An overview of grid-forming technology and its application in new-type power system

0 Introduction

With the exacerbation of climate change,accelerating energy transition is imminent,and it is essential to vigorously develop renewable energy generation.The International Renewable Energy Agency (IRENA) predicts that renewable energy will generate over 70% of the global electricity output by 2050 [1].To achieve the“dual carbon”goal,China is actively promoting the establishment of a new-type power system,entailing the large-scale grid connection of renewable energy sources with strong randomness and volatility,as well as the significant integration of electric vehicles,distributed power sources,and other interactive devices [2].In 2023,China added 290 GW of renewable energy.By the end of 2024,China’s total installed capacity of renewable energy is estimated to reach 1.3 TW,accounting for approximately 40% of the total installed capacity;by 2060,the share of renewable energy generation will exceed 65% of the total electricity output,and the share of electricity in terminal energy consumption will reach 70% [3].

Currently,most installed renewable energy facilities,such as wind and solar PV,use grid-following control,which mainly injects active power into the system with weak voltage and frequency regulation capabilities.With the large-scale integration of renewable energy sources,the system exhibits characteristics such as low inertia,low disturbance withstand capacity,and weak voltage support capability,which lead to prominent problems such as system voltage stability,frequency control,and wideband oscillation,and pose new challenges to system security and stability [4-5].To address this challenge,various grid-forming inverter-control technologies have been proposed.Grid-forming converters emulate the features of synchronous generators,that is,they establish their own reference voltage phasor through power exchange with the grid to realize synchronization with the grid.This effectively solves the voltage and frequency stability problems in power systems,improves the local renewable energy accommodation capacity and power supply quality of local grids,and can be applied in weak grids or even isolated grid scenarios to provide a stable AC voltage and strengthen the grid.

This article addresses the principles,control strategies,equipment types,and application scenarios of grid-forming technology and validates the role and effectiveness of gridforming technology in power systems with an extremely high proportion of renewable energy.Research has shown that this technology provides a promising solution to the security and stability challenges caused by the high proportion of renewable energy integration.

1 New-type power system and grid-forming technology

1.1 Challenges faced by new-type power system

Using renewable energy as a primary energy source is a defining feature of a new-type power system and its fundamental difference from traditional power systems.As a consequence of high renewable energy penetration,the two main challenges of energy adequacy and security are presented.

1.1.1 Adequacy

In traditional power systems,conventional generating units can track the power load curve with the power source following the load,maintaining the real-time balance of the system,whereas the new-type power system is oriented towards power source-grid-load-storage coordination.As renewable energy generation exhibits strong intermittency and volatility,a higher share of installed renewable capacity in the power system increases the probability of an imbalance between electricity generation and consumption.This will rapidly deplete the system’s flexible resources,making it more challenging to maintain the power balance.

1.1.2 Security

Currently,with the rapid increase in the installed capacity of wind power,photovoltaics,energy storage,and DC converter stations in power systems,most gridconnected converters use grid-following control,which has poor overcurrent withstand capability,weak voltage support capability,low mechanical inertia,and low damping.This results in insufficient voltage-and frequency-regulation capabilities of the power system.At the same time,the power grid’s power angle stability becomes more and more complex,and broadband oscillations have been reported in certain number of projects with wind power worldwide [6],which pose a more detrimental threat to the secure and stable operation of the system.Relevant research indicates that when the penetration rate of grid-following equipment reaches 60–70% or more,the system faces significant security and stability problems [7].Especially for weak grid scenarios with an extremely high proportion of renewable energy,security problems become more prominent.

1.2 Role of grid-forming technology in new-type power system

Grid-forming technology can establish the electric potential necessary for the stable operation of a power system before,during,and after a disturbance,thereby playing a crucial role in improving the frequency and voltage stability of the system.

In terms of frequency support,the grid-forming inverter introduces virtual inertia and primary frequency control to participate in the rapid adjustment of tsystem frequency,so as to make up for the lack of inertia in power systems with a high proportion of renewable energy,thereby improving the frequency stability of the system.In terms of voltage support,a grid-forming inverter can quickly provide a shortcircuit current during a fault.There is no control delay or slow reactive power recovery issue associated with the gridfollowing equipment after the fault,which can effectively alleviate the voltage deviation of the system during the fault and the post-fault transient overvoltage.When the penetration rate of renewable energy reaches 75% and the short-circuit ratio is around 1.5 or below,applying gridforming inverter control technology makes it easier to achieve extremely high proportions of renewable energy integration [8].

