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Isochronous Load Sharing and Control for Inverter-based Distributed Generation

Authors:
  • South Westphalia University of Applied Sciences, Branch Soest

Abstract and Figures

Distributed generation technologies such as photovoltaic, fuel cells and micro turbines are emerging. However, these sources require interfacing units to provide the necessary crossing point to the grid. The cores of these interfacing units are power electronics technologies such as inverters since they are fundamentally multifunctional and can provide not only their principle interfacing function but various utility functions as well. This paper shows the possibility of adapting the isochronous control methodology used by synchronous generators in conventional power systems to provide load sharing and control in inverter-based distributed generation.
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Isochronous Load Sharing and Control for Inverter-
based Distributed Generation
A. Mohd
1
, E. Ortjohann
1
, W. Sinsukthavorn
1
, M. Lingemann
1
, N. Hamsic
1
, D. Morton
2
1
South Westphalia University of Applied Sciences/Division Soest, Lübecker Ring 2, 59494 Soest, Germany
E-mail: Ortjohann@fh-swf.de, Alaamohd@gmail.com
2
The University of Bolton, Deane Road, Bolton, U.K.
Abstract -
Distributed generation technologies such as
photovoltaic, fuel cells and micro turbines are emerging.
However, these sources require interfacing units to provide the
necessary crossing point to the grid. The cores of these
interfacing units are power electronics technologies such as
inverters since they are fundamentally multifunctional and can
provide not only their principle interfacing function but various
utility functions as well. This paper shows the possibility of
adapting the isochronous control methodology used by
synchronous generators in conventional power systems to
provide load sharing and control in inverter-based distributed
generation.
I. I
NTRODUCTION
A new trend in power systems is developing toward
distributed generation (DG), which means that energy
conversion systems (ECSs) are situated close to energy
consumers and large units are substituted by smaller ones.
For the consumer the potential lower cost, higher service
reliability, high power quality, increased energy efficiency,
and energy independence are all reasons for interest in
distributed energy resources (DERs). The use of renewable
distributed energy generation and "green power" such as
wind turbines, photovoltaic solar systems, solar-thermo
power, biomass power plants, fuel cells, gas micro-turbines,
hydropower turbines, combined heat and power (CHP)
micro-turbines and hybrid power systems can also provide a
significant environmental benefit. This is also driven by an
increasingly strained transmission and distribution
infrastructure as new lines lag behind demand. It can reduce
overall system losses in transmission and distribution. Other
motives are the increased need for reliability and security in
electricity supply, high power quality needed by an
increasing number of activities requiring UPS like systems
and to prevent or delay the expansion of central generation
stations by supplying the growing loads locally.
The inverter is considered an essential component at the
grid side of such systems due to the wide range of functions it
has to perform. It has to convert the DC voltage to sinusoidal
current for use by the grid in addition to act as the interface
between the ECSs, the local loads and the grid. It also has to
handle the variations in the electricity it receives due to
varying levels of generation by the renewable energy sources
(RESs), varying loads and varying grid voltages [1]. Inverters
influence the frequency and the voltage of the grid and seem
to be the main universal modular building block of future
smart grids mainly at low and medium voltage levels.
The main problem associated with that is the development
of general, flexible, integrated, and hierarchical control
strategies for DERs to be integrated into the dynamic grid
control and management procedures of electrical power
supply systems (primary control, frequency and power
control, voltage and reactive power control) through flexible
power electronics namely inverters.
This paper will start by taking a look at the concept of DG
and future power systems. Afterwards, the proposed system
architecture and control methodology are introduced. Finally,
a case study is carried out to validate the proposed strategy.
II. B
ACKGROUND
:
D
ISTRIBUTED
G
ENERATION AND
F
UTURE
P
OWER
S
YSTEMS
Energy plays a vital role in the development of any nation.
The current electricity infrastructure in most countries
consists of bulk centrally located power plants connected to
highly meshed transmission networks. However, a new trend
is developing toward distributed energy generation, which
means that ECSs will be situated close to energy consumers
and the few large units will be substituted by many smaller
ones. For the consumer the potential lower cost, higher
service reliability, high power quality, increased energy
efficiency, and energy independence are all reasons for the
increasing interest in what is called “Smart Grids”.
Although the “Smart Grid” term was used for a while,
there is no agreement on its definition. It is still a vision, a
vision that is achievable and will turn into reality in near
future. One of the best and general definitions of a smart grid
is presented in [2]. Smart grid is an intelligent, auto-
balancing, self-monitoring power grid that accepts any source
of fuel (coal, sun, wind) and transforms it into a consumer’s
end use (heat, light, warm water) with minimal human
intervention. It is a system that will allow society to optimize
the use of RESs and minimize our collective environmental
footprint. It is a grid that has the ability to sense when a part
of its system is overloaded and reroute power to reduce that
overload and prevent a potential outage situation; a grid that
enables real-time communication between the consumer and
utility allowing to optimize a consumer’s energy usage based
on environmental and/or price preferences [2].
