Virtual synchronous generator: an overview

Abstract

The continuous increase in the penetration of renewable energy (RE) based distributed generations (DGs) in the power system network has created a great concern on the stability of the existing grid. Traditional bulk power plants, which are dominated by synchronous machines (SMs) can easily support system instability, due to the inherent rotor inertia and damping characteristic, as well as voltage (reactive power) control ability. Nevertheless, converter based RE has some special characteristics, such as stochastic real and reactive power output, quick active and reactive power response, small output impedance, and little or no inertia and damping property thereby causing frequency and voltage instability in the system. To solve this problem, virtual synchronous generator (VSG) concept was proposed to emulate some of the features of conventional SG through converter control strategy in order to provide additional inertia virtually. Different control schemes for VSG has been proposed in literature. Surprisingly, an overview of these schemes is yet to be efficiently presented. This paper presents an overview of the VSG control schemes. It provides the concepts, the features of the control schemes and the applications of VSG. Finally, the crucial issues regarding VSG control schemes and the necessary improvement that need to be addressed are highlighted.

Full text

* Corresponding author, tel: +234 – 803 – 463 – 8575
VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW
O. O. Mohammed1,*, A. O. Otuoze2, S. Salisu3, O. Ibrahim4 and N. A. Rufa’i 5
1, 2, 4, DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING, UNIVERSITY OF ILORIN, ILORIN, KWARA STATE, NIGERIA.
3, DEPARTMENT OF ELECTRICAL ENGINEERING, AHMADU BELLO UNIVERSITY, ZARIA, KADUNA STATE, NIGERIA.
5, DEPARTMENT OF ELECTRICAL ENGINEERING, BAYERO UNIVERSITY, KANO. KANO STATE, NIGERIA
E-mail addresses
: 1
mohammed.oo@unilorin.edu.ng,
2
otuoze.ao@unilorin.edu.ng,
3
s.salisu@live.com,
4
reacholaibrahim@gmail.com,
5
nabilarufai@yahoo.com
ABSTRACT
The continuous increase in the penetration of renewable energy (RE) based distributed
generations (DGs) in the power system network has created a great concern on the stability of
the existing grid. Traditional bulk power plants, which are dominated by synchronous machines
(SMs) can easily support system instability, due to the inherent rotor inertia and damping
characteristic, as well as voltage (reactive power) control ability. Nevertheless, converter based
RE has some special characteristics, such as stochastic real and reactive power output, quick
active and reactive power response, small output impedance, and little or no inertia and damping
property thereby causing frequency and voltage instability in the system. To solve this problem,
virtual synchronous generator (VSG) concept was proposed to emulate some of the features of
conventional SG through converter control strategy in order to provide additional inertia virtually.
Different control schemes for VSG has been proposed in literature. Surprisingly, an overview of
these schemes is yet to be efficiently presented. This paper presents an overview of the VSG
control schemes. It provides the concepts, the features of the control schemes and the
applications of VSG. Finally, the crucial issues regarding VSG control schemes and the necessary
improvement that need to be addressed are highlighted.
Keywords: Distributed generation, Synchronous generator, Virtual synchronous generator, Power electronic
converter, Energy storage system, Frequency control
1. INTRODUCTION
The level of distributed generation (DG) resources
and renewable energy sources (RES) envisaged to be
integrated into the conventional grid is tremendously
being explored. The appetite to “Go Green’’ due to
concerns on the dwindling non-renewable energy
sources and the preservation of environment has
forced various countries to devise means of accessing
renewable energy for electric power generation. The
European Union (EU) has given the regulations to
realize a 20% objective from the entire share of
energy from RE sources by 2020 [1] and this has
resulted to an increase in the level of DG. DG has
been the impetus in the transformation of the
traditional vertical grid scheme to a much more
looped and alloyed grid scheme as explained in [2],
Fig. 1 [3] depicts the grid transformation.
The most commonly used renewable energies are the
photovoltaic and wind power; they are
environmentally friendly and abundant. RE has
experienced fast technological growth, which
contributes to their availability at considerably low-
cost [4]. This has resulted in a drastic reduction in
fossil fuel consumption of many countries, which
subsequently maintain a reduced cost than normal
prices and improve the living standard with a greener
environment. Another advantage is their key
participation in support of electrical network in
remote and rural areas electrification [5]. The global
investment in REs in current years as depicted in Fig.
2 shows that wind and solar energy sources have
become more popular renewables in recent years [6].
Nigerian Journal of Technology (NIJOTECH)
Vol. 38, No. 1, January 2019, pp. 153 – 164
Copyright© Faculty of Engineering, University of Nigeria, Nsukka,
Print ISSN: 0331-8443, Electronic ISSN: 2467-8821
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http://dx.doi.org/10.4314/njt.v38i1.20