2 Principles of grid-forming technology

2.1 Difference between grid-forming and gridfollowing control

The fundamental difference between grid-following and grid-forming converter controls is their synchronization mechanisms [9].Grid-following converters are designed to follow the grid voltage and frequency as the generator terminal voltage reference using a phase-locked loop (PLL),and then modulate their output current to feed the extracted active and reactive power into the grid.This gridfollowing behavior resembles that of a high-impedancecontrolled current source,as shown in Fig.1(a).Using gridforming converters,synchronization can be achieved in a manner similar to that of a synchronous generator.They can independently set and maintain the magnitude and phase of the output voltage through power exchange with the grid without requiring a PLL.Active support for grid voltage and frequency is usually achieved using a powersynchronization control strategy.The behavior of a gridforming inverter can be approximated as that of a controlled voltage source with a low series impedance,as shown in Fig.1(b).

Fig.1 Schematic diagrams of grid-following inverter and grid-forming inverter control principles

Grid-following converters have relatively simple control principles that enable them to quickly and accurately follow changes in the power grid and efficiently deliver active and reactive power.They offer the advantages of low cost and easy grid connections,making them widely used in existing power generation systems.However,grid-following converters need track grid voltages as references and rely on dedicated synchronizing units to provide the voltage and frequency reference values.Owing to their inherent currentsource characteristics,large overshoots and longer settling times inevitably arise from grid disturbances before the PLL is tracked back to the grid phase.In weak-grid scenarios,grid-following converters are prone to significantly reduce the system stability margin [10] and cannot operate independently in the islanded mode [11].

Compared with grid-following converters,grid-forming converters,which can form a voltage phasor at the point of common coupling (PCC) as a voltage source,can achieve synchronization without a PLL [12].Their voltage source characteristics and power synchronization strategies render them more resilient to system failures.In particular,in renewable-dominated new-type power systems,gridforming technology can provide a more effective frequency/voltage support for the system and virtual inertia,allowing high levels of renewable power generators to connect with and operate on weak grids.

2.2 Typical grid-forming control strategies

The grid-forming inverter control technology was first proposed for microgrid operation.With the increasing demand for renewable energy sources to participate in power system frequency and voltage regulation,many researchers have successively proposed and developed droop control,virtual synchronous generator control,matching control,and virtual oscillator control schemes [13-16].Table 1 summarizes the characteristics,functions,and primary application scenarios of the four grid-forming control strategies.

Table 1 Functions and common application scenarios of grid-forming control strategy

Overall,droop control is the earliest grid-forming control strategy with a fast response speed and is mainly used in islanded microgrids or as uninterruptible power supplies [17-18].The droop control cannot provide virtual inertia by itself;however,low-pass filters in series can be implemented in its power outer loop to provide an inertia function in practical applications.A virtual synchronous generator (VSG) uses the rotor motion equation of a synchronous generator to control the phase of the controlled voltage (inner electric potential) [19],which can provide virtual inertia and damping.This strategy requires an active power response to the system’s frequency deviation and rate of change of frequency and has higher requirements for stable control of the DC-link capacitor voltage.Currently,VSG is the most widespread [20-24].Matching control simplifies the control structure,simultaneously realizing self-synchronization and control of the DC-link voltage at the grid-side converter.Currently,mainstream renewable power generators (grid-following) control the active power at the generator-side converters and control the DClink voltage at the grid-side converters;matching control has the advantage of better coordination with the current control method of renewable generators [25-29].Virtual oscillator control reproduces the limit-cycle oscillation of nonlinear systems,obtaining a sinusoidal modulation from the oscillating voltage of the physical model [30-34].This strategy has a fast control response speed but is relatively difficult to achieve reactive power control [35-36],and early virtual oscillator control lacks overcurrent protection and fault handling ability,whereas some recent studies have proposed overcurrent protection for it [37].To date,this has rarely been observed in grid-connected renewable energy systems.

