other terms frequently used and describing the same subject are Fault-Ride-Through, response to voltage drops, performance in case of voltage dips, voltage dip- tests, transient stability, network faults, double-dip test, voltage drops, and performance during network disturbances and behaviour during network disturbances.
Examples of LVRT in Solar Power Plants
For short system faults (lasting up to 150ms or 300 ms) the inverter in the solar plant has to remain connected to the grid. For High voltage grids, voltage dips of longer durations like 500 ms or 1000 ms or higher, the inverter in the solar power plant have to remain connected to the grid up to more than 2 ½ minutes. As the curve shown in CEA says the inverter to be on top of the curve if voltage follows it. During grid faults or brownouts a solar power plant has to supply maximum reactive current to the grid without exceeding the transient rating of the plant. This will boost the voltage of the grid to maintain stability.
On HV grids, during voltage dips lasting more than 300 ms, the active power output of a solar plant has to be retained at least in proportion to the retained balanced HV grid voltage.
LVRT for Electric Vehicle Charging Infrastructure (EVCI)
Grid-connected inverters need to have LVRT feature in-built in them to support the grid. As electric vehicle supply equipment (EVSE)/ EVCI contains the grid-connected inverter for V2G power flow, the importance of LVRT is increased. EV loads are going to be fluctuating and this making the existing conventional load curve of each region or entire country different than the present scenario. EV loads may not increase the load in the near future but will vary the load curve. Multiple EVCI’s connected to the grid can really help the grid if they possess the LVRT feature in them apart from taking power from EV to the grid (V2G). LVRT based grid-connected inverters in the EVCI’s can play a major role in maintaining the grid stability and security of the country.