- Renewable energies such as wind turbines and solar photovoltaic have gained significance as a result of global environmental p ollution and energy crises. These sources of energy are converted into electrical energy and delivered to end -users through the utility system. As a result of the widespread use of power electronics -based grid - interfacing technologies to accommodate renewa ble sources of energy, the prevalence of converters h as expanded as well. Due to this , the power system's rotating inertia is decreasing, endanger ing the transient stability of the grid. The use of Virtual Synchronous Machine (VSM) technology has been purp osed to mitigate the grid stability problem due to low rotating inertia. The grid -connected inverter used in VSM can be controlled to emulate inertia, which replicates the external features of a synchronous generator. As a result, the rotating inertia is i ncreased to support the power system's stability. A power angle control strategy is developed and the model is simulated in MATLAB/Simulink software to analyze the effects of parameter disturbances on the active power and frequency for a VSM. The system co nsists of synchronous generator, which has been modeled in such a way that the frequency drops is beyond the acceptable range during transient condition due to lack of inertia when VSM is not used. Then, the suggested model incorporating VSM, emulates rota ting inertia, injecting a controllable amount of energy into the grid during frequency transients to enhance transient stability. Key Words : VSM, ROCOF, Transient stability, Inertia – constant, Damping Constant. 1. INTRODUCTION The trend of generation and distribution of power using the Renewable Energy Sources (RES) and Distributed Energy Sources (DES) is increasing rapidly in recent years. Distributed Energy Sources (DES) like Photovoltaic -hydro hybrid micro -grid system seems to be suitable in distant regions having enough hydro sources and to those areas lacking electric grid. However, in comparison to traditional bulk power plants, where the synchronous machine is dominant, the DES/RES units have either a very tiny rotational mass or none at all [1 -2]. The massive rotational weights of synchronous generators are crucial for delivering the necessary inertia to large power systems. However, when the percentage share of PV in the system increases, the system's overall inertia decreases as the size of the sy stem grows. In these PV -hydro integrated systems, the lower inertia will have a substantial influence on transient stability and dynamic performance [3 -4]. Therefore, a large change in load in these systems can cause large frequency perturbation with a significant rate of change of frequency (ROCOF) . As a result, more inertial response is required to give superior dynamic performance and stability during frequency deviation transient periods. The employment of a Virtual Synchronous Machine can give this add itional inertia. The goal of this research is to investigate and simulate VSM using the power angle control approach. It has a wide range of applications. It can be im plemented in the PV -Hydro micro -grid where the resultant inertia of the system is significantly lower. A VSM is a power electronics devic e that stores short -term energy and uses a dispatching algorithm to behave like a synchronous generator. VSM can emulate the characteristics of a synchronous generator by providing inertia and damping properties to the grid or electrical network that is connected with VSM. Whenever there is load perturbation there is an exchange of power between the grid and VSM (i.e. VSM either absorbs or injects power to the grid) [5 -8]. Having these features, a VSM c an minimize frequency changes during transient periods. The VSM consists of an inverter, frequency sensor , and the controller to generate the necessary switching patterns. In PV -Hydro Micro -Grid shown in fig -1, the resultant capacity of the plant wi ll get increased in comparison to that when PV is not added. However, H = H 1 + H 2 (PV contributes zero inertia) The system's total inertia has not risen. As a result, the PV - hydro micro -transient grid's stability will be weak. To enhance transient stabilit y, VSM can be utilized in combination with PV -hydro micr o-grids as demonstrated in fig -1. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 08 Issue: 10 | Oct 2021 www.irjet.net p -ISSN: 2395 -0072 © 2021, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 2 Fig -1: Micro -grid without VSM and with VSM 2. METHOD 2.1 Introduction to Strategy The technique that has been implemented is the power angle control techn ique. Here, the frequency of the grid is sensed and compared with a reference frequency. The change in frequency and ROCOF is then pa ssed to an angle controller. The angle controller produces the power (P_VSM) to be injected by VSM and based upon this cal culated power, the power angle of an inverter is calculat ed which is then passed to a sinusoidal reference signal generator. The th ree-phase sinusoidal reference signals are passed to the PWM generator and the gate signals are given to the inverter from it. The inverter produces the voltage which leads/lags the grid voltage by the same power angle as calculated above. Thus, this difference in phase angle between two voltages is responsible for active power transfer. Functionally, the operation of VSM i s similar to that of an ideal synchronous machine, which can create or absorb active power by adjusting the inverter output. If the load is increased, the frequency of the system will decrease, then the synchronous machine will