Performance of Combined Power System With Design of Energy Storage Controller System

Abstract -We investigated a small isolated hybridaccelerates when the input power is higher than
power system that used two types of powerthe electrical output power of the generator
generation; wind turbine and diesel generation. The(including losses).
interaction of diesel generation, the wind turbine,The diesel decelerates when the input power is
and the local load is complicated because both thelower than the electrical output power of the
load and the wind turbine fluctuate during the day.generator (including losses). An oversized diesel
These fluctuations create imbalances in powerengine does not have problems accelerating or
distribution (energy sources are not equal todecelerating, but an undersized diesel engine may
energy sinks) that can affect the frequency andcreate problems, during, for example, the start-up
the voltage in the power system. The addition ofof a wind turbine or large compressor. Figure 5
energy storage will help balance the distribution ofillustrates a condition where the diesel is
power in the power network. For this paper, weundersized with respect to the load. The genset
studied the interaction among hybrid powerfrequency and the terminal voltage of the wind
system components and the relative size of theturbine generator are shown on the top graph,
components. We also show how the contributionand the real power of the diesel, wind turbine,
of wind energy affects the entire power systemwater pump, and local load are shown on the
and distribution and the role of energy storagebottom graph. At start-up, the wind turbine uses
under the transient conditions caused by loadthe smaller, 40-kW generator to motor up and
changes and wind turbine startups.bring the induction machine up to speed. Because
Index Terms - wind turbine, diesel generator,the wind speed is low, the wind turbine operates
hybrid power system, renewable energy, energyat low output power, and the local load is set to
storage.200 kW. The diesel engine has a rated power of
Â400 kW. At t = 2 s, the wind turbine is turned on.
I. INTRODUCTIONAs we can see, the voltage dip and the
Windmills were used to pump water and mill grain,frequency dip are not very large, because the
along with many other uses [1, 2, 3, 4].wind turbine is started using a smaller generator
Today, wind turbines are used for similarÂ
purposes (i.e., water or oil pumping, batteryFigure 5. Voltage, frequency, and power to
charging, and utility generation). One importantillustratean undersized diesel genset
aspect of wind turbine applications, especially in anAt t = 10 s, the 80-kW water pump is started up.
industrial environment, is that wind turbinesThe startup time for the water pump is longer
generate electricity without creating pollution. Windthan that of the wind turbine because the wind
turbines are also well suited for generatingturbine is started when the rotor speed is close
electricity in isolated places with no connections toto the synchronous speed and the wind turbine
the utility grid [2,3,4]. However, in isolatedalso gets some help from the wind. The voltage
applications, especially very small applications, thedrop is not very significant, but the frequency of
power system components (sources and loads)the diesel drops about 3%. The diesel output
are limited, and the system networks are weak inpower increases to cover the real power needed,
many cases. Thus, any changes in the powerwhereas the contribution from the wind turbine is
input or output of one component may affect theinsignificant because the wind is low. For a short
rest of the system more dramatically than in atime, the induction generator enters the motoring
larger system where the smoothing effect ofrange between t = 10.8 s and t = 11.3 s. After
many components benefits the overall system. Inthe condition is restored, at t = 14 s, the
this paper, we analyze a hybrid power systemadditional local load (300 kW noncritical) is turned
consisting of a wind turbine, a diesel generator, aon, bringing the total load to 580 kW. Because the
local load, and energy storage. We also presentdiesel can carry only up to 400 kW and the wind's
the impact of energy storage on the powercontribution is very small at about 40 kW, the
system performance. The results and conclusionsvoltage and frequency start decreasing, and the
of this analysis apply to similar hybrid powervoltage and frequency sensors detect the change.
