Lab Modelling of Study System

Figure 5-1 depicts the single-line diagram of the lab setup for the study system. The short circuit impedance of the network and line impedance are represented by Rg and Lg. A 10kVA interface transformer is used for interconnection of the PV system to the PCC. The transformer configuration is Delta/Wye whereas the Delta winding connection is located on the inverter side. A 10 kVA IGBT-based full bridge is utilized to operate as a voltage source inverter. To remove the ripples due to the switching frequency, an LCL filter is installed after the inverter. The filter inductor is in series with the inverter and the filter capacitor is in shunt with Delta connected transformer winding. A 10 kVA PV Simulator is employed to simulate the behavior of the PV solar panels. The PV Simulator can generate variable real power based on the pre-settable temperature and irradiance profiles. Three types of loads, resistive, inductive and capacitive, having a total 10 kVA rating are used in the lab setup. Voltage and current sensors are designed to measure PCC voltage, inverter current, load current and DC link voltage. The sensor signals are delivered to dSPACE controller board through ADC channels. The smart PV inverter controller is implemented on dSPACE controller board, which generates appropriate firing pulses for the IGBT gates of the inverter based on control objectives and operation modes.

Figure 5-2 depicts the actual lab setup in Power Systems Laboratory of Western University. However, the inductive load and the interface transformer are not shown in this picture.

Power Supply iLoad Sensor V-I Load SW 1 SW 2 Voltage Sensor Cf Interface Transformer R-L-C Loads LC Filter Inverter 10 kVA 208V/208V ∆:Y PCC SSR Switch Lf SW 3 PV Simulator iinverter Gate Pulses dSPACE Rg Lg PCC Current Sensor Load Switch

5.2.1 Overview of PV Simulator

The PV solar simulator is an integrated specialized computer system with a PV simulation engine. The Elgar TerraSAS PV solar simulator from AMETEK Programmable Power Inc., consists of a rack mounted controller, control system software, keyboard and GUI interface with a unique PV simulation engine that controls the output DC power supply. It has the capability to simulate different solar array V-I characteristics along with the flexibility to simulate different series/parallel combination of modules needed to meet the

operating voltage and power levels for grid tied photovoltaic inverters [153, 154]. The power rating of the PV Simulator is 10 kW. Appendix C. 1 shows the 10 kW PV Simulator. Figure 5-3 (a) and (b) show the characteristics of the PV Simulator. Figure 5-3 (a) shows the irradiance change with a fast ramp function for an ambient temperature of 25ºC. The maximum irradiance is chosen to be 1000 W/m2_{. Also, Figure 5-3 (b) illustrates the current-}

voltage curve of the solar panels. The open-source voltage and short circuit current of the solar panels are 440 V and 17 A, respectively. The V-I curve of the solar panel also changes as a function of irradiance.

(a) (b)

Figure 5-3: Specified profiles of solar panel in PV Simulator a) Temperature and irradiance profiles

b) Current- voltage and power-voltage curves

5.2.2 Voltage and Current Sensors

The voltage and current sensors observe the value of PCC voltage, inverter current, load current and DC link voltage, instantaneously. A current transducer with capability of 50 A (rms) is chosen as the current sensor. The frequency bandwidth and response time of the sensor are the two vital factors that effect on the controller performance. Hence, a specific current transducer is used with 200 kHz bandwidth and 1 µs response time. A voltage transducer with maximum capability 500 V and 40 µs response time is used for sensing

PCC voltage and DC link voltage. The output of the both current and voltage sensor boards are designed to be ±10V to be compatible with the voltage level of analog-to-digital (ADC) channels of the dSPACE.

5.2.3 Inverter

A two-level three phase IGBT power module supplied by Powerex with a rating 600 V and 100 A is chosen as the PV system inverter. The pulses for the IGBT gates are provided by dSPACE controller based on the control objectives. It should be noted that the voltage level of the PWM unit of the dSPACE controller is ±5V whereas the gate driver operates with ±15V. Hence, an interface voltage level shifter is designed to provide suitable voltage for gate driver of the IGBT. Similar to simulation studies and HIL study, the switching frequency is chosen 10 kHz for the actual inverter. Since, the IGBT switches are not ideal switches, the turn-off time and turn-on time of the switch are not equal. Hence, it is recommended to add dead-band time to avoid bridge shoot through. The dead-band time ensures that one IGBT switch will be off before another one turns on. Typically, the dead- band time is calculated based on time difference of turn-on time and turn-off time [155]. The actual inverter turn-on time is 100 ns and turn-off time is 300 ns [156, 157]. Two times of the difference time ensures the safety margin for the IGBT switches. Therefore, the dead-band time is chosen 400ns for the PWM signals. Appendix C. 2 presents a description of the three-phase IGBT module used in this research.

5.2.4 Harmonics Filter

To remove the switching harmonics from the waveforms, an LCL filter is used between inverter and PCC. A three-phase inductor with 1.2 mH inductance and 30 A rating is connected in series with the inverter. The turn-on resistance of the IGBT switch is considered as the series filter resistor. The filter capacitor is connected in shunt with Delta configuration of the transformer. The leakage inductance of the interface transformer is considered as the second inductor for the filter unit.

5.2.5 Interface Transformer

A 10 kVA transformer with Delta/Wye-grounded configuration is connected between the PCC and entire PV solar system. The main purpose of the transformer is to provide isolation between the PV system and grid. The leakage impedance of the transformer is 0.05 pu, as given by the manufacturer.

5.2.6 PCC Switch

The PCC connection switch is a three-phase solid state relay (SSR) with capability of 50 A [158]. The PCC switch turns to “Connected” state via the user command from the designed GUI in dSPACE software. When the PLL controller detects the phase angle of the PCC voltage properly, the GUI software will indicate that the PV system is able to connect to the grid. The SSR switch controls with a command signal having 0-15 V DC voltage level. As the output of the digital-to-analog (DAC) channels of the dSPACE are up to 10 V, an interface level shifter with capability of high current injection is designed with an optocoupler IC [159]. Optocoupler IC isolates the dSPACE controller from the SSR switch and it reduces the risk of controller damage due to high current injection.

5.2.7 Passive Load

Similar to previous studies, passive loads are considered in the lab implementation study. Three 10 kVA resistive, inductive and capacitive loads are connected to the PCC in parallel. The resistive and inductive load are connected with Wye-grounded configuration whereas the capacitive load is with Delta configuration.