Furthermore, there is no consensus on how the DG should
be exactly defined [3]. A very good overview of the different
definitions proposed in the literature is given in [4]. In
general, DG describes electric power generation that is
geographically distributed or spread out across the grid,
generally smaller in scale than traditional power plants and
located closer to the load, often on customers’ property [5].
Distributed generation is characterized by some or all of the
following features:
Small to medium size, geographically distributed
power plants
Intermittent input resource, e.g., wind, solar
Stand-alone or interface at the distribution or sub-
transmission level
Utilize site-specific energy sources, e.g., some wind
turbines require a sustained wind speed of 20
km/hour. To meet this requirement they are located
on mountain passes or the coast
Located near the loads
Integration of energy storage and control with power
generation
Technologies those are involved in DG include but are not
limited to: photovoltaic, wind energy conversion systems,
mini and micro hydro, geothermal plants, tidal and wave
energy conversion, fuel cell, solar-thermal-electric
conversion, biomass, micro and mini turbines, energy storage
technologies, including flow and regular batteries, pump-
storage hydro, flywheels and thermal energy storage.
The idea behind DG is not a new concept. In the early days
of electricity generation, DG was the rule not the exception
[6]. However, technological evolutions and economical
reasons developed the current system with its huge power
generation plants, transmission and distribution grids.
In the last decade, technological innovation, economical
reasons and the environmental policy renew the interest in
DG. The major reasons for that are:
To reduce dependency on conventional power
resources
To reduce emissions and environmental impact
Market liberalization
To Improve power quality and reliability
Progress in DG technologies especially RESs
To reduce transmission costs and losses
To increase system security by distributing the
energy plants instead of concentrating them in few
locations making them easy targets for attacking
DG is becoming an increasing important part of the power
infrastructure and the energy mix and is leading the transition
to future smart grids. Even though many papers are
addressing forming an electric power supply system using
power electronic inverters, see [7-16], various issues are still
unsolved or not adequately investigated and standardized.
III. S
YSTEM
A
RCHITECTURE AND
C
ONTROL
M
ETHODOLOGY
A smart mini-grid is usually built up by combination of
RESs, conventional energy systems, and storage systems. In
general, the power produced by the ECS is DC power. This is
fed to the grid through the inverter. The inverter produces an
AC output of specified frequency and voltage. In this
philosophy shown in Fig. 1, the power flow from an ECS into
the grid may be driven by the grid or by the ECS itself.
Different components connected to the grid can be
classified into grid forming (GF), grid supporting (GS) and
grid parallel (GP) based on their contribution to the grid. The
types and function of the inverter are shown in Fig. 1.
The control strategy of an inverter in grid forming mode is
shown in Fig. 2. The inverter is responsible for establishing
and maintaining voltage and frequency of the grid. This is
done by adjusting its power production to keep the power
balance in the system. The inverter in this case determines the
voltage and the frequency of the grid. There is one inner
current control loop and a second voltage control loop. Both
loops use only the d-component. In some cases the v
q
is
regulated to zero which makes v
d
equal to the voltage
amplitude. The reference angle for the dq-transformation is
taken from the reference frequency.
If the load is frequency/voltage critical then isochronous
mode (zero droop) is the optimal solution for load sharing.
An inverter operating in the isochronous mode will operate at
the same set frequency/voltage regardless of the load it is
supplying as shown in Fig. 3.
Fig. 1. Feeding modes related to the grid side.
Fig. 2. Inverter in grid forming mode.
Fig. 3. a) Frequency vs. active power (isochronous mode) and (b) Voltage vs.
reactive power (isochronous mode).
The isochronous control scheme provides in comparison to
the droop scheme the possibility of precise control of the
voltage and the frequency. This needs communication in
order to measure the grid load and share this information with
all the other inverters in the system. However, the realization
of such a system needs low communication requirements and
is considered practical especially if the inverters are
connected to the same load bus and have no massive distance
between them. This is also needed if sensitive loads that can
not accept the voltage and frequency band used in droop
schemes exist. The proposed grid forming with isochronous
control strategy can be seen in Fig. 4. Here, the total
measured load is divided by the total rated power and
compared to the active power supplied by the generator
(inverter) divided by its rated power.
