Modelling and Implementation of Multi-source Isolated Microgrid Using Virtual Synchronous Generator Technology

，，，，*

1.China Construction Power and Environment Engineering Co.Ltd.，Nanjing 211100，P.R.China；

2.College of Automation Engineering，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China

Abstract:To improve the living standards，economical efficiency and environmental protection of isolated islands，remote areas and other areas with weak electric power facilities construction，a multi-source independent microgrid system is studied，including diesel generators，photovoltaic power generation system，wind power generation system and energy storage unit.Meanwhile，in order to realize the voltage and frequency stability control of AC bus of multisource microgrid，the virtual synchronous generator technology is introduced into the energy storage unit，and the charge and discharge control of the energy storage battery are simulated as the control behavior characteristics of synchronous motors，so as to provide damping and inertia support for the microgrid.The operation mode and control principle of each energy subsystem are expounded and analyzed.The algorithm principle of virtual synchronous generator and the control method of energy storage unit are given.Then，the working modes of the microgrid system under different environmental conditions are analyzed，and the multi-source microgrid system simulation model is built based on MATLAB/Simulink.The simulation results show that the microgrid system can run stably under different working modes and the energy storage unit using the virtual synchronous generator technology can provide good voltage and frequency support for the microgrid system.Finally，experiments verify the supporting function of energy storage unit on the voltage and frequency of the microgrid system.

Key words：isolated microgrid；wind/solar/diesel/storage；virtual synchronous generator；modelling

0 Introduction

With the rapid development of technology and economy，the level of electrification in China is continuously improving，but there are still tens of millions of people without electricity in remote mountainous areas or islands.At the same time，China has a large number of overseas construction projects along the overseas “the Belt and Road” route，and there may be negative factors，such as weak power grid capacity，high oil price cost，or even double disconnection of oil and electricity，which put great pressure on the duration of the project.These areas have the disadvantages of long transmission distance and high cost of grid extension，so it is necessary to make use of renewable energy according to local conditions and establish independent microgrid system［1］.

Microgrid is a power system composed of distributed energy and load，which is an independent controlled unit relative to the large power grid，and can be operated in grid-connected or independent mode［2］.However，due to the lack of large power grid support，the main power source is needed to provide stable voltage and frequency.The renewable energy（solar energy，wind energy，etc.）in the microgrid is influenced by environmental factors，and the output power is intermittent and random，which causes instantaneous power fluctuation in the operation of the microgrid system［3-4］.Therefore，in addition to the main power source，the energy storage unit is usually required to realize the fast response to instantaneous power fluctuation and power peak shaving and valley filling［5］.

In recent years，a number of independent microgrid demonstration projects have been built at home and abroad［6-8］.The EU-led More Microgrids Project builds small-capacity microgrids through photovoltaic systems and batteries on the Greek Island of Kythnos，while Japan has built independent microgrid demonstration projects on dozens of islands.In 2009，China’s first independent renewable energy power station was built in Dangan Island，Zhuhai，to provide domestic and desalination electricity using solar，wind and wave energy.In 2011，an independent microgrid system designed by Zhejiang Electric Power Research Institute was put into use in Dongfushan Island，Zhejiang Province，with the optimization goal of maximizing energy utilization and battery life.Many achievements have also been made in the theoretical research of independent microgrid system.Ref.［9］established a wind-solar storage microgrid system to realize seamless switching of off-grid operation，but did not give specific modelling methods for each module.Ref.［10］established the wind-solar-diesel storage microgrid system，gave the modelling process of each module，and verified the anti-interference ability of the system through simulation，but did not consider the different working modes of the system.Ref.［11］studied the coordinated control between diesel engine and energy storage system，and introduced droop control into the energy storage system to alleviate the long-term overcurrent problem of diesel engine.However，distributed power sources in the above mentioned literatures are all conventional grid-connected converters，lacking inertia and damping，and unable to provide excellent voltage and frequency support for microgrid systems.

The power electronic interface of conventional grid-connected converter weakens the inertia and damping characteristics of microgrid and brings stability problems［12］.Therefore，some scholars put forward the concept of virtual synchronous generator（VSG）［13］，which makes the converter simulate the inertia，primary frequency modulation and voltage regulation characteristics of synchronous motors.At present，VSG technology is mainly divided into current control type and voltage control type［14］.Compared with current-controlled VSG，voltage-controlled VSG is more suitable for distributed weak grid environment with high energy permeability［15］.Refs.［16-17］analyzed and verified the supporting effect of VSG strategy on the stability of microgrid.Ref.［18］introduced VSG technology into the charging pile of microgrid to make electric vehicles participate in primary and secondary frequency modulation and improve the dynamic performance of microgrid.