systems.If the frequency drops below 95% and the
Âvoltage drops below 90% for an elapsed time of
Â0.5 s, the controller will drop the additional load
Â(300 kW) and keep the critical load (200 kW) to
II. SYSTEM CONFIGURATIONregain the voltage and frequency. After the load
The system has two types of generation: theis shed at t = 14.5 s, the frequency and voltage
diesel generator and the wind turbine generatoreventually return to normal. When the frequency
(Figure 1). Theenergy storage can act as a load ordrops, the wind turbine's power contribution
as a generator depending on the need. The dieselsuddenly jumps because of a sudden increase of
generator provides smooth output power,generating slip. Eventually, the genset frequency
whereas the output power of a wind turbineincreases again for a short period and the
depends on the wind velocity. As the windinduction generator enters into the motoring
velocity varies, so does is the power generation.condition (between t = 14.5 s and t = 15 s). This
For example, if the wind speed changes verycondition worsens if the mechanical time constant
smoothly, the output power of the wind turbineof the wind turbine rotor (including the blade) is
will also change very smoothly. On the other hand,higher than the diesel genset time constant. In
wind turbulence causes the output power toother words, the changing of the genset rotor
fluctuate. Figure 1 is a single line diagram thatspeed is much faster than the changing of the
represents the analyzed power system. The windwind turbine rotor speed. The response to the
turbine has an induction generator with a capacityload change is shown by how fast the governor
ranging from 40 kW to 225 kW. At low windcorrects the frequency and how fast the
speeds, the generator operates at 900 rpm withgenerator's field excitation control reacts to the
a rated capacity of 40 kW. At high wind speeds,voltage changes. Undersized diesel engine with
the generator speed is 1,200 rpm with a ratedenergy storage: As shown in the previous
capacity of 225 kW. We used 150 kW of energysubsection, an undersized diesel engine cannot
storage as a buffer to operate as a load or asupply all energy needed, and it must shed some
source depending on the need. This paperof the non-critical load to retain power-system
discusses only fixed-speed wind turbine generationstability. To remedy this situation, a 150-kW
and does not cover variable-speed wind turbineenergy storage is installed to bring the combined
generation [5]. The diesel engine, which has aoutput of the diesel genset and energy storage
rated capacity of 400 kW, is operated in parallelup to 550 kW. Figure 6 shows the improved
with the wind turbine to supply the load. The localpower system after the energy storage is added.
loads are mostly residential and light loads. OtherThe same simulation is performed except it is
loads include water pumps, compressors, andnow equipped with an energy storage. There is a
heavy equipment. An 80-kW water pumpsignificant improvement in the frequency
represents the transient condition of a heavy load.regulation after the storage is installed to stabilize
Âthe system. The non-critical load (300 kW)
Fig 1. One line diagram of power systemsurvives even during low wind conditions. The
Âfrequency dips during the wind turbine start-up
Âand the water pump start-up, and when the 300
III. COMPONENTS OF POWER SYSTEMkW load non-critical load is switched, it is reduced
The system we discuss in this paper consists ofdramatically. Obviously, the capability of the
four major subsystems: a diesel generator, a windenergy storage to deliver a large amount of
turbine generator, heavy (industrial) loads, andpower instantaneously plays a major role in
energy storage. In the power system network,restoring the frequency of the power system. An
the balance of active power and reactive poweradditional benefit is noticed in the system voltage
must be maintained. The diesel-genset, then, mustbehavior of the wind turbine. Because the change
be able to keep the power balanced when thein the frequency deviation presented to the wind
wind turbine or local load varies. This task is easyturbine induction generator is small and smooth,
to accomplish provided the diesel genset isthe behavior of the stator current at the induction
sufficiently sized. Although they are important, wegenerator is also smooth. Thus it reduces the Ldi
will not cover the details of the dynamic modeldt and overall voltage drop across the line.
for electric machines used in the simulation. ManyOversized wind turbine:
good textbooks are available on this subject.When the wind power output exceeds the power
A. Diesel Generatorrequired by the load, the synchronous generator
In terms of an electrical system, a dieselof the diesel genset becomes a synchronous
generator can be represented as a prime movermotor that tends to accelerate the rotor speed
and a generator. Ideally, the prime mover isof the diesel engine. The excess energy from the
capable of supplying any power demand up towind power, then, tries to drive the diesel engine.
