Simulation and application analysis of a hybrid energy storage station in a new power system

0 Introduction

China’s renewable energy resources are abundant,with the potential for photovoltaic (PV) and wind power resources reaching approximately 22 billion kW and 35 billion kW,respectively.However,the current development of renewable energy sources in China is less than onetenth of that of non-renewable resources [1].Wind power,photovoltaics,and other renewable energy sources pose fundamental challenges to the electrical grid compared with conventional sources such as thermal power and hydropower.These challenges include intermittent and variable outputs,a low redundancy of grid components,and fast control responses.From a time-scale perspective,these challenges can be divided into issues related to power quantity balance,grid safety,and stability.Owing to the constraints imposed by natural conditions on the output of renewable energy sources,they are generally not actively involved in regulating the power-quantity balance of the electrical grid.Furthermore,owing to the limited design margins of current devices,the capacity of the grid to support disturbances is constrained during periods of grid perturbation.Converter systems,primarily based on components such as insulated-gate bipolar transistors (IGBTs),exhibit rapid control responses that depend on the system conditions,leading to potential oscillations in certain operating scenarios [2-8].Renewable energy sources,such as wind and solar power,are intermittent energy sources that cannot be stored and lack the capacity to meet electricity demand.However,this can impose an additional burden on the power grid.The substantial integration of renewable energy sources into the grid results in a decrease in the safety and stability of the main power grid;this issue becomes increasingly severe as their share in the grid energy mix increases [9].This paper describes gridforming (GFM) and grid-following (GFL) converters.The advantages of GFM and GFL energy storage converters are then described,and the ratio of these two types of energy storage converters that should be present in energy storage systems is discussed.

Regarding the issue of the ratio between GFL and GFM energy storage systems,several studies have been conducted both domestically and abroad.(“domestic”in this instance,i.e.,China.) In [10],based on the constrained range of the short-circuit ratio at the grid connection points of new energy,a small GFM power conversion system was introduced to enhance the overall short-circuit ratio of a hybrid energy storage system.The analysis determined the minimum proportion required for GFM energy storage devices in the system and concluded that the installed capacity ratio of GFM power sources should not be less than 30%.In [11],the relationship between the dominant oscillation mode damping ratio of a system under multiple frequency bands was investigated,and the quantifiable indicators of GFL or GFM support capacity were determined.In [12],an investigation was conducted to address the frequency deviation issue in islanded microgrids composed of GFL and GFM converters under fault disturbances.Using eigenvalue analysis,this study examined the variations in system eigenvalues and dominant state variables under different penetration rates.However,the study did not provide precise numerical values for the optimal ratio between the GFL and GFM components.Currently,there is limited literature on the optimal capacity ratio between GFL and GFM systems,indicating that research in this area is still in its early stages.

In this study,research methods for GFM and GFL hybrid energy storage power stations are proposed.Two different converters and energy storage systems are combined,and the two types of energy storage power stations are connected at a single point through a large number of simulation analyses to observe and analyze the type of voltage support,load cutting support,and frequency support required during a three-phase short-circuit fault under different capacity configuration scenarios.

1 Types of energy storage stations

There are many types of energy storage systems categorized by their media,including physical,chemical,electrochemical,thermal,and magnetic storage [10].Currently,the most used storage methods are physical and electrochemical.These classifications are shown in Fig.1.

Fig.1 Classification of energy storage technology in ESSs

Pumped hydrostorage and compressed air energy storage,which are the most common forms of physical energy storage,are relatively mature technologies.However,pumped hydrostorage require strict geographical conditions and is not highly efficient.In contrast,compressed air energy storage has lower geographical requirements but incurs higher costs than pumped hydrostorage.

Electrochemical energy storage stations use chemical reactions within batteries to convert energy through charging and discharging processes.They generally consist of battery management systems (BMSs),power conversion systems (PCSs),energy management systems (EMSs),cooling systems,fire protection systems,and lighting and monitoring systems,including collection circuits and transformers [14].A BMS is a control system that ensures safe,reliable,and efficient battery management.It can monitor and manage battery temperature,voltage,and state of charge,and it is compatible with protocols for the PCS to manage the charge and discharge of battery clusters [15].An EMS is used to monitor,control,and manage electrical energy.It primarily manages the use,supply,and distribution of electrical energy,monitors the state of grid operation,and enhances grid reliability.