3 Grid-forming equipment and application scenarios

3.1 Introduction to grid-forming equipment types

3.1.1 Grid-forming VSC-HVDC links

In VSC-HVDC transmission systems,the use of gridforming control technology is an effective means of supporting large-scale renewable energy transmissions [38].It has the characteristics of fast power regulation speed and flexible operation,and reduces the harmonics of transmission,which not only allows the large-scale grid integration of renewable energy but also provides voltage and frequency support services to the receiving-end grid.Hence,it can play an important role in enabling the centralized delivery of high-proportion renewable energy,fundamentally eliminating the dependence of traditional VSC-HVDC systems on grid strength.In July 2023,the Zhangbei Grid-Forming Flexible DC Grid Project was put into operation,significantly improving the system stability with isolated-grid renewable generation connected to the Zhangbei Flexible DC Grid and solving broadband oscillation problems.Currently,several grids have already seen the successful deployment of grid-forming VSCHVDC technology and have been widely applied in the Three Gorges Rudong Offshore Wind Power Project and several overseas offshore wind power projects.The German VDE/FNN standard,VDE-AR-N-4131,requires gridforming control to be implemented in all VSC-HVDC projects constructed after 2023.

3.1.2 Grid-forming energy storage

Grid-forming energy storage systems can provide frequency and voltage support.The frequency-support function requires controllable energy sources connected to the DC link of the equipment.Electrochemical energy storage,as a flexible and controllable power source with rapid response,is one of the best carriers for grid-forming energy storage applications.For the voltage support function,if the grid-forming equipment needs to provide voltage support to the system and maintain the integrity of the power system in the case of a short circuit or other fault,the short-time overload capacity of the grid-side converter should generally be increased to more than three times the rated value.Currently,grid-forming energy storage with a high overload capacity has been researched and applied [39].In China,relevant equipment has been operated in the power grids of over ten provinces,including Hubei,Inner Mongolia,and Xinjiang.Among them,Qinghai and Ningxia commissioned two 100 MW energy storage stations that use high-voltage direct-mounted energy storage devices and centralized energy storage systems,respectively,making them the largest grid-forming energy storage projects in China.The 250 MW grid-forming energy storage system in the West Murray region of South Australia was put into operation in 2023 and is currently the world’s largest gridforming energy storage system.

3.1.3 Grid-forming static var generator

Grid-forming Static Var Generator (SVG) can provide instantaneous synchronous reactive power support,effectively addressing problems such as slow fault response speed,insufficient support capability,and reactive power inversion of HVDC transmission systems due to commutation failure with traditional grid-following SVGs.The grid-forming static synchronous compensator incorporates short charge/discharge duration (in seconds) high-power-density energy storage devices such as supercapacitors in the DC link based on grid-forming SVGs.Integrating modular multilevel converter technology offers voltage and short-term frequency support through the inertia control of the power grid.Compared with traditional synchronous compensators,grid-forming SVGs have lower losses and lower operation and maintenance costs and are suitable for applications in scenarios with very low shortcircuit ratios,such as load-center substations,renewable energy collection substations,and islanded microgrids.

3.1.4 Grid-forming wind turbine

For grid-forming wind turbines without the addition of energy storage,there are generally two feasible ways to achieve frequency support capability.The first is to reserve a portion of the power generation capacity as an active power backup to provide additional adjustment capability,which requires sacrificing the utilization hours.The second is to continue using maximum power point tracking (MPPT) for the units,relying only on rotor inertia for inertia support.Obviously,this support effect is relatively limited,with the risk of a second frequency drop.Some wind turbine manufacturers have released grid-forming wind turbine version 2.0,which integrates the energy storage at the DC link of the wind turbine generators.The grid-forming wind turbine has better performance,particularly in terms of instantaneous active and reactive power response speeds,adjustable contribution to damping broadband oscillations,transient voltage regulation,and strengthening of weak grid/off-grid power systems.The first commercial gridforming wind turbine demonstration project in China was implemented in the wind power grid parity demonstration project in Kangbao County,Zhangjiakou,Hebei Province,China.

