The traditional application of energy storage in power distribution system is to provide emergency power supply for some important facilities in the power grid. Among them, the use of batteries in substations to provide emergency power supply for control/dispatching systems and relay protection devices is a typical application example. In the following sections, other innovative effects of installing energy storage in electrical distribution systems will be described.
1. The role of energy storage in grid planning
(1) Energy storage is used for load smoothing
From the perspective of asset optimization operation management, power grid companies believe that load smoothing is an important function of energy storage. Of course, on a larger scale, distributed energy systems (including distributed generation and load management) can also play a role in load smoothing.
When an increase in load is expected to result in insufficient capacity of some equipment in the grid in the near future, the usual solution is to build a new distribution facility or retrofit an existing facility. Because power facilities are designed and manufactured in a standardized sequence, the corresponding capacity additions are often much greater than actual demand in the short term, resulting in "underutilized" (i.e., low utilization) new power assets for extended periods of time.
The use of energy storage systems in the lower grid of a congested network is a flexible and temporary solution. As shown in Figure 1, the energy storage system is charged during the off-peak load period to form active power backup. When the peak load occurs, the energy storage system can inject energy into the grid, which can reduce the maximum current delivered by the upper-level grid. In this way, energy storage can avoid network congestion on the grid. By controlling the active power and performing local reactive power compensation according to a predetermined operating curve or closed-loop real-time measurement, the current flow can be greatly reduced.
Figure 1 Schematic diagram of energy storage applied to load smoothing
Eliminating short-term load peaks in the above-mentioned ways can avoid investment in power expansion, or at least delay the time of investment. For example, in 2006, the American Electric Power Company installed a 1MW/7.2h sodium-sulfur battery energy storage system in a 12kV power grid. The energy storage system can cut the peak value of the 20MV·A 46kV/12kV step-down transformer, so that the power distribution system that will be expanded will continue to be used for several years. In this way, the construction of those hugely invested substations can be postponed. In addition, energy storage technology can also reduce the thermal stress of power facilities and prolong their service life.
At the end of the delayed expansion construction phase, upgrades to power facilities will be unavoidable. In this case, the energy storage system will either remain in place or be moved to other places for continued use. The peak shaving effect of energy storage is particularly effective in the following situations:
1) When the construction of power projects is hindered or delayed due to the existence of some constraints (such as environmental issues, legality, local residents' opposition), resulting in potential risks of reduced power quality and/or power supply continuity.
2) For temporary power supply such as engineering sites, temporary power wiring is often required, and the construction of energy storage systems can avoid power grid upgrades. Since these temporary power needs last for a maximum of 5-15 years (which is comparable to the typical lifespan of an energy storage system), it is worth considering configuring the energy storage system to solve the power supply problem.
Energy storage systems configured to delay grid upgrades are generally installed downstream of nodes with limited power consumption, which also enables owners to plan the location of energy storage more flexibly, such as installing them in locations that can better perform other auxiliary functions. The place. However, there are many factors to consider, including land, ease of construction, local acceptance of communication needs, and possible resource sharing. Of course, these issues require more special-case special handling.
If energy storage equipment is installed as close to the user side as possible (to smooth out load fluctuations), a large number of new power assets will be added. However, if a more centralized energy storage installation method is adopted [such as installation near high-voltage/medium-voltage (HV/MV) substations], the smoothing effect brought by the clustering effect of the load can be fully utilized, and it is more efficient than a large number of decentralized energy storage units. Easier to manage. Centralized energy storage devices installed upstream of the grid can achieve the same grid upgrade delay effect while using less energy storage capacity .
When the operation of the grid reaches the upper limit of certain technical conditions, the distribution system operator (DSO) needs to take corresponding countermeasures without considering the economics of the scheme. However, if a number of different solutions are technically feasible, it will be possible to choose the optimal one from an economical point of view. Assuming that several different schemes can meet the increasing demand of the load (such as grid reconstruction, partial or complete reconstruction of the grid, double-circuit lines), it is often necessary to comprehensively consider a variety of different economic costs, such as investment, network loss of power assets Maintenance, power outage losses, etc., select the lowest cost plan in a specific period. Therefore, the economic benefits of peak shaving will also be defined within this framework, by comparing "storage" peak shaving with other possible options to choose the best one.
(2) The role of energy storage in voltage control
Frequency is a global quantity based on the instantaneous balance of power generation and power consumption in the power system, while voltage is a local quantity. The level of voltage is very important. The power system needs to constantly adjust the local voltage to ensure the normal operation of electrical equipment. Therefore, grid companies should comply with voltage-related specifications and operational constraints to ensure that users are provided with a qualified supply voltage. The power grid can have a variety of different technical means to meet the needs of voltage control, such as on-load tap-changers for medium-voltage/high-voltage transformers, and off-load voltage regulators for medium-voltage/low-voltage distribution substations.
Due to the impedance on the line, the transmission of electrical energy causes the voltage on the feeder to drop continuously, which can be calculated using the following simplified formula:
In the formula, U represents the voltage; R and X represent the resistance and reactance of the line, respectively; P and Q represent the active and reactive power transmitted in the line, respectively.
