Download II. Droop Controlling Method

Microgrid Control In Islanding And Connected Mode
Abstract—In common practice a Microgrid can operate while
connected to a MV network. When a preplanned or unplanned
event like holding or occurring fault occurs in the MV network it
is possible to cause the islanding state in the Microgrid. In this
Paper the operation of the MV network in the islanding mode
and how to control the Microgrid by using the controlling
structure are investigated. In this paper the conventional droop
method has been described and two new controlling methods for
controlling the Microgrid are also proposed which in the first
method the AC power theory and in the second one the Cascade
controller are being used for the controlling of the Microgrid.
For comparing these three methods with the controlling method
which is proposed in reference [14], these four methods have
been simulated with PSCAD/EMTDC software. In the performed
simulating the inverter has been used as the bridge between DGs
and Microgrid and these controlling methods are used to control
the output voltage and current of the inverter. In addition for
switching the inverter the SPWM and hysteresis methods are
being used.
generation in LV networks, similar to the one that is being
used to integrate distributed generation on MV networks [1],
may result in technical problems on LV and MV networks
(excessive voltages, increase in fault levels, voltage
unbalance, overloading, etc.), namely when the penetration of
micro generation becomes high [2][3]. The new concept of
Microgrid emerged as a way to ease this integration, but in
fact corresponds to an entirely new way of understanding LV
networks, with potential benefits far beyond the easy
integration of micro generation [5]-[8].
Keywords-Connected mode, DG, Droop method, Islanding
mode, Microgrid, SPWM, Hysteresis Switching.
A Microgrid can operate in grid-connected mode or
islanded mode and hence increase the reliability of energy
supplies by disconnecting from the grid in the case of network
faults.
The high penetration of DGs, along with different types of
loads, always raises concern about coordinated control and
power quality issues. In Microgrid, parallel DGs are controlled
to deliver the desired active and reactive power to the system
while local signals are used as feedback to control the
converters. The power sharing among the DGs can be
achieved by controlling two independent quantities frequency
and fundamental voltage magnitude.
I. INTRODUCTION
Micro-scale distributed generators (DGs), or micro
sources, are being considered increasingly to provide
electricity for the expanding energy demands in the network.
The development of DGs operated in a Microgrid also helps to
reduce greenhouse gas emissions and increase energy
efficiency. Distributed generation encompasses a wide range
of prime mover technologies, such as internal combustion
engines, gas turbines, micro turbines, photovoltaic, fuel cells,
and wind power. Most emerging technologies such as
Microturbines, photovoltaic, fuel cells, and gas internal
combustion engines with permanent magnet generators have
an inverter to interface with the electrical distribution system.
In the last decades, the interest on distributed generation has
been increasing, essentially due to technical developments on
generation systems that meet environmental and energy policy
concerns. The interconnection of distributed generation has
been predominately confined to MV and HV levels, but the
development of micro generation technology, the decline of its
costs and the public incentives to distributed generation lead to
an increased installation of micro generation in LV networks.
Although there is in general a lack of regulations to frame the
operation of micro generators [1], in some countries, Portugal,
there is already specific legislation about micro generation.
This legislation is intended to encourage the investment on
micro generation, namely by subsidizing the remuneration of
the electricity produced by these generators and partially
funding the investment. The simple integration of micro
If the grid has to go in islanding mode due to fault or lowvoltage, the network controllers must keep the system
frequency and voltage in a desirable area leads to sending the
qualified power to consumers. The Microgrid should also
recover the frequency and voltage in islanding mode rapidly.
When Microgrid goes to islanding mode, its last situation
should be considered: if it was importing energy then after
entering to the islanding mode, it has to increase the
generation level to compensate the lost power; therefore in this
mode the Microgrid frequency will be decreased. On the other
hand, if Microgrid was exporting energy and afterwards it has
gone to islanding mode, it should decrease the generation level
so the grid frequency will rise.
Now a considerable research has been undertaken on the
control strategy of the Microgrid. Flexible and fast controls of
real and reactive power are important requirements during
transient and steady-state operation of a Microgrid system in
both grid-connected and islanded modes [4].
Operation during network disturbances while maintaining
power quality in the islanded mode of operation, a more
sophisticated control strategy for Microgrid needs to be
developed. In order to warranty both quality of supply and
ensuring power management supervising critical and noncritical loads. Also, the basic issue on small grids is the control
of the number of microsources. Microgrid concept allows
1
larger distribution generation by placing many microsources
behind singles interfaces to the utility grid. Some key power
system concepts based on power vs. frequency droops
methodology, voltage control and can be applied to improve
