1. Introduction

Among numerous renewable energy sources, solar energy is considered as one of the most promising resources for large-scale electricity production [1]. In several countries including Australia, an increasing number of photovoltaic (PV) generation systems are connected to the distribution network as a result of strong government support. The PV market is growing rapidly (30–40%), and its price is constantly decreasing. Many countries are trying to increase the penetration of renewable energy.

The power electronics interface is essential for connecting renewable energy sources to the grid. This interface has two main functions such as extracting the maximum amount of power from the PV modules [2, 3] and conversion of direct current (DC) power to an appropriate

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

form of alternative current (AC) power for the grid connection. Renewable energy sources such as solar energy cannot be manipulated in the same way as conventional power sources, so the operating conditions of PV inverters vary according to the solar insolation [4]. However, utility standards and manufacturers' data sheets are only concerned with the full-load condition.

eliminated. However, the electrolytic parts have far more limited life than the applications [14]

Harmonic Distortion Caused by Single-Phase Grid-Connected PV Inverter

http://dx.doi.org/10.5772/intechopen.73030

53

A single-stage inverter is shown in Figure 1(a); an efficient maximum power point tracking (MPPT) process is realized by a large power decoupling capacitor. Hence, modeling the inverter can use adaptable constant DC-link voltage assumption in this linear model. However, as twostage inverter is shown in Figure 1(b), the power decoupling capacitor is placed at the highvoltage DC link. In this topology, a larger voltage ripple is allowed to present across a DC link in order to minimize the decoupling capacitor [15], hence the constant DC-link voltage assumption

The three-phase bridge converter for harmonic transfer is investigated in [16], the voltage second harmonic on a DC link producing a third harmonic on the AC side can be found. However, the DC-link voltage also causes output current frequency spectrum for the fifth, seventh, and a series of odd harmonics [17]. The explanation of this phenomenon cannot be found in the previous research. Many methods have been proposed to eliminate the current harmonics caused by the DC-link ripple without analyzing the harmonics generation process. A specifically designed pulse-width modulation (PWM) control algorithm [18] is proposed to compensate the DC-link voltage ripple. In [19], a control technique, which allows for 25% ripple voltage without distorting the output current waveform, has been proposed. The cutoff frequency of this design is 10 Hz, which could attenuate the voltage ripple in the control loop, but dynamic performance is decreased in this system. The main purpose of all these works is to eliminate the effects of the DC-link voltage ripple. However, an understanding of the relationship and the analytical model for qualitative information between the output current har-

In this chapter, for harmonic analysis studies, a new model of the single-phase full-bridge PV inverter is proposed by regarding its loading level and the ripple of the DC-side voltage. It is obtained by adding representation of the DC-link voltage ripple into the conventional linear model of a grid-connected PV inverter. Thus, it becomes a periodical time-varying model.

This chapter is organized as follows: a general model with harmonic information is introduced in Section 2. In Section 3, the double-line frequency voltage ripple on the DC link is identified as the cause of a series of odd harmonics. A time-varying model is proposed to analyze this phenomenon. Section 4 gives simulation and experimental results, which verify the validity of

the proposed model and solution. Conclusions are given in Section 5.

Figure 1. Block diagram of (a) single-stage inverter and (b) two-stage inverter.

which need to be avoided.

monics and DC-link voltage ripple is still missing.

is not valid.

PV systems incorporate power electronic interfaces, which generate a level of harmonics [5], potentially causing current and voltage distortions. The summations of various higher frequency sinusoidal components are the harmonics of current or voltage waveforms, which are an integer multiple of the fundamental frequency. These harmonics have a great influence on the operational efficiency and reliability of the power system, loads, and protective relaying [6]. Due to the rapid growth of PV installations, attention to harmonic distortion introduced by PV inverters to the grid is on the rise.

The degree of current total harmonic distortion (THD), as a ratio of the fundamental current and the real power output of the inverter, vary significantly [7]. At a low power output level, the current THD becomes higher, especially for generated power below 20% of the rated power, such as in the morning or evening. Many researchers have reported this phenomenon and tried to find out the causes. In the control system, the quantization and resolution effects of the measurement devices have been pointed out as one of the causes [8]. Another explanation is that the closed-loop current controls, which are intended to minimize the harmonic components, stop working at a low power output level [7]. Some researchers have suggested that the DC-link voltage regulation is highly related to the reference current resolution [9]. However, the comprehensive and systematic analysis of the generation process of the harmonics in the PV inverter output current is missing.

The conventional model of current control structure [10] is widely used to design the control loop and to analyze its stability. However, this model dose not including harmonic information, and the model cannot reflect the influence of the control schemes on the resulted harmonics. Section 2 introduces a general model modified from a conventional control structure diagram to analyze the harmonic generation process. The "harmonic impedance" concept [10] is used to quantitatively calculate the harmonic amplitude caused by each source. This is important because of the growing concern of harmonics generated by these devices and their effect upon other equipment.

A series of fund odd harmonics cannot be completely explained by the factors usually examined in such cases. These harmonics are caused by the DC-link voltage ripple, and a timevarying model is proposed to analyze this phenomenon in Section 4.

