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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.
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* 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
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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
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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
VIRTUAL SYNCHRONOUS GENERATOR: AN OVERVIEW O. O. Mohammed, et al
Nigerian Journal of Technology Vol. 38, No. 1, January, 2019
162
existing control scheme as stipulated in section 6 of
this paper.
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... To satisfy the Paris Agreement of being CO 2 neutral by 2050, there is an increasing amount of renewable energy sources (RES) being integrated into power systems worldwide [2]. The most represented RES are photovoltaic (PV) panels and wind turbines [3] since they are among the most environmentally friendly and abundant [4] sources. Additionally, the level of RES integrated in the distribution network systems have been continuously growing in the last decade making power systems decentralized [5]. ...
... VSM0H has no PLL and no inner current loop. Active and reactive power control loops generate the voltage amplitude and frequency based on reactive power -voltage and active power -frequency droops, as shown by (3) and (4). ...
Article
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... Two of the most common grid-forming controls are the virtual synchronous machine (VSM) and the droop-based control. The VSM can be found in a variety of control schemes and variations [25][26][27][28][29], and it is based on the emulation of the swing dynamics of the synchronous machines. The droop-based control was originally designed for converter-dominated microgrids and it provides grid-forming functionalities through droop regulators for voltage and frequency [30][31][32]. ...
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The operation of a power system with 100% converter-interfaced generation poses several questions and challenges regarding various aspects of the design and the control of the system. Existing literature on the integration of renewable energy sources in isolated systems mainly focuses on energy aspects or steady-state issues, and only a few studies examine the dynamic issues of autonomous networks operated with fully non-synchronous generation. A lack of research can be found in particular in the determination of the required amount of grid-forming power, the selection of the number and rated power of the units which should implement the grid-forming controls, and the relative locations of the grid-forming converters. The paper aims to address those research gaps starting from a theoretical point of view and then by examining the actual electrical network of an existing island as a case study. The results obtained from the investigations indicate specific observations and design opportunities, which are essential for securing the synchronization and the stability of the grid. Possible solutions for a fully non-synchronous operation of autonomous systems, in terms of dynamic characteristics and frequency stability, are presented and discussed.
... To satisfy the Paris Agreement of being CO 2 neutral by 2050, there is an increasing amount of renewable energy sources (RES) being integrated into power systems worldwide [2]. The most represented RES are photovoltaic (PV) panels and wind turbines [3] since they are among the most environmentally friendly and abundant [4] sources. Additionally, the level of RES integrated in the distribution network systems have been continuously growing in the last decade making power systems decentralized [5]. ...
Preprint
Full-text available
The share of renewable energy sources interfaced to the bulk power system through power electronic devices is continuously increasing. As a result, The power systems of the future face challenges of stability and reduced inertia and, to maintain the stability of the low-inertia systems, various concepts of grid-forming converter control schemes have been introduced, which enable converters to behave similarly to traditional synchronous machines. As the research in this area is gaining on importance, the sheer amount of papers with models for different kinds of simulations and the complexity of control algorithms may be daunting for newcomers in this field to understand them and to start modeling such devices. Moreover, different control schemes are often erroneously used interchangeably which adds to the confusion. This papers aims to give a clear understanding of the above aspects by explaining details of modeling and initialization of grid-forming converter control schemes for power system fundamental frequency dynamic simulations (root-mean-square or RMS simulations) by using an example of a virtual synchronous machine (VSM). Starting from comparison of traditional systems and systems with a high share of converters and continuing with a functional overview of virtual inertia based converters in modern power systems, a step-by-step initialization of the static and dynamic model is given, focusing on RMS simulations. Finally, selected simulation examples are shown which illustrate main characteristics of virtual synchronous machines and their comparison to real synchronous machines. The presented dynamic model is made freely available in DIgSILENT PowerFactory format.
... The kinetic energy of the SGs' rotor is released to suppress the change of power and frequency during the disturbance. However, renewable energy resource-based distributed generator has no or low inertia and damping, which is critical to stabilising the power and frequency of the power system [4,5]. Therefore, to suppress the instability of DGs, a virtual synchronous generator (VSG) control technique is proposed, which mimics the inertial and damping characteristics of SG. ...
