The large-capacity battery energy storage system has been used in power systems for more than 20 years. In the early days, it was mainly used for frequency modulation, hot standby, voltage regulation and backup of isolated power grids. The application of battery energy storage systems in the integration of new energy sources has also been carried out abroad. In the late 1990s, Germany's Herne 1MW photovoltaic power plant and Bocholt 2MW wind farm were equipped with a 1.2MWh battery energy storage system, providing peak clipping, uninterrupted power supply and improved power quality. Since 2003, Japan has installed a 6MW / 6MWh all-vanadium flow battery (VRB) energy storage system at the Hokkaido 30.6MW wind farm to stabilize output power fluctuations. On August 18, 2014, the 220 kV smart substation of the National Scenery Storage and Loss Demonstration Project was successfully launched.
As the first pilot project of the State Grid Corporation to build a strong smart grid, the National Scenery Storage and Loss Demonstration Soil Course is currently the largest grid-connected solar photovoltaic power station in China, the largest wind farm with the largest onshore capacity in China, and the largest chemical energy storage in the world. Power station, the new energy demonstration project with the highest level of intelligent operation and the most diverse operation modes.
The application purpose of energy storage power station (system) in the power grid mainly considers several major functional applications such as “load regulation, matching new energy access, making up for line loss, power compensation, improving power quality, isolated network operation, and cutting peaks and filling valleysâ€. For example: cutting peaks and filling valleys, improving the grid operation curve, a common explanation, the energy storage power station is like a reservoir, which can store the surplus water in the low valley period and use it again when the electricity peak is used. This reduces the waste of electrical energy; in addition, energy storage plants can reduce line losses and increase line and equipment life.
Since 2014, China began to develop energy Internet and energy storage systems on a large scale. This paper mainly introduces energy storage systems.
figure 1
Two off-grid energy storage system
Off-grid photovoltaic power generation system, also known as independent photovoltaic power generation system, is mainly composed of PV components, DC/DC charging controllers, off-grid inverters and loads.
figure 2
The off-grid system consists of the following components:
Battery components, photovoltaic charge and discharge controllers, battery packs, off-grid inverters, AC/DC loads.
The main function of the photovoltaic charge and discharge controller is to control the charging and discharging of the battery and protect the battery from overcharging and discharging. Off-grid inverter, the function of the off-grid inverter is to convert DC power into AC power and provide it to the device used by the load.
Our common off-grid energy storage system is the solar street light. Photovoltaic module, a cigarette box-sized controller, a tens of watts of LED lights, one or several sets of batteries. It can provide nighttime lighting.
The larger off-grid energy storage system is the “household systemâ€. When the author first entered the business in 2006, the domestic PV industry is in its infancy, and the country implemented several problems in order to solve the problem of herdsmen’s electricity use in Qinghai and Northwest Tibet. The second "bright project" is to send a set of photovoltaic "household systems".
(At that time, 150Wp polysilicon also bought 20 blocks and one watt.) A household system is about 300W, two battery boards, one inverter inverter, and 12V100AH ​​batteries. You can watch LCD batteries, LED lights, or some small motors at night (the Tibetans stir the butter, the milk machine)
The larger off-grid power station, the author has participated in more than one. One of the more classic ones is Beijing Huineng Sunshine “Qinghai Yushu Zongda Temple†100KW off-grid solar power station. There are more than 200 lamas in this temple, and electricity is used every day for 100 degrees. The construction of this power station solves the problem of electricity consumption of these lamas.
image 3
Triple grid energy storage system
Figure 4
3.1 System components
In the scheme of Figure 4, the energy storage power station (system) is mainly used in conjunction with photovoltaic grid-connected power generation applications. Therefore, the entire system includes a photovoltaic module array, a photovoltaic controller, a battery pack, a battery management system (BMS), an inverter, and corresponding Power generation system including energy storage power station joint control and dispatching system.
