Grid-interconnected Photovoltaic (PV) source is one of the fastest developing and most prominent renewable energy sources in the globe. The main reason behind this is the remarkable progress in the semiconductor manufacturing domain. Also, the reduction in price of PV modules helps in the starting of economic incentives or subsidies. Although, the core of a PV system is the PV cell (or PV generator), power electronics sector plays a major role as a cutting edge technology for an efficient photo voltaic system control, hence transferring the generated power to the grid supply.
The functions of the power converter of a PV system consists of Maximum Power Point Tracking (MPPT), DC/AC power converter, grid synchronisation, power quality, active and reactive power control – and anti-islanding detection power converter interface of grid-connected PV system. The system has a PV generation set-up, which can be a single module, a string of series-connected modules, or an array of parallely connected strings. PV inverters nowadays have high demand, which are manufactured in different topologies. The configuration of series/parallel connections of PV modules with 3-Ø central string inverter is common for PV plants (10 to 250 kW & more) that gives high efficiency.
The PV set-up has a passive input capacitive filter, which decouples the input voltage and current from the subsequent power stages by reducing current and voltage ripples at the PV cell side. The input capacitive filter circuit after filtering the ripples comes to DC/DC boost converter, where MPPT techniques of PV system are performed. Moreover, galvanic isolation are also introduced (when DC/DC converters with High Frequency (HF) transformers are employed). The DC/DC chopper block is connected through a DC link to a grid-tied DC/AC central inverter, commonly known as PV inverter. In PV systems – where no DC/DC converter is used, the input filter is equivalent to the DC-link capacitor. The PV inverter is connected to grid source through output filter, usually a combination of inductors (L) and capacitors (C). The AC side filter enables harmonic mitigation – and helps the converter–grid interface control. Depending on the PV system requirements and the grid connection, a Low Frequency (LF) transformer is used to increase the voltage and give isolation to the circuit.
Current market scenario
The trend of reducing cost of PV modules and the good support of government in enhancing the technology have increased the use of PV and solar thermal energy as important factors in the present and future renewable energy’s growth scenario, a generalised block diagram of grid-connected photo voltaic system is shown in Fig. 1.
Fig. 1: Generalized Block Diagram of Grid-Connected PV System…
- These installations are based on housing applications, where power requirement is (<5kW). Also, larger PV power plants are rapidly going in construction to achieve a nominal power level up to 250 MW.
- Currently, the main installed PV set-ups are grid-connected with the off-grid sector accounting for an estimated 2% of global capacity. The output of PV panels is a DC voltage, and photo voltaic central string inverter gives an AC output voltage.
- PV set-ups, where each photo voltaic panel has its own module inverter, are commonly used for low-power applications – where power levels are below 500 W.
The closed loop feedback topology for the control system consists of several current and voltage transducers at the photo voltaic input side (for MPPT), DC-link (voltage control) and grid side (for grid synchronisation and active/reactive power control). Grid-connected PV energy conversion can be submerged into four different types of configurations: centralised configuration for large-scale PV plants (3-Ø), string configuration for small/medium scale photo voltaic plants (1-Ø and 3-Ø), multi-string configuration for small to large-scale plants (1-Ø and 3-Ø) and AC-module configuration.
Grid connected systems
In grid-connected applications, the power is supplied directly to the grid – and the important blocks are photo voltaic modules and inverters. This decreases the overall price of the plant and also reduces the necessary maintenance required, as the batteries are the most maintenance-demanding parts.
The PV inverters for grid connection can be of different topology and operation than off-grid ones. They have to produce excellent quality sine wave outputs with low ripples i.e., less THD, which has to match the frequency and voltage of the grid for synchronisation – and extract maximum power from the PV modules through the MPPT algorithm. The inverter input finds from I–V curve of the photo voltaic string cell until the maximum power point is achieved.
- The PV grid inverter always controls the grid & output voltage and frequency. The most effective modulation technique is the Pulse Width Modulation, which can function at frequency ranging from 2 to 20 KHz.
