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A U.S. Department of Energy study assessed the reserve capacity, demand, that should be met by wind farms participating in system frequency regulation. The wind farms referenced in the study have a larger installed capacity (138 wind turbines, with a total generating power of 103MW), so that the impact of the clustering effect of wind farms on primary frequency regulation can be better seen.
All the units of the above wind farm are divided into four zones, and the standby energy required for each group to participate in frequency regulation is estimated. The results are shown in Table 1. It can be seen that if the four partitions participate in frequency regulation individually, the required spare capacity is 7.5MW, while the spare capacity required after the combined operation of the wind farm is 4.8MW, which is only 65% of the former. The authors of the study concluded that the clustering effect of wind farms has a positive effect on the frequency regulation of the system.
Table 1 - Influence of wind farm cluster effect on system frequency regulation
On this issue, on the one hand, the adjustment effect of wind power on the system frequency has little effect on a large regional grid. Therefore, the primary backup capacity of other power generation methods will not be affected by wind power fluctuations. On the other hand, the positive effect of the clustering effect of large wind farms (such as multi-wind farm clusters) can reduce the reliance on spare capacity. Therefore, the effect of wind power on primary frequency regulation should be evaluated for the entire power system, not for individual wind farms. However, this statement has encountered new problems, namely what kind of wind power operators are willing or must deploy energy storage systems (whether each wind farm builds a distributed energy storage system locally, or builds a centralized energy storage system according to the system plan) .
In addition, the author pointed out in the conclusion part of the study that the flywheel energy storage system can well meet the energy storage performance requirements of the wind farm participating in the primary frequency regulation of the system (long cycle life, fast response speed, suitable for short-term discharge, etc.). Since the most important thing for primary frequency modulation is the power output capability, the energy storage required should be a "power-based energy storage" technology.
2. Operation strategy of energy storage
The traditional generator set participating in primary frequency regulation should have the power/frequency characteristics as shown in Figure 1, and its power output is adjusted around a reference value. If the frequency remains unchanged at 50Hz, the power generation of the set will not change.
Obviously, this operation strategy cannot be verified in a single wind farm with strong output power fluctuations. In order to ensure that a single wind farm has the same frequency regulation efficiency, the energy storage system should have the following two functions:
1) Adjust the power output according to the frequency adjustment requirements.
2) Maintain the relevant operating reference value of the system (for example, the frequency is 50Hz) without being affected by the change of power generation.
These requirements are likely to be unrealistically large in capacity configuration for energy storage systems. We know that for a certain scale of regional grid, the instantaneous output of wind power can be considered to be almost constant for a few minutes.
In this way, the effect of large-scale wind farm clusters (combination of multiple wind farms) on the primary frequency regulation of the system can be achieved through the energy storage system. During operation, the energy storage system only adjusts the power at the grid-connected wind farm, while keeping the output power of each wind turbine unchanged (often operating in the maximum power output mode). Due to the natural dispersion of different wind farms in geographical locations, the overall equivalent smoothing effect is achieved, so that the power of the entire wind farm cluster follows the expected active power/frequency droop characteristics.
3. Management of the state of charge of the energy storage
The primary frequency modulation of wind power is not like the traditional generator set, which is limited by the mechanical conditions of the set, and can only provide variable power between the minimum power value and the rated power value, but the primary frequency regulation process of wind power will be restricted by the state of charge of the energy storage system.
Each type of energy storage has a minimum state of charge below which the energy storage device risks degrading too quickly. In addition, in order to have the same available capacity during charging and discharging, to maintain the state of charge of the energy storage system at an intermediate value as much as possible to avoid the extreme situation shown in Figure 2, which is necessary for the power regulation application of energy storage.
During the actual operation of the power system, its frequency generally fluctuates around the rated value (50Hz on average). Under the condition of slight frequency fluctuation and zero average frequency deviation, the state of charge of the energy storage system should be maintained in an effective state and close to the reference value. However, when the system frequency is significantly disturbed, the reference value of the state of charge of the energy storage system may need to be reset. Therefore, it is a very critical and necessary issue to determine how much spare capacity the wind farm should set in terms of power and energy.
In order to realize the effective participation of a wind farm in primary frequency regulation, there must be enough spare capacity to achieve the maximum power required for frequency regulation and maintain it for at least 15 minutes.