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
154
Figure 1: Power network structural transformation from vertical to mesh structure with DG
Fig. 2 Growth in renewable energy investment in the whole world from 2014 to 2015
However, in the near future, power electronic
converters (PECs) based DGs are expected to have a
remarkable influence on sizeable power systems as a
substantial part of the conventional synchronous
generators (SGs) in the systems will be substituted
by PECs based generations [7]. Traditional bulk
power plants, which are dominated by synchronous
machines (SMs) with speed governor and excitation
control, can automatically regulate speed governor to
support frequency instability events, due to the

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
155
inherent rotor inertia and damping characteristic, as
well as voltage (reactive power) control [8, 9].
Nevertheless, converter based RES have some
special characteristics, such as stochastic output, real
and reactive power supplied to AC networks, quick
active and reactive power response, small output
impedance, and little or no inertia and damping
property [10, 11]. These RE sources cause
fluctuations in power generation, system frequency
deviations, and voltage rise due to reverse power
from PV generation [8]. Therefore, the increasing
penetration level of DG will have enormous effects on
the dynamic response and power system stability.
The main concern of these DG technologies is
maximization of power supply to the grid while
delivering efficient and operability in case of system
faults and disturbances [7, 12]. Normally, the safe
operation of large centralised power stations is not
threatened by the influence of a few small-size DG
units thus their effects are negligible.
Nevertheless, with large number of higher capacities
DG units, the entire dynamics of power systems are
considerably affected [13]. This scenario has
instigated remarkable research and development on
the methods to control the grid-connected DG via
power converters. Some researchers proffer solution
to this problem through the evolution of droop-based
control systems for microgrids operation dominated
by PECs and converters in a stand-alone operation
[14-17]. In such a technique, the energy storage is
employed in the DG and the converters employ droop
control mechanism to mimic the primary frequency
and voltage regulation features of the synchronous
generator (SG). As such, the converters with DG and
Energy Storage Systems (ESS) can be viewed as
voltage source integrated into the network to support
system voltage and frequency. Converters with
droop control can rapidly share load power in parallel
operation, in that it has no inertia, it has poor
frequency regulation.
In order to enhance the frequency stability of the DG
systems, virtual synchronous generator (VSG)
concept was proposed [2] to emulate the external
features of the SG through converter control strategy
[18], in order to provide additional inertia virtually
[19, 20]. VSG can be developed and implemented for
DG systems by employing short-term ESS and PECs
with efficient control technique which then operates
like conventional SG by exhibiting some amount of
inertia and damping characteristics for short period of
time [8]. This eventually provides the necessary
supports for power system stability in the presence of
large integration of DG systems.
This paper presents an overview of VSG control
mechanisms, it provides the concept, features and
the review of the existing models as well as
highlighting the necessary improvement that needed
to be done for proper control of DGs. The remainder
of this paper is structured as follows. Section 2
discusses the mechanism of synchronous generator
Iinherent stability mechanism and section 3 presents
the concept of VSG. The review of the existing VSG
control mechanisms is highlighted in section 4,
section 5 gives a brief detailed application of VSG and
the related challenges are discussed in section 6
while section 7 concludes the study.
2. SYNCHRONOUS GENERATORS’ INHERENT
STABILITY MECHANISM
It is very important to discuss the essential inherent
properties of a SG that are established to be
very critical in the stability and reliability
operation of a power system, namely: the inertia due
to rotating masses, damping effect due to the
damper windings in the rotor and the speed-droop
characteristics for load sharing [21].
2.1 Inertia Due to Rotating Masses
As the SG rotates, the field and damper windings of
the rotor generate a sinusoidal flux in the air-gap,
which consequently creates an EMF in the armature
terminals. Considering the general swing equation of
the SG dynamics,

