AckermannTh. (ed) Wind Power in Power Systems
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In many ways, it regulates itself based on a few battery-specific parameters. This is not
the case with equipment connected to the AC grid, which is much less forgiving. For AC
systems the key issues are the balancing of power production, load and voltage regula-
tion on a subcycle time scale. This is done largely with the use of synchronous con-
densers, dispat chable load banks, storage, power electronics and advanced control
systems that carefully monitor the operating conditions of each component to ensure
that the result is power with a consistent frequency and voltage. In AC systems, the
inclusion of a battery bank can also provide the same power-smoothing functions,
although more care must be taken in the control of system power.
In the following subsections three figures are provided to describe the three general
types of wind–hybrid power systems. The first, Figure 14.1, illustrates a small conven-
tional DC-based power system providing AC power using a power converter; second,
Figure 14.2 shows a small power system focused around the AC bus; last, Figure 14.3
illustrates a larger AC-coupled power system.
Small wind turbines, up to 20 kW, have also been connected to power devices directly;
the most common of these is for water pumps in agriculture or for portable water,
although other applications such as ice making, battery charging and air compression
have been considered . Both mechanical and electric wind turbines are used widely.
14.2.1.1 DC-based hybrid systems for small remote communities
Figure 14.1 illustrates a small conventional DC-based power system providing AC
power using a power converter. The use of smaller renewable-based hybrid systems
Turbine disconnect
Guyed lattice tower
Inverter (bidirectional optional)
Turbine controller
DC source centre
Battery bank
DC loads AC loads
PV charge
controller
Wind turbine
Generator
PV array
Figure 14.1 DC-based renewable power system. Note:PV¼ photovoltaic
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has grown as small wind technology increases in usability and photovoltaic (PV)
technology decreases in cost.
Most of these systems use a topography where the DC battery bus is used as a central
connection point. Generally, small winds turbines generate variable frequency and
voltage AC current that is rectified and applied to the DC bus at the voltage of the
battery bank. Energy either is stored in the battery or is converted to AC through an
inverter to supply the load. The use of the battery bank smoothes out wind turbine
power fluctuations and allows energy generated when there is wind to supply a load at a
later point in time. In cases where guaranteed power is required, a dispatchable gen-
erator – typically diesel, propane or natural gas – can also be installed to provide the
load and charge batteries in the prolonged absence of renewable-based generation.
Control in small DC-based power systems is usually conducted through the voltage of
the battery bank. All generators have a voltage limiter, which reduces or shunts any
energy generated by the wind turbine if the battery is too full to accept additional power.
Recent research conducted at the National Renewable Energy Laboratory (NREL)
(Baring-Gould et al., 2001, 1997; Corbus et al., 2002; Newcomb et al., 2003) has shown
high potential losses associated with the interaction of DC wind turbines and the battery
bank because of premature limiting of energy by the wind turbine controller. For this
reason, care must be taken when designing such a power system in order to ensure that
proper matching between the different components is achieved. All inverters and load
devices also ha ve low-voltage disconnects that stop the discharge of the battery bank if
the voltage drops below some present value. The power supplied to the AC bus is
controlled through the inverter, which may provide additional system control depending
on the unit (for more information on systems using DC architecture, see Allerdice and
Rogers, 2000; Baring-Gould et al., 2001, 2003; Jiminez and Lawand, 2000).
14.2.1.2 AC-based hybrid systems for small remote communities
Recent improvements in power electronics, control and power converters have led to the
rise of a new system topology, and Figure 14.2 shows a small power system focused
around the AC bus. These systems use small, DC or AC generation components, PVs
and wind, connected through a dedicated smart inverter to the AC distribution grid. A
battery is used to smooth out power fluctuations but also includes its own dedicated
power converter.
The prime advantage of this topology is its modularity, allowing the connection or
replacement of modules when additional energy is needed. Second, it steps away from
the need to co-locate all components where they can be connected to a DC bus, allowing
each component to be installed at any location along the micro-grid. These systems
generally use system frequency to communicate the power requirements between the
different generation and storage modules. The two disadvantages of systems using this
topology are cost and the use of sophisticated technology that will be impossible to
service in remote areas.
An additional issue is that all energy being stored must pass first from the point of
generation to the AC bus and then through the rectifier of the battery dedicated
power converter. It must then be inverted again prior to use, resulting in three power
conversion cycles, compared with only one for systems using a DC-based topography.
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Thus in systems where large amounts of energy storage is expected, such as PV systems
designed to provide evening lighting loads, this technology may result in higher system
losses (more information on these types of power systems may be located in Cramer
et al., 2003; Engler et al., 2003).