add extra active power to th e system. On the contrary, when the load is decreased, the frequency of the system will increase, then the synchronous machine will absorb surplus power from the grid. From dc voltage source, the converter of the VSM system produces a set of adjustable 3 -phase output voltage (V 0) having frequency synchronized with the grid voltage. The active power exchange between the inverter and the ac system is done in a similar fashion to that of the synchronous condenser by adjusting the phase angle of the inverter output voltage (V 0). i) If f ref>fgrid, then the inverter generates the active power ii) If f ref<fgrid, then the inverter consumes the active power iii) If f ref = f grid, then there is no exchange of active power . Fig -2: Proposed Scheme of VSM 2.2 C ontrol Strategy A VSM needs to have an appropriate control mechanism so that it can be able to exchange power with the grid depending on the frequency deviation and ROCOF. The control mechanism should allow the VSM to deliver power when grid frequency dec reases and to consume power from the grid when grid frequency increases. The sensed frequency by the frequency sensor is compared with the reference grid frequency, and the change in frequency and the ROCOF are calculated. The power that needs to be exchan ged between VSM and the grid is given by, = + (1) Where K I and K p are emulated inertial and damping constants respectively [4]. They are calculated as, = (2) = (3) Where P NOM is the nominal power rating of the VSM, is the greatest permissible deviation in frequency, is the maximum permitted ROCOF in the system. The VSM rating shou ld be selected such that, under the worst possible transient condition, it can provide deficient power or consume surplus power set up by boundary condition s for frequency change and ROCOF . The nominal power rating of the VSM, ‘P NOM’ should be taken 10% of the PV – hydro system’s overall size. According to ISO standards , the greatest permissible deviation in frequency during the normal operating condition is ±2.5% in either direction, i.e. 1.25 Hz (for 50 Hz system) and the maximum permitted ROCOF in either direction is ±1% i.e. 0.5 Hz/s (for 50 Hz system). Also, the maximum available allowable frequency deviation during transient operating condition s is ±15% i.e. 7.5 Hz (for 50 Hz system). In order to imitate the inertia and damping properties of a synchronous machine, VSM requires the proper selection of the constants K I and K p which are calculated using (2) and International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 08 Issue: 10 | Oct 2021 www.irjet.net p -ISSN: 2395 -0072 © 2021, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 3 (3). Inertia and damping emulation control need to be closely analyzed which are described below briefly.  Inertia Emulation Control : The in ertial power is related to the rate of frequency change. Accordingly, the part ‘ ’ in (1) is responsible for the emulation of inertial property in the system. It directs the inverter to interchange the power with respect to ROCOF in the same wa y that rotating mass does in a synchronous machine. The rate of change in frequency of the system can be impeded by interchanging the power, and hence inertia can be said to be emulated. The amount of inertia to be emulated can be varied by changing the va lue of ‘K I’ [8-9].  Damping Emulation Control : Similarly, the damping effect is related to the deviation of frequency from the reference value. Hence, the part ‘K p(Δf)’ in (1) will be emulating the damping effect into the system. This allows the inverter to inject or absorb power into the system according to the change in frequency, in a similar way as the dam per winding would have done . By changing the value of ‘K p’ the amount of damping to be emulated can be varied. 2.3 Power Angle Control The primary concept behind power angle control is to change the phase displacement angle between the inverter's three -phase voltage and the grid's three -phase voltage. The volt age difference between the inverter's three -phase voltage and the grid's three -phase voltage will be applied to an inductor in each phase. The larger the phase displacement angle, the larger the voltage difference across an inductor, which results in a hig her current through the inductor. The exchanged active power between inverter and grid is given by: P = (4) Where, E= Inverter’s phase to phase rms voltage V= Grid’s phase to phase rms voltage = Phase displacement angle X= Rea ctance in each phase Various power angle control schemes for VSM are available. Although the basic principle is to generate the reference inverter power angle to produce the calculated P_VSM, the methods of generating this angle can vary from other s. The frequency of the grid is measured using the Phase Locked Loop method (PLL). The sensed frequency is compared with the reference frequency (50 Hz) and the error is produced. The error signal is then passed to the P_VSM calculation block which calculates the power to be injected by VSM. This block calculates the power depending on the change in frequency and rate of change of frequency (ROCOF). The P_VSM and the electrical power from VSM at the instant are then passed to the angle controller which generates t he proper amount of phase angle. The angle controller i s based on the linearized small -signal model of the swing equation. The phase angle is supplied to the three - phase sine gen erator which produces the three -phase refe rence sinusoidal signals and is pass ed to the PWM generator. The appropriate gate signals are generated from the PWM generator, w hich will be given to the three -phase inverter. Ultimately, the proper amount of power will be injec ted in to the grid as required . Fig -3: Power Angle Control Us ing Sm all-Signal Model Controller The p ower angle Controller is based on the swing equation given below: (5) Where H = inertia constant = VSM Power to be injected = (power output from VSM) D = D amping constant = power angle Using small -signal model, We get, (6) Implementing (6 ) in the block as shown in fig -4, we get as output. 