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

Abstract -We investigated a small isolated				engine and a synchronous generator. The
hybrid power system that used two types of				diesel engine is responsible for controlling
power generation; wind turbine and diesel				the frequency and keeping it constant through
generation. The interaction of diesel				its governor. The synchronous generator is
generation, the wind turbine, and the local				responsible for controlling the voltage via
load is complicated because both the load and				its field winding and voltage controller.
the wind turbine fluctuate during the day.				Undersized diesel engine: The ability of a
These fluctuations create imbalances in power				diesel engine to change speed is its
distribution (energy sources are not equal to				accelerating or decelerating power. The
energy sinks) that can affect the frequency				diesel accelerates when the input power is
and the voltage in the power system. The				higher than the electrical output power of
addition of energy storage will help balance				the generator (including losses).
the distribution of power in the power
network. For this paper, we studied the				The diesel decelerates when the input power
interaction among hybrid power system				is lower than the electrical output power of
components and the relative size of the				the generator (including losses). An
components. We also show how the contribution				oversized diesel engine does not have
of wind energy affects the entire power				problems accelerating or decelerating, but an
system and distribution and the role of				undersized diesel engine may create problems,
energy storage under the transient conditions				during, for example, the start-up of a wind
caused by load changes and wind turbine				turbine or large compressor. Figure 5
startups.				illustrates a condition where the diesel is
				undersized with respect to the load. The
Index Terms - wind turbine, diesel generator,				genset frequency and the terminal voltage of
hybrid power system, renewable energy, energy				the wind turbine generator are shown on the
storage.				top graph, and the real power of the diesel,
				wind turbine, water pump, and local load are
Ã‚				shown on the bottom graph. At start-up, the
				wind turbine uses the smaller, 40-kW
I. INTRODUCTION				generator to motor up and bring the induction
				machine up to speed. Because the wind speed
Windmills were used to pump water and mill				is low, the wind turbine operates at low
grain, along with many other uses [1, 2, 3,				output power, and the local load is set to
4].				200 kW. The diesel engine has a rated power
				of 400 kW. At t = 2 s, the wind turbine is
Today, wind turbines are used for similar				turned on. As we can see, the voltage dip and
purposes (i.e., water or oil pumping, battery				the frequency dip are not very large, because
charging, and utility generation). One				the wind turbine is started using a smaller
important aspect of wind turbine				generator
applications, especially in an industrial
environment, is that wind turbines generate				Ã‚
electricity without creating pollution. Wind
turbines are also well suited for generating				Figure 5. Voltage, frequency, and power to
electricity in isolated places with no				illustratean undersized diesel genset
connections to the utility grid [2,3,4].
However, in isolated applications, especially				At t = 10 s, the 80-kW water pump is started
very small applications, the power system				up. The startup time for the water pump is
components (sources and loads) are limited,				longer than that of the wind turbine because
and the system networks are weak in many				the wind turbine is started when the rotor
cases. Thus, any changes in the power input				speed is close to the synchronous speed and
or output of one component may affect the				the wind turbine also gets some help from the
rest of the system more dramatically than in				wind. The voltage drop is not very
a larger system where the smoothing effect of				significant, but the frequency of the diesel
many components benefits the overall system.				drops about 3%. The diesel output power
In this paper, we analyze a hybrid power				increases to cover the real power needed,
system consisting of a wind turbine, a diesel				whereas the contribution from the wind
generator, a local load, and energy storage.				turbine is insignificant because the wind is
We also present the impact of energy storage				low. For a short time, the induction
on the power system performance. The results				generator enters the motoring range between t
and conclusions of this analysis apply to				= 10.8 s and t = 11.3 s. After the condition
similar hybrid power systems.				is restored, at t = 14 s, the additional
				local load (300 kW noncritical) is turned on,
Ã‚				bringing the total load to 580 kW. Because
				the diesel can carry only up to 400 kW and
Ã‚				the wind's contribution is very small at
				about 40 kW, the voltage and frequency start
Ã‚				decreasing, and the voltage and frequency
				sensors detect the change. If the frequency
II. SYSTEM CONFIGURATION				drops below 95% and the voltage drops below
				90% for an elapsed time of 0.5 s, the
The system has two types of generation: the				controller will drop the additional load (300
diesel generator and the wind turbine				kW) and keep the critical load (200 kW) to
generator (Figure 1). Theenergy storage can				regain the voltage and frequency. After the
act as a load or as a generator depending on				load is shed at t = 14.5 s, the frequency and
the need. The diesel generator provides				voltage eventually return to normal. When the
smooth output power, whereas the output power				frequency drops, the wind turbine's power
of a wind turbine depends on the wind				contribution suddenly jumps because of a
velocity. As the wind velocity varies, so				sudden increase of generating slip.