   󰇛󰇜
In equation (1),  is moment of inertia of rotating
masses (turbine and generator rotor),  is the
angular speed of the rotor,  is the synchronous
speed,     ,  is the mechanical torque, 
is the electromagnetic torque and  is the damping
torque coefficient. Detail explanation about this can
be found in [22].
2.2 Damping Characteristic of the Damper
Windings in the Rotor
Damping (due to mechanical losses) of rotor is small
and can be ignored for all practical purposes.
Damping is mainly provided by damper or armature
windings in SG. Due to small disturbances, the
generator rotor undergoes speed deviations, the
damper winding plays a crucial role in the restoration
of the rotor synchronism, and it presents the

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
156
damping effects in the SG [3, 22]. Upon a fault, the
response of the generator is analysed by breaking
the total period of the fault into three stages termed
as sub-transient, transient and steady state. The
damping effect will only surface during the transient
stage. Whenever the rotor speed is different from the
synchronous speed, the air-gap flux, which is rotating
at the synchronous speed will penetrate the damper
windings thereby inducing an emf and current in
them. The damping torque is therefore produced by
the induced current and subsequently restores the
synchronous speed of the rotor. During sub-
transient state, the damper windings have a
screening effect opposing the changes in armature
flux to penetrate them [22], while the steady-state
stage is a stable state. It can be deduced from
equation (1) that the swing equation in terms of
power can be expressed as;

   󰇛󰇜
   
Upon any disturbance of the rotor from the steady
state point, the rotor either accelerate or decelerate
depending on the nature of the disturbance. If
  then  will be negative i.e. exclusively
rejecting the acceleration and if the  ,  will
be positive and supporting  in rejecting the
deceleration. Subsequently, the rotor will follow the
normal trajectory and synchronism is achieved
without perpetual oscillations [3, 22]. It is worthwhile
to note that the coefficient  in physical SMs is not a
constant value but depends on the condition of
operation of the machine. Therefore, using a fixed
value of  in a reduced order model will not capture
the performance of SM in the whole operating range
[7].
2.3 Speed-Droop Characteristics for Load
Sharing
Under steady state condition, the rotor speed of SG is
proportional to the frequency of the armature current
and subsequently to the frequency of the terminal
voltage [23] as evident from the following equation;
  
󰇛󰇜
where;  is the SG voltage frequency,  is the rotor
mechanical speed,  is the number of poles.
Furthermore, considering the swing equation, it is
evident that in an event of an imbalance between the
input mechanical power to the generator and the
electrical output power to the grid, the rotor speed
will change. The speed-load dependence of the SGs
connected in parallel can be represented by the
following curve.
Fig. 3 Speed-droop characteristics of a synchronous
generator
When two generator sets operate in parallel in an
islanded mode, they share the same frequency as
shown in the diagram. If the load in the system
increases remarkably, the additional load will be
shared according to the droop settings of the two
generator sets and their frequency will change to.
Nonetheless, substantial drops in frequency below a
particular threshold values can be dangerous to the
system components. In this case, the adjustment of
the governor reference is required and subsequently
leads to a total generation increase of 
󰆒 and 
󰆒 due
to governor response of the two generator sets
respectively [23, 24].
3. VIRTUAL SYNCHRONOUS GENERATOR
CONCEPT
The VSG principle is based on integrating the
advantages of dynamic converter technology with
those of the static and dynamic operating
characteristics of electromechanical SMs [2]. The
pictorial representation of VSG concept is as shown in
Fig. 4. The three distinct components of VSG are PEC
(which comprises of two power conversion stages,
namely a DC to DC stage and a DC to AC stage), an
energy storage device (battery, supercapacitor,
flywheel, etc.) and the control scheme that controls
the power exchange between energy storage and
power system. This power exchange supports power
system by preventing frequency fluctuations similar
to SG rotational inertial [8, 25]. The VSG is
commonly placed in-between a DG (or DC source)
and the grid [8]. The DC source that goes to the VSG
algorithm performs the function of SG by providing
inertia and damping supports to the grid system.