PCSs are crucial electronic devices in an ESS.PCSs control battery charging and discharging,perform AC/DC conversion,and are capable of rapid adjustment.Therefore,these systems can address the issues related to the fluctuation of renewable energy power and enhance the quality of electrical energy in the grid.Depending on their control modes,energy storage converters can be classified into two types:GFL and GFM.The PCS of a GFL connects to the grid and can adjust the grid frequency and voltage as required while controlling the output load.The PCS of a GFM can enhance the voltage and frequency support and serve as an emergency backup power source during disasters and emergencies.The PCS supports the independent operation of microgrids,helps balance loads,facilitates renewable energy production,and provides electricity to remote areas.

2 Power Conversion Systems

2.1 GFL Converter

Currently,most energy-storage devices in renewableenergy facilities utilize GFL converters for power input and output.These converters regulate the input current to achieve their operational characteristics,and in terms of their working behavior,GFL converters can be approximated as controllable current sources in parallel with high impedance,as illustrated in Fig.2.The control structure is shown in Fig.3 [16].The structure of the GFL converter consists of two stages.The DC-DC converter is used to maximize the power point tracking of renewable energy sources,whereas the voltage source converter (DC-AC conversion) transforms DC energy into a form that can be fed into the AC grid [16].The control structure of a converter typically consists of the following components:sampling,a phase-locked loop (PLL),outer-loop power control,inner-loop current control,and a pulse-width modulation (PWM) generator.

Fig.2 GFL converter simplified into a controlled current source

Fig.3 GFM converter structure

Synchronization with the grid requires phase information measurements at the point of common coupling (PCC) using a PLL,as illustrated in Fig.3.By transforming the voltage vector into a dq frame and regulating the q-axis voltage using a proportional-integral (PI) controller,the phase angle of the PCC voltage is obtained through a feedback control loop.

Currently,GFL converters are well established compared to GFM converters.GFL control allows for independent control of active and reactive power but requires a certain level of grid strength support.These converters can only operate while connected to the grid and cannot operate in off-grid mode.GFL converters rely on phase-locked loop synchronization.However,when the grid strength is low,there is a strong coupling between the phase-locked loop and grid impedance,significantly reducing the smalldisturbance stability of the GFL converter.Energy storage converters operate in two modes:charging and discharging.Variations in charging and discharging can significantly alter the steady-state operating point of the grid.These changes affect the small-disturbance synchronization stability of the grid through the PLL.

2.2 GFM Converter

GFM and GFL converters differ fundamentally.GFM converters can be represented as an ideal AC voltage source with low output impedance,as shown in Fig.4 [17-18].

Fig.4 GFM converter simplified into a controlled voltage source

The voltage amplitude E and frequency ω can be set using control loops.GFM converters are designed to be the primary sources of voltage and frequency control,effectively shaping the grid itself,rather than simply following it [19].This concept and functionality are particularly critical,as the grid expands to incorporate a higher proportion of renewable energy sources with variable outputs.In contrast to GFL converters,which require a phase-locked loop for grid synchronization,GFM converters are not affected by the oscillations or instability introduced by the phase-locked loop.

GFM achieves flexible output power adjustment by controlling the phase and magnitude of its internal potential,exhibiting voltage-source characteristics.The expressions for the active power P and reactive power Q of the output are given by (1) and (2),respectively.

Here,E and U represent the virtual internal potential of the GFM converter and external grid voltage,respectively.φ is the phase angle difference between the GFM converter and the external grid. Z and θ represent the magnitude and angle of the interconnection impedance between the GFM converter and the synchronous generator [20].

The synchronization strategy of a GFM converter is similar to that of a synchronous generator.By referencing the power expression of a synchronous generator,the expression for the output power P of the GFM converter in an inductive grid is obtained,as shown in (3).

Here, Ug represents the grid voltage, Uout is the converter output voltage,and X denotes the line reactance.

GFM converters have the following advantages:

1) They can simulate the external characteristics of rotating machinery,enabling multiple power sources to synchronize and operate without PLLs.They automatically distribute power and provide system inertia and short-circuit capacity.

2) They offer high-performance primary frequency control,voltage regulation,and damping control,thereby enhancing the grid frequency,voltage,and power angle stability.

3) They improve the harmonic impedance characteristics of the system,reducing the risk of subsynchronous resonance and mid-to-high-frequency oscillations in the grid.

GFM emulates the control of conventional generators while addressing the limitations of conventional generator control.This includes improving the damping control,speed control,and excitation control performance.Depending on the system requirements,it can further incorporate functions,such as increasing the DC components and actively suppressing short-circuit currents.