The photovoltaic module array uses the photovoltaic effect of the solar panel to convert the light energy into electrical energy, and then charges the lithium battery pack, and converts the direct current into the alternating current to supply the load through the inverter; the intelligent controller continuously changes according to the intensity of the sunshine and the load. Switching and adjusting the working state of the battery pack: on the one hand, the adjusted electric energy is directly sent to the DC or AC load. On the other hand, the excess power is sent to the battery pack for storage. When the power generation cannot meet the load demand, the controller sends the battery power to the load to ensure the continuity and stability of the whole system. The grid-connected inverter system consists of several inverters, which turns the DC power in the battery into The standard 380V mains access is connected to the high voltage grid via the user side low voltage grid or via a step-up transformer. The lithium battery pack plays the two roles of energy regulation and balance load in the system. It converts the electrical energy output from the photovoltaic power generation system into chemical energy for storage in case of insufficient power supply.
3.2 Battery selection
As a storage power station that cooperates with photovoltaic power generation to achieve peak-cutting, valley-loading, load compensation, and power quality application, energy storage batteries are a very important component and must meet the following requirements:
It is easy to realize multi-mode combination to meet higher working voltage and larger working current; battery capacity and performance can be detected and diagnosed, so that the control system can realize dispatch control of power station load under the condition of predicting battery capacity and performance; High safety and reliability: Under normal use conditions, the normal service life of the battery is not less than 15 years; in the limit case, even if the fault occurs within the controlled range, there should be no explosion, combustion, etc. that endanger the safe operation of the power station. With good fast response and large rate charge and discharge capacity, generally requires 5-10 times charge and discharge capacity; higher charge and discharge conversion efficiency; easy to install and maintain; has better environmental adaptability, wider working temperature range.
Comparison of several battery performance
From the perspective of initial investment cost, lithium-ion batteries have strong competitiveness. Sodium-sulfur batteries and all-vanadium flow batteries have not been industrialized, and supply channels are limited and expensive. In terms of operating and maintenance costs, sodium and sulfur require continuous heating. The all-vanadium flow battery requires a pump for fluid control, which increases operating costs, while lithium batteries require little maintenance. According to the application status and battery characteristics of energy storage power stations at home and abroad, it is recommended that the battery selection of energy storage power stations is mainly lithium iron phosphate batteries. The reason why the lead-acid battery is not recommended is the battery life problem. The big-brand lead-acid battery has a life of about 2.5-3 years under frequent charge and discharge conditions, and the life of the lithium battery will be much longer.
3.3 Energy Management System
In energy storage power stations, energy storage batteries often consist of dozens of strings or even hundreds of strings of battery packs. As the battery is in the process of production and use, it will cause inconsistencies in the internal resistance, voltage, capacity and other parameters of the battery. This difference is manifested in the fact that the voltage between the series cells is different or the energy is different when the battery pack is full or discharged. This situation will lead to partial overcharging, and the battery with too low voltage during discharge may be over-discharged, so that the discreteness of the battery pack is significantly increased, and overcharge and overdischarge are more likely to occur during use, and the overall capacity is increased. A sharp drop, the capacity of the entire battery pack is the capacity of the battery with the worst performance in the battery pack, which eventually leads to premature failure of the battery pack. Therefore, for a lithium iron phosphate battery pack, a balanced protection circuit is necessary. Of course, the battery management system of the lithium battery is not only the balanced protection of the battery, but also more requirements to ensure the stable and reliable operation of the lithium battery energy storage system.
1 cell voltage equalization function
This function is to correct the dispersion of voltage or energy caused by the process variation of the battery cells in the series battery pack, and to avoid the occurrence of battery performance deterioration or even damage due to overcharge or overdischarge of individual cells. All individual battery voltage differences are within a reasonable range. The error between the batteries is required to be less than ±30mv. (Electric vehicles have just broken through this bottleneck,)
2 battery pack protection
Single cell overvoltage, undervoltage, overtemperature alarm, battery pack overcharge, over discharge, overcurrent alarm protection, cutoff, etc.
3 The collected data mainly include:
The cell voltage, the cell temperature (actually the temperature of each cell module), the group terminal voltage, and the charge and discharge current are calculated to calculate the internal resistance of the battery.