3.1.5 Grid-forming photovoltaics

Similar to grid-forming wind turbines,grid-forming photovoltaic power generation systems can also support grid frequency by maintaining operating reserves through load shedding.In 2017,the U.S.National Renewable Energy Laboratory and the solar PV panel developer First Solar conducted tests on grid-forming photovoltaics,showing that they have the same or better grid support capability than synchronous generators.However,compared to wind turbines,photovoltaic systems lack rotational inertia and have very limited energy storage in DC-link capacitors,making it impossible to provide inertial support without the addition of energy storage.Therefore,the current gridforming photovoltaic systems in China are mainly based on a combination of photovoltaics and energy storage.

3.1.6 Transmission and storage combined gridforming VSC-HVDC system

A transmission and storage combined grid-forming VSCHVDC transmission system solution (Fig.2) was proposed to mitigate the impact of the volatility of grid-connected renewable energy sources,enhance the active power control capacity and inertia support of DC transmission systems on the grid,and reduce the power impact of sending-end faults on the receiving-end grid [40].Compared with the technical route of integrating batteries into the modular multilevel converter (MMC) of the converter station,the scheme of installing independently controlled energy storage devices on the DC link of the DC transmission system is more advantageous in terms of power decoupling control,online deblocking and blocking,storage capacity allocation,system operation,and maintenance [41].Currently,the transmission and storage combined grid-forming VSCHVDC system has been applied in 10 kV and 35 kV smallscale and lower-voltage scenarios for the power collection network and local consumption of electricity from largescale renewable energy bases,such as offshore wind power bases and those in desert,Gobi,and wilderness areas.

Fig.2 Schematic of transmission and storage combined grid-forming VSC-HVDC system

In general,the frequency support properties of various grid-forming equipment are primarily constrained by the energy source connected to the DC link,whereas the voltage support capability is primarily constrained by the overcurrent capability of grid-side converters.It is important to emphasize that although the grid-forming control itself does not strictly rely on energy storage,energy storage will play a crucial role in grid-forming applications owing to its better responsiveness and stability.

3.2 Grid-forming equipment application scenarios

3.2.1 Large-scale delivery from renewable energy bases

With a high proportion of renewable energy sources connected to the receiving-end grid,large-scale delivery from renewable energy bases faces the technical difficulty of a weak receiving-end power grid.Owing to the voltage and frequency support capabilities of the grid-forming equipment,this problem can be solved by introducing gridforming control technology into VSC-HVDC systems.BorWin6,a VSC-HVDC grid integration project of an offshore wind power plant in Germany,will connect to the northern German power grid with“dual high-penetration”characteristics,i.e.,the high penetration of renewable energy sources and the high penetration of power electronic devices.The DC voltage control of multi-terminal MMCHVDC based on virtual synchronous generator technology is deployed to achieve“dual-end”grid-forming control including sending-end and receiving-end grids,and works as a“voltage source”under all conditions.They can autonomously provide millisecond-level fast active/reactive power support under various grid disturbances.This serves as a reference for the large bases in“desert,gobi and wilderness”in Inner Mongolia,Qinghai,etc.,as well as the delivery from offshore wind power bases.

3.2.2 Off-grid islanding operation

Off-grid islanding power supply is an important application area of grid-forming control technology for remote areas without utility grids and is mainly powered by a combination of renewable energy sources and energy storage units,including remote villages and islands.When disturbances occur in the system,the islanding system lacks frequency regulation resources and reactive power compensation capabilities,which can lead to significant fluctuations in frequency and voltage,posing a threat to the stability of the system.To address these problems,gridforming inverter control devices possess various capabilities such as autonomous active power-frequency control,autonomous reactive power-voltage control,virtual inertia and oscillation damping control,and black start capability,which can significantly enhance the reliability of the power supply for islanded microgrids [42].Grid-forming energy storage has become the main power source that supports power grids on islands such as Hawaii and Saint Eustatius.

In June 2023,the world’s first medium-and longterm off-grid operation test of a power system with a high proportion of renewable energy was conducted at the Ejina Power Grid in Inner Mongolia.The Ejina Power Grid is weakly connected to the main grid of Western Inner Mongolia through a 400 km 220 kV single-circuit transmission line,and only 80 MW of solar energy and 30 MW of wind energy are supplied to the grid.The grid connections are shown in Fig.3.