In this context，this paper establishes an independent multi-source microgrid system，including diesel engine，photovoltaic power generation system，wind power generation system，energy storage unit and load system.Among them，the output power of photovoltaic system and wind power generation system is greatly affected by the environment，and the battery as the main power source needs large capacity，so diesel engine is chosen as the main power source to provide stable voltage and frequency for the system.In the microgrid system，the photovoltaic array of 30 k W is used in the photovoltaic system，the doubly-fed induction machine is used in the wind power system，the storage battery is used in the energy storage unit，VSG technology is introduced into the front bi-directional AC/DC converter，and the voltage-controlled synchronverter scheme［15］is adopted，which can comprehensively simulate the electromagnetic characteristics，rotor inertia，frequency regulation and voltage regulation characteristics of the synchronous motor.The main load characteristics are also taken into account in the load system.In this paper，each subsystem of the microgrid is theoretically studied，and the MATLAB/Simulink simulation model is established.At the same time，the simulation verification is carried out according to different operation modes of the independent microgrid，and the response and support performance of the energy storage unit to the voltage and frequency of the microgrid system is verified by experimental results.

1 Microgrid System Architecture

The microgrid system studied in this paper is an AC independent microgrid system，which does not participate in grid connection.The microgrid system makes full use of local renewable energy and establishes multiple power modules to cooperate with each other to realize the stable operation of the microgrid.The structure of the microgrid system is shown in Fig.1.The main modules include diesel power generation system，photovoltaic power generation system，wind power generation system，energy storage unit and load system.Diesel generator system is the main power source，which provides stable voltage and frequency for the microgrid system.Photovoltaic power generation system and wind power generation system are operated under the maximum power point tracking（MPPT）mode.The energy storage unit stores electricity when renewable energy sources are abundant and provides power to the microgrid system when renewable energy sources are scarce.The load system is composed of active and reactive loads.

Fig.1 Structure of microgrid system

1.1 Diesel power generation system

Diesel power generation system operates as the main power source，balancing power supply and load consumption，and outputting 380 V/50 Hz AC power.Fig.2 shows the control diagram of the diesel generator system，wherefrefandfare the grid frequency reference and actual grid frequency，andVrefandVthe grid voltage reference and actual grid voltage.The system consists of a diesel engine，a governor，a synchronous generator and an excitation regulator.The governor detects the frequency signal，adjusts the output power of the synchronous generator by controlling the output torque of the diesel engine，and realizes the frequency regulation.The excitation regulator detects the voltage signal and keeps the voltage amplitude of the microgrid stable by regulating the excitation voltage of the synchronous generator.Diesel power generator system maintains the quality of microgrid AC bus voltage by closed-loop control of voltage frequency and amplitude.

Fig.2 Control diagram of diesel power generation system

Fig.3 shows the mathematical model of diesel engine speed regulation system，whereωrefandωare the speed reference and actual speed，respectively.Tmis the mechanical torque.T1，T2andT3are the time constants of the controller.kis the proportional parameter of governor.T4，T5andT6are the time constants of accelerator actuator.Tdis the engine lag time.

Fig.3 Mathematical model of diesel engine speed regulation system

Fig.4 shows the mathematical model of excitation regulation system.In Fig.4，Uref，UandUfare the voltage reference，the actual voltage and the excitation voltage，respectively.TaandT bare the time constants of the phase compensator.ke，Te，kfandTfare the proportional parameter of excitation regulator，the time constant of the excitation regulator，the differential parameter of the excitation stabilizer and the time constant of excitation stabilizer，respectively.

Fig.4 Mathematical model of excitation regulation system

1.2 Photovoltaic power generation system

The photovoltaic power generation system converts solar radiant energy into electrical energy under the photovoltaic effect.The output power of photovoltaic cells is non-linear under the influence of light intensity，junction temperature and load.TheP-Vcurve has the maximum power point，and MPPT is used to improve the generation efficiency.As shown in Fig.5，the photovoltaic power generation system adopts a two-stage topology，whereUpv，Ipv，Udc，UgandIgare the terminal voltage of the photovoltaic array，the output current of photovoltaic array，the DC side voltage of the inverter，the AC bus voltage and the grid-connected current，respectively.

