Capacitor energy storage-working principle, system structure and existing problems

In recent years, with the rapid development of new energy, the energy structure of power system is becoming increasingly complex, and the random fluctuation of load is frequent, which aggravates the frequency fluctuation of power grid.

The combination of energy storage and automatic generation control of thermal power units can greatly improve the frequency regulation performance of the units and extend the life of thermal power units to a certain extent.

The frequency regulation service of energy storage participating units has been rapidly promoted and applied in the energy storage market in China, among which lithium battery energy storage is the most common.

In addition, there is a super capacitor energy storage to assist thermal power units to optimize the primary frequency regulation technology. Reasonable capacitor selection and connection control can effectively reduce the coal consumption of units and improve the flexibility of frequency regulation of units.

In China, the frequency modulation of thermal power units assisted by energy storage is still dominated by lithium batteries, and the super capacitor energy storage is in the feasibility study stage. This article will introduce the working characteristics of super capacitor energy storage.

Principle of super capacitor energy storage

At present, the frequency modulation task in China’s power system is mainly undertaken by thermal power, hydropower and other units. Taking the thermal power unit as an example, due to its large rotational inertia and the need for secondary conversion of energy, the frequency modulation response of the unit is slow and the control accuracy is low.

With the deterioration of the frequency environment of the power grid, the frequency modulation burden of thermal power units increases, which seriously affects the operating life of units. Therefore, it is urgent to improve the existing frequency modulation methods.

With the exploration of frequency modulation technology, the combined fire storage system has shown excellent results when used in AGC frequency modulation. The power difference of AGC frequency modulation command can be given to the energy storage part for response, which can greatly improve the regulation performance of thermal power plants.

How does super capacitor energy storage improve the AGC frequency modulation performance

The improvement of super capacitor energy storage on AGC frequency modulation performance of the unit is as follows:

● Adjust the speed

The charging and discharging time of the capacitor energy storage system is seconds or even shorter. Taking into account the time delay of the control system (measured as 2~5s), the regulating speed of the unit can also be greatly improved.

● Adjustment accuracy

The regulation deviation of the capacitor energy storage system is generally 3% PN (PN is the rated power of the capacitor energy storage system), which is far less than that of the thermal power unit; And because the energy storage system has a fast regulation speed, it can also correct the steady-state regulation error of thermal power units, further improving the unit regulation accuracy.

● Adjust the time

Compared with thermal power units, the response speed of the capacitor energy storage system is faster. Even considering the delay of sampling, control and communication, it is easy to make the overall response time of the unit less than 20s.

In addition, when appropriate energy storage capacity is configured, fatigue damage caused by depth or frequent regulation of the unit can also be reduced.

What are the capacitor energy storage methods suitable for thermal power

The super capacitor energy storage has a very high charge discharge rate and a relatively high voltage retention rate (open circuit 72h, ≥ 98%).

In addition, super capacitor energy storage has higher safety characteristics and longer cycle life, so it is more suitable for short time high power output, as well as the frequency modulation field of thermal power units that require rapid response and long service life.

The system structure of capacitor energy storage assisted thermal power frequency modulation

The super capacitor energy storage unit is a DC power system. First, the DC power is converted into AC power through a converter, and then the voltage is raised to a certain level through a box transformer. Finally, it is connected to the auxiliary power system from the low voltage side of the high voltage auxiliary transformer through the grid connection switch.

The starting point of capacitor energy storage auxiliary frequency modulation is to improve the unit regulation environment and improve the unit’s performance in responding to AGC commands. Therefore, all units have been put into operation. In the initial design of the unit, the access of the capacitor energy storage system was not considered.

At present, the capacitor energy storage auxiliary thermal power frequency modulation projects are all large capacity energy storage connected to the unit auxiliary power system, so it is necessary to study the related problems caused by large capacity energy storage connected to the auxiliary power system.

Problems of super capacitor energy storage connected to auxiliary power

Capacity limitation of high voltage station transformer

The capacitor energy storage system increases the load proportion of high voltage auxiliary transformer to a certain extent. It is generally required that during the charging and discharging process of the capacitor energy storage system connected to the auxiliary power supply, the high voltage auxiliary transformer shall not be overloaded, nor shall reverse power transmission occur

The super capacitor energy storage is connected from the low-voltage side of the auxiliary power. At this time, the total capacity of the high voltage auxiliary transformer can be equivalent to the sum of the auxiliary capacity and the super capacitor capacity. Compared with the auxiliary power system, the capacity and equivalent impedance of the capacitor energy storage system are relatively small, which will not affect the capacity of the high voltage auxiliary transformer.

