dynamic model of wind power balancing in hybrid power system · power generation of the system with...
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Turk J Elec Eng & Comp Sci
(2017) 25: 222 – 234
c⃝ TUBITAK
doi:10.3906/elk-1410-163
Turkish Journal of Electrical Engineering & Computer Sciences
http :// journa l s . tub i tak .gov . t r/e lektr ik/
Research Article
Dynamic model of wind power balancing in hybrid power system
Audrius JONAITIS, Renata MILIUNE, Tomas DEVEIKIS∗
Department of Electric Power Systems, Faculty of Electrical and Electronic Engineering,Kaunas University of Technology, Kaunas, Lithuania
Received: 27.10.2014 • Accepted/Published Online: 11.12.2015 • Final Version: 24.01.2017
Abstract:The paper presents a dynamic model of a hybrid power system composed of a wind park, a diesel generator, and
an electrochemical energy storage system. The purpose of conventional generating units and energy storage equipment
in the hybrid power system is the balancing of fluctuating power generated by renewable energy sources as well as an
increase in supplied power quality. The composed dynamic model describes characteristics of the power governor of the
diesel generator and dynamic behavior of the energy storage system based on a vanadium redox battery. Simulations
are based on sampled data of real wind park power installed in the Lithuanian power system. The effectiveness of wind
park power balancing while using different capacities of diesel generator and electrochemical energy storage system is
investigated. Optimal capacities of the balancing equipment are estimated.
Key words: Dynamic model, wind power, hybrid power system, storage system
1. Introduction
Renewable energy sources are an alternative to conventional organic fossil fuel. In the European Union, local
energy sources are sufficient to meet only a half of consumers’ demand [1,2]. This situation creates economic
dependence on countries exporting energy sources. The second aspect is the growing air pollution from burning
fossil fuel and the subsequent global climate change. The forecasted increase in global energy consumption
means that the emission of harmful pollutions will increase also. Renewable energy sources can solve these
problems; however, their development involves new technical and economic aspects. One of the problems of
renewable energy sources is a variable supply of electrical power and energy that depends on natural conditions:
sun irradiance, wind power etc.
Different solutions are being developed for balancing renewable power; however, these solutions should be
implemented considering conditions of individual cases. For solving these problems, hybrid power systems are
being developed that integrate conventional as well as renewable energy sources. In such hybrid systems, the
conventional generating units follow the load shape of the renewable energy sources and balance their power.
Additional equipment in hybrid systems is energy storage systems that can accumulate excess energy from
renewable energy sources and generate it when the supply of power is insufficient or does not match scheduled
generation. In this way, a steady load pattern is ensured and power supply reliability and quality are increased.
Solving of the latter issue becomes very important in small power systems, especially in microgrids, whose
concept is based on optimal operation of distributed energy sources, representing a viable option to increase the
share of dispersed and renewable generation [1]. Coordination of the microgrid systems (Figure 1) covers the
∗Correspondence: [email protected]
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control of microsources, storage devices, and controllable loads connected to a low voltage or medium voltage
feeder [3].
Measurement and control system
Energy
storage
system
Wind
power
plant
Balancing
power
plant
Other
feeders
Main grid
Priority
loads
Non-priority
loads
AC DC
Microgrid
Figure 1. The simplified diagram of microgrid.
The microgrid with a hybrid power generation allows the use of different kinds of fuel more flexibly, to
obtain better efficiency, to increase power supply reliability and quality, to reduce emissions, to improve economic
factors, and to increase the flexibility of energy sources by adjusting for peculiarities of energy demand.
The typical hybrid power system contains:
1. Technologies using fossil fuel for power generation.
2. Technologies using renewable energy sources for power generation.
3. Energy storage systems.
4. Auxiliary systems: power electronics for power transformation, control equipment etc.
Power generation of the system with wind power plants depends on wind speed and might be variable
and discontinuous. This is the reason why wind power plants or wind parks should have balancing generating
units and energy storage systems allowing smoothly and more effectively operating power supply system. Such
a system would achieve higher reliability and the load pattern would be steadier and would not be affected by
changes in primary energy sources during operation. For efficient balancing of wind power, the balancing unit
must have a fast response for increasing or decreasing power. As implementation of conventional combined heat
and power plants [4,5] is possible, the increase in fuel consumption and maintenance costs and reduced lifetime
of the units may be not suitable for fast responses.