Fig.5 Structure of two-stage photovoltaic power generation system

The front-stage boost circuit increases the terminal voltage of the photovoltaic array，and MPPT is used to control the output power.MPPT control adopts disturbance observation method to detect the change of output voltage and current of photovoltaic array in real time and adjust the duty ratio of driving signal.

The DC/AC stage realizes grid-connected control，the DC into alternating current，through the filter circuit access to the AC bus.The mathematical model of inverter in thedqcoordinate system is

The voltage and current double closed-loop decoupling control diagram is shown in Fig.6，whereV*dcandVdcare the DC voltage reference and actual DC voltage.i*qandiqare theq-axis current reference and actualq-axis current.i*dandidare thed-axis current reference and actuald-axis current.eqandedare the feedforwardq-axis voltage andd-axis voltage.vqandvdare theq-axis and thed-axis voltages of inverters.KuIandKuPare the proportional and integral parameters of the voltage loop.KiIandKiPare the proportional and integral parameters of the current loop.

Fig.6 Control diagram of double closed-loop decoupling of inverters

1.3 Wind power generation system

The wind energy is utilized to drive the blades of the wind turbine to rotate to generate mechanical energy，which drives the doubly-fed induction generator（DFIG）to rotate.The gearbox regulates the speed of the wind turbine and DFIG，and the mechanical energy is converted into electrical energy output through DFIG.The AC excitation converter consists of two back-to-back PWM converters，and the grid-side converter adopts voltage and current double closed-loop control.The wind power generation system is shown in Fig.7.In Fig.7，nwandnrdenote the speeds of fan and DFIG.Pwindis the mechanical energy generated by the fan.PsandQsare the active and reactive power outputs of the DFIG stator.f1andfrare the frequencies of microgrid and DFIG.PrandQrare the active and reactive powers absorbed by the DFIG rotor.PgandQgare the active and reactive power inputs of the microgrid.

Fig.7 Structure block of doubly fed wind power generation system

The mechanical power of the wind turbine is

whereρ，R，λ，β，ω，Cp（β，λ），Vωare the air density，the fan radius，the blade tip speed ratio，the pitch angle，the wind wheel speed，the wind energy utilization coefficient，and the air velocity，respectively.The active power and reactive power emitted by the stator are

whereLmdenotes the mutual inductance in stator winding，Lsthe inductance of the stator winding，Usthe microgrid voltage，andω1the frequency of microgrid.irdandirqare thed-axis current andq-axis current at the rotor side.

1.4 Load system

In recent years，permanent magnet synchronous motor（PMSM）has been widely used in industry such as pumps and fans，which has the advantages of soft start，smooth speed regulation and low harmonic pollution.The motor load system is established to simulate the characteristics of the main power load in the microgrid system.Fig.8 is the control diagram of the permanent magnet motor servo speed regulating system，which adopts double closed-loop control of speed and current.In Fig.8，ia，ibandicare the three-phase currents，nrefandnthe motor speed reference and actual speed，iqrefandidreftheq-axis current reference andd-axis current reference，uqandudtheq-axis and thed-axis voltages of inverters，andsa，sbandscthe drive signals.Space vector pulse width modulation（SVM）is adopted in the system.

Fig.8 Control block of permanent magnet motor servo speed control system

In general，for ideal pump type load，the load torque and speed are quadratic.However，the pump works in a liquid environment actually，so in the starting process and low speed operation，it needs to overcome great fluid resistance.In addition，it also needs to overcome certain resistance in stable operation.The relationship between load torqueTLand speednis about 1.5 power function in stable operation.Its load torque characteristics are shown in Fig.9.

Fig.9 Load torque characteristic curve of electric pump

In the simulation modelling analysis，the resistive load and series RLC load are used to represent the active and reactive loads，respectively，for other equipment.

2 Energy Storage Unit with VSG Technology

When the unbalance between the supply and demand of active and reactive power in the microgrid system causes frequency and voltage offset rating problems，the energy storage unit can play an important role in suppressing the power fluctuation.The energy storage unit adopts a two-stage structure，and the front stage is connected to the grid through a full bridge converter.Because the rated voltage of the battery is low，the three-phase PWM rectifier is a Boost-type converter.Charging needs Buck converter for power conversion，discharge with Boost for power conversion.Therefore，the post-stage adopts Buck/Boost bi-directional converter to connect the battery.Fig.10 shows the circuit topology of the energy storage unit.va，vb，vcare three-phase voltages of the microgrid.LSandRSare the inductors and additional resistors of grid-connected filtering，respectively.Q1—Q6are full bridge circuit switches.Q7andQ8are post-stage DC/DC circuit switches.C1andC2are the voltage stabilizing capacitances.

