Combined photovoltaic panel simulator with impedance adjustment

L. Iliev1, N. Panteleev2, N. Mihailov3, D. Todorov3

1. University of Ruse, 8 Studentska Str, 7017 Ruse, Bulgaria.

e-mail: lailiev@uni-ruse.bg

2. ICCS, EMC Testing Laboratory, Bulgarian Institute of Metrology

3. University of Ruse, 8 Studentska Str, 7017 Ruse, Bulgaria

Abstract - The article describes the design steps of a photovoltaic (PV) panel simulator device intended for the designers and manufacturers of PV inverters, to serve as a pre-compliance tool and simulator of a real PV panel. The simulator is divided into 3 parts - Maximum Power Point (MPP) simulator, impedance simulator and a 50 MHz spectrum analyzer. MPP simulator adjusts the output DC voltage and current according to the sun radiation, shading, temperature and other parameters. The impedance simulator is a circuit which impedance can be adjusted to match the one of a real PV panel. It acts as a variable impedance DC LISN for conducted interference measurements.

Keywords – photovoltaic, EMI, simulation, high frequency.


1. Introduction

The number of photovoltaic panels installed worldwide is growing, but there are no official standards so far specifically for DC measurements of photovoltaic (PV) panels connected to an inverter. A number of PV DC LISNs are used for high frequency measurements, but they have fixed impedance of 150 ? for common mode interferences and 100 ? for differential mode in the defined frequency range from 150 kHz to 30 MHz [1]. Such PV DC LISNs are available on the market for different voltages and currents [2][3]. The PV panels have different values of their electrical parameters depending on the model, voltage, current, solar radiation, HF disturbances, temperature, humidity, aging and other factors. The values may vary in large limits and experience fast changes. Every inverter must be able to function correctly in the vast changing parameters and also it should not exceed the electromagnetic interference levels [4]. The current article proposes a device, which combines an adjustable impedance DC LISN, unlike the already available DC LISNs, which have fixed impedance, and voltage and current adjustable MPP source.

2. Design

The simulator is divided into 3 parts - Maximum Power Point (MPP) simulator, impedance simulator and a 50 MHz spectrum analyzer. The system separated into 3 PCBs - Power PCB, Control PCB and Spectrum analyzer PCB which have separated grounds and are isolated by optoisoltors. The block-diagram is presented on Fig. 1. Voltages up to 46V and currents up to 9.5A can be simulated by the current device, which covers most of the available PV panels today. Impedance simulator consists of capacitors and inductors which can be added or removed from the circuit, by switching relays, so that real electrical parameters can be set by the user. The device can be connected to a PC by USB port and all the adjustments and data reading is done by the PC using a special software. There is a possibility to connect an external spectrum analyzer for measurements. When all impedance adjustment components and the spectrum analyzer are switched OFF, the device can be used only as a MPP simulator with default parameters [5].

Fig. 1. Block-diagram of the system

The power input of the simulator is chosen to be a torodial transformer with 230V input and 50V/10A AC output. A more compact switching power supply can be used, but the disturbances are orders of magnitude higher than the ones of the transformer. An input multi-stage EMI Filter is used in order to lower the disturbances from the power line. The filter is directly connected to the metal housing so that no disturbances are flowing into the simulator and the output is connected to the transformer. The 50V AC output is then rectified and fed to the power management circuits. The protection circuit includes surge and overcurrent protection, reverse input protection.

2.1. MPP Simulation

The MPP simulation is designed using voltage source and current source circuits, which are separately controlled using a microcontroller. The outputs of the two sources is mixed by a dual high voltage schottky rectifier diode. An important feature using this diode is that it outputs the input that has a higher voltage. In that way voltage an current sources can be switched constantly in order to maintain the MPP voltage and current characteristic of a real PV panel. The voltage controlled voltage source (VCVS) is implemented using a positive adjustable voltage regulator, which is drives 3 NPN bipolar power transistors (Fig. 2). At the source of every transistor there is a 0.1 ? resistor connected in order to unify the flowing currents. Otherwise the current through one of the transistors may rise uncontrollable and lead to damages. The regulator is controlled by a high-voltage operational amplifier, which input is a voltage produced by a 12 bit Digital-to-Analog converter (DAC) with output voltage in the range of 0 to 2V. The DAC communicates with the MCU via SPI interface isolated with high speed optocouplers in order to prevent any HF disturbances from the digital interface to flow into the power circuit.

