The main electrical energy measurement problems in electric vehicle charging pile

introduction

The contradiction between people’s growing material and cultural needs and limited non-renewable energy is an important reason for the promotion and development of new energy and related industries. China’s aggressive push for electric vehicles has also boosted demand for charging piles. Promoting the development of new energy charging piles and improving and balancing the layout of charging infrastructure are important measures to meet the needs of electric vehicle users, greatly alleviate the range anxiety of electric vehicles, further promote the development and use of clean energy, and alleviate the demand for future energy shortages.

1 Electric vehicle charging pile

Charging piles are usually placed in charging stations, parking lots, highways and other places to provide DC and AC power charging services for electric vehicles. The interior includes man-machine interaction interface, charging standard interface and other components, but also includes protection, measurement and control, time-sharing metering, charging history information query and storage, operating status monitoring and other functions. The charging pile is mainly composed of metal shell, human-computer interactive touch screen interface, metering and charging system, charging electrical equipment, etc. The metering and charging system can be divided into metering energy meter, billing management platform, data reading and writing module. Charging electrical equipment can be divided into charging standard interface, charging interface switching module, charging cable, safe operation protection module and so on.

 

2. Main electrical energy measurement problems in electric vehicle charging pile

 

2.1 Environment required for collecting load waveform during construction

In order to determine the specific problems existing in the power measurement of the charger, the acquisition conditions that meet the requirements of the load waveform acquisition should be formed first, and the waveform recorder should be used. The frequency parameter of the device is 500khz, which can complete the dual acquisition tasks of current signal and voltage at the same time, realize the phase synchronization function, measure the phase and range of each auxiliary current and voltage wave, and meet various measurement requirements. When using a field waveform recorder, an external wiring voltage probe is used to obtain and collect voltage values. When the current value is obtained, the clamp current transformer must be used, and four phases and three wires can be selected to connect different electric energy measurement points. After the current and voltage data collection is completed, the waveform logger converts the data to MEM format for storage, reads the data with the device software, then converts the format, converts the data to TXT or CSV format, and relies on Mataab software for further analysis of the data. The reading speed of data in CSV format is slow and the range of data effectively displayed is limited. Matlab needs to be imported in about 1.05 million hours of data processing, editing is difficult, and the number of files is too large; TXT format data reading speed is slow, data processing efficiency is low, it is difficult to achieve some processing behavior; Although the reading speed of Matlab data format is higher than the other two formats, the amount of data that can be displayed is larger, more convenient, and the data processing efficiency is higher.

 

2.2 Difficulty in measuring electric energy of charging pile

 

The difficulty of measuring electrical energy is not only technical, but also operational. DC charging pile is widely used, it is difficult to ignore the influence of harmonics directly. If the DC harmonics are too large, the acquisition transformer will distort the measurement; The charger itself belongs to the whole metering device, and its functions include billing and metering functions; The metering counter of the charger application should not only meet the basic charging function, but also ensure the needs of orderly discharge and charge control, and strengthen the communication technology and application security. From an operational point of view, different from the daily electricity usage scenario, entities such as power supply companies, electricity sales companies, and owners must fully consider the charging operation and asset side when using chargers to charge electric vehicles, maximize the use of resources in complex multilateral trade scenarios, form asset management, determine the charger as a metering asset, implement management, and choose the appropriate inspection method. Pay attention to lifecycle management.

 

3. Electric vehicle charging station optimization

 

3.1 Replace the energy meter with the corresponding pulse output

GB/T29318-2012 has an interface for on-site inspection of energy metering devices in the charger. At present, non-vehicle chargers are usually equipped with DC energy meters, some have pulse output interfaces for metering errors, and some do not. Electric vehicles with fault metering pulse output interfaces are not vehicle chargers. When a working fault occurs, the charger cover of the electric vehicle must be opened to measure the working fault from the pulse output interface of the DC energy meter; Validation cables can be caused by many power counter cable terminals and hidden mounting positions, and validation errors may require suppression of validation cables from the shut-off plate. In response to this problem, the author suggests that strong manufacturers increase the photoelectric output on the display screen, so that it matches the energy generated, which is not only easy to verify, but also does not affect the overall protection level of electric vehicle chargers. For electric vehicle rechargeable batteries without pulse output interface fault metering, it is recommended to purchase and install electric vehicle rechargeable batteries that meet the requirements of national standards.