system stability, enhance active and reactive support. Flexible
and fast controls of real and reactive power are important
requirements during transient and steady-state operation of a
Microgrid system in both grid-connected and islanded modes.
A Microgrid when subjected to disturbances can experience
angle instability and poor voltage quality due to the presence
of DG units with slow response rotating machines and DG
units with power electronic converters as the interface to the
utility system. To ensure stable operation during network
disturbances while maintaining power quality in the islanded
mode of operation, a more sophisticated control strategy for
Microgrid needs to be developed.
load voltage are measured. The voltage magnitude at the load
bus and the active power injected are then calculated. When
controlling active power, there is a choice of regulating the
power coming from the unit or the power flowing in the feeder
where the source is connected. If the power in
The feeder is regulated then there is a need to bring in the
measure of the current flowing in that branch.
The voltage is calculated from the stationary frame
components of the filtered instantaneous voltages. The desired
and measured values for the voltages are then passed to a
dynamic block that implements the P-I response of the voltage
control. The output is the desired voltage magnitude to be
implemented by the gate pulse generator block. This version
includes low pass filters on the P, Q, and V calculated
quantities to reduce the propagation of the effects of noise.
These filters have also a secondary desirable effect: they
attenuate the magnitude of the 120Hz ripple that exists during
unbalanced operation. This allows the control to survive
conditions of load unbalance without having to take any
corrective action.
The work described in this paper regards the evaluation,
through numerical simulation, of the inverter-fed MG
behavior under islanded operation and connected operation for
different load conditions and using different control strategies
with limiter for power. The impact of considering the primary
energy source dynamics and the inverters contributions to
faults are issues addressed in this paper. PSCAD Simulink was
employed in order to develop a simulation platform suitable
for identifying MG control requirements and evaluating MG
dynamic behavior under several operating conditions.
Different Micro Source (MS) technologies were modeled and
are considered to operate together in the simulation platform.
The Micro sources are combination of fuel cell and
microturbine is considered. This paper is organized as follows,
in section II shows droop controlling method, In Section III,
was described proposing the suggested controlling methods.
Section IV shows case study system. In Section V the
Microgrid simulation and results was shown. In Section VI,
the conclusions of this paper are given.
II.
This final version of the control focuses on the
configuration that regulates the power injected by the unit.
This is not a loss of generality, since to implement the load
tracking configuration one needs just two modifications: the
first is to introduce the measure of the line current to calculate
feeder flow, still retaining the inverter current measure to
calculate the reactive power injected by the unit. The second
modification is to invert the sign of the droop coefficient of
the power- frequency characteristic to take into consideration
that now it is the line power that is regulated and not the
power injected by the unit.
DROOP CONTROLLING METHOD
In the Microgrid, the DG supply can be connected to a
network by using an inverter. An inverter can be tuned by
frequency and voltage control.
For controlling the inverters usually the drop method is
used. The active and reactive power of inverter can be
controlled by means of this method in order to control the
frequency and to produce voltage in a specific level. This
section gives the details of the final form that the control
assumes during the implementation. Hardware realization of
the control has been plagued by issues of noise propagation
from the analog to the digital word inside the controller. The
fundamental frequency selective filter is a first tool to handle
some of the higher harmonics of the noise, but the calculated
values of active and reactive power, as well as the voltage
magnitude still suffered from oscillations determined by
random spikes in the measured quantities.
Figure1. Micro source Control
In this method frequency changes is affected by changing
in active power. Fig. 2 shows the Droop of power versus
frequency changes.
The complete control of the micro source is shown in
Fig. 1. The inputs are either measurements (like the voltages
and currents) or set points (for voltage, power and the nominal
grid frequency). The outputs are the gate pulses that dictate
when and for how long the power electronic devices are going
to conduct. The inverter voltage and current along with the
Figure 2. Power versus Frequency
2
According to Fig. 2 it can be deduced that whenever
frequency decreases in a network, controllers make the
supplies to increase their production and it means the
production level changes from p0 to p1.
calculated by using the voltage and current amount. In
equation 3 the voltage and current are in α and β coordinate
which result in the amounts of p and q.
This state will happen to a Microgrid whenever a fault
occurs and it changes a Microgrid status into islanding state.
The Microgrid reconnects to principle network meanwhile the
frequency increases then controllers make the DG to decrease
the production level. Equation 1 represents this controlling
structure:
 p (t )   v  (t ) v  (t )  i  (t ) 
q (t )    v (t ) v (t )  i (t ) 


