PV Inverters-Types and Selection

PV inverter are one of the most important component of PV power generation system. PV inverters can be categorized in to different types. In generally, PV modules can be categorized into the following types.

1. String Inverters

The most commonly used type of inverters are string inverters. In a string inverter PV module are arranged in series also called string. Then these strings are connected to DC input side of inverter. Now, market available string inverters comes with multiple MPPT. Strings inverters from different manufacturers and capacity have different number of string inputs and similarly different number of MPPTs. Normally power capacity range of string inverters is 5kw-150kw. The system diagram with string inverter looks like as shown below.

2. Central Inverters

Central inverters are similar to string inverters but they are much larger and can support more strings of panels. Instead of strings running directly to the inverter, as with string inverter, the strings are first connected together in a common DC combiner box that runs the DC power to the central inverter where it is converted to AC power. Central inverters require fewer component connections, but require a combiner box. They are best suited for large installations with consistent production across the array. Their power ratings are higher staring from few hundred kws to tens MWs. The system diagram with central inverter is given below.

3. Microinverters

Microinverters are installed underneath each panel. Each of these micro-inverters is about the size of an internet router. The number of inverters in a system is equal to number of panels in this case.The main advantage of is that in case of shading the losses are minimal and they can produce more power and have higher yield.

4. Hybrid Inverters

Hybrid inverters are normally used for offgrid applications. Hybrid inverters are combination of string inverter and small battery inverter(pcs). They are very good choice for offgrid application where the energy can be stored in batteries during daytime and then released to supply power to load at night time. The system diagram and working principle is as shown below.

5. Battery Inverter (PCS)

Battery inverter as name suggests are used with battery energy storage system (BESS). They are also called bidirectional inverters as they can convert AC power to DC power and store in batteries and similarly DC power can be converted to AC power to supply power to load or grid. PCS normally have capability to work in four quadrants.

Difference & Selection between String and Central Inverters.

Central inverter “centralize” the plant's power into one place. Central inverter units are physically much larger than string inverters, use longer wires and can convert more power per unit. String inverters are of distributed architecture for solar plants. They are small, and each unit converts a much smaller amount of power compared to a central inverter. In string inverters, if one string fails or affected by shading the whole array's energy is not lost, just the power from that string. In contrast with a central inverter, where more strings are affected if there is problem with one string.

Central inverters are less expensive(cost/watt) than string overall for large utility-scale installations because fewer number of inverters are required per site. But for smaller utility-scale projects, string inverters could win out because of their higher yield and easy servicibility.

Cost/watt and energy yield calculations

PV system based on string inverters have higher cost per watt compared with central inverter which has less cost per watt. On the other hand, PV system based on string inverters have higher energy optimization. So, during the design of the project one must take care of all aspects to choose a particular type of inverter.

Manufacturers:

According to wood mackenzie power and renewables report Huawei ranks first in market share in 2018. Other top suppliers include Sungrow, SMA, Power electronics, ABB, Sineng, Goodwe and others as shown in the chart below.

Price:

It is very difficult to give exact price per watt for PV inverters. However, we can use this data from NREL report to estimate the cost of different inverters. The price of PV inverters are also decreasing on yearly basis as of PV modules.

Yield:

The yield of PV system depends upon site condition, type of PV modules, PV inverters and other parameters. If we compare different type solutions, the microinverter solution has higher yield but it is also the most expensive solution.

Microinverters > SolarEdge-Optimizer solution > String Inverters > Central inverters

Where to buy:

Most of all the above companies have their footprint in each country either directly or through their distribution partners. It is suggested to buy the inverters directly from the parent company because it can help in future maintenance/services.

Some interesting facts:

1. Inverters with Optimizers: Solar edge provides inverters which are specifically designed to work with power optimizers. In this kind of solution each pair of PV modules uses one optimizer and then connects with inverter. This system results in higher optimization and hence higher yield upto 25%. However, it is also expensive method.

2.  DC/AC ratio:  It is also known as inverter load ratio is an important parameter when designing a solar project. For example, a 6-kW DC array combined with a 5-kW AC rated inverter would have a DC/AC ratio of 1.2 (6 kW / 5 kW = 1.2). The key driver here is the “clipping loss”: when the DC power feeding an inverter is more than the inverter can handle, the resulting power is “clipped” and lost.

3. DC/AC ratio should be chosen based on the design calculation. Keeping in mind both clipping losses and economical factor. If we choose higher DC/AC ratio it means we will be using smaller number of inverters and hence it is an economical solution.

4. 1500V/1000V inverters: Nowadays all manufacturers have started shifting to 1500V inverters. The main advantage is overall lower cost of project with high voltage inverters. While the current per-watt cost of 1500 V components is more expensive than 1000 V components, overall costs are lower. 1500 V systems allow for 50% longer strings and larger inverters, with an output 10 to 40% higher. These increases allow solar arrays to deliver a higher yield while reducing costs; fewer inverters and shorter wire runs save money, while higher voltage improves the DC yield.