Fig. 2. VCVS Schematic

The voltage can be controlled in the range of 2V to 46V, with maximum output of 9.5A (Fig. 3).

Fig. 3. VCVS output diagram according to DAC voltage

Linear voltage regulation dissipates a large amount of power during the regulation cycle. Simulation with LTSpice software from Linear Technology shows that the peak dissipated power can reach 120W for the linear voltage regulator. In order to distribute the dissipated power into more components, 3 NPN bipolar transistors are used [6]. The dissipated power for a single transistor is lowered to less than 40W. Linear regulators are chosen, because of their low disturbances in comparison with the switched mode regulators. The other part of the power control is the Voltage Controlled Current Source (VCCS) which is implemented by a current sense amplifier (CSA), high voltage operational amplifier and 3 N-Channel power MOSFET transistors (Fig. 4). The CSA converts the current flowing through a shunt resistor into voltage, which is compared with a voltage produced by the DAC (Fig. 5). The high voltage operational amplifier is used as a comparator driving the gates of the MOSFET transistors according to the desired current output. Three transistors are used, as in the VCVS circuit, because of the heat dissipation, which was simulated to be 120W peak total. Thermal protection is also included to avoid over temperature conditions.

Fig. 4. VCCS Schematic

Fig. 5. VCCS output diagram according to DAC voltage

The voltage and current at all stages is monitored by the Current and Voltage Monitor with I2C interface. The IC has an integrated CSA and voltage sense which is encoded by a 12 bit ADC transmitted by I2C interface to the MCU. The speed of communication is 400 kbit which allows around 3000 reads of all sensors per second, which is enough for the MPP adjust algorithm to function properly. The interface is isolated by high speed optoisolators so that no HF disturbances flow into the power circuits.

2.2. Impedance control

The impedance control module is located on the power PCB and consists of relays, which switch ON and OFF capacitors and inductors. 12 capacitors are connected to each of the two power conductors and their values are chosen to be in the range of 220pF to 470nF, surface mount type [7]. By switching several relays simultaneously, different equivalent capacitance value can be set. Changing the values can simulate the variations of the capacitance of a real PV panel under different ambient conditions and also technical parameters [8]. The capacitors are connected to the metal enclosure of the device. For frequencies up to 30 MHz standard relays are suitable to use. There are also 6 inductors connected in series to the power conductors and they can be switched ON and OFF also in order to include or exclude them from the circuit. The inductors are air coils in order to minimize the parasitic effects of the ferrite cores. The values are chosen to be as follows: 1µH, 2 µH, 5 µH, 10 µH, 20 µH and 30 µH. Combining several inductors can be used to simulate different cabling of the PV panels. The relays are controlled by a MCU with transistors and the control circuits are separated from the power circuit .

2.3. Control PCB

The core of the control PCB is the LPC1768 MCU from the ARM Cortex M3 family. It has a high speed system clock (up to 100 MHz), a large number of input/output pins and it also supports many different interfaces (I2C, SPI, USB, CAN, etc.) [9]. The simulator uses 36 pins for the relay control, USB interface for connection with a PC, SPI interface for control of the DAC and connection with a spectrum analyzer and I2C interface for voltage and current reading. The power supply of the control PCB is provided from a separate transformer and rectifier. The PCB is mounted into a smaller metal enclosure directly attached to the large enclosure of the whole device in order to eliminate any radiated disturbances from the fast digital interfaces. The cables used for connecting the control PCB and the power PCB are shielded.

2.4. Spectrum Analyzer

A separate PCB for spectrum analyzer in the range of 150 kHz - 50 MHz is planned to be designed and included into the PV simulator. Analyzer with maximum frequency of 50 MHz does not require high precision and expensive components. It is possible to build one with wide available and relatively cheap components on the market. Such spectrum analyzer will be an option for the designers in order to be able to make pre-compliance tests without the need of additional spectrum analyzers, which in most cases are expensive and require special qualification to use. The integrated spectrum analyzer will transmit the measurements directly to the PC, where they will be visualized by a special software.