14.2.1.3 Wind–diesel systems
Larger power systems are focused around the AC bus and incorporate both AC-
connected wind turbines and diesel engines. Figure 14.3 shows a larger AC-coupled
wind–diesel power system.
A technically effective wind–diesel system suppli es firm power, using wind power to
reduce fuel consumption while maintaining acceptable power quality. In order to be
economically viable, the investment in the extra equ ipment that is needed to incorporate
wind power, including the wind turbines themselves, must be recouped by the value of
the fuel savings and other benefits. As the ratio of the installed wind capacity to the
system load increases, the required equipment needed to maintain a stable AC grid also
increases, forcing an optimum amount of wind power in a given system. This optimum
is defined by limits given by the level of technology used in the system, the complexity of
Wind turbine
Guyed lattice tower
Turbine disconnector
Generator
Turbine inverter
and controller
PV inverter
and controler
PV array
Bi-directional converter
Battery bank
AC loads
Figure 14.2 AC-based renewable power system. Note:PV¼ photovoltaic
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the layout chosen and the power quality required by the user. For this reason the
optimal design must be based on careful analysis, not simply the maximum amount of
wind energy possible. Although not the focus of this book, it should also be noted that
other diesel retrofit options do exist and should be investigated. This includes the use of
other renewable technologies, such as biomas s, or simply better control of the diesel
generators (Baring-Gould et al., 1997).
Because of the large number of isolated diesel mini-grids both in the developed world
and in the developing world, the market for retrofitting these systems with lower cost
power from renewable sources is substantial. This market represents a substantial
international opportunity for the use of wind energy in isolated power supply. To meet
this market, an international community has formed, c ommitted to sharing the technical
and operating experience to expand recent commercial successes.
A typical isolated diesel power supply system can be characterised by the following:
.
It has only one or a few diesel generating sets (gense ts). By using a number of diesel
gensets of cascading size an optimal loading of diesel gensets may be obtained, thus
increasing the efficiency of the diesel plant.
.
The existing power system has quite simple system co ntrollers, often only the govern-
ors and voltage regulators of the diesel generators, possibly supplemented by load-
sharing or self-synchronising devices.
.
The local infrastructure may be limited and there may be no readily available
resources for operation, maintenance and replacement (OM&R).
AC wind turbines
AC bus
DC
Rotary converter
Battery
DC bus
Control
system
Controlled
dump
load
AC
AC diesels
Dispatched
load
Figure 14.3 A typical large wind–diesel power system
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.
Fuel is generally expensive and is sometimes scarce and prone to delivery and storage
problems.
.
The diesel engines provide adequate frequency control by the adjustment of power
production to meet the load and voltage control by modifying the field on the
generator.
14.2.2 Basic considerations and constraints for wind–diesel power stations
In technical terms, an isol ated power system for a large community that incorporates
wind power will be defined as a wind–diesel system when both the syst em layout and
operation are significantly influenced by the presence of wind power in terms of:
.
frequency control, stability of system voltage and limited harmonic distortion;
.
the operating conditions of the diesel generators, especially with regard to minimum
load;
.
provisions for the use of any surplus wind power;
.
OM&R of any system components.
Wind–diesel power systems can vary from simple de signs in which wind turbines are
connected direct ly to the diesel grid with a mini mum of additional features, to more
complex systems (Hunter and Elliott, 1994; Lundsager and Bindner, 1994). The import-
ant complication of adding wind power to diesel plants is that the production of energy
from the wind turbines is controlled by the wind, meaning most turbines cannot control
either line frequency or voltage and must rely on other equipment to do so. With only
small amounts of wind energy the diesel engines can provide this control function, but
with larger amounts of wind energy other equipment is necessary.
With this in mind, two overlapping issues strongly influence system design and its
required components: one is the amount of energy that is expected from the renewable
sources (system penetration), the other is the ability of the power system to maintain a
balance of power between generation and consumpt ion (the primary use for energy
storage).
14.2.2.1 Wind penetration
When incorporating renewable-based technologies into isolated power supply systems,
the amount of energy that will be obtained from the renewable sources will strongly
influence the technical layout, performance and economics of the system. For this
reason, it is necessary to explain two new parameters – the instantaneous and average
power penetration of wind – as they help define system performance.
Instantaneous penetration, often referred to as power penetration, is defined as the
ratio of instantaneous wind power output, P
wind
(in kW), to instantaneous primary
electrical load, P
load
(in kW):
Instantaneous (power) penetration ¼
P
wind
P
load
: ð14:1Þ
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Thus, it is the ratio of how much wind power is being produced at any specific instant.
Instantaneous penetration is primarily a technical measure, as it greatly determines the
layout, components and control principles to be applied in the system.