rated power at constant frequency, and theBecause the diesel engine has only a small braking
synchronous generator connected to it must becapability resulting from engine compression, the
able to keep the voltage constant at any loadfrequency control can be lost when the extra
condition. Figure 2 is a block diagram of the dieselpower generated by the wind turbine is
generator. The diesel engine keeps the frequencysufficiently high.
constant by maintaining the rotor speed constantÂ
via its governor. The synchronous generator mustFigure 6. Voltage, frequency, and power to
control its output voltage by controlling theillustratean undersized diesel genset with storage
excitation current. Thus, as a unit, the dieselIn Figure 7, the diesel generator has a rated
generating system must be able to control itspower of 400 kW, the local load is initially set to
frequency and its output voltage. The inertia of280 kW and at t = 4 s, and the local load is set
the diesel genset, the sensitivity of the governor,to 100 kW. When the diesel is started, there is
and the power capability of the diesel engine allonly a local load of 280 kW. The wind turbine is
affect the diesel generator's ability to respond tothen started at t = 2 s with a 225-kW induction
frequency changes. The ability of the synchronousmachine. Although the diesel genset is rated at
generator to control its voltage is affected by theonly 400 kW and the wind turbine is started with
field winding time constant, the availability of thea 225-kW induction machine, the effect of wind
direct current (DC) power to supply the fieldturbine start-up on the power system is very
winding, and the response of the voltage controlmild, mostly because the induction machine
regulation mechanism.current is limited by a soft start. A soft start is a
Âdevice that limits starting current during start-up.
Figure 2. Diesel generator control block diagramIt consists of a pair of back-to-back thyristors
Âinstalled in series with each phase of the motor
Âwinding. Because the firing angle of the thyristor
B. Wind Turbinecan be controlled, the size of the starting current
The main components of a wind turbine are thecan be adjusted by controlling the firing angle of
rotor of the turbine, which is the prime mover,the thyristors. As we can see (Figure 5), the
and an induction generator. In general, the rotor issame wind turbine (225 kW) draws a starting
connected to the generator via a gearbox thatpower of 300 kW, but after the soft start is
matches the rotational speed. The simplestinstalled (Figure 7), the power surge during
system uses a fixed-speed turbine. A fixed-speedstart-up drops to about 100 kW. After the wind
turbine must rely on the blade-stall condition toturbine enters generating mode (at about t = 2.5
limit the output power when the winds are at highs), the local load (280 kW) is shared between the
speed. Note that, although the rotor speed of andiesel genset (55 kW) and the wind turbine (225
induction generator varies with wind speed, thekW). The voltage and frequency are maintained
speed range is within a 1% to 2% slip. On theconstant, and the diesel genset
other hand, the wind speed variation may rangeÂ
from 5 m/s to 25 m/s; thus, in terms of theÂ
wind turbine, the induction generator operates atÂ
a relatively "fixed speed" compared to the rangeFigure 7. Voltage, rotor speed, and power of
of wind speed variation.anoversize wind turbinegenerates only a small
C. Induction Machinespercentage of its rated load (about 13%). This
Most electric machines used in industry as primemakes a significant contribution to fuel savings
movers are induction motors. Two applications offrom the wind energy. At t = 4 s, the local load is
induction machines in the power system networkreduced from 280 kW to 100 kW; the wind
fall within the scope of this study: one as thespeed stays the same. As a result, the wind
generator on a wind turbine and the other as aturbine tries to supply 225 kW, but the only load
motor driving large pumps and compressors. Byavailable is 100 kW. As a result, the synchronous
its nature, an induction machine is an inductivegenerator of the diesel genset turns into a motor
load. This machine absorbs reactive power either(negative power), the governor loses its speed
as a motor or generator. The reactive powercontrol, and frequency runaway is triggered. This
absorbed by the induction machine comes fromis an example of the wind turbine being oversized
the line to which it is connected. In a hybrid powercompared to the local load. In such a case, a
system, the reactive power comes from thedump load (water heater, water pump, battery
synchronous generator of the diesel genset. In acharger, etc.) is usually deployed to keep the
wind turbine generator, a fixed capacitor is usuallydiesel genset generating, which prevents it from
installed to supply some of the reactive powermotoring. Minimum power generation of the diesel
that the induction generator needs. Figure 3genset is usually pre-set (for example, 15%-40%
shows the equivalent circuit of an inductionof the rated load). If the generated power of the
machine connected to a power system. Thediesel genset is less than the preset value, the
power system is represented by infinite bus Esdump load should be deployed. The dump load
and the line impedance is represented bymust be sized so that the diesel genset will
reactance Xs.always generate power above its minimum set
Âpoint. The dump loads are normally non-critical