Communication interface: Adopt digital communication protocol IEC61850. In the energy storage power station system, communication with the dispatch monitoring system is required to send data and execute instructions.
4 diagnostic function
BMS should have the analysis and diagnosis function of battery performance. According to the real-time measurement of battery module voltage, charge and discharge current, temperature and cell terminal voltage, calculated battery internal resistance and other parameters, the diagnostic model can be analyzed to obtain the current battery cell. Diagnosis of capacity or residual capacity (SOC), diagnosis of cell health status (SOH), battery state assessment, and estimation of sustainable discharge time in the current state at the time of discharge. According to the requirements of the relevant standards of electric vehicles, "General Requirements for Lithium Ion Battery Assembly" (currently there is no relevant standard for energy storage power stations), the diagnostic accuracy for residual capacity (SOC) is 5%, and the diagnostic accuracy for health status (SOH) is 8 %.
5 thermal management
During the charging process, the lithium battery module generates a large amount of heat energy, which causes the temperature of the entire battery module to rise. Therefore, the BMS should have a thermal management function.
6 Troubleshooting and fault tolerance
In case of an abnormality, the BMS shall give a fault diagnosis alarm signal and send it to the upper control system through the monitoring network. Real-time monitoring of each string of batteries in the energy storage battery pack, through the monitoring and analysis of parameters such as voltage and current, calculating the rate of change of internal resistance and voltage, and referring to the relative temperature rise and other comprehensive methods, immediately check whether some of the battery packs have been A battery that cannot be reused or may be damaged soon, judges the faulty battery and positioning, gives an alarm signal, and takes appropriate measures for these batteries. When the fault accumulates to a certain extent, and a malignant accident may occur or start, an important alarm signal is output, and the charging and discharging circuit bus or the branch battery stack is cut off, thereby avoiding a malignant accident. Fault-tolerant technology using energy storage batteries, such as battery bypass or energy transfer technology, when a single battery fails, to avoid affecting the operation of the entire battery.
The management system has a self-test function on the system's own software and hardware, and even if the device is damaged, it will not affect the battery safety. Ensure that the energy storage system does not malfunction due to management system failure, or even cause battery damage or a serious accident.
7 Other protection technologies
For fault conditions such as overvoltage, undervoltage, and overcurrent of the battery, the circuit is cut off to protect it. For an overcurrent condition of an instantaneous short circuit, the delay time of the overcurrent protection is generally at least several hundred microseconds to milliseconds, and the delay time of the short circuit protection is microsecond, and the circuit is cut off at the moment of the short circuit. Avoid large damage to the battery caused by short circuit. In the bus circuit, a fast fuse is generally used, and in each battery module, high-speed power electronic devices are used to achieve fast cutting.
8 Battery online capacity assessment SOC
Based on the measurement of dynamic internal resistance and true voltage, etc., using the corresponding relationship between charging characteristics and discharge characteristics, a multi-mode segmentation method is adopted to establish a mathematical analysis diagnostic model to measure the remaining battery SOC. The discharge characteristics of the lithium battery are analyzed. Based on the integral method, the method of dynamically updating the battery power is adopted. Considering the self-discharge phenomenon of the battery, the online current, voltage and discharge time of the battery are measured; and the remaining power of the battery under different discharge conditions is predicted and calculated. The power consumption prediction is corrected according to the battery usage time and the ambient temperature, and the predicted value of the remaining power SOC is given.
In order to solve the influence of the battery power change on the measurement, the method of dynamically updating the battery power may be adopted, that is, the power discharged last time is used as the reference power of the current discharge, so that as the battery is used, the battery power is reduced as the reference power. The reduction of the reference power also needs to be corrected according to changes in the ambient temperature.
Four suggestions
Energy storage systems and microgrid systems invest heavily, and the cost of batteries is quite high. The author has done the calculation, a plant energy storage system (at night low trough from the grid to take power storage, daytime peak release) peak and valley electricity price gap to reach 0.5-0.7 yuan lithium battery energy storage to achieve a small profit. The energy storage system technology is complex, and non-professional design institutes cannot design. It is necessary to closely cooperate with various equipment manufacturers.
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