A Novel Adaptive Frequency Control Strategy for
Micro Grid
Priya Singh Bhakar & G. Kesava Rao
Dr. Saumendra Sarangi
Dept. Of Electrical Engineering, NIT Uttarakhand
Srinagar, Uttarakhand, India
Email: priyabhakar1995@gmail.com
Kesava551@gmail.com
Asst professor, Dept. Of Electrical Engineering
NIT Uttarakhand
Srinagar, Uttarakhand, India
Email: usomsam@gmail.com
Abstract—Modern strategy like use of Electronic Load
Controller (ELC) to improve the system frequency is an
attractive solution. However such an approach leads to decrease
in efficiency of the system as part of the generated power during
light load condition is dumped in resistive loads. Several
approaches which are developed to improve the performance of
the ELCs still have a void where they achieved a solution with a
compromise of reduced efficiency. In this work, we have
proposed a central control strategy to control the frequency
without reducing the efficiency, where ELC and pump loads are
connected in parallel. The pump load and ELC are controlled by
observing the rate of change of frequency. When the ratio is
positive, loads will on and the frequency will be maintained. The
proposed method is tested for IEEE 7 bus system using
MATLAB/SIMULINK and test results show that the scheme is
able to maintain the frequency without losing the extra power.
employed for the frequency regulation in micro grids, which is
a basic concept drawn from micro hydro systems[2]. These
controllers use a resistive load that dissipates the power in the
form of heat energy which ultimately results in energy
wastage. Such an approach can regulate the frequency
compromising the efficiency of the system which is not
desired. However in a competitive environment which
demands highly efficient system, requires new development in
such techniques to improve the efficiency.
In the presence of renewable sources, simultaneous
variation in generation and loads are observed which results
into frequent voltage and frequency fluctuations. With limited
performance of conventional frequency controllers, new
schemes have been proposed to control the frequency which
uses Electronic load controller. ELCs are fast in operation and
best cost effective measures for active power balance in the
system. The proposed model uses a chopper to dump the extra
power (using resistive loads) which leads to energy wastage
[3]. These ELCs has been continuously modified with time to
achieve better performance [4]-[9]. Integrated or decoupled
configuration of ELCs with STATCOM has improved the
voltage and frequency of the system [4], where the authors
have neglected improvement in efficiency. The efficiency of
the system was compromised in few configurations that
included control algorithms like least mean square algorithm,
proportional resonant control algorithm, motor flux estimation
algorithm [10]-[12].These algorithms provide harmonic
compensation, voltage regulation and mitigation of triplen
harmonics (with the use of transformers) with rejection of
disturbances. The proportional resonant control algorithm
improves the power quality of the system and also reduces the
total harmonic distortion [10].The least mean square algorithm
or Adaptive linear element based algorithm provides
frequency control, load balancing and neutral current
compensation [12].The available methods have left a corner
where development is required to improve the efficiency and
with respects to this the characteristic of the loads have been
observed.
In the distribution section several loads are available like
heaters, water pumps, etc. Water being an essential part of the
society, the pump is utilized daily to lift the water for its
supply in all the cities. Water pump loads can be used in a
Keywords—Distributed Generation; Micro Grid; Water Pump
Load; Voltage and Frequency Regulation ; Electronic Load
Controllers
AbbrevationsSTATCOM-Static Synchronous Compensator
ELC- Electronic Load Controller
DG- Distributed Generator
IGBT- Insulated Gate Bipolar Transistor
FACTS- Flexible Alternating Current Transmission system
BESS- Battery Energy Storage System
HESS- Hybrid Energy Storage System
I.
INTRODUCTION
The fast degradation of fossil fuel reserves has lead to the
development in numerous renewable power generation
techniques. The dearth of power has been lessen with the
advancement in such power sources. With the penetration of
small capacity renewable generations in distribution grid,
existence of micro grid has evolved which can be operated in
both grid and off grid modes. However, simultaneously the
variation in both load and generation in such a micro grid has
affected the voltage and frequency stability of the system [1].