Figure 3. Equivalent circuit of an induction machineloads used to store excess electrical energy in
connected to power systemanother form, such as heat (water or space
D. Various Loadsheater), electric charge (battery charging), or
In the power system considered, there are twopotential energy (water pump). Oversized wind
major loads. The first is a large water pumpturbine with energy storage: As shown in the
representing a typical industrial load. The second isprevious subsection, an oversized wind turbine can
a collection of loads for which the size and powerdrive the system into an unstable condition
factor can be programmed throughout the day tobecause of the inability of the diesel engine to
represent a typical village load. The voltage at thekeep the frequency constant. An energy storage
terminal of the load varies as a result of a voltageinstalled in the power system network is not only
drop across the line impedance. The voltage dropuseful to remedy the undersized diesel engine but
across the line impedance varies depending on thealso for cases where there is an excess power
size of the current and the power factor of theproduced by the wind turbine. Without energy
load. The terminal voltage for a wind turbinestorage, the wind turbine can drive the
generator (VS), as the output current of inductionsynchronous machine into motoring region and the
machine, varies from start-up to generating mode.frequency output will be out of control. With a
During start-up, voltage drops significantly at thepower converter to interface between the
terminal voltage of the induction machine. Theenergy storage and the power network, the
voltage drop across line impedance is caused byenergy storage is capable of quickly absorbing
the current surge during start-up. In addition, theexcess power generated by the wind turbine and
phase angle of the stator current is very largehold the generator rotor speed from a runaway
and lagging. The combination of a poor powercondition. As shown in Figure 8, the frequency
factor and a lagging, large current surge creates arunaway can be prevented by using energy
voltage dip at the terminal of the inductionstorage to capture the excess power in the
machine during start-up. Thus, a start-up of shortpower network.
duration is preferable to a prolonged oneFigure 8. Voltage, rotor speed, and power of
E. Energy Storageanoversize wind turbine with energy storage
The energy storage can be of different types (i.e.B. Case Study II: Charging the Storage Under
flywheel, battery, hydrogen/fuel-cell, hydropowerNormal Condition
etc.). In this paper, we assumed energy storageThe energy storage will be charged only when
with a power converter interface to the powerthere is an energy surplus from the wind and the
network. The power converter is connected torequired network load is very light. Because the
the energy storage at one end. With variability ofgovernor of the diesel generator will always
wind resource, energy storage is an excellentmaintain the frequency constant, the output
contributor to the power system. The energypower of the diesel generator is an indicator of
storage behaves like a large buffer tothe power within the system available to charge
accommodate the unequal instantaneous energythe energy storage. One benefit of charging the
in the power system. Ideally, at any instant ofenergy storage during this condition is that the
time, there should be a zero net exchangeefficiency of the diesel engine is at its peak when
between the energy sources and the energyit is operated near its rated power. Thus, when a
sinks (both real and reactive power). If thissurplus of power is detected within the system,
balance is not achieved, the voltage andthe energy storage will be charged and some
frequency of the system changes to maintainenergy will be stored within the system. The
equilibrium. At any instant, the energy storageamount of energy and the size of charging power
behaves either as an energy source or energydepend on the size of the surplus power. The
sink depending of the mode of operation.charging process will be stopped when the energy
Figure 4. Energy Storage control block diagramstorage reaches its limit. Maximum charging
Âcurrent is also limited by the energy storage and
It is assumed that the energy storage has aby the power converter interface. Figures 9
power converter interfacing the power network.shows the charging process. Initially there is
Although it is possible for the power converter toenough wind speed to start the wind turbine. The
function as a reactive power compensator, thediesel generator is supplying a constant load of
cost of a power converter is very expensive280 kW (power factor = 0.995 lagging) all the
compared to other means of reactive powertime. As the wind turbine generates full power
compensation currently available in the market.(225kW), the diesel governor redistributes the
Keep in mind that the size of the powerload and there is a load sharing between the wind
semiconductor in the power converter is limitedturbine and the diesel generator. As the transient
by its current limit and its voltage limit. Thus,settles out, it is shown that the diesel generator is
minimizing the current passing through the powercontributing a very small amount of power to the
switches will minimize the current rating of theload, thus the charging mechanism is started. The
power converter and will lower the cost. For thisenergy storage is charged slowly until it reaches
paper, we only used the power converter toits limit.