3 Comparison of Control Methods

3.1 GFL Converter Control Methods

GFL technology is widely used in various types of ESSs for wind and solar power generation.It offers a fast response and grid synchronization,making its operation straightforward.However,it relies on the PLL detection of PCC voltage during grid connection,which results in a significant phase lag in the observation system and an inability to provide transient inertia support during faults.Much research on GFL converters has been conducted,and many control strategies and optimization methods have been developed.A distributed control strategy is described in [21].Here,a distributed control system architecture was adopted for modular multilevel PCSs.AC/DC power-decoupling control is performed through the central control unit,and the submodule control unit performs State of Charge (SOC) sag control according to information from the local submodules to achieve a battery energy balance in the submodules.A resonant controller system is described in [22].In the case of an unbalanced three-phase load,based on the resonant controller system control,the system is simplified and the output voltage distortion caused by the unbalanced load is suppressed.Various strategies and control methods have been proposed to improve the performance and stability of GFL converters under various operating conditions.

3.2 GFM Converter Control Method

GFM control has four main strategies:1) droop control,2) virtual synchronous generator (VSG) control,3) matching control,and 4) virtual oscillator control [23-27].Among these,the droop control and VSG strategies have been extensively researched.Based on the operational principles of VSG,matching control and virtual oscillator control represent nonlinear control amplification systems.They were proposed relatively later than the first two control strategies,and they have not yet been adequately researched.

Droop control,which is one of the most used control strategies in GFM control,selects frequency droop characteristics similar to those of traditional synchronous generators as the source control method.It adjusts the frequency based on changes in the output active power,eliminating the need for mutual coordination among units,resulting in simple and reliable control with a fast response.However,droop control has limitations,such as its tendency to induce system oscillations,low stability,and the potential for control coefficients and grid impedance to significantly affect operational characteristics.

The control strategy for a VSG involves referencing and simulating the electromagnetic and mechanical equations of a synchronous generator,primarily focusing on its rotor and primary frequency control processes.It adds inertial support to the converter to regulate the voltage and frequency of the grid.GFM control technology,primarily structured around the VSG control strategy,can be implemented in the GFM converter,significantly enhancing system stability.This control strategy has a wide range of applicable scenarios.However,it also has disadvantages,such as a strong reliance on AC-side voltage,current,and power measurements,as well as limited control over DC voltages.

The matching control strategy is an extension and improvement to the VSG control strategy.Unlike traditional VSG control,matching control utilizes the DC capacitor voltage instead of a synchronous machine rotor to achieve power and voltage synchronization.Therefore,this is referred to as DC capacitor voltage control.The advantage of matching control is that it allows for a controlled DC voltage,unlike VSG control.However,it involves a nonconstant DC voltage.This control strategy requires sufficient DC capacitor capacity and is suitable for the parallel connection of multiple wind turbines on the DC side or in DC grid systems;however,it is not suitable for controlling individual wind turbines.Currently,research on matching control is limited.

In highly advanced power systems driven by renewable energy sources,GFM technology can provide voltage and active inertia support and simulate the electromechanical transients of synchronous machines for power synchronization,control damping,and output impedance.It has the advantage of operating independently without a grid connection.However,GFM technology also has limitations,as it may lose synchronization during strong grid operations and face issues related to voltage-source paralleling.The advantages and disadvantages of GFL and GFM ESSs are compared in Table 1.

Table 1 Advantages and disadvantages of GFL and GFM ESSs

Table 2 Two energy storage capacity ratio scenarios

To maximize the advantages of both types of converters,the concept of a hybrid energy storage station was introduced.In this approach,energy storage units with different capacities are configured within a storage station to satisfy various grid requirements.This paper provides a brief analysis of their capacity ratios and dynamic response characteristics through simulations.

4 Modeling of Equivalent Circuits of GFM and GFL Converters and Short-circuit Ratios

The hypothetical scenario contains n energy-storage power stations,m passive nodes of the AC grid,and k infinite bus bars.The frequency-domain method was used to analyze the voltage-source characteristics of the GFM converter.The impedance model is as follows [27]:

In (4):

Kpi and Kii are the proportion and integral constants of the current inner loop transfer function ;Kpv and Kiv are the proportion and integral constants of the voltage outer loop transfer function ; TVF is the time constant of voltage feedforward filtering ;kF is the current feedforward coefficient;J and D are the inertia and damping coefficients,respectively;LF and CF are the filter inductance and filter capacitance,respectively;and ICd0 and Vd0 are the steadystate values of the d-axis current and voltage,respectively.The model of the GFM converter connected to the power grid through the transformer is a branch of the series equivalent reactance Zi of the voltage source.

Let the ratio of GFM energy storage capacity and total capacity of energy storage power station in the system PGFM be defined as follows:

Here,SGFM,i is the total storage capacity of the GFM and SGFL,i is the total storage capacity of the GFL.

