Impact of Photovoltaic Tied to Electrical Grid System on Power Quality

The High Penetration of PV power into the existing electricity grid needs more study and analysis to enable safe operation. One of the major concerns is the impact of PV system on grid power quality .Poor power quality could cause disturbance and financial losses due to the use of power inverters. Harmonic distortion is a serious power quality problem that reduces power quality and consequently causes a number of problems to consumer and network. This Paper proposes a new strategy for power quality improvement in a three-phase grid connected PV Inverter with central and string configuration and controlling on them using Sinusoidal pulse width modulation and space vector modulation methods with LCL Filter .This paper investigates PV Inverter configuration and impact of them on PV Plant Performance and harmonics that produced by PV inverter. The design, modeling, and analysis of a grid-tied PV system were performed in the PSCAD software simulation environment. The simulations of Harmonic Distortion (Total Demand Distortion & Total Harmonic Distortion) were applied at Point of Coming Coupling and the results obtained were compared with the limits specified by IEEE STD 519-1992 standards.


Introduction
Due to increases in energy demand, fossil fuel problems such as fossil fuel depletion, CO 2 emissions and the use of solar energy is growing fast. This Paper Studies the effect of photovoltaic tied to Electric grid System on the power quality under weather conditions of Egypt by using realistic temperatures and solar irradiance data from The ASHRAE International Weather for Energy Calculation. Also, this Paper investigates the impact of PV Inverter Configuration (Central and String) with different controlling methods on inverter such as (Sinusoidal Pulse width Modulation and Space vector Modulation) from point of view PV plant efficiency and Electric Grid Quality. A Photovoltaic tied to grid system is modeled in PSCAD. The model is described in details as shown in Figure 1.

PV Array Model
Photovoltaic is a method of generating electrical power by converting solar Radiation Into direct current electricity using semiconductors that exhibit the photovoltaic effect [1]. There are input parameters to determine PV Component in PSCAD model. First, the weather condition Includes (solar Irradiance and Temperature). In this paper the weather data used for Cairo of Egypt measured in December from The ASHRAE International Weather for Energy Calculation. Second, the default parameters that determine the Electrical Data for PV module for central inverter as shown in table1, and subsequently, the Performance of PV-Array is shown in Figure 2. In table1 the output power for one module is650 watt and the nameplate capacity for PV Array1 ≈ 200 KW with total number of modules 300.

DC-Link Capacitor
DC link capacitor is an important component to reduce output power ripple [2]. Capacitance's equation of the DC link capacitor is Therefore DC-link capacitor set to 0.021715734 F to minimize the ripple output power [3].

DC-DC Converter (MPPT)
As a result of changing weather condition, the output power PV Array will vary. So Maximum Power Point Tracking (MPPT) is very important component to track the maximum power point (MPP) and obtain it for any changing weather condition and achieved high-possible PV system efficiency.
DC-DC Buck converter is used for (MPPT) by controlling the terminal voltage of PV array and this is obtained through two ways. First, creating a reference voltage. Second, creating switching signal to control IGBT Switching in DC-DC BUCK Converter (as shown in Figure 3) and make the terminal voltage of PV array to follow reference voltage. With regard to creating a reference voltage, there are many types of Maximum Power Point Tracking (MPPT) which responsible for creating reference Voltage such as Perturb & Observe (P&O) method, Incremental Conductance (INC), and Hill Climbing Method) as explained [4,5,6]. In This Paper Incremental Conductance is selected because the better performance of this method in regard to rapid changing weather. This method is based on the fact that the slope of power curve is zero at MPP calculating the slope of power curve, positive on the left of the MPP, and negative on the right. The instantaneous conductance (I/V) and the incremental conductance (∆I/∆V) are compared when both conductance are equal MPP is met. Where dI/dV=-I/V (at Mpp) Based on the previous three cases, the MPPT generates a reference voltage (Vmppt), the PSCAD model used for this stage is shown in Figure 4.