In order to analyze and design the PV inverter, the DC-link voltage is assumed as constant in the traditional model of a PV inverter. However, this is not always the case. The AC instantaneous output power exhibits a pulsation at the double-line frequency for single-phase gridconnected inverters. Under stable insolation conditions, the DC output voltage of the PV modules is controlled as constant at the maximum power point (MPP). Therefore, the power pulsation caused by single-phase power generation is converted into the static stored energy on the decoupling capacitor, and the double-line frequency of voltage ripple can be found at the DC link [11–13]. By using large electrolytic capacitors, the ripple can be reduced but not eliminated. However, the electrolytic parts have far more limited life than the applications [14] which need to be avoided.

form of alternative current (AC) power for the grid connection. Renewable energy sources such as solar energy cannot be manipulated in the same way as conventional power sources, so the operating conditions of PV inverters vary according to the solar insolation [4]. However, utility standards and manufacturers' data sheets are only concerned with the full-load condition.

52 Power System Harmonics - Analysis, Effects and Mitigation Solutions for Power Quality Improvement

PV systems incorporate power electronic interfaces, which generate a level of harmonics [5], potentially causing current and voltage distortions. The summations of various higher frequency sinusoidal components are the harmonics of current or voltage waveforms, which are an integer multiple of the fundamental frequency. These harmonics have a great influence on the operational efficiency and reliability of the power system, loads, and protective relaying [6]. Due to the rapid growth of PV installations, attention to harmonic distortion introduced by

The degree of current total harmonic distortion (THD), as a ratio of the fundamental current and the real power output of the inverter, vary significantly [7]. At a low power output level, the current THD becomes higher, especially for generated power below 20% of the rated power, such as in the morning or evening. Many researchers have reported this phenomenon and tried to find out the causes. In the control system, the quantization and resolution effects of the measurement devices have been pointed out as one of the causes [8]. Another explanation is that the closed-loop current controls, which are intended to minimize the harmonic components, stop working at a low power output level [7]. Some researchers have suggested that the DC-link voltage regulation is highly related to the reference current resolution [9]. However, the comprehensive and systematic analysis of the generation process of the harmonics in the

The conventional model of current control structure [10] is widely used to design the control loop and to analyze its stability. However, this model dose not including harmonic information, and the model cannot reflect the influence of the control schemes on the resulted harmonics. Section 2 introduces a general model modified from a conventional control structure diagram to analyze the harmonic generation process. The "harmonic impedance" concept [10] is used to quantitatively calculate the harmonic amplitude caused by each source. This is important because of the growing concern of harmonics generated by these devices and their

A series of fund odd harmonics cannot be completely explained by the factors usually examined in such cases. These harmonics are caused by the DC-link voltage ripple, and a time-

In order to analyze and design the PV inverter, the DC-link voltage is assumed as constant in the traditional model of a PV inverter. However, this is not always the case. The AC instantaneous output power exhibits a pulsation at the double-line frequency for single-phase gridconnected inverters. Under stable insolation conditions, the DC output voltage of the PV modules is controlled as constant at the maximum power point (MPP). Therefore, the power pulsation caused by single-phase power generation is converted into the static stored energy on the decoupling capacitor, and the double-line frequency of voltage ripple can be found at the DC link [11–13]. By using large electrolytic capacitors, the ripple can be reduced but not

varying model is proposed to analyze this phenomenon in Section 4.

PV inverters to the grid is on the rise.

PV inverter output current is missing.

effect upon other equipment.

A single-stage inverter is shown in Figure 1(a); an efficient maximum power point tracking (MPPT) process is realized by a large power decoupling capacitor. Hence, modeling the inverter can use adaptable constant DC-link voltage assumption in this linear model. However, as twostage inverter is shown in Figure 1(b), the power decoupling capacitor is placed at the highvoltage DC link. In this topology, a larger voltage ripple is allowed to present across a DC link in order to minimize the decoupling capacitor [15], hence the constant DC-link voltage assumption is not valid.

The three-phase bridge converter for harmonic transfer is investigated in [16], the voltage second harmonic on a DC link producing a third harmonic on the AC side can be found. However, the DC-link voltage also causes output current frequency spectrum for the fifth, seventh, and a series of odd harmonics [17]. The explanation of this phenomenon cannot be found in the previous research. Many methods have been proposed to eliminate the current harmonics caused by the DC-link ripple without analyzing the harmonics generation process. A specifically designed pulse-width modulation (PWM) control algorithm [18] is proposed to compensate the DC-link voltage ripple. In [19], a control technique, which allows for 25% ripple voltage without distorting the output current waveform, has been proposed. The cutoff frequency of this design is 10 Hz, which could attenuate the voltage ripple in the control loop, but dynamic performance is decreased in this system. The main purpose of all these works is to eliminate the effects of the DC-link voltage ripple. However, an understanding of the relationship and the analytical model for qualitative information between the output current harmonics and DC-link voltage ripple is still missing.

In this chapter, for harmonic analysis studies, a new model of the single-phase full-bridge PV inverter is proposed by regarding its loading level and the ripple of the DC-side voltage. It is obtained by adding representation of the DC-link voltage ripple into the conventional linear model of a grid-connected PV inverter. Thus, it becomes a periodical time-varying model.

This chapter is organized as follows: a general model with harmonic information is introduced in Section 2. In Section 3, the double-line frequency voltage ripple on the DC link is identified as the cause of a series of odd harmonics. A time-varying model is proposed to analyze this phenomenon. Section 4 gives simulation and experimental results, which verify the validity of the proposed model and solution. Conclusions are given in Section 5.

Figure 1. Block diagram of (a) single-stage inverter and (b) two-stage inverter.