does is the power generation. For example, if				Eventually, the genset frequency increases
the wind speed changes very smoothly, the				again for a short period and the induction
output power of the wind turbine will also				generator enters into the motoring condition
change very smoothly. On the other hand, wind				(between t = 14.5 s and t = 15 s). This
turbulence causes the output power to				condition worsens if the mechanical time
fluctuate. Figure 1 is a single line diagram				constant of the wind turbine rotor (including
that represents the analyzed power system.				the blade) is higher than the diesel genset
The wind turbine has an induction generator				time constant. In other words, the changing
with a capacity ranging from 40 kW to 225 kW.				of the genset rotor speed is much faster than
At low wind speeds, the generator operates at				the changing of the wind turbine rotor speed.
900 rpm with a rated capacity of 40 kW. At				The response to the load change is shown by
high wind speeds, the generator speed is				how fast the governor corrects the frequency
1,200 rpm with a rated capacity of 225 kW. We				and how fast the generator's field excitation
used 150 kW of energy storage as a buffer to				control reacts to the voltage changes.
operate as a load or a source depending on				Undersized diesel engine with energy storage:
the need. This paper discusses only				As shown in the previous subsection, an
fixed-speed wind turbine generation and does				undersized diesel engine cannot supply all
not cover variable-speed wind turbine				energy needed, and it must shed some of the
generation [5]. The diesel engine, which has				non-critical load to retain power-system
a rated capacity of 400 kW, is operated in				stability. To remedy this situation, a 150-kW
parallel with the wind turbine to supply the				energy storage is installed to bring the
load. The local loads are mostly residential				combined output of the diesel genset and
and light loads. Other loads include water				energy storage up to 550 kW. Figure 6 shows
pumps, compressors, and heavy equipment. An				the improved power system after the energy
80-kW water pump represents the transient				storage is added. The same simulation is
condition of a heavy load.				performed except it is now equipped with an
				energy storage. There is a significant
Ã‚				improvement in the frequency regulation after
				the storage is installed to stabilize the
Fig 1. One line diagram of power system				system. The non-critical load (300 kW)
				survives even during low wind conditions. The
Ã‚				frequency dips during the wind turbine
				start-up and the water pump start-up, and
Ã‚				when the 300 kW load non-critical load is
				switched, it is reduced dramatically.
III. COMPONENTS OF POWER SYSTEM				Obviously, the capability of the energy
				storage to deliver a large amount of power
The system we discuss in this paper consists				instantaneously plays a major role in
of four major subsystems: a diesel generator,				restoring the frequency of the power system.
a wind turbine generator, heavy (industrial)				An additional benefit is noticed in the
loads, and energy storage. In the power				system voltage behavior of the wind turbine.
system network, the balance of active power				Because the change in the frequency deviation
and reactive power must be maintained. The				presented to the wind turbine induction
diesel-genset, then, must be able to keep the				generator is small and smooth, the behavior
power balanced when the wind turbine or local				of the stator current at the induction
load varies. This task is easy to accomplish				generator is also smooth. Thus it reduces the
provided the diesel genset is sufficiently				Ldi/dt and overall voltage drop across the
sized. Although they are important, we will				line.
not cover the details of the dynamic model
for electric machines used in the simulation.				Oversized wind turbine:
Many good textbooks are available on this
subject.				When the wind power output exceeds the power
				required by the load, the synchronous
A. Diesel Generator				generator of the diesel genset becomes a
				synchronous motor that tends to accelerate
In terms of an electrical system, a diesel				the rotor speed of the diesel engine. The
generator can be represented as a prime mover				excess energy from the wind power, then,
and a generator. Ideally, the prime mover is				tries to drive the diesel engine. Because the
capable of supplying any power demand up to				diesel engine has only a small braking
rated power at constant frequency, and the				capability resulting from engine compression,
synchronous generator connected to it must be				the frequency control can be lost when the
able to keep the voltage constant at any load				extra power generated by the wind turbine is
condition. Figure 2 is a block diagram of the				sufficiently high.