process real power in and out of the energyÂ
storage. Figure 4 shows a block diagram ofÂ
energy storage control algorithm. It usesFigure 9. Real power flow in the power system
frequency deviation to indicate a real powerÂ
imbalance in the system. The frequency deviationIn Figure 9, the charging of energy storage during
is also used as the feedback to control thenormal condition is limited to 75 kW, which is
energy storage output. If the load power demandabout 50% of the rated power of the capacitor.
is higher than the power supply available, theThis limit ensures that the power converter still
frequency of the diesel generator will slowly drop.has enough headroom to deliver or absorb power
Other energy stored in the system includes theduring an emergency. For example, if there is
kinetic energy in the turbine blades, the dieselsome loss of the loads in the power systems, the
generator inertia, and energy in the inductors andenergy storage must absorb the loads loss to
capacitors, etc.avoid a sudden change in frequency. Similarly, to
Âcompensate for a sudden load increase to the
F. Balance of Energy in the Systempower systems (e.g. the water pump is started),
In the isolated system we studied, the balance ofthe energy storage must release energy to the
real and reactive power must always bepower system to keep constant frequency at
maintained. The balance of real power isthe diesel generator. As shown in Figure 9, the
maintained by the governor of the dieselreal power used by the energy storage to
generator. The balance of reactive power isstabilize the frequency takes precedence over the
maintained by the exciter of the diesel'scharging power used to charge the storage. This
synchronous generator. When the load demandscan be seen especially when the water pump is
more power than the diesel and the wind turbinestarted at about t = 15 seconds.
can produce, and the diesel engine has reached itsV. CONCLUSION
highest limit, as the loads continue to increase, theAfter presenting an overview of the components
governor of the diesel cannot push more power,of the power system under investigation, we
and the rotor speed of the diesel will start todescribed the operating characteristics of the
drop. The frequency of the generator will thencomponents as they relate to voltage and
drop until balance is reached or the systemfrequency variations in the power network. The
collapses. The voltage in the system is also ananalysis shows the dynamic interaction among the
indicator of the balance in the system. When thewind turbine, diesel engine, large loads, and energy
reactive power demand from the loads is higherstorage. It also demonstrates the dynamics of
than what can be provided by the dieselreal power balance and how the system is
generator, the capacitor, and other means ofstabilized with the controlled energy storage. The
compensation, the system voltage will drop.voltage regulation is very minimal and the
Although the size of output and input of thefrequency regulation is controlled very closely. The
energy storage is adjustable, it is limited by itsvoltage regulation is controlled mostly by the
ratings. For this paper, we assumed that thebalance of reactive power in the system and the
energy storage is capable of storing and providingtime constant of the excitation system of the
long-term energy to the power network togenerator. The frequency regulation depends on
maintain system balance. In reality, only a limitedthe energy storage control, the size of the
amount of energy can be stored. We will notenergy storage, the total inertia in the system
discuss energy analysis in detail in this paper. In(temporary energy storage).Many technical
practice, the energy will be stored when the windsolutions can be implemented to remedy the
turbine produces enough power and the diesel isshortcomings covered in this paper. However, as
operating under light load. The actual loads arein any power generation system, the economic
divided into critical and non-critical loads. Criticalimplications of the solutions must be carefully
loads are supplied at all times and non-critical loadsconsidered.
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