Average penetration, often referred to as energy penetration, is defined as the ratio of
wind energy output, E
wind
(in kWh), to average primary electrical load, E
load
(in kWh),
measured over days, months or even years:
Average (energy) penetration ¼
E
wind
E
load
: ð14:2Þ
Average penetration is primarily an economic measure as it determines the levellized
cost of energy from the system by ind icating how much of the total generation will come
from the renewable energy device.
14.2.2.2 Power balance
As an illustration of the balancing of generation and consumption of power, Figu re 14.4
shows the power balance of a basic wind diesel system, where the instantaneous con-
sumer load is shown as 100 % load. In order to highlight the issues, the system
illustrated in Figure 14.4 has an unusually high proportion of wind energy.
At low wind speeds, the diesel generator produces all the power required, but as the
wind speed increases the increased power production from the wind turbine is reflected
by a decreased production from the diesel generator. Eventual ly, the diesel generator
reaches its minimum load defined by the manufacturer and the surplus power must be
taken care of either by limiting the power production from the wind turbine or by
diverting the surplus power in a controllable load. If the system configuration and
–150
–100
–50
0
50
100
150
200
Power (%)
0 5 10 15 20 25
Wind Speed (m/s)
Wind turbine power Consumer load
Diesel generator power Power surplus
Figure 14.4 Simple power balance of a wind–diesel power system
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control allow it, the diesel could also be shut off, allowing the load to be supplied
completely by wind energy.
14.2.2.3 Basic system control and operation
The simplest large wind diesel system configuration is shown in Figure 14.5, in which a
standard grid-connected wind turbine with an induction generator is connected to the
AC bus bar of the system. This represents a simple low-penetration power system.
When the wind turbin e power output is far less than the consumer load minus the
diesel minimum load, the diesel governor controls the grid frequency and the voltage
regulator of the synchronous generator of the diesel set controls voltage.
If the designed wind turbine output is about equal to the consumer load minus the
diesel minimum load, controllable resistors, typically referred to as a dump load, may be
included to absorb possible surplus power from the wind turbine, in which case the
dump load controller helps to regulate the frequency. This would be termed a medium-
penetration power system.
If the wind turbine can produce more power than is needed by the load a more
complicated system is required, which may also incorporate energy storage to smooth
out fluctuations in the wind energy, as was shown in Figure 14.3. In this case, some
surplus energy is saved for later release when the need arises. If the wind production is
more than the load, it then becomes possible to shut off all diesel generators altogether.
Although this maximises fuel saving s it requires very tight control of all power system
components to ensure system stability. Additionally, the fluctuating nature of the wind
power may pose extra problems for the control and regulation of the system. This last
configuration would be characteristic of a high-penetration power system.
AC wind turbines
AC bus
Community load
Simple control
Diesel engines
Figure 14.5 Simple, low-penetration wind–diesel power system
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As can be intuitively understood, the higher the wind penetration, the more energy that
cannot be used by the electric loads is generated. Instead of just dissipating this surplus
power in a dump load the power can be used to satisfy additional community loads such as:
.
production of fresh water (e.g. by desalination, purification and so on);
.
ice making;
.
water heating;
.
house or district heating;
and much more. Usually, these loads are broken into two categorises:
.
Optional loads: loads that will be met only if and when surplus power is available as
other energy sources can be used when excess energy is not available (e.g. space
heating);
.
Deferrable loads: loads that must be met over a fixed period of time, for example on a
daily basis, and if not supplied by surplus power will be served by primary bus bar power.
Additionally, some loads can be control led by the power system instantaneously to
reduce the requir ed power demand, thus saving the system from having to start an
additional generator to cover what is only a momentary defect of power. This is often
referred to as demand-side management.
14.2.2.4 Basic system economics
As with any power system, economics play a key role in the selection of power system
design and the technology to be applied. As has been discussed previously (Section 14.2),
the reason to consider wind energy as an addition to diesel-based generation is because
of its potential lower cost. However, as the level of system penetration increases, we
have seen that the complexity of the power system also increases and subsequently so
does the cost (Section 14.2.1). Therefore, the optimal level of wind penetration depends
on the relative cost difference between increasing the penetration of wind and the
difference between generation using wind and diesel technology. We have also seen that
if excess energy from the wind can be used in place of other expensive fuels to provide
additional services, such as heating, the economics of using alternative sources also
improve. Determining this optimal system design is not a trivial problem, but luckily
tools that will be introduced later in this chapter (Section 14.6) have been developed to
assist with this process.
14.2.2.5 The role of storage in wind–diesel power stations
Storage may be used in a high-penetration power system to smooth out fluctuations in
the load and wind power in cases where the diesel generators are allowed to shut off.