New techniques like Electronic Load Controllers (ELC) are
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micro grid (work autonomously and supply water to a
particular locality) that has used wind or micro hydro as
distributed generator. In literature, various control strategies
are developed for these pump loads [13]-[18]. Mostly such
pumps are being operated for some fixed time in a day, either
in morning or evening. However it is not required to operate
the pump at regular fixed time and it can be done at any time
in a day which may not affect water distribution. So why not
operate the pump at light load condition and this can help us to
maintain the frequency and manage the power which is
dumped in ELC. This thought has driven us to design an
automatic frequency controller for micro grid using water
pump in parallel with ELC.
In this paper, an electronic load controller along with a
water pump load is employed for the frequency regulation.
The control strategy uses the concept of rate of change of
frequency for improving efficiency of micro grid. The
proposed scheme is tested on a 7 bus distributed system and
test results show the method is highly efficient in maintaining
the system frequency where the surplus power is effectively
utilized to improve the efficiency.
II.
SURPLUS POWER UTILIZATION BY WATER PUMP LOAD
Electronic load controllers are connected near distributed
generators in order to maintain the frequency constant. Earlier
ELCs with resistive load were used. The surplus power was
dissipated in resistor in the form of heat energy [19]. Dumping
extra power in resistor was a wastage of energy and the system
needed to overcome this energy wastage to improve the
efficiency. Loads like battery energy storage systems (BESS)
were used with ELCs to control the frequency and utilize the
excess power. The use of BESS has improved efficiency to
some extent. A further improvement in technology was needed
as the batteries produced toxic disposals [20]. There was a
need of a load that can be used in day to day life in almost
every distribution grid and also consume the excess power to
enhance the efficiency. Water pump is such a load that is used
to raise water to certain height for distribution of water in
societies, buildings, etc.
Pumping water is required in every urban or rural areas.
These pumps consume electrical energy to bring the water up
to some height for distribution, where the energy required may
be taken from grid or from renewable generations. With
penetration of distributed generations in the grid, during light
load condition to control the frequency, water pump can be
used as loads which improve the frequency and efficiency of
micro grids. However use of pump loads at light load
conditions to eliminate the frequency variation will not affect
normal water distribution. By consuming the excess power,
the frequency is regulated along with the maintenance of the
active power balance and power is also utilized without its
wastage.
As transient response of such system are not fast. In order
to improve the frequency response, control combination of the
Electronic load controller (ELC) which is faster in operation
and pump load may be one of the possible option. However, in
a micro grid system, loads are continuously varying in nature
and this demands multiple electronic load controllers near
distributed generators to maintain the active power balance of
the system using adaptive control strategy. Such a control
scheme regulates operation of ELC and pump load by
considering the frequency and active power of the system.
III.
PROPOSED CONTROL STRATEGY
A. Description of System
An IEEE 7 bus system with distributed generators as
shown in Fig. 1 is considered to observe the variations in
frequency with variation in load generation of micro grid. The
system consist of two DGs connected at bus number 1 (wind
generator) and 6 (micro hydro generator) to provide 1.2MW
power. As both the generated and load power are continuously
varying, voltage and frequency fluctuations are observed in
the system. To compensate these frequency variations, the
system is modified with ELCs near by the DG connected to
buses 6 and 1. Water pump as load is selected to mitigate the
frequency fluctuations. It is tested at different buses of the
system and its response is studied. For better performance of
the system in order to decrease the losses and improve the
system transient response, multiple water pumps are
connected to bus 3 ( maximum voltage is obtained at bus 3 for
connecting the pump loads after performing the load flow
analysis) of the system as shown in the Fig.1.
Fig. 1. Single line diagram of proposed micro grid using water pump load
Control block shown in Fig. 1 may be placed at substation
which is preferred to be at the centre of any region. The input
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signal to this controller is active power and frequency
collected from each bus and after every 4 second, this data is
revised. Using the data, rate of change of frequency is
computed and accordingly the electronic load controllers are
triggered. The pictorial diagram of the proposed scheme with
communication channels is shown in Fig.1.
Step 5- οܲᇱ will be the difference in the total excess power
and the power fed to the water pumps that results into
the power which is dissipated in ELC using (3).