- Grid connected inverters are classified as Voltage Source Inverters (VSIs) and Current Source Inverters (CSIs). However, in PV applications, VSI inverters are used. The complete diagram of PV panels & VSI with grid integration is provided in Fig. 2.
Fig. 2: Grid Connected Solar PV Fed VSI…
The up gradation and fast development in power electronics technology has led to manufacturing of solar photo voltaic inverters with different modern control topologies, which not only are efficient but also synchronises the grid. Central inverters in past days are used for most PV applications. The PV modules were divided into series connections (called strings), each generating a sufficiently high voltage to avoid further amplification. Then all the strings were connected in parallel through string diodes in order to reach high power levels. The use of central inverter has many drawbacks – such as MPPT power losses, losses from differentiations between the modules and high voltage DC cable lines from the photo voltaic panels to the inverter.
- The inverter efficiency in 1988 was about 85 to 90%, in the 1990s, it increased to 90 to 92%, and currently attained 98%. The most demanding is string inverter with transformer-less one, because the transformers that operated at grid frequencies are bulky, expensive and incur losses. Furthermore, the transformers impose limitations in the control of grid current by the inverter. Especially at low load, the reactive power for the magnetisation of the transformer leads to a lower power factor. Hence, using transformer-less connection will improve the plant’s efficiency as losses are reduced.
- The Vmax-in of the string inverters kept rising from 600V up to 900V in 2009, while in the year 2010 inverters with Vmax-input = 1000V allowing even bigger strings came into the market. So, higher the Vmax-in is, the less strings of more modules are used, so the losses are further reduced as less cables are being used.
The IGBTs and MOSFETs with high frequencies give improved power quality in compliance with the regulations of the utility grid. The high frequency used has led to the use of high frequency transformers with lower weight. Since, as frequency increases the size of equipment reduces. Hence, total weight of the inverters significantly (up to 20%) is less nowadays.
The present string inverters vary from 22 to 65 kg. The lower the weight is, the easier the installation and the lower the transportation costs are. String inverters are now available in the market with power ranging from 2 to 30 kWp. Until 2008, string inverters were not produced at more than the 5kWp. 1-Ø ones are used in PV stations of even 2MWp. However, a more recent technology is the development of the 3-Ø string inverters, at almost the same power range.
Development in PV sector
In the year 2010, the first 3-Ø inverters became available in the market providing easier design and electrical connections, as well as a completely symmetric power output, an important factor for the utility
grid operators.
- The multi-string inverter is a development of the string inverter. A combination of strings are connected to separate DC/DC converters and then to a common DC/AC PV inverters. This is beneficial and advantageous in comparison to the central inverter – because each string can be controlled individually. This results in higher efficiency, flexibility and reliability of the plant.
- Central inverters are used in larger scale applications, offering Operation and Maintenance (O&M) contracts for the plant owners. The operation availability of such inverters is warranted up to 99% throughout a complete year of operation. Until 2008 the power range of the central inverters was from 100kW but not more than 500kW.
- The efficiency of central inverters has increased from 92% since the 1990s to 98.8% in the year 2010, hence providing high reliability and maximum operational life.
- The recent trend is to use Central Station Inverters (CSI), which consist of the house, the transformer, the medium voltage switchgears, the monitoring system, and the cooling, heat sinks for inverter switches to minimise losses and the wiring channels that come on the installation preassembled, hence reduces all the required tasks and connections in less time. Such inverters are used mostly in PV parks higher than 2MWp, however lower size installations are also preferred.
Conclusions
With the present accelerated efforts on the part of manufacturers, designers & utilities with adequate government support, PV systems will occupy a place in country’s power sector in the next few decades. Grid-connected solar PV systems can provide some relief towards future energy demands.
Solar PV is the technology that offers a solution to a number of issues associated with fossil fuels. It is clean decentralised, indigenous and environmentally friendly. On top of that, India has among the highest solar irradiance in the globe which makes Solar PV more attractive for India.
