Report on Inverter

Inverter:

Introduction of Inverter Technology:

In the grid-interconnected photovoltaic power system, the DC output power of the photovoltaic array should be converted into the AC power of the utility power system. Under this condition an inverter to convert DC power into AC power is required. Apart from the solar panels, the core technology associated with these systems is a power-conditioning unit (inverter) that converts the solar output electrically compatible with the utility grid. [78]

Most inverters in the mid 1990 have consisted of a central inverter of dc power rating above 1 kW. They connect several solar panel strings in parallel via a dc bus. However, the concept has the drawbacks of causing a complete loss of generation during inverter outage and losses due to the mismatch of strings [79]. Later, string inverters, which are designed for a system of one string of panels, were used to lessen the problems and have become popular nowadays. With further system decentralization, concept of “AC-module” was introduced. Every solar panel has a module-integrated inverter of power rating below 500 W mounted on the backside [80]–84]. This panel inverter integration allows a direct connection to the grid and provides the highest system flexibility and expandability. It also offers the possibilities to overcome problems with respect to high dc voltage level connection, safety, cable losses, and risk of dc arcs, and to achieve high-energy yield in case of system suffering from shading effect, due to the lack of mutual influence among modules’ operating points .Typical structures of the AC-module consist of several power conversion stages (Fig. 1) [84], [85].

Figure (37): Typical structures of grid-connected PV systems. (a).with voltage-fed

self-commutated inverter switching at high frequency.

(b) current-fed, grid- commutated inverter switching at the grid frequency.

The line commutated inverter uses a switching device like a commutating thyristor that can control the timing of turn-on while it cannot control the timing of turn-off by itself. Turn-off should be performed by reducing circuit current to zero with the help of supplemental circuit or source. Conversely, the self-commutated inverter is characterized in that it uses an switching device that can freely control the ON-state and the OFF-state, such as IGBT and MOSFET. The self-commutated inverter can freely control the voltage and current waveform at the AC side, and adjust the power factor and suppress the harmonic current, and is highly resistant to utility system disturbance. Due to advances in switching devices, most inverters for distributed power sources such as photovoltaic power generation now employ a self-commutated inverter. The front stage has a maximum power point (MPP) tracker for maximizing the output power of the panel, because the maximum power drawn from the panel varies with temperature and insolation. The grid-connected stage uses a full-bridge inverter toward the grid, either self-commutated with a high switching frequency [Fig. 37(a)],

or grid-commutated at the grid frequency [Fig. 37(b)]. In the former structure [Fig. 37(a)], the panel voltage is firstly boosted to the grid level together with the tracker. The dc/ac conversion stage, which is usually a pulse-width-modulated (PWM) voltage-source inverter, shapes and inverts the output current. A high-frequency filter is used to eliminate the high-frequency component at the inverter output. In the latter structure [Fig. 37(b)], the tracker, voltage boost, and output current shaping are performed in the front stage. The full bridge is switched at the grid frequency for inverting the shaped output current [85]. There are various types of inverters as shown in Fig. 2

Figure (38): classification of inverter type

The Self-commutated inverters include voltage and current types. The voltage type is a system in which the DC side is a voltage source and the voltage waveform of the constant amplitude and variable width can be obtained at the AC side. The current type is a system in which the DC side is the current source and the current waveform of the constant amplitude and variable width can be obtained at the AC side. In the case of photovoltaic power generation, the DC output of the photovoltaic array is the voltage source, thus, a voltage type inverter is employed. The voltage type inverter can be operated as both the voltage source and the current source when viewed from the AC side, only by changing the control scheme of the inverter.

When control is performed as the voltage source (the voltage control scheme), the voltage value to be output is applied as a reference value, and control is performed to obtain the voltage wave form corresponding to the reference value. PWM control is used for waveform control. This system determines switching timing by comparing the waveform of the sinusoidal wave to be output with the triangular waveform of the high-frequency wave, leading to a pulse row of constant amplitude and a different width. In this system, a waveform having less lower-order harmonic components can be obtained. On the other hand, when control is performed as the current source (the current control scheme), the instantaneous waveform of the current to be output is applied as the reference value. The switching device is turned on/turned off to change the output voltage so that the actual output current agrees with the current reference value within certain tolerance. Although the output voltage waveforms of the voltage control scheme and the current control scheme look substantially same, their characteristics are different because the object to be controlled is different. Table 1.2 shows the difference between the voltage control scheme and the current control scheme. In a case of the isolated power source without any grid interconnection, voltage control scheme should be provided. However, both voltage-control and current-control schemes can be used for the grid interconnection inverter. The current-controlled scheme inverter is extensively used for the inverter of a grid interconnection photovoltaic power system because a high power factor can be obtained by a simple control circuit, and transient current suppression is possible when any disturbances such as voltage changes occur in the utility power system. Fig. 1.2 shows the configuration example of the control circuit of the voltage-type current-control scheme inverter.

Table: Difference between the voltage control scheme and the current control scheme inverter

Figure (39): configuration example of the control circuit of the voltage-type current-control

Scheme inverter