The hybrid power system researched in the paper contains a wind park, an electrical generator driven by
an internal combustion diesel engine, and an electrochemical vanadium redox energy storage system (Figure 2).
The hybrid power system can operate connected to an external power grid as well as autonomously, and has
advantages for residential, commercial, or industrial buildings and industrial utilities.
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P
t
P
t
P
t
Measurement and control system
ACDC
ESS
WPP BGU
Electrical
grid
Figure 2. The functional diagram of a hybrid power system wind-diesel generator-electrochemical energy storage system
(WPP wind power park; BGU balancing generating unit; ESS energy storage system).
Balancing of wind parks’ power in a microgrid and a bulk grid differs in the amount of power that has to be
compensated. While power balance between total demand and total generation should exist in the power system,
the balancing concept of renewable power plants may differ in a microgrid and a large-scale power system. The
design of a microgrid includes the balancing units and energy storage systems, the purpose of which is to follow
the load as well as random variation in renewable power sources’ output power. Historically, the composition
of conventional generating units in the bulk power system is optimized to follow the load pattern and the
balancing possibility of them is limited to compensate the forecast errors of load. The penetration of renewable
energy sources such as wind power plants increases uncertainty in power disbalance. As forecasted wind park
generation can be approached as forecasted negative load, the scheduled generation of fully controllable power
plants follows the load pattern as well as output of wind parks. The forecasting errors of wind parks’ output
increases the demand in power balancing resources. In this paper, the case of a wind park with balancing units
connected to the bulk power system is analyzed.
2. Dynamic model of hybrid power system
2.1. Dynamic model of diesel generator power governor
Simplified or detailed dynamic models can describe the power and speed governor of the generator driven by
an internal combustion engine. The simplest dynamic model [6] describes the speed governor by PI link that
accounts for the droop 1/R , the integrating circuit, and the first order phase lag link that describes the operation
of the fuel actuator. The more detailed model [7] of the diesel generator’s power control accounts for transfer
functions of the electric control block, the actuator, and the internal combustion engine (Figure 3). In the case
of operation in the isolated system or if the generator participates in the primary frequency control, the input
signal of the governor should be the deviation of frequency. Otherwise, if the generator operates in a large
power system, the input signal should be the variation in active power caused by a change in wind park power.
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Σ1+T3s
1+T1s+T2T1s2
K(1+T4s)
s(1+T5s)( 1+T6s)e-sTD Π
Kf
Pmax
Pmin
(1+Δω)
PmechPd.e.PaPcu
Pref
+
––
Figure 3. Dynamic model of diesel generator.
The change in mechanical power ∆Pmech produced by the diesel generator equals the product of rotational
speed and power change ∆Pd.e. of the diesel engine:
∆Pmech =
(1 +
∆ω
ω0
)·∆Pd.e., (1)
where ∆ω is the deviation in the angular speed of the generator from the synchronous angular speed ω0 .
The following equations describe the engine power in such way:
∆Pd.e. = e−sTD ·∆Pa, (2)
∆Pa =(1 + T3s)
1 + T1s+ T2T1s2· K (1 + T4s)
s (1 + T5s) (1 + T6s)∆Pc, (3)
∆Pc = ∆Pref −∆u−Kf∆Pa, (4)
where ∆Pa is the change in hydromechanical actuator power, which corresponds to a valve position of the fuel
supply; ∆Pc is the change in a control signal; ∆Pref is the change in a power reference; ∆u is the change
in input control signal of the diesel generator; TD is the fuel burning time lag; T1 , T2 , and T3 are the time
constants of the electric control block; T4 , T5 , and T6 are the time constants of the hydromechanical actuator;
1/K is the time constant of the engine control signal; Kf is the feedback gain; and s is the “s” transformation
operator.
In the case when the hybrid system operates in a nonisolated power system, it is assumed that the shaft
speed of the generator is constant, i.e. ∆ω = 0.