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Conference Paper
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Energy sustainability and reliability issues have created great concerns globally, instigating various utilities to create interconnections with one another for system security and techno-economic benefits. Capacity benefit margin (CBM) is the amount of transmission capacity reserved between interconnected systems for emergency power exchange between utilities. It is usually estimated by evaluating the reliability of the generating units of interconnected systems to know the amount of external generation required during emergency supply shortage. Various techniques have been employed in literature to evaluate CBM. However, despite the global awareness on the continuous shift from conventional power generation sources to renewable energy sources, the assessment of CBM in the presence of renewable energy has not been addressed. Moreover, existing techniques for CBM calculation are based on iterative optimization and may not scale well to more than one deficient area. To solve these issues, this article incorporates wind power (WP) generation in CBM computation. The proposed technique is based on graph theory and can be used to calculate CBM in the presence of multideficient areas and renewable energy sources, in particular, WP. Simulations using the IEEE 24-bus reliability test system in MATLAB show that the proposed technique is significantly better in terms of scalability and accuracy as compared to existing techniques. The proposed technique can be employed by utilities for CBM estimation in the presence of renewable energy sources.
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Energy sustainability and reliability issues have created great concerns globally , instigating various utilities to create interconnections with one another for system security and techno-economic benefits. Capacity benefit margin (CBM) is the amount of transmission capacity reserved between interconnected systems for emergency power exchange between utilities. It is usually estimated by evaluating the reliability of the generating units of interconnected systems to know the amount of external generation required during emergency supply shortage. Various techniques have been employed in literature to evaluate CBM. However, despite the global awareness on the continuous shift from conventional power generation sources to renewable energy sources, the assessment of CBM in the presence of renewable energy has not been addressed. Moreover, existing techniques for CBM calculation are based on iterative optimization and may not scale well to more than one deficient area. To solve these issues, this article incorporates wind power (WP) generation in CBM computation. The proposed technique is based on graph theory and can be used to calculate CBM in the presence of multi-deficient areas and renewable energy sources, in particular, WP. Simulations using the IEEE 24-bus reliability test system in MATLAB show that the proposed technique is significantly better in terms of scalability and accuracy as compared to existing techniques. The proposed technique can be employed by utilities for CBM estimation in the presence of renewable energy sources. K E Y W O R D S capacity benefit margin, graph theory, multi-area power systems, reliability, wind power generation List of Symbols and Abbreviations:
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In this paper, we propose an idea to integrate more renewable energy into the existing China power grid and to provide remote islands with a reliable power supply. The idea is to expand the existing point-to-point line-commutated converter (LCC) HVDC transmission link into a hybrid multiterminal HVDC (hybrid MTDC) system, which contains LCC stations and voltage-source converter (VSC) stations. Accordingly, this paper proposes a novel control strategy for this system called active voltage feedback control, which does not need any high-speed communication system under various disturbances, such as wind speed variations, load fluctuations, faults, etc. This paper also adds secondary frequency regulation to the synchronverter, which is one way of combining the VSC converter with synchronous machine behavior. Then, this paper applies an approach to limit its current under fault conditions. Finally, the entire hybrid MTDC system is modeled in PSCAD/EMTDC. Simulation results show that the improved synchronverter is able to achieve secondary frequency control, and the presented active voltage feedback control works very well when exposed to various disturbances.
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Control structures containing cascaded loops are used in several applications for the stand-alone and parallel operation of three-phase power electronic converters. Potential interactions between these cascaded loops and the complex functional dependence between the controller parameters and the system dynamics prevent the effective application of classical tuning methods in the case of converters operating with a low switching frequency. A tuning approach guided by the eigenvalue parametric sensitivities calculated from a linearized model of the converter and its control system is proposed in this paper. The method is implemented in the form of an iterative procedure enforcing the stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in the dynamic performance of the system when tuned with the presented algorithm compared with a conventional rule-based tuning method.
Book
This book discusses relevant microgrid technologies in the context of integrating renewable energy and also addresses challenging issues. The authors summarize long term academic and research outcomes and contributions. In addition, this book is influenced by the authors’ practical experiences on microgrids (MGs), electric network monitoring, and control and power electronic systems. A thorough discussion of the basic principles of the MG modeling and operating issues is provided. The MG structure, types, operating modes, modelling, dynamics, and control levels are covered. Recent advances in DC microgrids, virtual synchronous generators, MG planning and energy management are examined. The physical constraints and engineering aspects of the MGs are covered, and developed robust and intelligent control strategies are discussed using real time simulations and experimental studies.