  
  
(3)
This method compensates the reactive power by active
filter. So that if by using equation 4 the current harmonic
values which are effective in generating the reactive power
could be obtained, it would be well possible to compensate the
reactive power and to eliminate harmonically current by
inserting the aforementioned obtained current to system by an
inverter.
(1)
Reactive power control versus voltage
This control mode, changes in voltage have an interaction
with changes in reactive power. According to Fig. 3, this
controlling mode is represents by Equation 2:
1
i  (t )   v  (t ) v  (t )   0 
i (t )    v (t ) v (t )  


   
 q (t ) 
(4)
This theory can also be used for controlling the level of
generating in the Microgrid. If the values of active and
reactive power which should be generated by DG are set in the
Equation 5, the DC source of the inverter is compelled to
supply these values of p and q through inverter. Therefore the
source current which is built by these power values could be
given to the current controlling section to insert it to the
system by switching. By inserting the appropriate current to
system, the active and reactive power essential for the load
would be supplied and distributed. The needed DC source for
generating the power is the aforementioned DG which is
connected to the system through the inverter.
(2)
1
i  (t )   v  (t ) v  (t )   p (t ) 
i (t )    v (t ) v (t )  


   
  q (t ) 
Now we consider the two situation of islanding state and
connected in the DG. In each situation if the source power
values of for generating and distributing to the network are put
in Equation 5, the inverter output could be changed according
to system demand. In this situation with the change of the
demand of the load (change of state from connected to
islanding state or vice versa), the power value and
consequently the source current value in Equation 5 will
change and thus for doing this change in the inverter output,
the level of DG generating connected to it under the
appropriate controlling method, would be changed.
Figure 3. Voltage versus Reactive Power
Where E is the amplitude of the inverter output voltage; is
the frequency of the inverter; ω*and E* are the frequency and
amplitude at no-load, respectively; and n is the proportional
droop coefficient.
III.
PROPOSING THE SUGGESTED CONTROLLING METHODS
In this chapter first of all the two proposed controlling
methods for controlling the Microgrid will be introduced.
Then by implementing these methods on a Microgrid by using
the PSCAD/EMTDC software, the results obtained by using
the proposed methods will be compared with the OFT
controlling method.
A.
(5)
After that for switching the inverter the hysteresis
controlling method could be used which has advantages like
god stability, simple and economically designing and
implementing and quick dynamical response. For doing so the
appropriate hysteresis band in the both sides of the source
current value will be considered. If the output current of the
inverter crashes hits the above band, the output voltage of the
inverter will diminish and if the output current crosses the
under band, the inverter's switches will be activated and the
output voltage will increase. Thus the current will remain in
Applying the instantaneous active power theory for
controlling the Microgrid:
In this method AC powers AGAGI are used for supplying
the essential amount of TAVAN of load. AC powers theory
was proposed by AGAGI in 1984 and is still being used in the
industry. According to this theory AC powers can be
3
the interested band and the interested wave shape will be
constructed and inserted to the system.
system which is connected to a 1/1 KV system. The
distribution system is consisted of feeder 5, 2 DG unit and 3
load of 100 KVA which are modeled as RLC. The DG units
are consisted of one Fuel cell unit and one micro turbine unit
which are connected to the system through a DC-DC converter
and an inverter.
By using the described controlling method, the active and
reactive power generated by DG in Microgrid can be
controlled and the supply and demand in the power network
with Microgrid in two situations of connected and islanding
state will be adjusted.
B.
The advantages of using the Fuel cell and micro turbine
are high reliability and low emission. The dynamical structure
of the fuel cell and micro turbine is described as follows.
Using the cascade controller
One of the major advantages of using the PI controller is
using two PI controller simultaneously which led to a better
closed-loop dynamical operation and these controllers are
called cascade controller. In cascade controller two controllers
are connected in such a way which one controller produces the
other controller's input. The major controller in the outward
loop controls the primary physical variable which in this case
is frequency. The secondary controller in the inward loop
reads the outward loop which is power and so controls the
powers which is changing. Since the changes in the network
depends on the costumers and is changing constantly, with this
controlling system these changes enters the inward loop and
there will be compensated and these changes will then less
affects the outward loop. It could mathematically be proved
that the working frequency of the controller will increase and
time constant will decrease dramatically.
Level 1
f*
Figure 5. Case Stusy
1) DC-DC inverter: Fuel cell has a slow response and
generates little voltage. Thus it cannot be connected to the
load directly and output voltage before transforming DC to
AC should be improved. In addition the DC output of fuel cell
in is not controlled and an inverter for transforming the
uncontrolled DC voltage to controlled DC voltage in an
appropriate level is needed. Thus a buck-boost inverter,
something between SOFC and inverter is needed. This
convertor can have more or less output voltage according to
Duty cycle (D). If D is less than 0.5, the inverter acts like a
buck inverter, otherwise it will act like a boost inverter.
In Fig. 6 the simulated model of Buck-Boost converter in the
PSCAD/EMTDC is shown.
Level 2
PI
V
PI
f net
PLANT
Pout
Figure 4. Control Structure of Cascade
The advantages of using the cascade controller are:

Better controlling of the primary variable

Noise decrease of the primary variable

Increase of the retrieve speed after noise
occurring

Increase of the natural frequency of the system

Dramatically decrease of the time constant of the
system

Improving of the dynamical operation of the
system
IV.
A.
CASE STUDY
The structure of the Microgrid
In the Fig. 5 the single line diagram of the tested Microgrid
is shown. The Microgrid is consisted of radial distribution
Figure 6. Simulation model of Buck-Boost Converter
4
2) Inverter: The convertors of the direct current to AC
current (DC-AC) are known as inverter. The task of an
inverter is to transform a direct input voltage or current to an
AC Symmetric output with the desired amplitude and
frequency. In inverters the switching techniques are being
used so that by giving on and off orders to switches, the
desired output wave shapes will be generated.
There are two kinds of switching techniques which can be
seen in Fig. 7. The first kind which is known as open loop and
are using for voltage controlling is consisted of methods like
SPWM and SVM. The second one which is known as closed
loop is being used for current controlling which in this method
a feedback is taken from the output current for decreasing the
current error. This kind contains hysteresis band control, The
linear PI, Approximating regulators and et cetera. The current
controlling method has some advantages over the voltage
controlling method which are:
 Instantly control of the current and high accuracy
 Protecting the peak current
 Very good dynamical response
 Compensating the effects of load characteristic
changing
Figure 7. Inverter Switching Technique
Here two kinds of switching are being used. In the first section
which the Droop controlling method was used, the SPWM and
in the second section, the hysteresis band controlling have
been used.
Figure 8. SPWM Switching
b) Hysteresis switching
a) SPWM Switching
Hysteresis switching model in PSCAD/EMTDC was
shown in Fig. 9.
Ia
SPWM switching is used for the inverter which is applied
here. In this kind of inverter the desired output wave is
obtained by comparing the desired source wave with the high
frequency triangular wave which is shown schematically in
Fig. 8, considering the voltage is bigger or lesser than the
carrier wave, one of the positive or negative voltages would be
used.
Ia1
D
+
Ib
Ib1
F
g2
D + -
*
1000
Ic
Ic1
g1
-
*
1000
D
*
1000
g3
F
g4
+
g5
F
g6
Figure 9. Hysteresis Switching
5
B.
The controlling method used in reference [14]
applying the AC power theory for controlling the Microgrid
have the best response and had the ability to stabilize the
transient state of the micro turbine quickly. Also it can be
deduced from the diagrams that using the PID controller and
the AC power theory control method has the better voltage
profile compared to droop method.
In this method SPWM is used for inverter's switching. The
switching signals are obtained by using two control PI loops
which are used for controlling the voltage and frequency
values. The phase voltages and currents are moved to d-q
space. Id-ref is the reference current value of the d axis and
Iq-ref is the reference current value of the q axis. These
currents are being compared to the real currents and the
difference value is being sent to the two PI controllers. The d
axis current is used for controlling the inverter current value
and the q axis current is used for controlling the frequency.
The outputs of the controllers are moved from the p-q space to
abc space and are sent to PWM.
a) Active & Reactive power for DG1 & DG2
b) Voltage Profile Of Unit 1 & 2
Figure 11. distinct operation of each DG unit with Cascade
Controlling Method
Figure 10. Controlling Method used in refrence [14]
V.
SIMULATION AND ANALYSIS
In this section the operation of DG sources in Microgrid by
using proposed controlling methods in this Paper will be
investigated and the obtained results are compared to obtained
results in reference [14]. Here three tests have been carried out
on the Microgrid which is respectively as follows:
1) The distinct operation of each DG unit : in which the
DG1 and DG2 are alone responsible for supplying the load of
L1 and L3 in the steady state.
2) The connected state : in this situation some of the
required power in Microgridis supplied by the major network.
3) Islanding Mode: In this situation the Microgridis
separated from major network and goes to islanding mode.
a) Active & Reactive power for DG1 & DG2
A. The results from the distinct operation of each DG unit