Typical parameters of the dynamic model are [7]: TD = 0; T1 = 10 s; T2 = 0.2 s; T3 = 5 s; T4 = 15 s;
T5 = 5 s; T6 = 0.2 s; K = 0.02 s−1 ; Kf = 1. The response of the diesel generator’s power governor to a step
change is presented in Figure 4.
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0 2 4 6 8 100
0.05
0.1
0.15
0.2
0.25
Po
wer
ch
ange
(p.u
.)
Time (s)
Figure 4. Response of diesel generator to step change of reference signal of 0.01 p.u.
2.2. Dynamic model of energy storage system
The majority of studies related to the operation of electrochemical energy storage systems are based on analysis
of electrochemical reactions or volt/ampere characteristics of the storage systems [8,9]. The task of the presented
model is the balancing of active power. The model accounts for the main energy characteristics of the VRB
battery: power, state of charge level, and charge and discharge duration as well as power losses in the storage
system.
The power losses in the VRB battery consist of initial or electrochemical losses and external or parasitic
losses: auxiliaries, operation of pumps, power losses in electronic equipment etc. If the VRB system is used for
balancing of active power, the battery must be ready for immediate response at every moment, i.e. the pumps
of electrolyte must be operate continuously.
The input signal 1 of the VRB battery dynamic model (Figure 5) is the value of unbalanced power (a
positive value means that the excess power is accumulated and a negative value means that a power deficit
exists). The input signal 2 is rated power of the storage system. The output signals are the value of charging
(consumed) or discharging (supplied) power and the amount of accumulated energy.
Figure 5. Dynamic model of VRB energy storage system.
The model allows evaluation of variable and permanent losses in the storage system and the charge/discharge
time ratio, which can vary from 1:2 to 1:1 (in commercial systems, the ratio varies from 1:1.6 to 1:1 [8]). It is
assumed that the energy storage system responds to the power signal immediately and the lag depends only
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on the operation of power electronics. In the model, aperiodic links describe the performance of the power
electronic equipment.
2.3. Model for balancing of wind park power
The model for balancing of wind park power presents the operation of a hybrid power system with a wind park,
a diesel generator, an energy storage system, and a control algorithm of the equipment’s power. Two cases for
balancing of wind park power are analyzed:
a) wind speed and wind park power are forecasted. In this case, the wind park must follow the forecasted
power pattern (Figure 6a). The diesel generator covers the difference between the actual power of the wind park
and forecasted power. Additionally, fast variations in power difference are compensated by the energy storage
system VRB. In the model, the initial power of the diesel generator P DG and rated power of the VRB battery
P VRB should be defined. Rated power of the diesel generator and capacity of the energy storage system are
settled in the blocks DG and VRB, correspondingly.
b) wind speed and wind park power are not forecasted. In this case, the power pattern of the hybrid
system must be as smooth as possible. Wind park power variation is damped by the diesel generator DG and
energy storage system VRB. Conversely to the first case, the high-pass filter with time constant Tfl is added
in order to eliminate the component of slow power variation from the signal (Figure 6b).
a)
b)
Figure 6. Dynamic model of hybrid power system: following the forecasted wind power pattern (a) and smoothing the
power pattern of the hybrid system (b). (Block PDG denotes reference output power of diesel generator and block PV RB
denotes rated power of VRB battery).
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JONAITIS et al./Turk J Elec Eng & Comp Sci
3. Simulation results
For research of the efficiency of wind park power balancing, the sampled data of active power of the wind park
with 16 MW capacity installed in the Lithuanian power system are analyzed. The power variation per 24 h is
presented in Figure 7. Wind park power varies between 0 and 15.8 MW and the average value is 7.4 MW. There
are some sharp fronts of power increase and decrease reaching from –4.9 MW/min to +5.2 MW/min in the
power pattern. For high accuracy of wind power forecasting, a large amount of meteorological data is required.
The forecasted data of wind power generation were not available for the analyzed period of time. Because of
this, the fictitious short-term forecasted power pattern is simulated. It is assumed that the forecast of the wind
park generation was performed for each hour and the forecasted power output of the wind park is constant
during one hour and equals the mean generated power value of the wind park during the specified hour. The
simulated forecasted power pattern is shown in Figure 7, black line.