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
157
Fig. 4 Basic configuration and concept of the VSG [8, 25]
This is achieved by active power regulation of the
inverter in inverse proportion of the rotor speed. The
electrical features of the VSG are the same with that
of conventional SG from the network viewpoint, apart
from the high-frequency noise from the power
transistors in the inverter. Due to the presence of an
energy storage system (ESS), the VSG is able to
absorb or inject (charge or discharge) power into the
system. The nominal state of charge (SOC) of the
ESS is suggested to be 50%, and the lower and
upper limits should be 20% and 80% respectively
[7]. When the state of charge of ESS is within the
limits, the VSG is working in its active operation,
when there is excess energy in the system; the VSG
is working on the virtual load operation [8].
The main idea of VSG is to mimic the important
features of a traditional SG by using PEC control.
Therefore, any VSG application involves
approximately a direct mathematical model of a SG
[7]. The choice of any SG model and its parameters
are mainly based on the random choice of design as
demonstrated by several solutions proposed in
literature. Although, the mimicry of the inertial
characteristics and damping features of
electromechanical oscillations are the usual
characteristics of every VSG implementation.
Depending on the required extent of complexity and
accuracy in replicating the SG dynamics, the transient
and sub-transient dynamics of an SG model can be
added or ignored [7]. Moreover, parameters’
selection for VSG applications is not restricted by the
physical design of any conventional SG model,
consequently, the VSG parameters can be chosen to
mimic a particular behaviour of SG model or can be
defined in the course of control system development
to obtain the required characteristics [7]. The power
output of a VSG can be solely represented as follows:
  
 󰇛󰇜
Where;  is the output power of VSG, = 
,  is the nominal grid frequency,  represents
the initial power to be transferred to the inverter,

 represents the power regulation term of the
VSG, depending on the initial rate of change of
frequency (
 ), if positive or negative, power will
either be absorbed or injected into the system, 
denotes the inertia emulation characteristic of VSG.
Since the rate of change of frequency gives an error
signal, power exchange takes place during the
transient event and stabilizes the frequency. This
brings about the third term in the equation (3), 
mimics the damping effect of damper windings in an
SG, and its value must be properly selected to
commensurate with the fluctuations in system
frequency [26].
Although, the system frequency and rotational speed
dip can be minimized by increasing the virtual mass
(), however, the synchronous units may likely
increase the oscillation [26]. In an event of a
disturbance,  minimizes the maximum variation of
the rotor speed; but the natural frequency and
damping ratio may be reduced [27]. In effect, 
suppresses the system frequency dip and  (virtual
damper) counteracts the frequency oscillation of the

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
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grid.  and  are negative constant gains and
should be fixed for maximum active power
interchange [8]. In an event of maximum frequency
excursion and rate of change of frequency, more
power will either be injected or absorbed into/from
the system depending on the frequency signal. In
conventional SG, energy is absorbed by the damper
winding resistance, which is subsequently dissipated
as heat. However, in VSG the ESS is employed to
absorb power fluctuation to balance the system.
These characteristics symbolize the emulation of
electromechanical SGs.
4. REVIEW OF THE EXISTING VIRTUAL
SYNCHRONOUS GENERATOR CONTROL
MECHANISM
Beck and Hesse were the first to propose virtual
synchronous machine (VSM) based control technique
in 2007 [2], which was named VISMA where they
modelled the two windings of the stator in d-q
frames and the inertia without any current loops.
Similarly, researchers [28-30] employ only swing
equation in their SM model. Likewise, authors in [31,
32] presented models without current controllers,
however, in later work [33], the current controllers
were added and swing equation was derived with the
magnitude of the virtual back EMF generated by
some reactive power command. Ashabani and
Mohamed [34] propose a modified swing equation
incorporating DC bus voltage balance and droop
control. The same technique was used in [35] with
the addition of current controllers. In all the
techniques, the virtual back EMF is employed simply
to produce PMW signals, which correlates with the
dynamic operation of a conventional SG.
In these schemes, the output current is uncontrolled
(remains unbounded) and limited either by virtual
inductors or by actual boost inductors. Nevertheless,
these VSG control strategies are the easiest
techniques, however, the probable overcurrent
problem affects its deployment in case of large
transients. Fig. 5 summarizes the control strategy,
where a common grid-connected converter connects
to the grid with an inductor, which could be a step-
up transformer or output filter. From the Fig, the grid
voltage  and current  are measured to
compute P and Q, and the same voltage may be
employed by PLL to get the frequency of the grid
(). The output from either Q-control or V-control
and the phase θ from the VSG model are fed to the
PWM for the control of the converter, with this
control scheme different implementation was
employed by different authors.
Similar control strategy was proposed in [38], in
which the traditional cascaded controller with inner
current loops and outer voltage loops was employed;
only the inertia emulation was implemented using
swing equation. The scheme was enhanced in [39,
40] in which they control the output current rather
than the virtual back EMF. structure as an enhanced
model of synchroverter.
Fig. 5 VSG model without a current control loop [7, 9, 36, 37]

VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
159
The current control loop was incorporated to mitigate
the problems. Authors in [41, 42] improve the
current controller by incorporating virtual impedance
in order to enhance the model performances such as
elimination of harmonics and negative sequence
compensation. Authors in [9, 43] improve the control
scheme to realize autonomous coordination while
[44, 45] and [46] also employed the same control
Fig. 6 shows the general control structure for the
VSG control strategies with an inner current loop.
The major discrepancy from the previous type is that
the output voltage from the VSG model passes
through a current controller rather than PWM
directly, where the voltage vector can be termed as
virtual back EMF to differentiate it from actual output
voltage vector. The current references from the
control block can either be incorporated into the VSG
model or as a separate block termed as virtual
impedance. The virtual impedance has the
functionalities to regulate current at a different
frequency and different sequence to ease the
transformation from islanded mode to grid-connected
mode, and to presents more damping effect, etc.
implementation of an advanced current control
scheme can be employed to simplify the virtual
impedance. The current controllers are simply
derived from d-q frame for simplicity of the control.
Moreover, the phase information required by the park
transformation is obtained from PLL in some
literature for parameter optimization.
Some different cascaded schemes as shown in Fig 7
were presented in [47, 48]. In those schemes, the
current reference is not from the VSG model (inertia
emulation block), and VSG model simply act as an
alternative synchronization strategy replacing the
PLL, making the scheme more homogenous to the
traditional cascaded controller. The current reference
is conventionally derived from the DC voltage
regulator in the d channel and AC voltage regulator
in q channel. In this case, more functionalities can be
added through modifications. Therefore, the full
control scheme becomes very easy; however, based
on our knowledge, comparison with the previous
schemes has not been investigated.
Fig. 6 VSG model incorporating current control loop [7, 9, 36, 37]

* Corresponding author, tel: +234 – 803 – 463 – 8575
Generally, emulation of inertia characteristic using
swing equation is enough to supply system frequency
information. Hence, PLL in the traditional d-q channel
control can be substituted or used as an alternative
back up during start-up or contingencies events.
Authors in [50] proposed an interesting model in
which PLL is employed to obtain the system
frequency that is different from the VSG frequency in
order to perform automatic tuning of the controller
parameters.
Although each type has different variations, several
researchers tend to focus more on cascaded
controller having only inertia emulation. This is
because virtual inertia gives the converters the
potential to expend their energy storage to increase
the entire system inertia for frequency stability [51]
while maintaining synchronism with other rotating
units, in addition to accessibility to droop control to
operate without centralized control. The overall
control schemes can be grouped into two types:
direct back EMF and current loop controller;
classification by SM models are also of two kinds: a
simple model with only inertia emulation and a
complete model with both inertia and dynamics in
flux.
There are few papers, which focus on modelling and
design, while others only presented control schemes
theoretically without giving reasons to back their
proposed schemes. In [9, 43, 52], the full state
space model of VSG controlled converter was derived
to realize the pole-zero map to observe the
influences and the sensitivity from each state to each
mode was discussed. A similar investigation was
discussed in [47, 53, 54]. This technique is capable
of accomplishing control over the entire system
provided the system is controllable which is usually
characterized by the power system structure.
Nevertheless, the disadvantage of the method is
large computations requirement and some
parameters or states may not be feasible. A reduced
model was proposed in [55] to present some
fundamental stability limit analysis.
Fig. 7 VSG model with modified current control loop [7, 9, 37, 49]