diesel generator. The diesel engine keeps the
frequency constant by maintaining the rotor				Ã‚
speed constant via its governor. The
synchronous generator must control its output				Figure 6. Voltage, frequency, and power to
voltage by controlling the excitation				illustratean undersized diesel genset with
current. Thus, as a unit, the diesel				storage
generating system must be able to control its
frequency and its output voltage. The inertia				In Figure 7, the diesel generator has a rated
of the diesel genset, the sensitivity of the				power of 400 kW, the local load is initially
governor, and the power capability of the				set to 280 kW and at t = 4 s, and the local
diesel engine all affect the diesel				load is set to 100 kW. When the diesel is
generator's ability to respond to frequency				started, there is only a local load of 280
changes. The ability of the synchronous				kW. The wind turbine is then started at t = 2
generator to control its voltage is affected				s with a 225-kW induction machine. Although
by the field winding time constant, the				the diesel genset is rated at only 400 kW and
availability of the direct current (DC) power				the wind turbine is started with a 225-kW
to supply the field winding, and the response				induction machine, the effect of wind turbine
of the voltage control regulation mechanism.				start-up on the power system is very mild,
				mostly because the induction machine current
Ã‚				is limited by a soft start. A soft start is a
				device that limits starting current during
Figure 2. Diesel generator control block				start-up. It consists of a pair of
diagram				back-to-back thyristors installed in series
				with each phase of the motor winding. Because
Ã‚				the firing angle of the thyristor can be
				controlled, the size of the starting current
Ã‚				can be adjusted by controlling the firing
				angle of the thyristors. As we can see
B. Wind Turbine				(Figure 5), the same wind turbine (225 kW)
				draws a starting power of 300 kW, but after
The main components of a wind turbine are the				the soft start is installed (Figure 7), the
rotor of the turbine, which is the prime				power surge during start-up drops to about
mover, and an induction generator. In				100 kW. After the wind turbine enters
general, the rotor is connected to the				generating mode (at about t = 2.5 s), the
generator via a gearbox that matches the				local load (280 kW) is shared between the
rotational speed. The simplest system uses a				diesel genset (55 kW) and the wind turbine
fixed-speed turbine. A fixed-speed turbine				(225 kW). The voltage and frequency are
must rely on the blade-stall condition to				maintained constant, and the diesel genset
limit the output power when the winds are at
high speed. Note that, although the rotor				Ã‚
speed of an induction generator varies with
wind speed, the speed range is within a 1% to				Ã‚
2% slip. On the other hand, the wind speed
variation may range from 5 m/s to 25 m/s;				Ã‚
thus, in terms of the wind turbine, the
induction generator operates at a relatively				Figure 7. Voltage, rotor speed, and power of
"fixed speed" compared to the range of wind				anoversize wind turbinegenerates only a small
speed variation.				percentage of its rated load (about 13%).
				This makes a significant contribution to fuel
C. Induction Machines				savings from the wind energy. At t = 4 s, the
				local load is reduced from 280 kW to 100 kW;
Most electric machines used in industry as				the wind speed stays the same. As a result,
prime movers are induction motors. Two				the wind turbine tries to supply 225 kW, but
applications of induction machines in the				the only load available is 100 kW. As a
power system network fall within the scope of				result, the synchronous generator of the
this study: one as the generator on a wind				diesel genset turns into a motor (negative
turbine and the other as a motor driving				power), the governor loses its speed control,
large pumps and compressors. By its nature,				and frequency runaway is triggered. This is
an induction machine is an inductive load.				an example of the wind turbine being
This machine absorbs reactive power either as				oversized compared to the local load. In such
a motor or generator. The reactive power				a case, a dump load (water heater, water
absorbed by the induction machine comes from				pump, battery charger, etc.) is usually
the line to which it is connected. In a				deployed to keep the diesel genset
hybrid power system, the reactive power comes				generating, which prevents it from motoring.
from the synchronous generator of the diesel				Minimum power generation of the diesel genset
genset. In a wind turbine generator, a fixed				is usually pre-set (for example, 15%-40% of
capacitor is usually installed to supply some				the rated load). If the generated power of
of the reactive power that the induction				the diesel genset is less than the preset
generator needs. Figure 3 shows the				value, the dump load should be deployed. The
equivalent circuit of an induction machine				dump load must be sized so that the diesel
connected to a power system. The power system				genset will always generate power above its
is represented by infinite bus Es and the				minimum set point. The dump loads are
line impedance is represented by reactance				normally non-critical loads used to store
Xs.				excess electrical energy in another form,
				such as heat (water or space heater),
Ã‚				electric charge (battery charging), or
				potential energy (water pump). Oversized wind
Figure 3. Equivalent circuit of an induction				turbine with energy storage: As shown in the
machine connected to power system				previous subsection, an oversized wind
				turbine can drive the system into an unstable
D. Various Loads				condition because of the inability of the
				diesel engine to keep the frequency constant.