PV Inverter Topology and Configuration
The PV inverter consists of three pairs of power electronics switches (commonly implemented with an insulated gate bipolar transistor, or IGBT) [7]. As shown in Figure 1. The DC input to the inverter is applied to the top and bottom of the three parallel branches. The three phase AC output is produced in between the two switches in each of the three branches. [3]. PV Inverter can be categorized into four types according to configuration central, string, multi-string and Ac module [8]. This paper investigates and compare between (central and string) inverter as shown in Figure5 When all PV modules are connected to single inverter this is central. As for string inverter, there are many numbers of inverters and each one is responsible for certain number of PV modules with its own MPPT control. String inverter is the best choice when PV system subjected to shading condition because certain number of PV modules affected by shading condition In contrast Central inverter all PV modules affected by Shading and output power decrease.

Controlling IGBT Switching
Controlling IGBT switches range from simple to complex. In this paper two pulse width modulation methods are presented for controlling PV Inverter sinusoidal PWM and space vector PWM.
I. Sinusoidal pulse width modulation Generation of the desired output voltage is achieved comparing the desired reference waveform by (modulating signal) with a high-frequency triangular 'carrier' wave with magnitude ranging between -1 and 1.there are three variables that control desired reference wave form (phase shift, The magnitude and Frequency) phase shift equal to the output of real power control modeled. With additional shifting of -120 and 120 degrees. Or 120° apart from each other. The magnitude equal to output of reactive power control modeled. Real power control and reactive power control are responsible for setting dc bus at constant voltage.5 KV and unity power factor of inverter that produce sinusoidal voltage and current which are in phase. When reference waveform is greater than triangular carrier waveform, the output of comparator equals 1, otherwise, 0. So switching signals of gate 1, 3, 5 are generated and by inverting them, the switching signals will be 4, 6, 2 respectively as shown in Figure 6, to produce the desired sinusoidal output voltage with a frequency of 50 Hz.

II. Space vector modulation
The waveform of the output voltage depends on the switching states of the switches used in the inverter. Space Vector Modulation is a modulation technique that calculates duty cycles of switches to synthesize a desired output voltage on average, without the use of a carrier waveform. The Space Vector Modulation component utilizes a two-level converter shown in Figure 1.with this converter, there are eight possible states, it is described in table 2 .By specifying a reference voltage (magnitude and phase), a voltage vector can be reconstructed on the average by using the 8 possible states of the converter.
Note: the respective Voltage should be Multiplied Vdc The reconstruction is done by sampling the reference voltage at a given period Ts computing periods of time to be in certain states so that on the average, the reference voltage is attained as shown in Figure 7.  The phase angle and amplitude of reference vector are input parameter to SVM component that generate switching signals gate (1, 2, 3, 4, 5, 6) as shown in Figure9   Figure 9. Calculation of firing pulses of IGBT.

LCL Filter
A LCL filter is often used to interconnect an inverter to the utility grid in order to filter the harmonics produced by the inverter. The LCL filter achieves a higher attenuation along with cost savings, which give the overall weight and size reduction of the components. LCL filters have been used in grid connected inverters. Where L1 is the inverter side inductor, L2 is the grid-side inductor; Cf is a capacitor with a series Rf damping resistor [10]. The inverter side and Grid inductor, and capacitor with series damping resistor Values for LCL filter were chosen as 1.449852 mH. 0.0490282mH. 220.5uF, 0.154610389Ω respectively. The resonant frequency is 1556.6 Hz that must range in-between 500 Hz and 1875 Hz.

Transformer and Utility Grid Equivalent
A transformer is used in the PV system to step-up voltage to The utility grid system that is represented only as an equivalent 11 kV and 50 Hz source behind the system inductive impedance as The power rating of the transformer is designed according to the capacity of generated power from the PV array which has a peak generation of 0.2 MW. Taken the reactive power generation and absorption into account, 0.4MVA rating is selected for the transformer. The primary and secondary of any three-phase transformer can be independent connected in either a wye (Y) or a delta (d). This gives a total of four possible connections for a three-phase transformer bank: In this paper the winding connection types, delta-wye is chosen for the transformer. Delta-wye connection eliminates the third order harmonic effectively. The primary side voltage winding is designed as 380V. Therefore, the modulation index of the inverter under normal operation is 0.44387.