* Corresponding author, tel: +234 – 803 – 463 – 8575
Different design process focused on transfer function
and Bode plots was presented in [56, 57] to reduce
the computation problem in the previous model. In
[50, 58], some simplification was employed by tuning
control parameters online, in order to realize better
performance and efficient damping effect. Full state
space model is rarely used due to the nonlinearity of
the model. Energy functions and Lyapunov method
are considered as an efficient technique as in [28,
30, 35, 59], however, it is not feasible for complex
systems.
If the entire dynamic characteristics of SG is to be
reproduced by VSG, the SG model incorporating a full
order features of SG has to be modelled [7, 22, 24]
and this would results in 7th order model. However, if
the objective of the VSG implementation is to mimic
the inertia and damping characteristics of the SG,
using full order model of SG will add nonessential
complexity in the model. The two properties (inertia
and damping characteristics) can be captured by the
general swing equation [7].
5. APPLICATIONS
The VSG control strategy can be used for all types of
generation units, e.g. PV farms [41], electric vehicles
[45, 53, 54, 60, 61], STATCOMs [25, 32, 34, 56, 57],
and conventional DGs [9, 31, 33, 48], because of its
inherent features which enables it to participate in
frequency stability support.
It can also be employed to perform some
conventional power system functions, such as
oscillation damping [28, 39, 50, 62] and low voltage
ride through [42] by adjusting the control loops,
usually incorporating feedback and increase the order
of compensators. Although, these have not been
investigated much because the VSG control schemes
are still under development.
The highlighted control schemes are based on
simulations and experimental studies, VSG is still
under developmental stage. Most of the existing
literature on VSG is about proposals of various
control schemes, modelling, design and application.
However, there is still a great deal of investigations
that need to be done as highlighted in the next
section because the concept is still new and to the
best of our knowledge, no practical implementation is
under operation yet.
6. CHALLENGES THAT ARE NEEDED TO BE
ADDRESSED
 It is very important to present a quantitative
design process with respect to known system
parameters and the corresponding sensitivities in
order to have a robust controller, which can
withstand various system operating points.
 Efficient and robust control scheme can still be
achieved through improvement of the existing
models by further study of the mathematical
derivation of the equivalency between the VSG
concept and SG where only the preferable parts
are utilized.
 More research is required regarding the control
of ESS in the VSG control scheme, a robust
technique to supervise the state of charge of the
ESS system in response to system instability.
 More real-time experiments need to be
conducted to see the influence and performance
of VSG controller.
 An important characteristic of VSG is its fastness
in counteracting power system deviations to
support frequency stability. In an event of power
imbalance, VSG injects or absorbs power
into/from the system to mitigate the frequency
excursion and this takes place within few
seconds, the traditional SG needs to react to a
huge change in power by adjusting its generation
to balance the system. However, the fastness of
the VSG in counteracting the frequency excursion
may affect the response of the SG. Therefore, a
robust coordination between VSG and SG is
crucial for effective power control.
 The system reliability should also be emphasized,
the generation reliability of the grid integrated
DGs should be constantly assessed and the
reliability assessment techniques for this type of
alloyed systems should be standardized and
explicit enough in order to have a robust and
efficient system.
7. CONCLUSION
The continuous growth in the integration of DGs in
the power system network, for the reason of stability
and sustainability, has contributed to the imbalance
in traditional power system structure. The DGs
systems have little or no inertia and damping
property as found in the conventional SGs, thereby
causing a total decrease in the entire system inertia.
This paper has presented an overview of the crucial
issues regarding the influence of the DGs in power
system network, the VSG control schemes and their
applications, the challenges that are needed to be
addressed and the necessary improvement in the

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162
existing control scheme as stipulated in section 6 of
this paper.
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