,
1
,[%]
, ,
,
1
n
Grid
Inv i
i
n
i
r Inv i
r i
i
P
P
P
S
S
=
=
=
(1)
This difference is amplified and added to the summation
point of the actual/reference angular frequency. The
difference out of that summation point is passed to the q
current controller. The output of the controller is compared to
the actual current value. The output of that comparison is
given to the voltage controller, this will calculate V
q
which is
transformed to the αβ frame and used by the SVM to generate
the switching states. The reactive power is also controlled in
the same manner. The total measured reactive power load is
divided by the total rated power, and then is compared to the
active power supplied by the generator divided by its rated
power.
,
1
,[%]
, ,
,
1
n
Inv
i
i
n
r Inv i
r i
i
Q
Q
Q
S
S
=
=
=
(2)
Fig. 4. Grid-forming with isochronous control function
This difference is amplified and added to the summation
point of the actual/reference voltage. The difference out of
that summation point is passed to the d current controller.
The output of the controller is compared to the actual current
value. The result of that comparison is given to the voltage
controller, this will calculate V
d
which is transformed to the
αβ coordinator and used by the SVM to generate the
switching states. The frequency used by the controller is
measured from the grid using PLL and then integrated to get
the needed angle.
An example of such a grid can bee seen in Fig. 5. The
network consists of three inverters from different power
classes working in isochronous mode (modified grid
forming). The inverters will work at the same
frequency/voltage in steady state regardless of the load they
are supplying.
The same principle can be also implemented in four wire
systems since one of the desirable characteristics of inverters
in three-phase systems is the ability to feed unbalanced loads
with voltage and frequency nominal values.
Four-wire inverters are developed to power
unbalanced/nonlinear three-phase loads. They can also feed
three phase and single phase AC loads simultaneously.
Furthermore, four leg inverters can be also used as shunt
active power filters to reduce the zero and negative sequence
current components generated by unbalance loads. By
compensating these current components the efficiency of
power transmission can be maximized which means less line
losses and better power quality.
The proposed grid forming scheme for the four-wire power
systems including isochronous load sharing control can be
seen in Fig. 6. More information about the control strategy
can be found in [17, 18].
L
L
R
L
L
L
R
L
L
R
L
R
L
R
L
R
L
R
V
α
V
β
-
-
-
-
L
V
dc
C
SVM
PLL
I
q_act
I
d_act
V
act
V
ref
∆V
∆ω
ω
ref
ω
act
φ
φ
P
total
-
∆P%
P-Factor
10-99%
1
Q
total
Q
gen,1
-
∆Q%
Q-Factor
10-99%
P
gen,1
1
1
1
-
-
-
-
L
V
dc
C
SVM
PLL
I
q_act
I
d_act
V
act
V
ref
∆V
∆ω
ω
ref
ω
act
φ
φ
P
total
-
∆P%
P-Factor
10-99%
1
Q
total
Q
gen,2
-
∆Q%
Q-Factor
10-99%
P
gen,2
1
1
1
-
-
-
-
L
V
dc
C
SVM
PLL
I
q_act
I
d_act
V
act
V
ref
∆V
∆ω
ω
ref
ω
act
φ
φ
P
total
-
∆P%
P-Factor
10-99%
1
Q
total
Q
gen,n
-
∆Q%
Q-Factor
10-99%
P
gen,n
1
1
1
V
α
V
β
V
α
V
β
Power
Calculation
Power
Calculation
Power
Calculation
Fig. 5. Modular grid using grid-forming inverters with isochronous control
function.
This is done for each component separately (+, -, 0). This
difference is amplified and added to the summation point of
the actual/reference angular frequency for the positive
component. The difference out of that summation point is
passed to the q current controller. The output of the controller
is compared to the actual current value. The result of that
comparison is given to the voltage controller. This will
calculate V
q
which is transformed to the αβ coordinator and
used by the SVM to generate the switching states.
The reactive power is also controlled in the same manner.
The total measured reactive power load is divided by the total
rated power and compared to the active power supplied by
the generator divided by its rated power.
This is done for each component separately. This
difference is amplified and added to the summation point of
the actual/reference voltage. The difference out of that
summation point is passed to the q current controller. The
outputs of the controller (I
pd_ref
, I
nd_ref
, I
0d_ref
) are compared to
the actual current value (I
pd_act
, I
nd_act
, I
0d_act
). The output of
that comparison is given to the voltage controller. This will
calculate V
d
which is transformed to the αβ coordinator and
used by the SVM to generate the switching states. The
frequency is measured from the grid using PLL and then
integrated to get the needed angle.