Fig.3 Schematic diagram of the power grid in ejina banner

Owing to the lack of stable and reliable power sources,the grid cannot maintain reliable and stable operation during interconnected-line maintenance (accidental/planned maintenance) in the grid-connection mode.Therefore,a 20 MW/25 MWh grid-forming energy storage device and a source-grid-load-storage coordinated control system were constructed.This enables a smooth transition from grid-connected to island-mode operation and subsequent real-time resynchronization with the grid,validating the characteristics of grid-forming equipment as a voltage source and its disturbance withstanding capability,especially in off-grid microgrid scenarios.

3.2.3 Integration of renewable energy sources into weak grids

In a weak grid with low short-circuit capacity,gridforming technology enhances the short-circuit capacity and supports the integration of more renewable energy sources.This can provide the system with the necessary virtual inertia and damping [43],making the frequency fluctuations smoother and reducing the changes in the voltage magnitude.

In January 2024,the 10 MW/40 MWh grid-forming energy storage system in Suoxian County,Tibet,was the first grid-forming energy storage system implemented in accordance with the T/CES 243-2023 Technical Specifications for Grid Connection of Grid-Forming Energy Storage Systems and was tested according to the T/CES 244-2023 Test Specifications for grid connection of grid-forming energy storage systems.The results showed that grid-forming energy storage can increase the system strength of the power grid and effectively improve the accommodation capability of renewable energy sources.In February 2024,the world’s first grid-forming static synchronous compensator was successfully connected to a wind power plant in Jilin,which has better inertia and damping characteristics than conventional synchronous condensers and can increase the short-circuit capacity of a weak grid.

4 Application of grid-forming technology in new-type power system

4.1 System overview and energy storage configuration

4.1.1 System Overview

Figure 4 shows a diagram of the power grid.The power supply includes 40 MW of thermal power,15 MW of hydropower,350 MW of photovoltaic power,and 38 MW/190 MWh of electrochemical energy storage.The local weak grid mainly consists of 220 kV and 110 kV substations and transmission lines and is connected to the main grid through a 220 kV lines approximately 1,000 km apart.By the end of 2023,the maximum load of the system was approximately 88 MW,and a sharp increase in the mining load was predicted.Currently,the share of the installed new energy capacity in the local area is 86.4%,representing a typical new-type power system dominated by new energy sources.

Fig.4 Diagram of the power grid architecture

4.1.2 Energy storage configuration and gridforming control strategy

The energy storage configuration of the system was as follows:20 MW/100 MWh at Station b,4 MW/20 MWh at Station c,4 MW/20 MWh at Station d,6 MW/30 MWh at Station A,and 4 MW/20 MWh at Station B,all of which were grid-following energy storage systems.

In this study,we plan to transform the aforementioned energy storage device into a grid-forming energy storage system using the PSDEDIT simulation platform developed by the China Electric Power Research Institute and a newly developed grid-forming energy storage module based on this platform to simulate the power system and energy storage device.Figure 5 shows the control block diagram of the energy storage system,including components such as the virtual frequency control,virtual inertia and damping control,virtual excitation control,outer-loop virtual circuit control,inner-loop current control,and pulse control.

Fig.5 Block diagram of grid-forming inverter-based energy storage system

4.2 Simulation &analysis of grid-forming technology on enhancing overall system performance

To study the role of grid-forming equipment in renewable-dominated new-type power systems,this study mainly selected the energy storage at Station b for transformation into a grid-forming system.The main conclusions of the simulation are as follows:

4.2.1 Enhancing the utilization rate of renew ables

The multiple renewable energy station short-circuit ratio (MRSCR) is an important indicator for measuring the support strength of power systems and is also a crucial factor affecting the utilization of renewable energy sources.In the calculation specifications for power system security and stability,the MRSCR is defined as the ratio of the shortcircuit capacity at the PCC of a renewable energy station to the equivalent power of renewable energy,considering the influence of other renewable energy stations.