 

3.2 Verification methods

The system composition of verifying charging pile includes electric load, power supply and standard watt-hour meter. By connecting an adjustable electronic load to an electric vehicle, an efficient simulation of AC charging can be achieved. The AC charging gun is plugged into the electric vehicle, and the charger uses the battery BMS and the charging interface for handshake communication. The battery BMS sends a power generation request according to the characteristics of the charger and the status of the power battery, and the AC charger automatically adjusts the parameters to form the most appropriate charging current. During the charging process, the measurement module measures the actual output power, and the measurement module uses the communication module of the system to transmit the information such as battery status, charging time and electrical parameters to the collection interaction module and the charging module. After the battery is fully charged, the BMS will inform the charger to power off and complete a charge. When charging, the output parameters of the charger will change with time. After the charger is started, the charging power is not necessarily the same as the power formed before the battery is charged. If measured by watt-second method, it is easy to produce errors. Chargers require constant data exchange, collection and communication activities, which will increase power consumption. If analog detection of the electronic load is selected, the heat will reduce the impedance. Therefore, in the application of this charger, the pulse verification method can form the best verification results.

 

3.3 Detection Control Guide

In this work link, inspectors usually use non-on-board chargers to focus on checking the connection of the electric vehicle plug and clarifying the voltage value of the charging pile. If it is found that the plug is not connected to the socket, the inspector should check the current charging situation and the actual voltage, manually open the charging gun switch, and then use the power of the digital voltmeter to obtain data information, and check the current and voltage with the oscilloscope. If the plug is connected to the socket, make sure the switch is off and repeat to check the actual charging. At this point, pull out the button, keep the switch off state, when the vehicle plug and socket are connected, check the current, voltage value and the corresponding charging situation. When the inspection personnel check the electronic lock, they should pay attention to observe its closed state, and close the electronic lock as soon as possible after the charging connection returns to normal.

 

3.4 It is recommended that the minimum electric energy variable modified in the verification procedure should not be greater than 0.01kWW·h

The minimum power variable displayed on the charging pile screen does not reach 0.001kW·h, which is difficult to solve this problem. According to the principle of electric vehicle charger, the electrical energy displayed on the charger screen will be collected from the metering module of the charger. The pulse constant of the energy meter installed on the electric vehicle charging pile is less than 1000imp/kW·h, and the minimum electric energy variable displayed on the screen of the electric vehicle charging pile is less than 0.001kW·h. Electric vehicle AC charger adopts single-phase AC watt-hour meter and three-phase AC watt-hour meter; Non-on-board chargers for electric vehicles use DC meters. Single-phase AC watt-hour meter specification is 220V, 5(60)A, pulse constant is generally 1200imp/kW·h, can meet the requirements; The three-phase AC watt-hour meter specifications are 3. ×220V, 5 (60) A, the pulse constant is generally 400imp//kW·h, can not meet the requirements; DC energy meters such as DC750V have more specifications, and the pulse constant of 100a/75mv is 200imp/kW·h, and the pulse constant of most other specifications is also less than 1000imp/kW·h, which cannot meet the requirements. Therefore, only replace the meter whose pulse constant is greater than 1000imp/kW·h.

Closing remarks

The working mode of the battery pack is based on Internet ideas and new technologies, such as private battery pack sharing, double or multiple charging, wireless fast charging and other new methods. It is evolving, creating more and more problems for measurement verification. With the emergence of these new technologies and the resolution of these problems, the specifications of the measurement technology should be updated.

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