Total Demand Distortion and Total Harmonic Distortion Analysis
Sinusoidal terms (harmonics) whose frequencies are whole multiples of the fundamental frequency. In order to connect PV system to grid utility using inverter which is responsible for add harmonic distortion to grid that degrade power quality of it. For better performance of PV system connected to grid with good power quality there are standards that organize and regulate this problem. One standard is IEEE Std 929-2000 "IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems" which ensures compatible operation of photovoltaic (PV) systems that are connected in parallel with the electric utility [13]. It is recommended by the previous standard that the harmonic distortion at the Point of Common Coupling (PCC), which is the point at which the PV system is tied with the grid, should comply with IEEE Std 519-1992 [11]. In this PSCAD model, PCC lies between the transformer and the grid [12]. IEEE 519-1992 standard sets specific limits for allowable current and voltage harmonics in power systems, in tables 3, 4.

Tests and Results
In order to explain the power quality problems that occur in the PV grid-tied system, there are several tests of central and string inverters using sinusoidal pulse width modulation controlling method and space vector modulation controlling method. Then, the results are analyzed from different point of views related to types of inverter configuration and the effect of them on Power quality of grid and efficiency of system, and also related to two pulse width modulation control and effect of them on each inverter. The weather test conditions are listed as in table 5.the per unit system is performed into the PV grid-tied as in Table 6.   Figure 11. THD % of the Voltage at PCC of central inverter using SPWM.  Figure 13. TDD % of the current at PCC of string inverter using SPWM. Figure 14. THD % of the Voltage at PCC of string inverter using SPWM.  Figure 15. RMS-output voltage of string inverter using SPWM.

RMS Output Voltage
In Figure (10, 11, 13, and 14) are shown the TDD of the current and THD of the voltage in phase A at PCC for Central and String Inverter by using SPWM. It is observed that TDD &THD of central inverters are less than string inverter and this is rational because Central Inverter has only one Converter and String Inverter has more than one. But String inverter has good performance than central inverter as shown in Figure (12, 15) that Illustrates RMS output voltage of string inverter higher than central inverter. TDD, THD, individual current and voltage harmonic amplitude for central and string inverter are shown in tables 7, 8, 9.  Figure 17. THD % of the current at PCC of central inverter using SVM.  Figure 19. TDD % of the current at PCC of string inverter using SVM. Figure 20. THD % of the current at PCC of string inverter using SVM. Figure 21. RMS-output of string inverter using SVM.

RMS Output Voltage
Comparing between Central and string Inverter Using Space Vector Modulation proving that central Inverter less than string inverter in total harmonic distortion and total demand distortion and string inverter better performance than central under effect of changing weather condition as shown in Figure (16,19,17,20,18 and 21). TDD, THD, individual current and voltage harmonic amplitude for central and string inverter by using SVM are shown in tables10, 11, 12.

Conclusion
This paper focuses on the impact of PV system tied to grid on power quality problem. This paper investigates PV inverter configuration such as Central and string with twopulse width modulation methods (SPWM-SVM) and the effect of them on power quality from view point of Harmonic. Also, the effect of two types selected of inverter on PV system efficiency is determined. Lastly analysis of Total Demand Distortion and Total Harmonic Distortion at PCC by using FFT block and the value obtained were compared with limit specified through IEEE-std. the results proved that TDD and THD of central inverter less than string Inverter in both cases of using SPWM, SVM Controlling method and string inverter is better performance than central when subjected to shadow effect. Also, TDD and THD of Inverter using SVM less than Inverter using SPWM.