οܲ ᇱ ൌ ο െ 
(3)
There is possibility of multiple distributed generation
sources which requires multiple electronic load controllers. If
multiple pumps are available the complexity of the control
algorithm will increase. In this work we have selected ELC at
two locations where distributed generations are available (bus
1 and bus 6). However ELC may be placed at any point of the
system. For simplicity the pump load is assumed to be
connected at middle of the both the DG connected in the
system. Using the generated power and load power
information, total excess power is calculated and the controller
determines the power to be utilised in water pump and in the
electronic load controller to mitigate frequency deviation
problem and accordingly the signals are generated. The ELC
is operated using firing angle control of isolated gate bipolar
transistor chopper. The power is dissipated in the electronic
load controller as soon as the signal is given to the gate of the
IGBT. This control strategy is completely automatic and no
human interference is required to switch on the pump load and
ELC.
B. Proposed Algorithm
The control algorithm is designed for the system provided
in Fig.1. However it is not system specific. As system size
increases the requirement of communication channel increases
which in turn increases the data collected at the distribution
center. This increases the computational burden which can be
solved using high speed computational devices. Incase failure
of communication channel, using previous data and observing
the local frequency at distributed generation buses ELCs can
be operated to reduce the frequency deviation. The flow
diagram of the proposed method given in Fig. 2 and steps of
the control strategy are given below:
Step 1- Active power and frequency from all the buses are
collected and the rate of change of frequency is
computed.
Step 2- The control algorithm is triggered for
ௗ௙
൐ Ͳ
(1)
ௗ௧
ௗ௙
Whenever is less than zero, after four seconds the
ௗ௧
data from all the buses is revised.
Step 3- The total excess power is calculated by equation (2) as
¨P which is the difference between generated and the
load power.
¨P = ୥ െ ୐
(2)
Step 4- ¨P is divided among all the pumps and the total motor
pump load demand is nP (where n is the number of
pumps) which is given to the controller. The motor
loads are simultaneously switched ON.
Fig. 2. Flow chart of control algorithm
Step 6- These ELCs are controlled by the firing angle control
of the IGBT chopper.
ௗ௙
Step 7- The parameter is again compared with zero. The
ௗ௧
control algorithm is repeated whenever the rate of
change of frequency is greater than zero. The
ௗ௙
is less than zero.
algorithm will stop when
ௗ௧
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IV. RESULTS AND DISCUSSION
An IEEE 7 bus system is selected and modified as wind
connected at bus 1 and micro hydro connected at bus 6 is
varying in nature. The distributed generators are varied
separately and also simultaneously to observe the efficiency of
the proposed algorithm. Few special cases in this section are
presented and simulated in MATLAB/SIMULINK to show the
capability of the system.
A. WITHOUT PUMP LOAD
Case: Performance of proposed method for variation in wind
unit
To test the accuracy of the proposed algorithm, wind
generation is varied at a light load condition keeping the hydro
generation constant. As load varies, the frequency and active
power at the buses are collected and the distribution generator
bus frequency is plotted in the Fig. 3. As observed from the
figure, the frequency has increased with decrease in loading.
ௗ௙
is greater than zero it will trigger the control
Whenever
ௗ௧
algorithm. The surplus power in this case is 20kW and is
calculated using (2). The signal is transferred to the ELC
connected at the bus 1 which consumes total 20kW.
Fig. 3. Generation variation of wind generator without ELC
Point A represents starting of the wind generation
variation. The frequency is increasing with variation in wind
power output at 6 seconds as seen in Fig. 3.
Fig. 4. Generation variation of wind generator with ELC
Point B represents starting of the frequency fluctuations. It
is observed that when ELC is switched on at 60.5Hz, the
variation in frequency is minimized and achieved within 0.35
seconds as observed from the Fig. 4.
B. WITH PUMP LOAD
Case I: Performance of proposed method for variation in
wind unit
The performance of the proposed method is also tested for
variation in wind generation at a light load condition keeping
the micro hydro generation constant. As load undergoes
variations, the frequency and the active power at the buses are
collected and the distribution generator bus frequency is
plotted in the Fig. 5. As observed from the waveform, the
ௗ௙
frequency has increased with decreased loading. Whenever ௗ௧
is greater than zero it will trigger the control algorithm. The
surplus power is 40kW, dumped power in ELC is 10kW and
power fed to the motor pump loads is 30kW which are
computed using (2) and (3). Signal is transferred to the ELC
connected at bus 1 and water pumps connected at bus 3. This
shows that the efficiency of the system has improved on
comparing with previous case (without pump loads).
Fig. 5. Generation variation of wind generator without ELC and water pump
load
Point C represents the variation of wind generation
without employing electronic load controller and water pump
load. The frequency has started increasing with generation
variation at 6 seconds as seen in Fig. 5.