2.4 MATLAB Simulation The system consists of a synchronous g enerator of 250KVA (1pu) along with a hydraulic turbine and generator and excitation system for the generator. It also consists of 100KW (0.4pu) resistive load which is connected all the time and 50KW (0.2pu) load is connected to the system whose switching time is at 10 seconds. A VSM block is present in the proposed Simulink model that contains the PLL (Phase Locked Loop) block which measures the grid frequency and is supplied to the P_VSM calculation block. The P_VSM calculation block calculates the power to be injected by the inverter to the grid. Then, the calculated power would be sent to the power angle controller block which uses small signal model for swing equations and calculates the power angle δ. After this, the power angle would be sent to the three phase sine generator to create necessary sine waves for the PWM generator to create phase shift. The PWM International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 08 Issue: 10 | Oct 2021 www.irjet.net p -ISSN: 2395 -0072 © 2021, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 4 generator provides PWM to the gate of inverter so that inverter will inject the necessary power to the grid. Fig -4: Power angle controller As the VSM needs to be activated only during the transient period, there needs some activation logic to activate the VSM .This task is done by the a ctivator block as shown in fig -6. This block activates the VSM when the change of frequency is greater than 2 Hz. Fig -5: MATLAB Simulation model of VSM with hydro - generating unit Fig -6: Virtual Synchronous Machine (VSM) 3. RESULTS Simulink model of VSM has been developed in Matlab - Simulink. Simulation results are shown without and with activating VSM. From fig -7, it is observed that frequency drops to about 0.845pu (deviation is 15.5% from normal value) during transient , the condition which is unacceptable. Fig -7: Frequency without VSM From fig -8, it is observed that the frequency drop is reduce d to 0.93pu (deviation is only 7% from normal value) after activation of VSM during transient condition s. Fig -8: Frequency plot after using VSM From fig -9, the VSM injects power after 10 seconds as the extra load is switched on at this instant, and a fter the transient period, the power from VSM is zero where power has been solely provided by the generator. Fig -9: Active Power from grid and VSM From fig -10, the SOC of a battery needs to be 50% at normal condition. After 10 seconds of operation, t he SOC of the battery starts to decrease as the VSM is activated. During this time VSM is injecting power to the grid and the battery is in discharging mode. After some time, VSM starts to absorb International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 08 Issue: 10 | Oct 2021 www.irjet.net p -ISSN: 2395 -0072 © 2021, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 5 power from the grid. During this time, the battery is in charging mode and, thus the SOC of the battery starts to increase . Fig -10: Charging and discharging of battery (SOC) 3. DISCUSSIONS In this research, the VSM is made to improve the transient stability where the frequency deviation is 15.5%. The frequenc y deviation of 15.5% is decreased to 7% using VSM, thus improving the transient stability. Indeed, VSM can be accommodated to do so even if the frequency deviation is greater than 15.5%. Instead of the conventional current control technique to control the VSM, a novel strategy using the power angle control method has been used here. The system is modeled (with low inertia) using only the generator to simulate the effect of VSM in transient stability. However, in a real scenario, there will be a large number of PV arrays and other non -inertial sources along with a hydro - generator. Thus, the PV array can also be accommodated in the system during simulation. The SOC of the battery is to be kept at 50% in a normal state. So, there needs some control logic to br ing the SOC to this level after the transient condition. This can be incorporated in to the project. Thus, the power angle control strategy of VSM is a novel approach to control VSM which can be further explored by incorporating different circumstances. 5. CONCLUSION From this MATLAB simulation, it is possible to deduce that adding VSM helps to enhance the transient stability of the grid having low inertia. When 50KW load is added to the initial system after 10 seconds of operation, frequency changed is up to a maximum of 15.5% during the transient period without VSM which is unacceptable. With the inclusion of VSM in the system, the maximum frequency change of 15.5% is reduced to 7% (maximum). Hence, it improved the transient stability of the system. Thus, we can penetrate renewable energy sources in higher number s with the use of VSM which provides enough inertia to the grid. ACKNOWLEDGEMENT The Department of Electrical Engineering at the Pulchowk Campus supported this study. We would like to express ou r gratitude to all of the department's professors and lecturers for their valuable suggestions and unwavering support during the project's duration.