2017 2 nd International Conference on Test, Measurement and Computational Method (TMCM 2017) ISBN: 978-1-60595-465-3 An Energy Efficiency Measurement Scheme for Electric Car Charging Pile Chun-bing JIANG Beinong Road, Huilongguan Town, Changping District, Beijing, China Keywords: Charging equipment of electric car, Energy efficiency measurement, Loss distribution, Electric meters measurement. Abstract. As the rapid development of electric car, the charging equipment is constructed in a large number. The charging load has an effect on the grid. It s necessary to assess the level of charging equipment energy efficiency. This paper has designed an energy efficiency measurement scheme for electric car. It can measure the power loss of each part during charging and the result can trace to the source. Based on the loss distribution, it can be used for energy conservation of electric car. From the result of charging pile simulation, it can be seen that fundamental electric energy meters can be used in the grid and the charging piles just input harmonic current to the grid. Introduce In recent years, electric cars are prioritized and wide applied in many countries because of their clean, environment protection and high efficiency. Meanwhile, supporting facilities for electric cars such as charging pile or charging station are built in large quantities[1]. To some extent, the energy efficiency level of electric car charging equipments can reflect the capacity of electric car industry to save energy and reduce emission. So it is of great importance for realizing energy saving and energy utilization rate improvement to have comprehensive evaluation of electric car energy efficiency [2,3]. Charging piles are fundamental electric facilities for electric car charge. The DC charging piles contain high-power nonlinearity DC battery charger. The battery charger will generate harmonic during charging and discharging. There is residual harmonic polluting the grid even after the necessary harmonic elimination device processing. And it also will cause wrong measurement of electric quantity. Besides, the charging pattern of electric car is pulse. It exists a lot of ripple in charging process and will influence measurement instruments[4]. Therefore, the energy efficiency test for electric car cannot be conducted directly and completely. And internal meters cannot trace to the source directly. This paper puts forward a energy efficiency measurement scheme for electric car. Through setting appropriate measurement points, installing meters in the points and uploading metric information to energy efficiency measurement platform, it can calculate charging loss and energy efficiency by mathematical models. And external installing meter can trace to the source. By software simulation, it can analyze the harmonic of DC charging-pile and research the influence of charging load to grid or meters so that suitable energy meters can be chosen. Energy-consuming Analysis for Electric Vehicle According to the charging pattern, charging piles can be divided into AC charging pile and DC charging pile. Ac charging piles provide stable AC source and have measure meters, protection switching circuit and control units [5]. The monitor can display charging quantity and rates. The power loss of AC charging pile is mainly of control circuit. Excepting meters, protection circuit and control units, DC charging piles contain off-board charger. The charger can provide stable DC source for electric car directly. The piles' power loss includes control circuit and charger. Towards different charging pattern, it needs corresponding measurement scheme for charging piles. Figure1 shows the loss analysis of electric car charging equipments. The meters can be set in the measuring points presented in figure 1. 16
Figure 1. Loss analysis chart of electric car charging equipment. Energy Efficiency Measurement Scheme Design The energy efficiency measurement of electric car mend to define the loss of each consumption equipments. It includes distribution transformer, electric lines, control circuit and charger. As long as installing energy measurement device in input and output of consumption equipment, the efficiency measurement can be achieved. Selection of Measuring Points In theory, the measuring points for trade settlement should be set in boundary of dealing properties. For electricity settlement of charging station, the measuring point is point 4 in figure 1. For charging pile, it is point 5 or point 8. For user, it is point 7 or point 9. Allocating CT, PT and power meter in measuring point 1, 2 and 3, the loss of transformer can be measured. The power loss is: P tran =P 1 -(P 2 +P 3 ). Allocating CT, PT and power meter in measuring point 2 and 4, the loss of electric line can be measured. It is: P line =P 2 -P 4. In the same way, the power loss of DC charging pile is: P DC =P 6 -P 7 +P 控制. And the power loss of AC charging pile is: P AC =P 控制. In the formulas, P control is the loss of control unit. As for internal power loss of DC charging pile, it needs energy efficiency measurement platform to calculate the loss according to relevant parameters. About the choice of measuring points, the following two major factors should be considered: (1)Economy The difference of input power and output power is the power loss of equipment. In some situation, the cost is high to allocate measuring equipments in every points. If not all ports can be equipped meters, it can only consider the elements with high power loss. In other way, the mathematical method can be used to calculate the loss so that the cost can be reduced. In addition, installation condition and construction difficulty should be considered[6,7]. (2)Harmonic influence The harmonic characteristic of charging system is another factor that should be considered in measuring points selection. The point with small THD of voltage or current waveform should be chosen so that the loss can be measured by fundamental power meters. The cost will increase to use high performance meters if harmonic cannot be ignored. AC charging pile doesn't contain nonlinear element. The harmonic of its output is small. It can use fundamental meters to measure its efficiency. As to DC charging pile, it can accord to the simulation to choose suitable equipments. Design of Energy Efficiency Measurement Scheme AC charging piles have simple construction and the power loss is relatively small. The energy efficiency measurement scheme of AC piles can be completed through measuring point 8 and 9. As for DC charging pile, it needs energy efficiency measurement platform to receive internal loss distribution of charger. The relative measuring points information should be collected. Figure 2 is one measurement scheme for DC charging pile. The internal of source interface is equipped with voltage and current transform. The high voltage and current can be convert to low voltage and current. Then the electric parameters can be gathered to energy efficiency measurement device. In 17
