Inertia Problems in Power Grids
From a physical point of view, inertia is defined as: the ability of an object to resist external forces that disturb its state of motion.
In the power system, we often call it inertia, which means the kinetic energy stored in the rotor, the unit is MW S, the expression:
E is the rotational kinetic energy, J is the moment of inertia, ω is the rated angular velocity of the generator, r is the radius of rotation, and m is the mass of the rigid body. In addition, the inertia time constant M can also be used to express the inertia, which is related to the rated capacity. The specific relationship is
Here H is the inertia time
The total inertia of the system is the inertia of the starting unit and
In the new energy grid, wind power does store inertia, but since wind power is connected to the grid through an inverter, the stored inertia (expressed as the rotor speed of the wind turbine) is actually completely decoupled from the grid frequency of the grid. In addition, photovoltaics are also true, and there is no inertia in photovoltaics. Therefore, photovoltaic and wind power cannot participate in the frequency regulation of the power grid like synchronous machines. Then when the grid frequency fluctuates, due to the lack of inertia, the adjustment ability will be greatly reduced.
For new energy, there are two ways to provide inertia support for the grid, one is the current source type virtual inertia, and the other is the voltage source type virtual inertia. For a whole power grid, its source of inertia can be expressed as the following formula:
As the proportion of new energy connected to the grid increases, the inertia constant of the grid decreases:
The reduction of inertia will affect the frequency stability. The definition of frequency stability is: after the power system receives a small disturbance or disturbance, the system frequency can be maintained or restored to the allowable range without frequency oscillation or collapse.
Analysis of Frequency Stability Improvement Method for Low Inertia Power System
Energy storage is a fast-response resource that can input a large amount of active power to the system in a short period of time to make up for the gap, reduce the frequency change rate of the system, and reduce the maximum frequency deviation of the system. Due to the continuous change of the frequency signal, the traditional control method can not achieve the desired effect, you can try fuzzy control and model predictive control method to control.
The energy storage on the power generation side can be operated in conjunction with the generator set, and the frequency can be adjusted in conjunction with thermal power.
The main function of grid-side energy storage is to ensure power transmission and distribution and frequency regulation, and improve the stability of the power system.
User-side energy storage includes electric vehicles and charging piles, which can quickly charge and discharge when the load is disturbed to respond to system frequency changes.
Control strategy of energy storage participating in frequency regulation
1. Droop control: Participate in frequency regulation by simulating the droop characteristics of the synchronous machine through energy storage:
It can be seen that the droop control actually responds to the frequency fluctuation of the grid by setting a proportional coefficient, and when the frequency deviates, the energy storage converter transmits active power proportional to the frequency deviation to the grid. This method will also cause problems, that is, when the system frequency changes too much, the active power provided by the energy storage according to a fixed ratio cannot meet the frequency modulation requirements, which will cause the system frequency to continue to drop.
2. Virtual inertia control: simulate the inertia response of synchronous generators to participate in frequency modulation, the expression is:
Where M_E is the virtual inertia coefficient of energy storage. It can be seen that similar to droop control, this method controls the output power of the converter by changing the reference frequency. But like droop control, its disadvantage is that it cannot improve the steady-state value of frequency deviation when the frequency is greatly disturbed.
However, the above two methods have limitations, that is, the state of charge (SOC) of the energy storage is not considered. When the frequency deviation is large, the energy storage needs to provide a large amount of active power in a short time, but the frequency recovery is still slow, and the storage Insufficient energy capacity. After a long period of charging and discharging, the energy storage may be saturated or exhausted due to the absorption or release of active power. Once such a situation occurs, the high probability of energy storage charging and discharging power suddenly withdraws from frequency regulation, and the system may fail. A secondary active power deficit occurs, causing a secondary drop in the system frequency.
In order to solve this problem, there are some frequency modulation strategies that take into account the SOC state, such as giving different charging and discharging efficiencies at different stages of the SOC, so that the battery can be maintained in a state that can be charged and discharged for a long time.
as shown in the picture
Of course, in addition to this, there are some design game theory and shared energy storage strategies for optimal scheduling, which we will talk about later.
References for this article: [1] Ye Lin, Wang Kaifeng, Lai Yening, Chen Hao, Zhao Yongning, Xu Xian, Lu Peng, Jin Yifei. Analysis of power system frequency characteristics under low inertia and review of battery energy storage frequency modulation control strategy [J]. Power grid technology ,2023,47(02):446-464.DOI:10.13335/j.1000-3673.pst.2022.1269.