0 4 8 12 16 20 240
2
4
6
8
10
12
14
16
P
( M
W )
Actual
Forecasted
Act
ive
po
wer
(M
W)
Time (h)
Actua l
Forecasted
Figure 7. Variation in actual and forecasted wind park power.
The difference between forecasted and actual wind park power is shown in Figure 8. The forecasting
error varies from –5.8 to 7.9 MW. The presented data are common to separate wind parks. If several wind
parks distributed in large geographical area are observed simultaneously, the total dispersion of power would
decrease [10].
0 4 8 1610 20 24–10
–5
0
5
10
PA
ctiv
e p
ow
er d
i"er
ence
(M
W)
Time (h)
Figure 8. Difference between forecasted and actual wind park power.
For wind park power balancing, the diesel generator and energy storage system VRB are used. While
the diesel generator operates at minimal power close to zero, only a decrease (i.e. power deficit) in wind park
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power can be compensated. For compensation of wind park power increases, the diesel generator must operate
at nonminimal power, i.e. it must have capability to decrease its power. Otherwise, if the wind speed increases
and the diesel generator cannot decrease its power, wind park power should be restricted and not supplied to
the grid. The optimal initial operation point of the diesel generator is 50% of rated power. The energy storage
device must be able to store a sufficient amount of energy to cover the maximum possible shortage of energy.
The oversizing of the generator and storage system would lead to a significant rise in their cost [11].
3.1. Wind speed and wind park power are forecasted
It is assumed that the hybrid power system is composed of a wind park of 16 MW capacity, diesel generator of
2 MW capacity, and energy storage system VRB of 2 MW rated power (2 × 4 MWh capacity). The referenced
power of the diesel generator is 1 MW and the VRB battery is charged at 80%. The task of the hybrid system
is to follow the referenced (forecasted) power pattern. Because of the operation of the diesel generator, the
forecasted power is increased by 1 MW, i.e. the constant component of the diesel generator power is added to
the wind park forecasted power pattern.
Three cases of the hybrid power system are analyzed:
1) wind park power is balanced only by the diesel generator (VRB battery does not operate);
2) wind park power is balanced only by the VRB battery (the diesel generator does not operate);
3) wind park power is balanced by the diesel generator and VRB battery.
Simulated variation in hybrid power system power patterns is presented in Figures 9a, 10a, and 11a,
corresponding to cases 1, 2, and 3. The differences between forecasted and total power of the hybrid system are
shown in Figures 9b, 10b, and 11b, correspondingly.
Balancing efficiency of the hybrid power system power is analyzed when the diesel generator and energy
storage system of different capacities are used. It is assumed that the rated power of the diesel generator varies
from 0 to 4 MW and the power of the energy storage system varies from 0 to 2 MW. Efficiency is determined
by analyzing parameters of generated power and forecasted power patterns’ difference: amount of excess or
deficit energy per day (Table 1), maximum amount of excess or deficit energy per hour (Table 2), and maximum
difference in real and forecasted power (Table 3).
Analysis of the simulated results shows that the maximum difference in real and forecasted power depends
on the installed capacity of balancing equipment, i.e. on total rated power of the diesel generator and the energy
storage system. The high value of power difference is deceased by balancing equipment but if the power limits
of the balancing equipment are reached, this difference may not be compensated totally.
The excess or deficit energy amount per day or per hour depends not only on the total capacity of
balancing equipment but also on their dynamical characteristics. The diesel generator is capable of balancing
slow oscillations of power while the VRB battery with practically zero inertia compensates very fast power
changes. It is determined that balancing efficiency increases very slowly if the rated power of the diesel generator
is increased from 2 to 4 MW and the power of the energy storage system is increased from 1.5 to 2 MW (Figure
12). However, for instantaneous peak changes in wind park power, an energy storage system of higher power
(2 MW) should be installed.
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JONAITIS et al./Turk J Elec Eng & Comp Sci
0 4 8 12 16 20 240
2
4
6
8
10
12
14
16
18
P
( M
W )
Wind
DG
Total
Act
ive
po
wer
(M
W)
0 4 8 12 16 20 24–6
–4
–2
0
2
4
Time (h)
Time (h)
Act
ive
po
wer
d
i"er
ence
(M
W)b)
a)
Wind
DG
Total
Figure 9. Power variation in the hybrid power system when the diesel generator follows referenced power pattern (a)
and difference between forecasted and total power of the hybrid system (b).
