In the power system, the integration of energy storage devices may have an impact on the power factor of the grid, especially in certain operating scenarios, which may lead to a decrease in power factor and result in fines for electricity consuming enterprises. This is detrimental to the economic benefits of energy storage and the electricity consumption of enterprises. So why does energy storage integration reduce the power factor of the power grid, and how should it be handled?
1 The core reason for the low power factor caused by energy storage integration
Power factor is an important indicator for measuring the proportion of active power to total power in a power system. Power factor=P/S, where P is active power and S is apparent power. The power factor decreases after the energy storage system is connected, mainly related to the following factors:
1. Reactive power characteristics of energy storage inverters (PCS)
Energy storage systems are usually connected to the grid through energy storage inverters. PCS is essentially a power electronic converter, which may generate the following issues during operation:
The non ideal characteristics of switching devices: IGBT and other switching devices have transient reactive power demand during the commutation process, which may cause PCS to inject or absorb reactive power into the grid.
The impact of control strategy: If PCS adopts "constant active power control" without closed-loop regulation of reactive power, its power factor may deviate from 1. For example, when the system needs to respond quickly to active power fluctuations, PCS may temporarily absorb lagging reactive power due to current loop bandwidth limitations, resulting in a decrease in instantaneous power factor.
Harmonic pollution: PWM modulation of PCS generates harmonic currents (such as 5th and 7th harmonics), which increase the apparent power S of the system and indirectly reduce the power factor.
2. Operation mode switching of energy storage system
When the energy storage system switches between charging and discharging modes, there may be fluctuations in reactive power:
Charging mode: The energy storage system is equivalent to an "inductive load", which may absorb lagging reactive power (especially in the early stages of battery charging, where the current is large and the phase lags).
Discharge mode: If PCS control is improper, it may output advanced reactive power (such as when the battery discharges, the inverter may enter the capacitive working area due to DC voltage fluctuations).
Transient process: During mode switching, the phase-locked loop (PLL) of PCS may lose lock due to voltage fluctuations in the power grid, resulting in loss of control of reactive current and short-term lowering of power factor.
3. Grid impedance and system resonance
When the energy storage system is connected to the distribution network, if there is inductive impedance in the grid (such as long lines, transformer leakage reactance), it may form an LC resonant circuit with the filtering capacitor of the energy storage system:
Resonance amplifies harmonic currents of a specific frequency, leading to a surge in reactive power and a deterioration of power factor. When the output filter capacitor of the energy storage system resonates with the grid inductance at a certain harmonic frequency, the harmonic current may reach several times the rated current, significantly increasing the apparent power S
4. The synergistic effect of multiple energy storage units
In large-scale energy storage power plants, multiple energy storage units (such as battery clusters) connected in parallel to the grid may exacerbate power factor issues due to the following reasons:
Parameter inconsistency: The control parameters of each PCS (such as PI regulator parameters and dead time) have slight differences, resulting in uneven distribution of reactive current during parallel connection and overload of some units.
Circulating current problem: Parallel PCS may generate circulating current due to voltage phase or amplitude differences, which contains a large amount of reactive power components and further reduces the overall power factor of the system.
2 The impact of power factor deterioration
1. Increased power grid losses: Reactive power can lead to increased copper losses in transmission lines and transformers, reducing system efficiency.
2. Decreased voltage stability: Lag reactive power can cause a voltage drop in the power grid, especially at the end of the distribution network, which may affect the normal operation of other loads.
3. Risk of electricity penalty: Most power grid companies have assessment requirements for user side power factor (such as fines for power factor below 0.9). If the power factor does not meet the standard after energy storage is connected, it may increase operating costs.
4. Shortened equipment lifespan: Harmonics and reactive currents can cause increased heating of equipment such as transformers and cables, accelerating insulation aging.
3 Solution for power factor optimization
To address the power factor issue caused by energy storage access, a comprehensive strategy of "equipment improvement+control optimization+system collaboration" can be adopted:
1. Static Var Generator (SVG)
SVG generates the required reactive current (lagging or leading) in real time through a voltage source inverter, quickly compensating for the reactive power demand of the energy storage system (response time can reach ms level). The dynamic adjustment range is wide (-1 to+1 power factor), which can simultaneously suppress harmonics and is suitable for high-power fluctuation scenarios (such as new energy distribution and storage systems).
The configuration method is to install centralized SVG at the grid connection point of the energy storage power station, or integrate distributed small capacity SVG in each PCS module to achieve on-site reactive power compensation.
2. Control optimization of energy storage converter (PCS)
Add a reactive power outer loop to the control strategy of PCS, and calculate the required reactive power reference value in real time by detecting the grid voltage and current, so that PCS can actively output or absorb reactive power and maintain a power factor of 1.
3. Hardware design and parameter matching
Optimize filtering parameters: Design PCS filter parameters (such as inductance and capacitance values) based on the impedance characteristics of the power grid, avoiding resonance frequencies;
Choose high power factor devices: use wide bandgap semiconductor devices such as silicon carbide (SiC) and gallium nitride (GaN) to reduce switching losses and reactive power requirements;
Distributed compensation for distributed energy storage: For distributed energy storage (such as user side photovoltaic storage systems), small reactive power compensation devices (such as thyristor switching capacitors TSC) can be installed on-site in each energy storage unit to reduce the transmission of reactive power in the power grid.
4 Implementation suggestions
1. Large scale energy storage power plants
Adopting a combination scheme of "inverter reactive power control+SVG dynamic compensation+active filter harmonic control".
2. User side energy storage system (industrial and commercial energy storage)
Priority should be given to optimizing the inverter control strategy (such as setting a fixed power factor value), in conjunction with small capacitor banks for static reactive power compensation.
3. Microgrid scenario
Adopting droop control and adaptive reactive power compensation, combined with local load characteristics to dynamically adjust reactive power output, to ensure stable power factor above 0.9 during off grid operation.
5 Conclusion
The integration of energy storage systems into the power grid leads to a decrease in power factor, which is essentially the result of the combined effects of reactive power characteristics, harmonic pollution, and system parameter matching issues of power electronic equipment. By using dynamic reactive power compensation devices (such as SVG), optimizing PCS control strategies, and coordinating power grid scheduling, the power factor can be effectively improved.