In the power system considered, there are two				An energy storage installed in the power
major loads. The first is a large water pump				system network is not only useful to remedy
representing a typical industrial load. The				the undersized diesel engine but also for
second is a collection of loads for which the				cases where there is an excess power produced
size and power factor can be programmed				by the wind turbine. Without energy storage,
throughout the day to represent a typical				the wind turbine can drive the synchronous
village load. The voltage at the terminal of				machine into motoring region and the
the load varies as a result of a voltage drop				frequency output will be out of control. With
across the line impedance. The voltage drop				a power converter to interface between the
across the line impedance varies depending on				energy storage and the power network, the
the size of the current and the power factor				energy storage is capable of quickly
of the load. The terminal voltage for a wind				absorbing excess power generated by the wind
turbine generator (VS), as the output current				turbine and hold the generator rotor speed
of induction machine, varies from start-up to				from a runaway condition. As shown in Figure
generating mode. During start-up, voltage				8, the frequency runaway can be prevented by
drops significantly at the terminal voltage				using energy storage to capture the excess
of the induction machine. The voltage drop				power in the power network.
across line impedance is caused by the
current surge during start-up. In addition,				Figure 8. Voltage, rotor speed, and power of
the phase angle of the stator current is very				anoversize wind turbine with energy storage
large and lagging. The combination of a poor
power factor and a lagging, large current				B. Case Study II: Charging the Storage Under
surge creates a voltage dip at the terminal				Normal Condition
of the induction machine during start-up.
Thus, a start-up of short duration is				The energy storage will be charged only when
preferable to a prolonged one				there is an energy surplus from the wind and
				the required network load is very light.
E. Energy Storage				Because the governor of the diesel generator
				will always maintain the frequency constant,
The energy storage can be of different types				the output power of the diesel generator is
(i.e. flywheel, battery, hydrogen/fuel-cell,				an indicator of the power within the system
hydropower etc.). In this paper, we assumed				available to charge the energy storage. One
energy storage with a power converter				benefit of charging the energy storage during
interface to the power network. The power				this condition is that the efficiency of the
converter is connected to the energy storage				diesel engine is at its peak when it is
at one end. With variability of wind				operated near its rated power. Thus, when a
resource, energy storage is an excellent				surplus of power is detected within the
contributor to the power system. The energy				system, the energy storage will be charged
storage behaves like a large buffer to				and some energy will be stored within the
accommodate the unequal instantaneous energy				system. The amount of energy and the size of
in the power system. Ideally, at any instant				charging power depend on the size of the
of time, there should be a zero net exchange				surplus power. The charging process will be
between the energy sources and the energy				stopped when the energy storage reaches its
sinks (both real and reactive power). If this				limit. Maximum charging current is also
balance is not achieved, the voltage and				limited by the energy storage and by the
frequency of the system changes to maintain				power converter interface. Figures 9 shows
equilibrium. At any instant, the energy				the charging process. Initially there is
storage behaves either as an energy source or				enough wind speed to start the wind turbine.
energy sink depending of the mode of				The diesel generator is supplying a constant
operation.				load of 280 kW (power factor = 0.995 lagging)
				all the time. As the wind turbine generates
Figure 4. Energy Storage control block				full power (225kW), the diesel governor
diagram				redistributes the load and there is a load
				sharing between the wind turbine and the
Ã‚				diesel generator. As the transient settles
				out, it is shown that the diesel generator is
It is assumed that the energy storage has a				contributing a very small amount of power to
power converter interfacing the power				the load, thus the charging mechanism is
network. Although it is possible for the				started. The energy storage is charged slowly
power converter to function as a reactive				until it reaches its limit.
power compensator, the cost of a power
converter is very expensive compared to other				Ã‚
means of reactive power compensation
currently available in the market. Keep in				Ã‚
mind that the size of the power semiconductor
in the power converter is limited by its				Figure 9. Real power flow in the power system
current limit and its voltage limit. Thus,
minimizing the current passing through the				Ã‚
power switches will minimize the current
rating of the power converter and will lower				In Figure 9, the charging of energy storage
the cost. For this paper, we only used the				during normal condition is limited to 75 kW,
power converter to process real power in and				which is about 50% of the rated power of the
out of the energy storage. Figure 4 shows a				capacitor. This limit ensures that the power
block diagram of energy storage control				converter still has enough headroom to
algorithm. It uses frequency deviation to				deliver or absorb power during an emergency.