IV. C
ASE
S
TUDIES
R
ESULTS
Two case studies are carried to investigate the control
behaviour of modular isolated grid controlled inverters in
isochronous mode. The first one is for a three-wire system
and the second is for a four-wire system.
In the first case study, the topology is shown in Fig. 7. The
network consists of three inverters from different power
classes working in isochronous mode. At the beginning of the
simulation, the grid demands an apparent power of 69 kVA.
At t = 3 s, a load step of 17.24 kVA is included. Later, at t =
5 s, the extra load is switched off and the grid is back to its
original status. Having a look at the system frequency of the
simulated system shown in Fig. 8, it can be seen that the
frequency is rapidly restored back to 50 Hz (the nominal
frequency) after any load step. This is the advantage over the
droop concept where a frequency gap stays due to the droop
response. The speed of the frequency restoration is related to
the control loop’s parameters and is adjustable. The system
load is shown in Fig. 9 for comparison. The swinging
response at the beginning is normal behaviour of the starting
phase since the voltage is still not stable at the load.
Fig. 7. Topology: isochronous modular grid (three wire).
Frequency [Hz]
Fig. 8. The system frequency.
L
L
R
L
[V
pn0_dq_act
]
V
dc
[V
pn0_dq
]
v
α
v
β
v
γ
[I
pn0_dq_act
]
Q
P
Fig. 6. Grid-forming inverter with isochronous control function.
Fig. 9. The system total load.
At the inverter side, the inverters outputs are shown in
Figs. 10, 11 and 12 it can recognize that they are supplying
fixed voltage output and respond by changing their current to
the different load steps. At t = 3 s, the load is decreased. The
currents supplied by the inverters are decreased, but the
voltage is be kept constant by the controllers all the time.
10
20
30
40
50
2.9 2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4
-80
-60
-40
-20
0
20
40
60
Current
I
[A]Current
I
[A]
t [s]
I
1
I
1
Fig. 10. The current response of inverter one to load step at t = 3.0 s.
100
200
300
400
500
2.9 2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35
-200
0
200
400
Voltage
V
[V]Voltage
V
[V]
t [s]
V
2
V
2
Fig. 11. The voltage response of inverter two to load step at t = 3.0 s.
2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4
-400
-200
0
200
400
-50
0
50
100
150
83 A 69 A
323V
323V
Current
I
[A]Voltage
V
[V]
t [s]
I
3
V
3
Fig. 12. The voltage and current response of inverter three to load step at t =
3.0 s.
In this second case study, the control behaviour of modular
isolated grid controlled in isochronous mode is tested. The
topology is shown in Fig. 13. The network consists of three
inverters working in isochronous mode.
At the beginning of the simulation, the grid demands an
apparent power of 33.8 kVA. At t = 2 s, a load step of 16.8
kVA is included. Later, at t = 3 s, the extra load is switched
off and the grid is restored to its original status. Having a
look at the system frequency of the simulated system shown
in Fig. 14 it can be seen that the frequency is rapidly restored
back to 50 Hz (the nominal frequency) after any load step.
This is possible because of the isochronous load sharing
method where all the loads are fed back to the controller to
allow precise sharing. The system load is shown in Fig. 15
for comparison. It is also shared equally between the units;
this can as well be adjusted through the controller even
though normally the load is shared based on the inverters
rated power.
Fig. 13. Topology: isochronous modular grid (four-wire).
Fig. 14. The system frequency response.
Fig. 15. The active power.
Having a look at the reactive power sharing shown in Fig.
16, it can be seen that the three units are sharing it equally
when any load step occurs in the grid. When looking at the
load, see Fig. 17, it is clear that the voltage is kept constant
and symmetrical at any load step while the current is
changing to compensate for that which is wished-for. The
controller strength can be seen at t = 2 s when the additional
load at one phase is switched on. The voltage has a small
distortion but will be restored rapidly.
Fig. 16. The reactive power.
Fig. 17. The load voltage and current response at the first load step.
This can be seen as well from the response of the inverter
to load steps as shown below. It will maintain the voltage
constant and adjust the supplied current to the grid. Finally,
having a look at the neutral current it can be noticed that it is
shared by the three units equally.
Fig. 18. Second grid forming inverter voltage and current response at the
second load step.
Fig. 19. The neutral current response at the first load step.
V. C
ONCLUSION
This paper shows the possibility of adapting the
isochronous control methodology used by synchronous
generators in conventional power systems to provide load
sharing and control in inverter-based distributed generation.
The control tasks (voltage/frequency control) are done
locally at the inverters to guarantee modularity and to
minimize communication requirements. The simulation
results show the ability of the proposed concept for DG to
supply high quality power. The total load is distributed
among the different inverters according to their capacity to
guarantee flexibility.
