An inverter is the heart of the solar PV system. It converts solar array-generated DC power into AC power in synchronisation with the grid. The inverter has additional functions such as maximum power point tracking, grid monitoring, and anti-islanding protection in addition to DC to AC conversion. Solid-state switching circuits are responsible for DC to AC conversion. The inverter contains magnetic circuits, capacitors, AC and DC disconnects, electromagnetic and radio frequency interference filtering equipment, a cooling system, a ground fault detection interruption circuit, and command, control and communication components to reliably and safely feed high quality AC power into the grid.
The variable output voltage of the solar array is connected to the maximum power point tracker (MPPT) circuit of the inverter. The DC current is made to flow through switches that are turned on/off with certain frequencies and times using a pulse width modulation (PWM) technique that creates a square wave. The square wave is modified to a pure sine wave using digital signal processors. In an inverter, DC power from the PV array is inverted to AC power via a set of solid-state switches such as insulated gate bipolar transistor (IGBT) that essentially flips the DC power back and forth, creating AC power.
Both the input and output circuits of the inverter are protected with fuses or circuit breakers and are accessible. To protect against nearby lightning strikes or any other transient voltage spikes, a surge protection device on the inverter input is recommended. Inverter manufacturers provide various features in an inverter such as DC and AC overvoltage protection, DC and AC short circuit protection, out of voltage range protection, and DC inverse polarity protection. Also, the inverter will have DC fuses at the input, an AC output breaker, a ground-fault detector interrupter device, insulation monitoring, high voltage ride through (HVRT), low voltage ride through (LVRT), and anti-islanding features for reliable and safe performance.
There are two-six types of solar inverters. These are central inverters, string inverters, hybrid inverters and storage inverters. Optimiser-based inverters and micro-inverters are also used, which are directly mounted on the solar modules.
Central inverters are designed to meet 1,500 V system voltage. The design has been changed from 600 V to 1,000 V and from 1,000 V to 1,500 V to save balance of system (BoS) costs of solar PV plants. It may further change to 2,000 V system voltage in future. The capacity of inverters has evolved from 1 MW to 5 MW, and they have become more modular. There is a single MPPT in the central inverter. The present trend is to go for more than one MPPT for central inverters to minimise mismatch losses of the strings. There is an increasing demand to design inverters to take care of a DC to AC power ratio of more than 1.5.
Present-day inverters are designed to operate in outdoor conditions to reduce the cost of civil works required for inverter. Inverters use forced cooling or liquid cooling with heat pipe technology or with phase transition material. The current trend in the thermal design of inverters is to make them work effectively even at a temperature of 50 °C. Inverters are designed to support reactive power during the day and night and the latest trend is to minimise power loss occurring due to reactive power compensation.
All central inverters support LVRT, HVRT and grid support features effectively. Central inverters are single-stage type and with multilevel topology. The present trend is to use three-level H-bridge with neutral point clamping topology. Silicon carbide (SiC)-based switches give better efficiency and lower losses compared to IGBT switches but are costlier. So, IGBT switching and PWM technology are expected to remain relevant. The DC link capacitor has been changed from electrolytic to thin film-based. Efforts are being putinto selecting the right components to withstand stresses caused due to more DC power loading and minimise losses. Central inverters are equipped with built-in auxiliary power supply and DC UPS systems with battery for smooth operations. The inverters are enabled with negative grounding fuse protection to take care of potential induced degradation (PID) issues.
String inverters come with multistage topologies and not only convert DC power to AC power, but also provide anti-islanding protection, AC leakage current fault protection, DC reverse-polarity protection, AC surge protection, DC surge protection with a metal oxide varistor, DC overvoltage protection, ground fault protection and IP-65 ingress protection. These inverters are provided with an RS 485/232/WIFI/ GPS/Ethernet or a LAN-based communication interface for remote monitoring.
In recent times, the power capacity of inverters has improved significantly, but for low voltage 600-800 V on-grid inverters, the maximum capacity could reach up to 200 kW. Also, the inverter has to manage more and more devices such as solar PV modules, DC cables, and even AC distribution boards and grids. Further, the communication functions have become more powerful. An inverter needs multiple USB, RS485 and RS232 ports to connect with the computer, the flash disk, the data logger, the meter, and the current transformer and it has the option of GPRS/ethernet/direct communication.
The increasing power not only brings higher current but also increases the complexity of the control algorithm. Consider, for example, the 200 kW inverter with 12 MPPTs. The inverter needs to control 12 different circuits and the controlling difficulty has been increased by 12 times compared to the single MPPT-based inverter. The present-day inverter has the following functions:
- String-level or zone-level monitoring and mobile communications through wired or wireless network
- Insulation monitoring device with digital measurement of insulation value
- Ground fault detector and indicator with adjustable pre-warning alarm and level of tripping
- Arc fault current interrupter detection
- Solar panel PID healing with anti-PID kit
- Provisions for two-way communication with utility control centres
Smart inverters
- They are equipped with voltage and frequency sensors that allow detection of grid abnormalities and send the feedback to utility operators
- They have more DC overloading options and flexible MPPT channel
- Wireless communication systems that can be easily connected to mobile-friendly applications
- Ability to supply reactive power both during the day and night
- Intelligent and flexible power plant controller for PV plant regulation
- Integration of two-way communication interfaces
- These inverters support grid management services such as grid error recording, power factor adjustment, grid harmonics adjustment, output power throttling capabilities and compliance with complex HVRT and LVRT requirements of large-scale PV plants.
String inverters
- These inverters have a fuseless option
- They are equipped with shade-tolerant hill climbing MPPT tracking algorithms, which allows for tracking of dynamically changing global power peaks with no significant decrease in traditional static and dynamic tracking, resulting in maximum efficiency and maximising output power
- I/V curve monitoring and diagnosis at string level for individual DC inputs – This technology allows the inverter to export I/V curves, deploys algorithms on the management system, and analyses data. It also identifies modes to scan all strings of a PV plant and detects hidden PV module faults. The smart I/V curve diagnosis can carry out online I/V curve analysis on entire strings with advanced diagnosis algorithm. The scanning helps find and identify the strings with low performance or faults, which enables proactive maintenance, in turn, increasing O&M efficiency and reducing operation cost
- String inverters come with new technologies – harmonics, active surge suppression and Class II SPD.
String and central inverter technologies have been developed to handle high current from high wattage solar modules based on M10 and G12 wafers.
The technology advancement in solar inverters makes them smart and helps provide all information of the plant to the operator. A smart inverter can generate an alarm or warning, which enables the plant operator to address the fault before it can become permanent. The inverter operates in a manner such that the threshold values are not crossed, and the energy generation loss is minimised. Technology advancements are expected to continue in the near future and help realise the overall goal of minimisation of downtime, enable quick action in terms of O&M activities and module cleaning, and predict faults so as to improve plant efficiency and operations.