Fig.10 Topology of energy storage unit

In order to make the energy storage module have inertia and damping to support the microgrid，the VSG algorithm is adopted in the front stage bidirectional AC/DC converter.VSG algorithm introduces the synchronous generator mathematical model into the converter control algorithm，which can simulate the frequency and voltage regulation characteristics of the synchronous motor.

The control block diagram is shown in Fig.11.In the two-stage structure of the energy storage unit，the DC side voltage is stabilized by the front stage，and the primary voltage regulation function is realized.iis the AC input current.vis the voltage of microgrid.VdcrefandVdcare the reference and actual values of the DC voltage.Tmis the virtual mechanical torque.Teis the virtual electromagnetic torque.Jis the virtual moment of inertia.Dpis virtual damping coefficient.QrefandQare the reference and actual value of reactive power respectively.Kis the reactive power regulation coefficient.Mfifis the virtual excitation parameter.vrefis the reference value of the effective value of the microgrid voltage.Kvis the voltage-reactive power regulation factor.KpandKiare the proportional and integral parameters of the voltage loop.PWM denotes pulse-width modulation.RMS denotes root mean square.PLL denotes phase locked loop.The VSG algorithm is

Fig.11 Control diagram of VSG algorithm

The primary frequency regulation is introduced into the post-stage bidirectional DC/DC converter to support the active power of the microgrid.The battery can be charged and discharged with constant current or constant power according to different needs.Charging current in constant current mode is set as

3 Working State and Simulation Analysis of Independent Microgrid System

Based on the MATLAB/Simulink platform，a model of the microgrid system is built，as shown in Fig.12，to verify the stable operation of the microgrid system under different modes.The system works in different modes under different natural environment and battery state of charge.

Fig.12 Simulation model of microgrid system

3.1 Mode 1

When the intensity of wind and light is weak and the renewable energy generation system almost provides no electric energy，the electric energy required by microgrid is provided by the diesel engine generation system and energy storage unit，and the battery works in the discharge mode.If the battery state of charge（SOC）reaches the lower limit，SOC＜SOCmin，the battery cannot discharge and only the diesel engine supplies electricity at this time.

Initially，the diesel engine runs with an active load of 30 kW.Whent=5 s，the active load increases by 10 k W，and the microgrid frequency drops.Fig.13 shows the frequency and voltage magnitude waveform of the microgrid.After a sudden load increase，the frequency immediately drops to about 0.15 Hz，and then returns to 50 Hz after about 5 s.Whent=10 s，the inductive reactive power load of 10 k W is increased to simulate the voltage drop of microgrid.After the sudden increase of reactive load，the voltage drops instantly about 2.5 V，and the voltage recovers after about 2 s.

Fig.13 Waveform of microgrid voltage magnitude and frequency under Mode 1

3.2 Mode 2

When the renewable energy is sufficient and the total electric energy provided is higher than the load demand，there will be surplus system energy in addition to the supply to the load.If the battery is not fully charged，i.e.SOC＜SOCmax，the energy storage unit works under the charging mode and transmits part of the electric energy to the battery for storage.If the battery has reached the maximum state of charge，the energy storage unit does not work.At the same time，the photovoltaic power generation system or wind power generation system should be changed from the MPPT mode to the constant power mode.

To verify the supporting effect of energy storage unit on the voltage and frequency of the microgrid，the diesel engine was set to work in the open-loop mode with the rated output power of 40 kW.The wind power system and photovoltaic power system both output power 30 kW.The energy storage unit works under the charging mode，and the charging power is 20 kW.Meanwhile，80 kW active load is connected as well.

Whent=1 s，the system runs steadily.Due to the open-loop operation of the diesel engine，the inductance in the line and the coil in the motor，the actual load is inductive and the voltage is lower than the rated value.Whent=1.5 s，the energy storage unit enables droop algorithm.The reactive power output of the energy storage unit makes up for the perceptual reactive power missing in part of the system，and the voltage amplitude of the microgrid rises.Att=2.5 s，10 kW active load is removed to simulate the frequency drop of the microgrid.At this point，the frequency-active droop algorithm of the energy storage unit responds，and the battery reduces its own charging power to compensate the required active power.As shown in Fig.14，the microgrid frequency gradually rises under the action of droop algorithm.The output active power and reactive power of the energy storage unit are shown in Fig.15.The current waveforms of the energy storage unit are shown in Figs.16，17.The current changes smoothly with the drooping instruction.