0 4 8 12 16 20 24-6
-4
-2
0
2
4
0 4 8 12 16 20 24-2
0
2
4
6
8
10
12
14
16
18
P
( M
W )
Wind
VRB
Total
Act
ive
po
wer
(M
W)
Time (h)
Time (h)
Act
ive
po
wer
di"
eren
ce (
MW
)b)
a)WindVRBTotal
Figure 10. Power variation in the hybrid power system when the VRB battery follows referenced power pattern (a)
and difference between forecasted and total power of the hybrid system (b).
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JONAITIS et al./Turk J Elec Eng & Comp Sci
0 4 8 12 16 20 24–6
–4
–2
0
2
4
0 4 8 12 16 20 24–2
0
2
4
6
8
10
12
14
16
18
Act
ive
po
wer
(M
W)
Time (h)
Time (h)
Act
ive
po
wer
di"
eren
ce (
MW
)b)
a)
Wind
DG
VRB
Total
Figure 11. Power variation in the hybrid power system when the diesel generator and VRB battery follow referenced
power pattern (a) and difference between forecasted and total power of the hybrid system (b).
Table 1. Unbalanced energy per day, in MWh, at different capacities of the diesel generator and the energy storage
system (the upper number is excess energy and the lower number is unsupplied energy).
hhhhhhhhhhhhhhhPV RB , MWPDG,N , MW
0 0.5 1 1.5 2 2.5 3 3.5 4
015.12 12.89 11.14 9.73 8.57 7.68 6.98 6.43 5.99–15.13 –12.74 –11.01 –9.65 –8.57 –7.70 –7.06 –6.56 –6.20
0.510.09 8.22 6.71 5.50 4.54 3.80 3.21 2.73 2.37–9.90 –8.06 –6.62 –5.47 –4.57 –3.87 –3.31 –2.91 –2.62
16.47 5.11 4.03 3.17 2.49 1.95 1.53 1.21 0.97–6.35 –5.05 –4.02 –3.22 –2.59 –2.13 –1.78 –1.51 –1.31
1.53.97 3.08 2.36 1.79 1.34 1.00 0.74 0.56 0.43–3.96 –3.12 –2.45 –1.96 –1.58 –1.30 –1.08 –0.90 –0.76
22.35 1.77 1.31 0.96 0.69 0.50 0.36 0.26 0.18–2.44 –1.94 –1.55 –1.26 –1.03 –0.84 –0.69 –0.57 –0.47
3.2. Wind speed and wind park power are not forecasted
In this case, the equipment of the researched hybrid power system is the same as that described in section
3.1. The task of the balancing equipment consisting of the diesel generator and the energy storage system is to
minimize oscillations of supplied power.
The power patterns of the hybrid power system when wind park power is balanced only by the diesel
generator, only by VRB battery, and by both diesel generator and VRB battery are shown in Figures 13a–13c.
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JONAITIS et al./Turk J Elec Eng & Comp Sci
Table 2. Unbalanced energy per hour, in MWh, at different capacities of the diesel generator and the energy storage
system (the upper number is excess energy and the lower number is unsupplied energy).
hhhhhhhhhhhhhhhPV RB , MWPDG,N , MW
0 0.5 1 1.5 2 2.5 3 3.5 4
01.93 1.74 1.56 1.40 1.25 1.11 0.99 0.88 0.79–1.81 –1.71 –1.62 –1.53 –1.46 –1.38 –1.31 –1.24 –1.16
0.51.53 1.36 1.19 1.04 0.90 0.78 0.68 0.58 0.50–1.58 –1.47 –1.38 –1.29 –1.20 –1.12 –1.04 –0.96 –0.88
11.18 1.02 0.88 0.75 0.64 0.54 0.45 0.38 0.31–1.36 –1.26 –1.17 –1.08 –0.99 –0.91 –0.83 –0.75 –0.67
1.50.88 0.75 0.64 0.53 0.44 0.37 0.31 0.25 0.19–1.16 –1.07 –0.98 –0.90 –0.82 –0.74 –0.66 –0.58 –0.51
20.64 0.53 0.44 0.37 0.31 0.25 0.19 0.14 0.10–0.98 –0.90 –0.82 –0.73 –0.65 –0.58 –0.51 –0.44 –0.37
Table 3. Maximum difference in real and forecasted power, in MW, at different capacities of the diesel generator and
the energy storage system (the upper number is excess power and the lower number is unsupplied power).
