oa Transient PV System Models for Power Quality Studies

In recent years, increasing concerns about climate change and the liberalisation of the energy market have provided the necessary impetus for a revolutionary restructuring of the electricity network. Traditional power networks are designed to operate in a passive and unidirectional way as their main functionality encompasses the transfer of energy from the power stations to the customer, with minimum loss. Increased electricity production from renewable energy sources (RES) coupled with energy efficiency lie at the heart of the ambitious targets set by Europe and globally in the quest to curb greenhouse gas emissions and to reach energy sustainability. As a result, complete restructuring of the electricity networks will have to take place in order to accommodate increased penetration of RES and distributed generation (DG) which is associated with electricity production from RES. Especially in regions with high solar irradiance, the penetration of photovoltaic (PV) systems is expected to increase in the near future as the technology becomes more competitive. High penetration of PV systems will definitely have serious consequences on the operation of the electricity grid and further challenges will arise as penetration levels increase. Thereafter the security and stability of the power system should be considered carefully to identify possible impacts due to uncontrolled deployment. The occurrence of power quality problems is not only negatively affecting utility customers but is also affecting the generated energy from Photovoltaic (PV) systems and the stability of the power system. The severity and frequency of occurrence of power quality issues can be the result of distribution grid topology/dynamics, arising due to high PV system concentration or even abnormal PV system operation. Analytical tools and accurate models of PV systems must be developed in order to evaluate their behaviour in the context of the full network. The utilization of accurate simulation models is of great importance in an attempt to assess the real consequences of localized energy production from distributed energy sources and in particular PV.

A common detailed PV system model is formed by a PV array, an inverter and a power grid interface as shown in Fig. 1.

Figure 1: General schematic of a Detailed Photovoltaic System.

The PV array is affected by the solar irradiance, the temperature and the specific characteristics of the chosen PV module technology. The PV array converts the solar irradiance into DC power which is then delivered to the distribution grid via the PV inverter. A Maximum Power Point Tracking (MPPT) Controller is used to absorb the maximum available energy. The MPPT Controller varies the duty cycle of a DC/DC converter to adjust the voltage at the output of the PV array (DC link A). The DC/DC converter is connected to a DC link (DC link B) of which the voltage is maintained constant by a DC/AC inverter circuit topology. In more detail, the DC/AC inverter is set to inject the power reaching the DC link B into the electricity grid and in that way the DC link B remains constant. A filter is always placed at the output of the inverter to eliminate undesirable harmonic currents produced by the switching operation of the inverter [1].

A generic PV system model for transient studies, the parameters of which can be tuned using transient data is developed in this work. The adopted analysis utilizes existing knowledge to formulate an accurate transient representation that considers the PV system control circuit and dynamics [2], [3]. The model is tuned and validated using transient data obtained from a detailed PV system circuit topology developed in Matlab Simulink via the Nelder-Mead simplex algorithm [4]. Its abstract representation is shown in Fig. 2. The proposed model is actually a three phase representation capable of simulating with sufficient accuracy normal/unbalanced operating conditions and voltage regulation. Harmonics are also incorporated into the model to reveal its capability for use in complete power quality studies.

Figure 2: Proposed Transient Photovoltaic System Model (TPVSM).

This PV system model is able to characterize the transient behaviour of PV systems in a generic way via a parameter estimation process. In addition, it enables the analysis of more aspects of power quality and voltage stability with higher accuracy under balanced and unbalanced conditions. It must also be pointed out that the proposed model is simpler and faster, thus allowing the computationally efficient simulation of complex problems.

Figure 3: Output current response to a step change in active power input.

The transient response of both the detailed and the proposed PV system model during a step change in active power input is shown in Fig. 3 (the reactive power input is kept at a zero value). In the next step, the developed model is used to assess the voltage transient response of a distribution grid busbar (point of common coupling of the PV system with the electricity grid) and the results are shown in Fig. 4.

The comparison is made with the “Theil inequality coefficient”. The specific inequality coefficient provides a measure of how well a time series of observed values compares to a corresponding time series of estimated values. A value of 0 indicates zero difference or perfect predictions, whereas a value of 1 indicates poor model performance. Values lower than 0.3 depict good agreement between estimated and observed data. As can be seen from the results and inequality coefficient in Figs. 3 and 4, good agreement has been obtained between the detailed and the proposed PV system model. It is important to stress that the proposed generic model can been tuned by using experimental data as well.

Figure 4: Transient reponse of voltage during a step change in reactive power reference.

In summary, the proposed generic model is in line with current DG standards as it can be used for studies of voltage regulation/power quality. The aim of the aforementioned research is to enhance the effort of assessing the consequences of high PV penetration and facilitate corrective action with appropriate technical solutions so as to enable the safe and unrestricted deployment of these technologies in electricity grids [5].

References

[1] S.-K. Kim, J.-H. Jeon, C.-H. Cho, E.-S. Kim, and J.-B. Ahn, “Modeling and simulation of a grid-connected PV generation system for electromagnetic transient analysis,” Sol. Energy, vol. 83, no. 5, pp. 664–678, May 2009.

[2] M. Liang and T. Q. Zheng, “Synchronous PI control for three-phase grid-connected photovoltaic inverter,” in 2010 Chinese Control and Decision Conference, CCDC 2010, 2010, no. 2, pp. 2302–2307.

[3] Z. Xueguang, X. Dianguo, and L. Weiwei, “A novel PLL design method applied to grid fault condition,” in Conference Proceedings-IEEE Applied Power Electronics Conference and Exposition-APEC, 2008, pp. 2016–2020.

[4] W. Bao, X. Zhang, and L. Zhao, “Parameter estimation method based on parameter function surface,” Sci. China Technol. Sci., vol. 56, no. 6, pp. 1485–1498, 2013.

[5] M. Patsalides, A. Stavrou, V. Efthymiou, and G. E. Georghiou, “A Generic Transient PV System Model for use in Power Quality Studies,” Renew. Energy, 2015, Accepted.