Inverters for Grid-connected Systems Basic structures of grid-connected PV systems

The basic structure of a typical grid connected PV system is shown in Figure 21.16. It consists of a PV generator directly coupled to the input side of an inverter without any means of storage. The output side of the inverter is coupled to the grid via safety devices, which typically are integrated into the inverter. A meter measures the energy fed into the public grid.

Based on this fundamental structure, three typical layouts of grid connected systems have emerged:

1. Central inverter/master-slave systems. This topology was used in the first grid – connected applications. A number of PV modules are connected in series building a string to achieve the input voltage required by the inverter. To attain the planned power, several of such strings are switched in parallel in a generator junction box. The collected DC power is then fed into a central inverter, converted to AC power and injected into the grid. As shown in Figure 21.17, the DC power can also be distributed to a number (typically two or three) of smaller inverters switched in parallel. In this so called master-slave configuration, the inverters are switched on and off according to the instantaneous DC power. Thereby, even at low input power, the inverters are operated in their optimum efficiency range which increases the overall efficiency of the system.

Today, central inverters with or without master-slave configuration are mostly used in high-power systems in the several hundred kilowatt or megawatt range.

2. String concept. In this concept, each string has its own dedicated small inverter in the power range of several hundred watts up to several kilowatts. Typically, a grid-connected rooftop

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pv generator inverter safety device meter

Figure 21.16 Fundamental structure of a grid connected PV system

system on a single family house consists of one to three strings each with its own assigned inverter, thus, in principal forming independent subsystems as shown in Figure 21.18.

This broadly used system layout offers many advantages:

• generator junction boxes are avoided;

• the strings can be of different length and orientation which is especially advantageous for building integrated (BIPV) systems;

• each of the strings has its own maximum power point tracker (MPPT), leading to reduced losses in case of partial shading or different orientation of the strings;

• high production quantities of small inverters reduce the production costs;

• handling and maintenance is much easier compared to bulky central inverters.

These numerous advantages lead to the use of this system layout also in larger PV plants where, for example, in a 100 kW system, 20 inverters, each of 5kW, work independently.

The layout described is often modified in such a way that two or three strings can be connected to one inverter unit. Inside the inverter, those strings are either simply switched in parallel, saving the costs for an external generator junction box at the expense of the major advantages of the original string technology. Or, each input has a dedicated DC/DC converter feeding an intermediate DC link, thus enabling individual MPP-tracking of each string. Typically, there are two or three inputs in those so called multi-string inverters.

3. Module integrated power conditioning units. From the very beginning of the use of terrestrial PV, there was the ambition to combine the power conditioning unit directly with the PV module as shown in Figure 21.19. Thus, each module has its own dedicated MPP-tracker reducing losses caused by deviant module parameters, different module temperatures or partial shading of the generator. Furthermore, it is claimed that it would be advantageous to have an AC cabling instead of DC cabling. A clear advantage of a PV plant based on AC-modules is the very simple system planning and later extension. Another approach to integrate power electronics is to build only a matching DC/DC converter into the junction box, thus enabling individual MPP-tracking. The DC energy is then collected on a (high voltage) DC bus and fed into a central inverter.

Besides some obvious advantages of the module integrated approach, there are some drawbacks.

The efficiency of a small inverter in the several hundred watt range will always be lower than that of a higher power unit. One of the main reasons is the self consumption needed to power the control systems, relays, monitoring and communication systems of the inverter, as well as, e. g. hysteresis losses in inductors and transformers. The self-consumption of a leading-edge inverter in the 5 kW range is less than 5 W, which means less than 0.1% of the nominal power.

Figure 21.18 Grid-connected PV system based on string connected inverters

Figure 21.19 System based on module integrated inverters (AC-modules)

As the self consumption for a low power inverter will not decrease linearly, this leads to a poor efficiency especially under part load conditions. In an unshaded PV plant, a noticeable reduction of the annual yield can be expected compared with conventional string or central inverter concepts. In building-integrated PV systems, where regular recurring partial shading cannot be avoided, module integrated power conditioning may lead to an increase in annual yield.

Further sticking points are reliability and lifetime of such module integrated power con­ditioning systems. They are exposed to harsh environmental conditions such as high temperature levels and variations, humidity and electromagnetic fields in the case of lightning. At the same time, they have to have a lifetime comparable to that of the PV module, e. g. 25 years. To achieve this goal, design and manufacturing quality must be on an industry or military level at the costs of commercial products.

If these fundamental problems are overcome, building-integrated photovoltaics (BIPV) would be the first and rapidly growing market for module integrated power conditioning systems.

Updated: August 22, 2015 — 6:04 am