Article
The Virtual Synchronous Generator (VSG) is an inverter control structure that supports power system stability by imitating a synchronous machine. Because of the restriction of inverter power and current, the VSG performance under disturbances should be evaluated and enhanced. In this paper, the response of the VSG unit to symmetrical and unsymmetrical voltage sags is assessed. A theoretical analysis that traces the trajectory of the state variable of the system during voltage sags is presented. The analysis confirms the effect of the characteristics of symmetrical and unsymmetrical voltage sags on the severity of their consequences. In addition, it is detected that two types of transients appear that must be mitigated: one is the transients during the voltage sag and the other one is the transients after voltage recovery. To prevent overcurrent during voltage sags, voltage amplitude control and output power control are implemented, and to suppress the transients after voltage recovery, virtual inertia control is implemented. The experimental results from a 10 kVA VSG-controlled inverter confirm the effectiveness of the additional controllers.
Conference Paper
To improve the stability of power system with a big portion of inverter based Distributed Generators (DG), Virtual Synchronous Generator (VSG) concept used to mimic Synchronous Machine(SM) characteristics through converter control algorithm is proposed, on the other hand, power oscillation and other issues after a change and disturbance similar to synchronous machine might happen. For this purpose, an improved VSG method is raised. By adding a differential term of the voltage angle frequency, the dynamic characteristics of the system can be improved, and the range of the virtual inertia can be increased. Simulation and experience is achieved to verify the effectiveness of the proposed algorithm.
Conference Paper
This paper introduces a new control strategy to realize a virtual synchronous generator. The proposed control strategy is based on virtual impedance and band-pass damping algorithms to achieve the electro-mechanical characteristics of synchronous generator for the grid converter. Differences between new and traditional method on impedance emulation, and differences between traditional pure proportional damping algorithm and new band-pass damping algorithm are discussed as well. Additionally, detailed analysis of the components of synchronous generator is given, also the roles of the various parts of synchronous generator in the power system stability control are explained. The behavior of designed control strategy is validated by both simulations and experiments made on a 2kW grid-connected inverter.
Article
Renewable energy sources are increasing their penetration in power systems, making necessary new control systems that offer services usually provided only by conventional generators. In this paper, an active power controller able to achieve synchronization with the grid and to control the DC link voltage is proposed. This controller allows identifying the converter with a virtual synchronous generator whose inertia can be modified online, considering that its virtual kinetic energy is stored in the DC link. Additionally, the resulting active power loop is a second order system whose damping factor can be defined freely.
Article
This paper investigates the use of a virtual synchronous machine (VSM) to support dynamic frequency control in a diesel-hybrid autonomous power system. The proposed VSM entails controlling the grid-interface converter of an energy storage system (ESS) to emulate the inertial response and the damping power of a synchronous generator. In addition, self-tuning algorithms are used to continuously search for optimal parameters during the operation of the VSM in order to minimize the amplitude and rate of change of the frequency variations and the power flow through the ESS. The performances of the proposed self-tuning (ST)-VSM and the constant parameters (CP)-VSM were evaluated by comparing their inertial responses and their damping powers for different scenarios of load variations. For the simulated cases, the ST-VSM achieved a similar performance to that of the CP-VSM, while reducing the power flow through the ESS in up to 58%. Moreover, in all the simulated scenarios, the ST-VSM was found to be more efficient than the CP-VSM in attenuating frequency variations, i.e., it used less energy per Hertz reduced.
Article
The virtual synchronous generator (VSG) is a control scheme applied to the inverter of a distributed generating unit to support power system stability by imitating the behavior of a synchronous machine. The VSG design of our research incorporates the swing equation of a synchronous machine to express a virtual inertia property. Unlike a real synchronous machine, the parameters of the swing equation of the VSG can be controlled in real time to enhance the fast response of the virtual machine in tracking the steady-state frequency. Based on this concept, the VSG with alternating moment of inertia is elaborated in this paper. The damping effect of the alternating inertia scheme is investigated by transient energy analysis. In addition, the performance of the proposed inertia control in stability of nearby machines in power system is addressed. The idea is supported by simulation and experimental results, which indicates remarkable performance in the fast damping of oscillations.