## 1 Introduction
Another factor that affects the harmonic emission level of a PV system is the system operating power, which is further determined by the solar irradiance absorbed by the PV panels. Due to the intermittent features of solar energy, the actual operating powers of PV systems can significantly vary from 10% to 100% rated power, while the performance of the inbuilt power converters degrades at low powers, resulting in lower efficiency, lower  
Figure 1: The block diagram of conventional rooftop PV systems with its voltage and current waveforms for each subsystem. power factor and increased harmonic emission. As in Figure 3, the harmonic emission level of PV-B at low powers is close to the value at rated power. Although an individual PV system has limited contribution to the overall grid supply voltage distortion, the harmonic aggregation effect of large-scale PV integration may result in nonnegligible adverse impacts on the supply voltage quality. Accordingly, suitable harmonic modelling methods have to be applied to represent the harmonic emission characteristics of PV systems operating under comprehensive conditions, which is the prerequisite for reasonably evaluating the potential harmonic impact of PV systems at high grid penetration levels. Harmonic source modelling for PV systems is generally conducted  
Figure 3: The total demand distortion (TDD) of two rooftop PV systems operating under a combination of different powers and supply conditions (WF1–3 refer to sinusoidal supply voltage, “flat-top” supply voltage and “pointed-top” supply voltage, respectively, while ZS1–2 refer to a close-to-zero source impedance and “flicker” source impedance, respectively). Figure 2: The voltage harmonic magnitudes and phase angles recorded at the PoC of a residential house. in either the time domain (e.g., [12; 13; 14; 15]) or the frequency domain (e.g., [16; 17; 18]). For example, average models are developed for PV inverters in [13; 14] to reduce the simulation time. In [12; 15], component-based models are developed for residential PV systems. The time-domain harmonic model is normally built on the equivalent circuit representation of the modelled device. However, developing an accurate time-domain harmonic model depends on a _priori_ knowledge of the circuit topology, control strategy and main parameters of the modelled PV system, which are hard to be obtained. As a result, it is mainly applied to investigate the harmonic generation and propagation mechanisms of PV systems. Regarding the frequency-domain harmonic modelling approaches (e.g., [16; 17; 18; 19; 20]), they can be further divided into three types, including the constant current source model (CCSM), the harmonic Norton model (HNM) and the harmonic admittance matrix model (HAMM), with their circuit representation illustrated in Figure 4. Specifically, CCSM is the most widely used harmonic source modelling technique, which represents the harmonic source as a series of parallel-connected individual harmonic current sources with a fixed harmonic spectrum. The supply voltage dependency of current harmonics is fully ignored here. In [16; 17], CCSM is applied to investigate the large-scale grid integration impact of PV systems on the distribution networks. Unlike CCSM, HNM represents the harmonic source as a Norton equivalent circuit, where each individual harmonic current source is connected in parallel with a harmonic admittance of the same order. With the aid of harmonic admittance, HNM takes into account the coupling between voltage and current harmonics at the same order. In [18; 19], the accuracy of applying HNM to PV systems is evaluated. To further consider the mutual coupling between voltage and current harmonics at different orders, HAMM is proposed and applied to PV systems in [19; 20]. Among all the three harmonic source modelling approaches, HAMM has the best accuracy. However, the parameter derivation of HAMM normally requires performing the individual voltage harmonic sensitivity tests.