Additionally, short-term energy storage can also be used to cover short-term lulls in
wind energy or allow diesel engines to be started in a controlled manner (Shirazi and
Drouilhet, 2003). Generally, energy storage will use either battery storage or flywheel
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storage. Both of these devices would require the addition of some power conditioning or
the use of a bidirectional power converter. Many other forms of storage have been
proposed or experimented with. However, the only other successful implementation has
been using pumped hydro storage, where excess power is used to pump water into a (high)
reservoir to be used later to drive the pump in reverse operation to generate electricity.
The use of hydrogen as a storage medium has received notoriety recently. In this case,
an electrolyser uses any excess energy to create hydrogen, which would then be stored for
later use. An internal combustion engine or fuel cell would be used to convert the
hydrogen into electricity when it was needed. Since the amount of potential storage is
dependent only on the size of the storage mechanism, the possibility of long-term or
season-based storage makes this option look very attractive. However, the exceedingly
high cost of both electrolysers and fuel cells, as well as the very low turn-around efficiency
of the system, which approaches 30 % compared with 80 % with lead–acid batteries, still
makes this option unattractive from an economic standpoint. Since neither fuel cells nor
hydrogen internal combustion engines can be started instantaneously, some form of
short-term energy storage will still be required (Cotrell and Pratt, 2003).
The economics of systems with energy storage depend strongly on the reliability and
cost both of power electronics, expected to improve significantly with time, and of the
storage medium itself. Prediction of the expected life of the battery under site-specific
conditions is an important economic and technical issue. The economics of systems
without storage are strongly influenced by operational restraints of the diesel system in
terms of diesel scheduling, the existence of a mandatory spinning reserve safety thresh-
old and the operational load limits of each generat or.
As shown in Figure 14.6, representative for all but very small diesel generators, when
diesel generator output decreases, presumably as a result of an increasing instantaneous
0
25
50
75
100
Fuel consumption (%)
0
0.25
0.5
0.75
1
Specific fuel consumption (l/kWh)
0 25 50 75 100
Power (%)
Fuel consumption Specific fuel consumption
Figure 14.6 Fuel consumption (as a percentage of the maximum) and specific fuel consumption
(in litres per kilowatt-hour) as a function of power (as a percentage of the maximum)
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penetration, the absolute fuel consumption (in litres per hour) decreases proportionally
with diesel power, but it does not reach zero at zero diesel load. Therefore, the specific
fuel consumption does not decrease proportionally with decreased load on the diesel.
For this reason, it is always better to shut down diesel engines when they are not needed
in place of having them operate at very low loading.
14.3 Categorisation of Systems
There is a clear difference between the wind turbines installed in a small isolated system
in a rural community and a wind turbine situated in an offshore wind farm in Denma rk.
Because of the differences in their design characteristics and performance it is useful to
introduce a categorisation of power systems according to the installed power capacity.
A su ggested categorisation is shown in Table 14.1 (Lundsager et al., 2001).
Typically, a micro system uses a small wind turbi ne with a capacity of less than 1 kW;
a village system typically has a capacity between 1 kW and 100 kW, with one or more
wind turbine(s) of 1–50 kW; an island power system is typically from 100 kW to 10 MW
installed power and with wind turbines in the range from 100 kW to 1 MW; a large
interconnected power system typically is larger than 10 MW, with several wind turbines
larger than 500 kW installed as wind power plants or wind farms.
The theoretical maximum wind penetration levels of the power systems presented in
Table 14.1 are plotted in Figure 14.7 as a function of the total installed capacity. To
provide an ind icator for very large power systems, the existing conditions of Denmark in
1998 and one projection for Denmark for the year 2030 are used. The dashed line shows
the current condition in which the level of wind energy penetration of actual power
systems with successful track records decreases as the power system size increases. The
dotted line indicates possible future development towards higher penetration levels,
which may be achieved in the next 20–30 years. The benchmark used for the future
village power system is based on the Frøya Island power system – a Norwegian resear ch
system designed to investigate maximum wind penetration (Table 14.2; see Section
14.4.1).
The theoretical feasibility of very high wind energy penetration is seen to change
dramatically in the 100 kW to 10 MW system size range. In this range, conventional
electricity generation is still based on diesel generation, which has a high cost of energy.
The main reasons for the existing low penetration levels in the largest systems is
primarily economic, even if at the present stage of development wind energy production
cost is on a par with most conventional sources. At any given configuration, there is
Table 14.1 Categorisation of isolated power systems
Installed power (kW) Category Description
<1 Micro systems Single point DC-based system
1–100 Village power systems Small power system
100–10000 Island power systems Isolated grid systems
>10000 Large interconnected systems Large remote power system
310 Isolated Systems