The branch of the GFM converter is equivalent to a part of the power grid,and its equivalent inductance is ΔB=SGFM·diag[PGFM/Zi],where ΔB=SGFM·diag[PGFM/Zi],diag[·] represents the diagonal matrix,and the linearized dynamic characteristics of the AC network can be expressed as [28]:

In (6):

Here,Ixi and Iyi are the X-and Y-axis components of the current injected into node i,respectively,and Vxi and Vyi are the X-and Y-axis components of the voltage injected into node i,respectively.⊗ represents the Kronecker product.

If the GFL converters in the system have similar dynamic characteristics (the external characteristics of the impedance matrix under its own capacity basis are the same),the dynamic model on the device side of the system can be expressed as follows.

YPLL(s) is the admittance matrix of the GFL converter,and its specific expression is described in [29].Based on multivariable frequency-domain control theory combined with (6) and (7),the closed-loop characteristic equation of the system is:

Equation (6) can be decoupled as:

where λi is the eigenvalue of the matrix According to the Schur complement property of a matrix,λi is also a generalized eigenvalue of a matrix about a matrix [In,0;0,0];therefore,it satisfies:

The small disturbance stability of multi-feed systems depends on the minimum characteristic root λ1,that is,the generalized short-circuit ratio [30].The generalized shortcircuit ratio can be used to reflect the small disturbance stability margins of the system,and it is defined as:

Here,Sac and PN are the short-circuit capacity of the nodes and the active power of new energy,respectively.The short-circuit ratio of multiple stations of the new energy interconnection points should not be less than 2.0 [31].

The simulation results show that the larger the GFM ratio,the larger the short-circuit ratio and the more stable the grid.However,with an increase in GFM,the cost will also increase significantly,and future studies should investigate the costs of this system.

5 Simulation Analysis

A distribution network model was established in a simulation environment using the PSDEdit simulation application intelligent integration platform,version 3.0.1.2.A wiring diagram is shown in Fig.5.The reactance of each line in the diagram is 0.01 Ω,and the capacity of the two transformers is 60 MVA,with a leakage reactance of 0.05 Ω.The generators are rated at 110 kV,and there are two 110 kV/10 kV transformers at points 1 and 5,as well as a single 10 kV/363 KV transformer between points 3 and 11.Points 4,6,and 7 have constant active loads of 100 MVA and constant reactive loads of 20 Mvar.GFM and GFL systems were added at point 3.A three-phase short circuit fault was introduced on the lines between points 1 and 2,occurring at 50 Hz (1 s) and lasting for 10 cycles (200 ms) before clearing.

Fig.5 Circuit diagram

By analyzing the effect of GFM on short-circuit ratio,ess of different types and capacities are inserted into the model in different proportions,which improves the fault recovery rate,reduces voltage fluctuation and alleviates overvoltage events.

Eight scenarios were established,as listed in table below.The total installed capacity of the ESSs was set to 150 MW/300 MWh.The allocation of the ESSs was set considering a simulation duration of 300 cycles (6 s),with Scenario H representing the absence of ESSs.

Figure 6 displays the voltage at point 3,which is the closest point to the energy storage system,for each scenario.After a three-phase short circuit fault occurred at 1 s,the voltage decreased rapidly.As indicated by the red circles in Fig.6,in Scenario H (where no ESS is present),the voltage drops to the greatest extent.In Scenario G,where the GFM system capacity accounts for 100% of the total capacity,the voltage drop is minimized.Examining the red circles above Fig.6,it is evident that in all scenarios with inserted ESSs,the voltage recovery rate is faster compared to the scenario without ESSs.This indicates that ESSs provide voltage support during fault conditions.However,in Scenario F,in which the GFL ESSs were fully deployed,overvoltage occurred during voltage recovery.This overvoltage can be reduced by adding a certain proportion of GFM.Table 3 lists the short-circuit current,capacity,and ratio of point 3 for cases A–H.

Table 3 Short-circuit current,capacity,and ratio of point 3

Fig.6 Scheme 1:(a) Voltage at point 3 in each case for a three-phase short circuit.(b) Partial magnification of (a)

With an increase in the GFM capacity,the short-circuit ratio gradually increased,and the system became more stable.There are two sets of GFL control methods used in the simulation,and the specific differences between the two schemes are as follows:

1) Active power control during fault and recovery:Scheme 1 sets the active power value during fault crossing as 200 MV and sets the initial value of the active climb as–1 MW after crossing,and the system is immediately restored.Scheme 2 sets the active current value during the fault crossing as–30 A and sets the initial value of active climb as–30 MW after the crossing,and the climb after the crossing is restored at 0.5 A/s according to the active current.