indicate a real power imbalance in the				For example, if there is some loss of the
system. The frequency deviation is also used				loads in the power systems, the energy
as the feedback to control the energy storage				storage must absorb the loads loss to avoid a
output. If the load power demand is higher				sudden change in frequency. Similarly, to
than the power supply available, the				compensate for a sudden load increase to the
frequency of the diesel generator will slowly				power systems (e.g. the water pump is
drop. Other energy stored in the system				started), the energy storage must release
includes the kinetic energy in the turbine				energy to the power system to keep constant
blades, the diesel generator inertia, and				frequency at the diesel generator. As shown
energy in the inductors and capacitors, etc.				in Figure 9, the real power used by the
				energy storage to stabilize the frequency
Ã‚				takes precedence over the charging power used
				to charge the storage. This can be seen
F. Balance of Energy in the System				especially when the water pump is started at
				about t = 15 seconds.
In the isolated system we studied, the
balance of real and reactive power must				V. CONCLUSION
always be maintained. The balance of real
power is maintained by the governor of the				After presenting an overview of the
diesel generator. The balance of reactive				components of the power system under
power is maintained by the exciter of the				investigation, we described the operating
diesel's synchronous generator. When the load				characteristics of the components as they
demands more power than the diesel and the				relate to voltage and frequency variations in
wind turbine can produce, and the diesel				the power network. The analysis shows the
engine has reached its highest limit, as the				dynamic interaction among the wind turbine,
loads continue to increase, the governor of				diesel engine, large loads, and energy
the diesel cannot push more power, and the				storage. It also demonstrates the dynamics of
rotor speed of the diesel will start to drop.				real power balance and how the system is
The frequency of the generator will then drop				stabilized with the controlled energy
until balance is reached or the system				storage. The voltage regulation is very
collapses. The voltage in the system is also				minimal and the frequency regulation is
an indicator of the balance in the system.				controlled very closely. The voltage
When the reactive power demand from the loads				regulation is controlled mostly by the
is higher than what can be provided by the				balance of reactive power in the system and
diesel generator, the capacitor, and other				the time constant of the excitation system of
means of compensation, the system voltage				the generator. The frequency regulation
will drop. Although the size of output and				depends on the energy storage control, the
input of the energy storage is adjustable, it				size of the energy storage, the total inertia
is limited by its ratings. For this paper, we				in the system (temporary energy storage).Many
assumed that the energy storage is capable of				technical solutions can be implemented to
storing and providing long-term energy to the				remedy the shortcomings covered in this
power network to maintain system balance. In				paper. However, as in any power generation
reality, only a limited amount of energy can				system, the economic implications of the
be stored. We will not discuss energy				solutions must be carefully considered.
analysis in detail in this paper. In
practice, the energy will be stored when the				REFERENCES
wind turbine produces enough power and the
diesel is operating under light load. The				[1] E. Muljadi, L. Flowers, J. Green, and M.
actual loads are divided into critical and				Bergey. 1996. "Electrical Design of
non-critical loads. Critical loads are				Wind-Electric Water Pumping." ASME Journal of
supplied at all times and non-critical loads				Solar Energy Engineering 118:246-252.
are served only if there is enough source and
it will be shed off the system when the				[2] J.T.G. Pierik and M. De Bonte. 1985.
voltage or frequency drops below the				Quasi Steady State Simulation of Autonomous
allowable limit. With the existence of				Wind Diesel Systems (Status Report). Report
sufficiently sized energy storage, it is				No. ECN-85-091. Petten, The Netherlands: The
possible to serve all the loads (critical and				Netherlands Energy Research Foundation.
non-critical) all the time.
				[3] A.J. Tsitsovits and L.L. Freris. 1983.
IV. DYNAMIC ANALYSIS				Dynamics of an Isolated Power System Supplied
				from Diesel and Wind. Proc.IEEE 130, Part A,
The case studies look at different aspects of				No. 9:587-595.
major power system components in the power
network. The first case study investigates				[4] J.T. Bialasiewicz, E. Muljadi, G. Nix,
the diesel power component. In the power				and S. Drouilhet. 1998. "RPM-SIM Simulator: A
network, a diesel generator must maintain				Comparison of Simulated versus Recorded Data.
system balance by responding properly to				"Proceedings of WINDPOWER '98." Bakersfield,
power changes.				California, 423-432.

A. Case Study I: Diesel-Wind Turbine				[5] E. Muljadi and C.P. Butterfield. 2001.
Interaction				"Pitch-Controlled Variable-Speed Wind Turbine
				Generation," Transactions of the
A diesel generator consists of a diesel				IEEE-Industry Applications Society.

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How pumps work

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