For distribution lines, in the absence of distributed generation, the voltage drops continuously from the high-voltage/medium-voltage substation to the end of the supply line. Therefore, it is especially necessary to avoid low voltage during peak electricity consumption when planning the power grid.
Figure 2 Voltage distribution diagram of medium voltage line
In recent years, some studies have begun to focus on the regulation of distributed generation on distribution line voltage, in order to improve the penetration rate of distributed generation or reduce the investment cost of new lines. Similarly, distributed energy storage systems can also inject active and reactive power into existing lines to ensure voltage quality during peak power consumption.
Therefore, energy storage can be an effective alternative in some places where power grid reconstruction must be carried out to make the power supply quality meet the contract requirements, such as increasing the wire diameter of some lines to reduce impedance and thus reduce line voltage drop.
An application example of Pacific Power Company. The company established a 350kV·A/8h vanadium flow battery energy storage system in 2003 to maintain a 25kV line voltage with a long transmission distance. In the past, the company often received complaints from local users due to power supply quality problems, and due to low voltage restrictions, the region could not meet the grid-connected access needs of new users. The grid company evaluated several different technical options, including adding new reactive power compensation devices, expanding existing lines, and rebuilding the system. In the case of very harsh environmental constraints (there is a nature reserve in the area), the construction of energy storage systems was finally chosen as the solution. The energy storage system is installed in the middle of the line, and the load is cut through a preset program to maintain the voltage at a specific level.
(3) The supporting role of energy storage when the power grid operating state drops
Generally, medium-voltage lines drawn from substations are often connected to other lines drawn from the same substation or other substations, so as to quickly restore power supply after a fault. This backup power supply method will change the topology of the power grid, thereby affecting the power flow (current, voltage, power) of the power grid. This issue should be taken into account during the grid planning stage. For medium voltage systems, verification of the "N-1 criterion" can be performed during peak load periods, focusing on the voltage profile and the currents flowing through the various power devices in the network.
From all perspectives, the use of energy storage can bring benefits to the grid. In addition to normal functional requirements, energy storage can also bring some additional ancillary services, such as being able to handle some rarer emergencies. It is necessary to invest more in this potentially important function, and research on new grid technologies such as energy storage is also reasonable.
Distributed energy storage provides support when grid operating conditions decline by relieving the stress of electrical equipment. It can do two things (and possibly both), reducing peak loads in degraded substations and providing local voltage support. In this way, in the process of performance degradation or fault recovery of the substation, the electrical load can continue to operate within the allowable voltage limit. Therefore, distributed energy storage can support the derating operation of the grid, similar to the aforementioned load smoothing and voltage regulation. Of course, it can also be seen as a special case of voltage regulation.
2. Other applications
Theoretically, the peak shaving and voltage regulation effects of energy storage can only be achieved by targeted operations during certain time periods of the year (peak loads), and these peak load curves can be predicted hours or days in advance come out. It is a relatively special case (difficult to predict accurately) for energy storage to support the grid when its operating state deteriorates, however, this can be done simply by maintaining sufficient energy storage in the long term to deal with the occurrence of unexpected events.
In fact, energy storage can maximize the benefits to the distribution system operator (DSO) beyond the backup needs of the several time periods mentioned above. For example, the following additional benefits can be obtained through energy storage.
1) In several functional applications of energy storage, valley electricity is used for charging and discharging at peak load, which means that the network loss decreases with a quadratic trend. It can be considered that the benefit brought to DSO is the reduction of the loss of the energy storage device itself, and this part of the benefit is very considerable when the energy storage capacity is large. Of course, this also depends on many different factors, such as load curve, network impedance, etc.
2) Distributed energy storage can play the role of reactive power compensator in an important part of the power distribution system through the power electronic conversion device, so as to avoid the investment in the reactive power compensation capacitor bank in the substation, so that the distributed energy storage can be evaluated. benefits that can be brought. In addition, the power electronic interface also helps the grid company to fulfill the contract regarding the quality of the power supply (acting as an active filter) to the electricity user.
3) Distributed energy storage can also restore voltage to part of the distribution network after a fault. Voltage recovery can use a mobile energy storage system, just like a traditional oil-fired generator, which can be transported to the site for power generation in time, or a static energy storage power station can assist in providing local voltage support. A typical application scenario is in an area that is prone to power supply reliability issues and where conventional solutions (such as multi-circuit power supply or grid reinforcement) are difficult to implement. If the area is in severe weather conditions and the outside world is difficult to access, the restoration of local power supply can be solved by the energy storage system or other local power sources. In this way, the benefits of using energy storage devices locally will be even more pronounced.
Having said that, the technical issues involved in the application of distributed energy storage to distribution systems are numerous and complex, in particular the following technical issues need to be covered: power quality, supply security, and grid companies' re-evaluation of current operations to determine Which users can be islanded by distributed generation to continue supplying power after grid failure.