Fig.14 Waveform of microgrid voltage magnitude and frequency under Mode 2

Fig.15 Waveform of active power and reactive power of energy storage unit under Mode 2

Fig.16 Current waveform of energy storage unit under Mode 2

Fig.17 Enlarged current waveform of energy storage unit under Mode 2

3.3 Mode 3

When the total electric energy provided by renewable energy and diesel engine does not meet the load demand，if the battery SOC＞SOCmin，the energy storage unit works in the discharge mode.If SOC＜SOCmin，the battery does not support discharge.To maintain the normal operation of the microgrid，the secondary load needs to be removed.

The diesel engine is set to work in open loopmode and the rated output power is 40 kW.Wind power system and photovoltaic power system output 30 k W power，respectively.The energy storage unit works in the discharge mode，and the discharge power is 20 kW，in which 120 kW active load is connected.

Whent=1 s，the system runs steadily.Whent=1.5 s，the energy storage unit enables droop algorithm.The reactive power output of the energy storage unit makes up for the perceptual reactive power missing in part of the system.As shown in Fig.18，the voltage amplitude of the microgrid rises.Whent=2.5 s，10 k W active load is removed to simulate the frequency drop of the microgrid.At this point，the frequency-active droop algorithm of the energy storage unit responds，and the battery increases the discharge power to compensate the required active power.As shown in Fig.18，the microgrid frequency gradually rises under the action of droop algorithm.The output active power and reactive power of the energy storage unit are shown in Fig.19.The output current waveform of the energy storage unit is shown in Fig.20.The current changes smoothly with the droop instruction.

Fig.18 Waveform of microgrid voltage magnitude and frequency under Mode 3

Fig.19 Waveform of active power and reactive power of energy storage unit under Mode 3

Fig.20 Waveform of current of energy storage unit under Mode 3

4 Experimental Results

In order to verify the response and support function of the energy storage unit using virtual synchronous generator technology to the frequency and voltage of microgrid，experiments are carried out by using CHROMA programmable three-phase AC power supply and a virtual synchronous rectifier.Limited by the power supply，the input AC voltage is 110 V，the energy storage unit works in the charging mode，and the steady-state power is 5 k W.

In order to verify the response and support function of the active power-frequency droop algorithm of the energy storage unit to the voltage frequency of the microgrid，the step value of voltage frequency is 0.25 Hz，and the sudden change of active power of energy storage unit with voltage frequency is 2.5 k W.As shown in Fig.21，the active power and current can change rapidly and steadily with the change of the microgrid frequency，and the frequency of the energy storage unit can quickly track the change.As shown in Figs.22，23，when the power grid frequency increases from 49.75 Hz to 50.25 Hz and decreases from 50.25 Hz to 49.75 Hz，the active power can respond quickly and takes about 80 ms.In order to verify the response and support function of the reactive power-voltage droop algorithm of the energy storage unit to the microgrid voltage，the step change of the microgrid voltage is set to 11 V（110 V×10%），and the reactive power output is 0 kVar.The sudden change value of the reactive power of the energy storage unit with the voltage is 5 k Var.As shown in Fig.24，when the microgrid voltage rises to 121 V，the energy storage unit emits capacitive reactive power 5 kVar.As shown in Fig.25，when the voltage drops to 99 V，the energy storage unit emits inductive reactive power 5 kVar.Reactive power can change rapidly with the change of microgrid voltage.In the process of voltage and reactive power change，the active power and the frequency of the energy storage unit return to the initial value after a small fluctuation，which remains basically unchanged.

Fig.21 Experimental result when microgrid frequency step changes

Fig.22 Experimental results when microgrid frequency step changes from 49.75 Hz to 50.25 Hz

Fig.23 Experimental results when microgrid frequency step changes from 50.25 Hz to 49.75 Hz

Fig.24 Experimental results when microgrid voltage step changes from 110 V to 121 V

Fig.25 Experimental result when microgrid voltage step changes from 121 V to 99 V

5 Conclusions

A multi-source independent microgrid system is studied in this paper.The diesel generator is used as the main power source to maintain the stability of the voltage and frequency of the microgrid，the energy storage unit adopts the VSG technology to support the microgrid together with the main power source，and photovoltaic and wind power systems work under the MPPT mode.Based on MATLAB/Simulink，the simulation model of multi-source microgrid system is built to verify the reliability of independent microgrid system and the supporting function of energy storage unit under different working modes.The response and support function of the energy storage unit to the voltage and frequency of the microgrid system is also verified by experiments.