In this situation each DG unit is responsible for supplying a
defined load of 100 KVA. The results of this simulation are
shown in Figs 11-14. As it could be seen in each of these
figures, the micro turbine has a transient state in the first
moment which is because of absorbing the reactive power.
The voltage profile of each unit is shown in the B part of each
figure which is showing the similar response of both units for
approaching the desired voltage level. As it could be seen
b) Voltage Profile Of Unit 1 & 2
Figure 12. distinct operation of each DG unit with
instantaneous active power theory Controlling Method
6
a) Active & Reactive power for DG1 & DG2
c) Voltage Profile Of Unit 1 & 2
Figure 14. distinct operation of each DG unit with Controlling
Method used in reference [14]
B. The results from the operation of each DG unit while
connecting to the major network
In this situation Microgridis connected to the major
network and is exchanging energy with it. In this situation
DG1 and DG2 are responsible for supplying the required
power for L1, L2 and L3 loads which 73% of the required
power in the connected situation is supplied by the generating
units of the Microgridand 23% of it is supplied by the major
network. And the controller could keep stabling the voltage
within 98% of the nominal voltage.
b) Voltage Profile Of Unit 1 & 2
Figure 13. distinct operation of each DG unit with Droop
Controlling Method
a) Active & Reactive power for SOFC
a) Active power for DG1 & DG2
b) Reactive power for DG1 & DG2
b) Active & Reactive power for Micro turbine
7
c) Voltage Profile of Unit 1 & 2
a) Active power for DG1 & DG2
Figure 15. Performance of DG units in connected mode
with Cascade Controlling Method
b) Reactive power for DG1 & DG2
a) Active power for DG1 & DG2
c) Voltage Profile of Unit 1 & 2
b) Reactive power for DG1 & DG2
Figure 17. Performance of DG units in connected mode
with Droop Controlling Method
c) Voltage Profile of Unit 1 & 2
Figure 16. Performance of DG units in connected mode
with instantaneous active power theory Controlling Method
a) Active power for DG1 & DG2
8
b) Reactive power for DG1 & DG2
b) Reactive power for DG1 & DG2
Figure 19. Performance of DG units in islanding mode
with Cascade Controlling Method
c) Voltage Profile Of Unit 1 & 2
Figure 18. Performance of DG units in connected mode
C. used in reference [14]
with Controlling Method
a) Active power for DG1 & DG2
D. Investigating the operation of Microgrid in the
islanding mode
In the fourth second the Microgrid in the state of B2 will
cut and the Microgrid goes to islanding mode. In this situation
the controllers command increase generation to the generating
units in Microgrid to supply the demand. The diagram of
generated power changes are shown in Figs. 19-22. In the
islanding mode, the reactive power of DG units diminishes
because the reactive power produced by the network is cut.
b) Reactive power for DG1 & DG2
Figure 20. Performance of DG units in islanding mode
with instantaneous active power theory Controlling Method
Main : Graphs
0.200
P(SOFC)
p(MTG)
p(MAIN)
0.150
0.100
y
0.050
0.000
-0.050
a) Active power for DG1 & DG2
-0.100
0.0
1.0
2.0
3.0
a) Active power for DG1 & DG2
9
4.0
5.0
...
...
...
Main : Graphs
q(SOFC)
0.030
q(MTG)
q(MAIN)
0.020
0.010
y
0.000
-0.010
-0.020
-0.030
0.0
1.0
2.0
3.0
4.0
5.0
b) Reactive power for DG1 & DG2
...
...
...
Figure 21. Performance of DG units in islanding mode
with Droop Controlling Method
References
[1]
[2]
[3]
[4]
[5]
a) Active power for DG1 & DG2
[6]
[7]
[8]
[9]
[10]
b) Reactive power for DG1 & DG2
Figure 22. Performance of DG units in islanding mode
with Controlling Method used in reference [14]
VI.
2) Applying the AC power theory as a new approach to
control the active and reactive power of Microgrid.
3) Applying the cascade controlling method for
improving the responding speed of the system to load
changes
4) Simulating the Droop controlling method and two
new controlling methods and comparing them with
each other and observing the superiority of these two
controlling methods in their speed to responding to
the changes in the load and reducing the time
constant of the system and improving the dynamical
operation of the system compared to Droop
controlling method.
[11]
[12]
CONCLUSIONS
In this paper we have investigate and simulated some
approaches for controlling Microgrid as follows:
[13]
1) A full description of the Droop controlling method
and using it for controlling the voltage and frequency
of the inverter as a bridge between DG and
Microgrid.
[14]
10
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