Protection and control

Considering the reliability and safety of the unit and auxiliary power system, it is necessary to analyze the impact of super capacitor energy storage access on the auxiliary power protection and control system.

● Influence on electric quantity protection of generator and excitation transformer

When the super capacitor energy storage is in discharge condition, it can be equivalent to a voltage source for the entire auxiliary power system.

When a short circuit fault occurs somewhere in the system, the capacitor energy storage provides the short circuit current to the short circuit point. Because the value of the short circuit current it can provide is very small, its influence can be basically ignored.

● Influence on electric quantity protection of generator transformer unit

The super capacitor energy storage is not within the differential protection range of the generator transformer unit, which has no impact on the differential protection; The zero sequence current at the high-voltage side of the main transformer is taken from the grounding zero sequence CT at the high-voltage side, and the protection setting is not affected by the energy storage system;

The rated current and terminal voltage of the generator are not affected by the energy storage system, so the capacitor energy storage has no impact on the compound voltage overcurrent protection at the high voltage side of the main transformer.

● Influence on electrical quantity protection of high voltage station transformer

The setting values of differential protection of high voltage station transformer and complex voltage overcurrent protection at high voltage side are not affected by the current increase after the capacitor energy storage system is connected;

In the low-voltage side zero sequence overcurrent protection, the current is taken from the low-voltage side neutral point zero sequence CT, independent of the capacitor energy storage system access;

In the low-voltage side complex voltage overcurrent protection, the setting values are the branch voltage and rated current of the high voltage station transformer, which have nothing to do with the access of the capacitor energy storage system, so they have no impact on the protection.

● Influence on low-voltage protection of auxiliary power

Considering the characteristics of the capacitor energy storage system, the energy storage system in the combined fire storage system can not be put into low voltage ride through function, which can effectively avoid the impact of the capacitor energy storage system.

The super capacitor energy storage system uses a bidirectional power converter based on high-frequency power electronic devices for grid connection.

When a short circuit fault occurs on the auxiliary power bus, the short circuit current component that the capacitor energy storage system can contribute is small, and the dynamic and thermal stable short circuit currents are within the allowable range, which will not affect the breaking capacity of existing circuit breakers.

Influence of super capacitor energy storage on power quality

Influence on current harmonic at access point

In the process of power conversion of grid connected inverter, the full control characteristics of power electronic devices will cause a certain degree of waveform distortion, resulting in harmonics.

The control algorithm adopted by PCS in the fire storage joint project makes the current waveform injected into the power grid by grid connected inverter approximate to sine wave, the total distortion rate of current harmonic is less than 5%, and the distortion rate of each harmonic is less than 3%, meeting the requirements of energy storage industry standard NBT 31016-2011.

Therefore, the harmonic content generated by the super capacitor energy storage connected to the auxiliary power system is very small, which will not affect the safe and reliable operation of the auxiliary power system.

Influence on access point voltage

The super capacitor energy storage is connected to the auxiliary power system through the PCS, which has a high degree of control freedom. With the application of some efficient control algorithms, the efficiency, power factor and output voltage stability of the converter have been greatly improved.

Among them, the power factor can even reach 0.99, and there is little reactive power exchange with the power grid, so the voltage connected to the bus section will not be greatly affected.

Conclusion

Based on the fire storage combined AGC frequency modulation system, this paper analyzes the impact of super capacitor energy storage connected to the auxiliary power system, and draws the following conclusions:

● Super capacitor energy storage will not overload in response to AGC frequency modulation command, and will not affect the capacity configuration of high voltage auxiliary transformer；
● The connection of the super capacitor energy storage system has little impact on the relay protection and control system；
● After the super capacitor energy storage is connected to the auxiliary power system, the dynamic and thermal stable short-circuit currents of the 6kV bus section are within the allowable range；
● The connection of super capacitor energy storage will not reduce the power quality of the auxiliary power system, and will not affect the normal operation of the auxiliary load.

According to the comprehensive analysis, the access of super capacitor energy storage, one of the potential energy storage technologies, will not affect the auxiliary power system, which has a good reference significance for its combined thermal power units to participate in the application of AGC frequency modulation technology.