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... The bias points are computed dynamically for active load sharing for each unit by load power measurement. These setpoints are rapidly varied as the system settles to a new steady state post-disturbance[158]. Isochronous load sharing is feasible for a microgrid with large load clusters and a precise number of grid-formers connected at the same bus (considering low electrical distance between loads and units) as it requires load measurement and a communication network between units. ...
Thesis
The electric power system has been traditionally energised by synchronous machines like steam turbines, hydro turbines, and diesel engines. These rotating machines inherently contribute to the system resilience by providing rotational inertia.The presence of an adequate inertia in the system provides the liberty of allowing a control delay for the governor-input valve controls to respond to the frequency deviation. With the displacement of synchronous machines by converter-connected sources, the reduction of inherent system inertia is evident. However, there is also a counterpoised observation that the required amount of inertia in the transformed power system is reduced, given the faster response of the converter-based DERs. Therefore, we resort to synthetic inertia to improve the resilience of a low-inertia grid.In this context, this thesis explores questions such as: What is the adequate synthetic inertia/frequency response capability for a stable power system? How can we quantify the flexibility required to provide this adequate inertia? Does synthetic inertia greater than the adequate level necessarily indicate a higher stability margin? How different is the effect of distributed synthetic inertia on the oscillatory stability compared to synchronous inertia?Firstly, the aspects of flexibility and methods to characterize them for an adequate synthetic inertia and fast-frequency response are addressed. A generalized virtual storage flexibility model has been proposed to quantify the heterogeneous bidirectional flexibilities and their combination to provide a certain level of synthetic inertia. As an illustration, a hybrid energy storage system has been sized to provide synthetic inertia and fast-frequency response for a standard power network.The subsequent chapters discuss synthetic inertia and fast-frequency control actuated by PV systems with hybrid energy storage. In this thesis, inverter control has been explored with a complete DC-side model takes into account the effects of PV intermittency, unlike most research works on inverter control that assume a sufficiently large DC source/sink. Synthetic inertia controllers are categorized as grid-following and grid-forming topologies, which significantly affect their impact on system stability. Conventionally, the inertia and damping parameters are tuned and fixed over a scheduled time slot based on the available flexibility. It has been identified that a higher inertia is required on the occurrence of a disturbance to limit the rate of frequency deviation and a higher damping is required for a faster settling time. Therefore, for each of the control topologies, a rule-based real-time inertia tuner has been proposed to optimize the frequency deviation, its rate, and the settling time. The algorithm has been improved through a model predictive control with a rate-based linearization. The rate-based linearization extends the model validity to the transient zones. For systems with multiple grid-formers and multiple frequency responsive units, a distributed optimization problem has been formulated and solved to collectively tune the inertia and damping parameters which are constrained by the available flexibilities.The efficacy of distributed grid-forming and grid-following synthetic inertia in replacing their synchronous counterpart in a microgrid has been compared. Microgrid regulation in grid-connected and islanded modes have been studied by modelling the DERs with discussed control strategies. The impact of the two types of synthetic inertia controls on the small signal stability of the system are examined by modal analysis and bifurcation plots to derive the conditions for oscillatory stability in a microgrid with distributed synthetic inertia reserves. The effectiveness of the proposed control strategies in restoring the frequency stability of low-inertia systems has been validated by power hardware-in-the-loop experimentation.
... The main goal of the isochronous control is to allow the PV sources (slaves) to supply their maximum power [33,50,51], controlling them in the so-called PQ (active and reactive power) control mode [29]. In this way, each load variation will be handled completely by the storage system that acts as a master. ...
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... 0.013j, 0.022j, 0.002j, 0].  Scenario 4: Islanded mode with isochronous operation [31]. In this mode, the largest DG units ( DGs 13,23,27) undertake the role of keeping the network frequency and their terminal voltage to pre-specified values. ...
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... however, DG units cannot be interfacing directly with high power loads. the heart of this interface is the inverter which plays a very important role in terms of energy conversion and adaptation between sources and loads to ensure efficient use of DG units [4]. The conversion principle in inverter is the use of Pulse Width Modulation or PWM technology to give a sinusoidal stable output voltage of 230V AC of the load. ...
... epletion of fossil fuels resources and increase in their costs, environmental concerns, transmission limits, financial intensive to utilize clean and renewable energies, as well as recent advances in small generation technologies have encouraged utilization of distributed generations units in distribution systems [1][2]. Additionally, from customer side of view, several advantages such as high power quality, uninterruptable and reliable service, high energy efficiency, better economy, and a reduced dependency on local grid are all motivations toward on-site generation [3]. ...
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