3. Simulations

3.1. Relay simulation

Every block of the circuit is simulated during the design stage in order to evaluate the parameters and results. First the circuits are simulated using LTSpice to ensure the correct electrical functionality. Some physical parts of the device are simulated using CST Studio software for 3D electromagnetic simulation [10]. One example is shown on (Fig. 6). A 3D model of the relay contacts is created in the CST Microwave studio with the dimensions and parameters measured from the relays that are used. On Fig. 7 the simulation results can be seen. The S-Parameters show that there is a resonance occurring at 240 MHz The PV simulator is intended to work with conducted emissions up to 30 MHz and in that case the graphics show that the relay is suitable to be used at these frequencies.

Fig. 6. 3D model of relay contcts in CST studio

Fig. 7. Relay contact HF simulation

A simulation with CST studio can show the exact impedances of the whole system with actual physical parameters. After a 3D model of every PCB is created, the entire device can be simulated for radiated and conducted emissions taking into account all of the components, including the metal enclosures and cabling.

3.2. PCB for impedance control simulation

The PCB for the control of the impedance consists of relays, which switch on/off certain capacitors, connected between the power lines and the protective earth conductor. Fig. 8 shows he PCB as it is imported in CST PCB Studio.

Fig. 8. Impedance control PCB imported in CST PCB Studio

The input terminals are connected to the MPP simulator board. Switching on capacitors in the PCB, changes the impedance between the power lines and the protective earth conductor. Fig. 9 and Fig. 10 show the simulated impedance with 220pF and 220pF + 470pF (690pF) capacitors switched on in the range from 9 kHz to 30 MHz. BNC connectors are placed on the PCB so that a spectrum analyzer or network analyzer can easily be connected. The simulation measurements are taken at the HF connectors

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Fig. 9. Power line to protective earth conductor impedance with 220pF capacitance added

Fig. 10. Power line to protective earth conductor impedance with 220pF+470pF capacitance added

At the next stage an inductance simulation PCB will be added in order to simulate the cabling from the PV panel to the inverter.

4. Conclusion

The designed device is intended to be used by developers and manufacturers of inverters and other devices related to photovoltaic panels. With its wide range parameters (2 - 46V; 0 - 9.5A), the PV simulator covers most of the panels on the market. The output voltage-current characteristics can be controlled according to the type of the panel, solar radiation and temperature. HF parameters can be changed according to the type of the module, type of frames, outdoor moisture, cable lengths and other. There is a possibility to connect external spectrum analyzer to the HF connectors or to use the integrated one for pre-compliance tests. All of the measurements and settings can be made using a PC with the special software, which allows the user to adjust a large number of parameters, to perform a parameter sweep, for simulation of changing environment and also to save the measurement results from the spectrum analyzer and the other sensors (voltage, current, temperature).

Acknowledgement

"The present document has been produced with the financial assistance of the European Social Fund under Operational Programme “Human Resources Development”. The contents of this document are the sole responsibility of the “Angel Kanchev” University of Ruse and can under no circumstances be regarded as reflecting the position of the European Union or the Ministry of Education and Science of Republic of Bulgaria."

Project ą BG051PO001-3.3.06-0008 “Supporting Academic Development of Scientific Personnel in Engineering and Information Science and Technologies”

References

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[2] DC AMN (LISN) PVDC 8300 Manual, Rev. D, Schwarzbeck

[3] Dual-line-V-Y-?-LISN for GCPC DC-LISN-M2-100, 82-243885 E01 Mar. 2013, Teseq

[4] H. Haeberlin, NEW DC-LISN for EMC-Measurements on the DC side of the PV systems Realisation and first measurements at inverters, 17-th European Photovoltaic solar energy conference, Munich, Germany, Oct. 22- Oct. 26, 2001.

[5] Y. Kim, W. Lee, M, Perdam, Dual-mode power regulator for photovoltaic module emulation, Applied Energy 101 (2013), pp. 730-739, 2013.

[6] LT Spice software, http://www.linear.com/designtools/software/

[7] M. C. Di Piazza, F. Viola, G. Vitale, High Frequency Model of PV Systems for the Evaluation of Ground Currents, International Conference on Renewable Energies and Power Quality (ICREPQ'12) Santiago de Compostela (Spain), 28-30.03.2012

[8] Capacitive leakage currents, Information on the Design of transformerless inverters, SMA Solar Technology AG

[9] NXP LPC1700 Datasheet, http://www.nxp.com/products/microcontrollers/cortex_m3/series/LPC1700.html

[10] CST Studio Website, https://www.cst.com/