the same way, dc voltage and current can be gathered[8,9]. On the basis of these data, the efficiency and power factor of charging pile can be measured. The measuring device is integrated with standard electric energy meters. The meters can conduct the verification of internal three phase ac power meters and dc power meters in charging pile externally. It can use low frequency pulse method when the charging pile has electric pulse output. Use electric energy comparison method for verification when there is no electric pulse output. Figure 2. Energy efficiency measurement scheme of DC charging pile. From loss calculation formula for electronic devices in reference [7] and [8], the conclusion can be drawn that the main loss source of charger is the conduction loss of diodes. On the industrial, decreasing the state impedance of diodes or choosing low-energy diodes can improve operating efficiency of charging pile, reduce the impact to grid of electric car charging and accelerate the development of electric cars. Simulation of Electric Car Charging Pile Charging piles connect to grid as load. When the quantity of charging piles increase to a certain number, the influence to the grid cannot be ignored. It is necessary to study the variation of port voltage and current when the charging piles connect to the grid. It can confirm the influence of nonlinear load to power quality. And also determine the type of power meters to increase measuring accuracy. DC Charging Pile Simulation The simulation adopts Matlab software. DC charging pile model uses three-phase-bridge rectifier circuit[10]. The effective voltage value of sine ac source is 220V to simulate the input of grid. The resistance of load is 10Ω. The smoothing reactor is 0.2H. After operation, analyze the line-side voltage and current waveform and energy measurement. 18
Harmonic Analysis and Influence to Meters Figure 3. Charging pile simulation model. In the simulation, the voltage waveform and current waveform in ac side showed in figure 4 and figure 5. It can be seen that the influence of charging load to grid voltage is not large and the voltage maintains sine rule basically. Under nonlinear load, the current in ac side distorts and transport harmonic current to grid system. Figure 4. Voltage waveform of ac side. Figure 5. Current waveform of ac side. Analyze the voltage and current waveform with FFT. The result of voltage and current amplitude, phase difference, power in each time frequency is showed in table 1. The harmonic component of voltage in ac side is zero because its waveform follow sine rule basically[11,12]. The current waveform in ac side distorts and its odd harmonic is not zero because of distortion. The harmonic mainly is fundamental wave part. 19
Value Frequency Table 1. Voltage and current amplitude, phase, power in each time frequency. Voltage amplitude (V) Current amplitude (A) Phase difference ( ) Power (W) 1 310.270 56.607 0 8781.724 3 0 0.033 0 0 5 0 11.301 0 0 7 0 8.098 0 0 9 0 0.033 0 0 11 0 5.126 0 0 13 0 6.368 0 0 15 0 0.033 0 0 Comprehensive power:p=8781.724w From table 1, the conclusion can be drawn that the ac-side current under charging load will distort and transports harmonic current to grid when the source is sine wave. According to the perpendicularity of trigonometric function, three-phase-bridge charging load does not transport harmonic power to grid system and electric energy meters measure fundamental wave power. Summary On the basis of charging equipments loss analysis and relative economic or harmonic factors, this paper has designed an energy efficiency measurement scheme. In measuring points, the meters can trace to the source. The charging pile output measuring device is equipped with standard meters. The internal ac meters or dc meters of charging pile can be traced. As a consequence, the energy efficiency measuring results can trace to the source. Background check software can analyze power loss distribution of charger by collecting relative operation parameters and apparatus parameters. Because of increasing demand for electric cars, the quantity of charging facilities increases rapidly. The influence of charging system to grid and power quality cannot be ignored. It can analyze voltage or current waveform in ac side and charging electric energy through the simulation of three-phase-bridge charging pile model. It can be seen that charging load has no influence on grid voltage but effects the grid current with distortion. The system will transport harmonic current to grid. Considering perpendicularity of trigonometric function, the charging piles do not convey harmonic power to the grid. The fundamental wave meters can be used to measure charging power. As for other types of charging pile, it needs further research. Reference [1] Bin Zhang, Fang Xu, Yun Shi. Application and analysis of electric energy information collection and monitoring system[j]. Hebei Electric power, 2012, 31(01): 12. [2] Xian Wang. Design and implementation of distributed energy acquisition and monitoring system[d]. Fudan University, 2009. [3] Yao Yang. Design and implementation of electric energy acquisition and energy efficiency analysis system[m]. Shanghai: Shanghai Jiao Tong University, 2014. [4] Yan Wang, Zhaohui Zu. Reflection on the tracing method of measuring instruments in legal metrology technology[j]. Metrology and measurement technique, 2013, 40(12): 36-39. [5] Xinqi Chen, Peng Li Wentang Hu, et al. Analysis of the influence of electric vehicle charging station on power system harmonic[j]. Electric power, 2008, 41(9): 31-36. [6] Sustanto D., Fok C C. Harmonic load flow study for electric vehicle chargers[j].power Electronics and Drive Systems,1999,l:495-500. [7] Zhigang Li, Shunag Mei, Shaojie Wang, Fang Yao. A summary of the calculation methods for switching losses of IGBT modules[j]. Application of electronic technique, 2016, 42(1): 10-15. 20
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