hhhhhhhhhhhhhhhPV RB , MWPDG,N , MW
0 0.5 1 1.5 2 2.5 3 3.5 4
05.86 5.61 5.36 5.11 4.86 4.61 4.36 4.11 3.86–7.93 –7.68 –7.43 –7.18 –6.93 –6.68 –6.43 –6.18 –5.93
0.55.36 5.11 4.86 4.61 4.36 4.11 3.86 3.61 3.36–7.43 –7.18 –6.93 –6.68 –6.43 –6.18 –5.93 –5.68 –5.43
14.86 4.61 4.36 4.11 3.86 3.61 3.36 3.11 2.86–6.93 –6.68 –6.43 –6.18 –5.93 –5.68 –5.43 –5.18 –4.93
1.54.36 4.11 3.86 3.61 3.36 3.11 2.86 2.61 2.36–6.43 –6.18 –5.93 –5.68 –5.43 –5.18 –4.93 –4.68 –4.43
23.86 3.61 3.36 3.11 2.86 2.61 2.36 2.11 1.86–5.93 –5.68 –5.43 –5.18 –4.93 –4.68 –4.43 –4.18 –3.93
0 0.5 1 1.5 2 2.5 3 3.5 4–20
–15
–10
–5
0
5
10
15
20 P VRB
0 MW
0.5 MW
1.0 MW
1.5 MW
2.0 MW
Un
bal
ance
d e
ner
gy (
MW
h)
Capacity of DG (MW)
0 MW
0.5 MW
1.0 MW
1.5 MW
2.0 MW
Capacity of VRB
Figure 12. Unbalanced energy per day at different capacities of the diesel generator and energy storage system.
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JONAITIS et al./Turk J Elec Eng & Comp Sci
Simulation results show that the diesel generator with 2 MW capacity and the energy storage system with 2
MW capacity are capable of balancing the wind park power with sufficient efficiency.
0 4 8 12 16 20 24–2
0
2
4
6
8
10
12
14
16
18
0 4 8 12 16 20 24–2
0
2
4
6
8
10
12
14
16
18
VRB
0 4 8 12 16 20 240
2
4
6
8
10
12
14
16
18
Wind
DG
Act
ive
po
wer
(M
W)
Time (h)
a)
Act
ive
po
wer
(M
W)
Time (h)
b)
Act
ive
po
wer
(M
W)
Time (h)
c)
Total
WindVRBTotal
WindDGVRBTotal
Figure 13. Variation in power of the hybrid power system when wind power is not forecasted. The system consists
of wind park and diesel generator (a), wind park and energy storage system (b), and wind park, diesel generator, and
energy storage system (c).
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4. Conclusions
The created dynamic model of the hybrid power system composed of a wind park, a diesel generator, and
an energy storage system allows analysis of power balancing efficiency when different balancing equipment is
used. The model describes the dynamic characteristics of the diesel generator power governor and the main
characteristics of the electrochemical energy storage system. The wind park is not modelled; only active power
generated by the wind park is used as the input signal of the model.
The simulation is based on sampled data of active power of the wind park with 16 MW capacity. The
analysis of simulation results shows that the best power balancing efficiency can be achieved when different kinds
of balancing equipment are used, e.g., slowly operating diesel generator and fast operating electrochemical energy
storage system. Different capacities of the diesel generator and energy storage system were used in simulations
in order to estimate the optimal capacities of balancing equipment. It is determined that the optimal rated
power of the diesel generator is 2 MW and the power of the energy storage system is 2 MW for the studied case.
The suggested model can be applied for research of the operation of various kinds of hybrid power systems
including renewable power sources.
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