The formula for calculating short-circuit ratio of multiple stations is as follows:

where Ui,UNi,and Ii represent the operating voltage,nominal voltage of node i and nominal current injected into node i in the grid,respectively;SREi represents the apparent power injected by renewable energy at the ith bus with renewable energy injection;ZEQii and ZEQij represent the selfimpedance and mutual impedance between nodes i and j,respectively.

According to the GB 38755-2019 Code on Security and Stability of Power Systems and the GB/T 40581-2021 Calculation Specification for power system security and stability,the MRSCR should reach a reasonable level.The MRSCR on the low-voltage side of the step-up transformer of the renewable energy generation unit should not be less than 1.5,and that on the high-voltage side should not be less than 2.0,preferably greater than 3.0.

Table 2 presents the calculation results of the multistation short-circuit ratios for different control strategies.The results showed that the MRSCR decreased as the penetration of renewable energy sources increased.Following the requirement that the MRSCR should not be less than 2.0,when the grid-following control strategy is adopted for energy storage at Station b,the proportion of renewable energy output at Station b should not exceed 20% of the existing installed capacity.In contrast,with gridforming control,the maximum output of renewable energy at Station b can reach 40%,doubling that of the former.

Table 2 Comparison of MRSCR at photovoltaic stations b/c

When the proportion of each renewable energy station output was maintained at 40%,an analysis was conducted on the MRSCR improvement of Stations b and c before and after the transformation of Station b towards the gridforming mode.The results show that grid-forming energy storage at different grid connection points has different effects on the MRSCR of the station.Direct installation at the renewable energy station node (i.e.,local transformation) had the best effect on that node.

4.2.2 Enhancing system frequency and voltage support capability

Figure 6 depicts the system frequency response curve and energy storage active power output curve on the event that the thermal power generating unit trips and power output sharply drops by 30 MW.It can be seen from the figures that under the grid-following control strategy,the output power of the energy storage system remains relatively stable when a fault occurs,leading to very small power fluctuations,with a maximum frequency drop of 0.27 Hz.Conversely,under the grid-forming control strategy,the active power output of the energy storage system rapidly increases from 13 MW to 22.3 MW within 0.15 seconds.The maximum frequency drop is only 0.19 Hz,and the frequency quickly returns to a stable value,effectively enhancing the system’s frequency stability.

Fig.6 System frequency deviation and active power output curve of the energy storage system

Figure 7 depicts the bus voltage curve of the photovoltaic-energy storage booster substation and the reactive power output curve of the energy storage system when a three-phase short-circuit fault occurs in the transmission line of Station b.It can be observed from the figures that during the fault,the grid-forming energy storage provided more reactive power.The bus voltage at Station b was slightly higher than that of the grid-following control strategy.After the fault is cleared,under the grid-forming control strategy,the bus voltage at the photovoltaic-energy storage booster substation recovers quickly with a delay of 0.1 seconds,whereas under the grid-following control strategy,the bus voltage drops greatly and recovers with a delay of 0.25 seconds.The grid-forming control strategy exhibits a stronger voltage support capability.

Fig.7 Bus voltage curve and reactive power output curve of the energy storage system

4.2.3 Increase tie line transmission power

To improve the power supply capacity of the local system,the grid company constructed a transmission line with a total length of approximately 1,000 km.Tie-line power transmission capacity is severely limited because of the excessive length of the line.With grid-following energy storage at Station b,Fig.8 shows the power angle curve of a small terminal generating unit under different tie-line power transfers when a single-phase transient fault occurs on the line.It can be observed from the figure that with grid-following control,the tie-line power transmission limit is 45 MW.Under grid-forming energy storage at Station b,Figure 9(a) shows the power angle curve for a small terminal generating unit under different tieline power transfers when a single-phase transient fault occurs on the line.It is evident that the maximum power transmission capability is 70 MW.As shown in Fig.9(b),by upgrading the existing grid-following energy storage of the other stations (18 MW/90 MWh),the maximum power transmission capability can reach 92 MW.The use of grid-forming energy storage can significantly enhance the maximum power transmission capacity of the tie lines in weakly interconnected systems.