2) Reactive power control during fault and recovery:Scheme 1 adopts the specified reactive power value under a three-phase symmetric condition,where the reactive power is 0.1 Mvar.Scheme 2 adopts the voltage-controlled reactive current mode under a three-phase symmetric condition,and the reactive power adjustment coefficient is 5.0 pu.Both schemes adopt the same control mode as the positive sequence when a three-phase asymmetric fault occurs.

The curves of the different energy storage capacity ratio schemes for the two GFL converters are as follows:

Case 1:Three-phase short circuit.The simulation results are presented in Fig.6.and Fig.7.The voltages in the figure are per-unit values.

Fig.7 Scheme 2:(a) Voltage at point 3 in each case for a three-phase short circuit.(b) Partial magnification of (a)

Case 2:Load cutting.The simulation results are presented in Fig.8.

Fig.8 Voltage at point 3 in each case during load cutting

The two schemes had no effect on the load-cutting test,and the simulation results were identical.

In the analyzed system,frequency variations at point 6 during load shedding were simulated under different capacity proportions between the two types of ESSs.At 1 s (50 Hz),a load-shedding event was initiated at point 6,removing 30 MW of the active power load and 6 Mvar of the reactive power load.The frequency response at this point was observed in various scenarios.The simulation duration was 3000 cycles (60 s),following the same conditions as those of the three-phase short-circuit experiments listed in Table 1.Compared to Scenario H (with no ESS),scenarios with inserted ESSs exhibited faster frequency recovery and a shorter time to achieve stability.Scenario G,in which the GFL ESS was fully integrated,performed the best,exhibiting a faster frequency recovery at point 6.This also demonstrates that the GFL ESS accelerates the response of the system during load shedding,resulting in minimal frequency variations.

Utilizing hybrid ESSs with the two types of energy storage converters can simultaneously harness the advantages of both systems,serve the needs of a large power grid,and may be used in future substation installations.Hybrid energy storage combines the benefits of GFL and GFM,enabling a flexible control switchover based on the fault conditions of the grid.GFL energy storage offers rapid grid integration and a fast PLL response,whereas GFM can operate independently,providing autonomous inertia support.Hybrid energy storage combines the strengths of both GFL and GFM.GFM can provide reactive power support,establish a stable voltage and frequency,and achieve self-sustained operation without relying on the grid,whereas behind-the-meter energy storage enhances grid integration and response rates.

In [10],the ratio of GFL to GFM energy storage was established based on the constraint of the short-circuit ratio.It was found that GFM should account for no less than 30% of the total capacity.However,this study did not provide exact numerical values.With an increase in the proportion of GFM energy storage,there is stronger support for voltage during fault occurrence and Duration of the fault.Nevertheless,during the fault recovery phase,a high proportion of GFM energy storage leads to overvoltage issues.Therefore,as the proportion of GFM energy storage increases,other issues arise along the enhanced voltage support capability during faults.

6 Conclusion and Outlook

This study introduced structural principles and grid integration methods for GFM and GFL converters,analyzed their current research status,provided their respective advantages and disadvantages,and conducted a comparative analysis of these systems.Finally,a hybrid ESS was proposed.Using PSD-BPA simulation software,a preliminary study was conducted on the capacity ratio between GFL and GFM systems.The analysis examined the impact of different capacity ratios on the response rates during faults and their effects on the voltage and frequency.This study validated the voltage support capability and frequency recovery speed of GFM systems during faults and disturbances.Through simulations,it was demonstrated that the addition of GFM can significantly improve the shortcircuit ratio of the system and enhance its stability.A GFM to GFL ratio of 1∶1 for a three-phase short-circuit fault is desired,and an overvoltage will occur when the capacity of the proportional GFM is too large.Load-cutting simulations were conducted,demonstrating that a larger GFM capacity results in a faster system frequency recovery rate.This study provided an advanced analysis of GFM and GFL hybrid energy storage simulation analysis,and an analysis and comparison of multiple scenarios based on a mathematical model and short-circuit ratio of the energy storage converters was presented.Further research is required to explore coordinated control methods for hybrid ESSs with multiple GFL and GFM ESSs integrated into different grids.

Acknowledgments

This study was supported by the National Key Research and Development Program of China (Gigawatt Hour Level Lithium-ion Battery Energy Storage System Technology,NO.2021YFB2 400100;Integrated and Intelligent Management and Demonstration Application of Gigawatt Hour Level energy storage station,NO.2021YFB2400105).

Declaration of competing interest

We declare that we have no conflict of interest.