Fig. 6. Generation variation of wind generator with ELC and water pump
load
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Point D represents the starting of frequency variations. It is
observed that when ELC and motor pump loads are switched
on after 60.5Hz, the variation in frequency is minimized and
achieved within 0.45 seconds as observed from the Fig. 6.
Case II: Performance of proposed method for variation in
micro hydro unit
The performance of the proposed method is also tested for
variation in micro hydro generation at a light load condition
keeping the wind generation constant. As load undergoes
variations, the frequency and the active power at the buses are
collected and the distribution generator bus frequency is
plotted in Fig. 7. Surplus power in this case is 50kW, power
consumed by ELC is 20kW and pump loads are consuming
30kW.
Case III: Performance of Proposed method for variation in
both Hydro and Wind geneartion
Similarly the accuracy of the method is tested for variations
in both micro hydro and wind generation. As the load
changed, the frequency and active power at all the buses are
collected and the distribution generator frequency is plotted in
the Fig. 9. As observed from the figure, the frequency has
increased with decrease in loading. The algorithm will be
ௗ௙
is greater than zero. The surplus power
triggered whenever
ௗ௧
is 60kW, power fed to ELC is 30kW and the power consumed
by pump loads is 30kW which are computed using (2) and (3).
The signals are transferred to the ELCs connected at bus 1, 6
and pump loads connected at bus 3.
Fig. 9. Generation variation of both wind and micro hydro without ELC and
water pump load
Fig. 7. Generation variation of micro hydro generator without ELC and water
pump load
As observed from Fig. 7, the frequency has started
ௗ௙
increasing (point E) with decrease in loading. Whenever ௗ௧
is greater than zero, the control algorithm is triggered
automatically and the signal is transferred to the ELC
connected at bus 6 and pump loads connected at bus 3.
The frequency increases (starts from point G) with
variation in generation of both wind and micro hydro
generators without ELC and motor pump as shown in Fig. 9.
Fig. 10. Generation variation of both wind and micro hydro with ELC and
water pump load
Fig. 8. Generation variation of micro hydro with ELC and water pump load
It is observed that when motor load and ELC are triggered
at 60.5Hz, the frequency is controlled within 0.45 seconds as
observed from fig 8.
It is observed that when motor loads and ELCs are
switched on, the frequency variation is controlled within 0.5
seconds as observed from Fig. 10.
Different cases of power distribution between ELC and
water pump loads are shown in Table I that hasTotal number of pumps in the system (n) - 8
Capacity of each pump- 5 Hp
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TABLE I
POWER DISTRIBUTION BETWEEN ELC AND PUMP LOADS
Frequency
Variation
(in Hz)
Number
of pumps
working
61.6
62.07
63
63.5
5
Power
consumed by
motor pump
loads (kW)
17.45
Power
consumed
by ELC
(kW)
2.55
7
8
8
24.43
30
30
5.57
20
30
Efficiency of the system increases with the use of water
pump loads as seen in Table I. The frequency variations are
minimized with the use of electronic load controllers and also
the efficiency is improved with increasing number of pump
loads.
V.
CONCLUSIONS
This work presents a novel technique to control the
frequency in a micro grid. System frequency and active power
are collected at all the buses and the rate of change of
frequency is computed to drive the control algorithm.
ௗ௙
Whenever is positive, the algorithm starts and accordingly
ௗ௧
the electronic load controllers are triggered. The water pump
along with ELC will consume excess power and also improve
the system efficiency. The proposed method is tested for
various cases considering variations in single and multiple
distributed generators and found to be accurate. Test results
shows that the frequency can be stabilized with improved
efficiency within minimal time. It has been observed that
both ELC and water pumps work satisfactorily for generation
variation of different distributed generators.
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APPENDIX
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11kV, 1.2MW, 60Hz system data are provided as followsGenerators- PMSG and asynchronous generator
Load- 1.2MW, 60Hz
Water pump loads- 30kW (each of 5Hp)
Line parametersLine
1-2
2-3
2-5
3-4
4-5
5-6
6-7
R (ohm)
1.35309
1.17024
0.8411
1.19702
1.52348
1.0882
1.25143
L(mH)
0.003516
0.003037
0.0021823
0.002141
0.0027279
0.001946
0.001946
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