Fig.8 Power angle curve of the small generating unit (grid-following energy storage)

Fig.9 Power angle curve of the small generating unit (with a capacity of 20 MW/100 MWh (a);all grid-forming with a capacity of 38 MW/190 MWh (b))

4.3 Simulation analysis of grid-forming technology to enhance the power supply capacity of large mining load

Considering that the new load of the system mainly originates from mining,this study investigates the effect of grid-forming energy storage on enhancing the power supply capability for large mining loads in both grid-connected and off-grid states.Figure 10 shows a diagram of the power grid.There is a new 50 MW mining load in the area near Substation J,which is 50 MW and approximately 300 million kWh of the annual electricity consumption.The proportion of induction motor to the total mining load was 90%.This load was connected to the grid through Substation J,which was linked to 220 kV Substation A through more than 300 km of a 110 kV line,forming a typical long-chain power supply structure.The load substation was equipped with 200 MW of photovoltaic power and 40 MW/160 MWh of grid-forming energy storage.

Fig.10 Diagram of grid-connected power supply system

4.3.1 Grid-connected state

As shown in Fig.11(a),the photovoltaic output dropped sharply by 30 MW,resulting in a decrease in the system frequency.The system frequency variation curve indicates that the grid-forming energy storage can increase its own inertia and perform fast frequency regulation.In Fig.11(b),it can be seen that the active power response speed and support capability of grid-forming energy storage are significantly better than those of grid-following energy storage,which effectively improves the system frequency stability.

Fig.11 System frequency deviations and active power output curve of photovoltaic and energy storage

Figure 12 depicts the bus voltage of the load substation and the reactive power curve of the energy storage system when a three-phase short-circuit fault occurs in the line where renewable energy is collected and connected to the load substation.It can be observed from the figures that during the fault and after the fault is cleared,the gridforming energy storage system provides more reactive power and quickly raises the bus voltage of the load substation to 0.95 pu after the fault is cleared.4.3.2 Off-grid microgrids

Fig.12 Bus voltage of the load substation and reactive power output curve of the energy storage system

The operation of grid-following equipment requires a voltage source within the system to provide reference values for frequency and voltage,thereby achieving control synchronization;therefore,the equipment cannot operate in island mode.As shown in Fig.13(a),the photovoltaic output dropped sharply by 30 MW,resulting in a decrease in the system frequency.Under the grid-forming control technology,the frequency can quickly recover and remain stable.As shown in Fig.13(b),using grid-forming energy storage can provide better primary frequency control services for the system,sharing the grid’s unbalanced power naturally instantly,and the frequency returns to a stable value more quickly.Research has shown that under sufficient capacity conditions,grid-forming energy storage technology can support the stable off-grid operation of mining loads with 100% renewable energy.

Fig.13 System frequency deviations and active power output curve of photovoltaic and energy storage

As shown in Fig.14,when a three-phase short circuit fault occurs in the line where renewable energy is collected and connected to the load substation,the bus voltage of the load substation decreases instantaneously and recovers quickly after the fault is cleared.The grid-forming energy storage system responds instantaneously by increasing the reactive power output to maintain rapid voltage recovery.

Fig.14 Bus voltage curve of the load substation and reactive power output curve of the energy storage system

5 Conclusions and outlook

The renewable-dominated new-type power system faces the challenges of adequacy with respect to electric power and energy balancing and difficulties in grid security and stability owing to the lack of system inertia.Grid-forming technologies are essential for building new-type power systems based on renewable energy sources.

Grid-forming technology gives full play to its role of fast frequency and voltage regulation,system inertia and short-circuit capacity support in new-type power system with an extremely-high proportion of renewable energy.This improves the MRSCR and enhances the stability and reliability of the power supply capability of the mining load.Research also indicates that under sufficient capacity conditions,grid-forming energy storage devices can support stable off-grid operation of mining loads powered by 100% renewable energy.

The grid-forming technology has wide application prospects for new-type power systems.However,at present,grid-forming technology is still in the initial stage of development,and corresponding research needs to be carried out in the aspects of cooperative control of multiconverter equipment,improvement of the overcurrent capacity of converters,corresponding standards and specifications of grid-forming technology,and dynamic power output allocation of synchronous machines,gridforming equipment,and grid-following equipment in a hybrid power system after transient disturbances to better support the development of new-type power systems.

Declaration of competing interest

We declare that we have no conflict of interest.