AckermannTh. (ed) Wind Power in Power Systems
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.
Customer requirement 1 (CR1): the voltage level at the connection point has to
stay within an acceptable range [see Equation (3.10)], as most customer appliances
(e.g. lighting equipment, motors, computers, etc.) require a specific voltage range
(e.g. 230 V 10 %) for reliable operation.
.
Customer requirement (CR2): the power should be available at exactly the time the
consumers need it in order to use their various applia nces (i.e. when a customer
switches on a certain device).
.
Customer requirement 3 (CR3): the consumed power should be available at a reason-
able cost (this may also include low external costs to reflect the low environmental
impact of electricity production). CR1 and CR2 also concern the reliability of the
power supply. Greater reliability leads to higher costs and hence a conflict arises
between CR1 and CR2 and the demand for reasonable costs.
3.7.2 Requirements from wind farm operators
Similar to consumers, wind farm owners or operators have certain demands on the
existing power system in order to be able to sell the wind power production:
.
Wind power requirement 1 (WP1): similar to consumer appliances, wind farms
require a certain voltage level at the connection point as wind turbines are usually
designed to operate within a specific voltage range (e.g. nominal voltage 10 %). For
the consumers, the requirements are rather homogeneous, since all consumers use the
same type of equipment. For wind farms the requirements can sometimes be softened,
since wind farms can be designed to handle different quality levels.
.
Wind power requirement 2 (WP2): in addition, wind farm owners want to be able to sell
their power production to the grid when wind power production is possible (i.e. when
the wind speed is sufficient), otherwise, the production has to be spilled, which means
that the wind farm owner loses possible financial income.
.
Wind power requirement 3 (WP3): WP1 and WP2 also concern the reliability of the
power system at the connection point of the wind farm. As mentioned in Section 3.7.1,
there is always a trade-off between costs (i.e. low system costs) and reliability. The
higher the reliability the higher the costs.
3.7.3 The integration issues
The challenge that the integration of wind power poses is to meet CR1 and CR2 and
WP1 and WP2 in an economically efficient way (i.e. CR3 and WP3), even in the case of
significant wind power penetration in the system.
(5)
This challenge will now be discussed
in more detail using the simplified power system in Figure 3.9.
(5)
In this context, it is also important to consider that the amount of the losses, P
L
in a power system depend
on the size of the loads and the wind power production (see Chapter 9).
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3.7.3.1 Customer requirement 1: voltage level at the connection point of the consumer
First we will assume that no wind power is installed in the power system shown in Figure 3.9
and that the voltage U
0
is kept constant by the P
G
generators. Now, if the load P
D
varies, the
currents I
3
and consequently I
1
will vary. Hence, there will be a voltage drop [Equation (3.7)]
over the corresponding impedances Z
3
and Z
1
. If impedances Z
3
and Z
1
are large (e.g. in the
case of long lines or a comparatively low voltage) voltage U
3
will vary substantially when P
D
varies. Possible measures to avoid large voltage variations in U
3
, are as follows.
.
Measure (a): use a stronger grid (i.e. impedances Z
1
and Z
3
are small). This is possible
by using higher voltages in the lines and transformers that are not too small.
.
Measure (b): control the voltage U
3
by using controllable transformers close to U
3
.
.
Measure (c): control voltage U
1
by using controllable transformers and/or some
voltage controlling equipment such as shunt capacitors and/or shunt reactors.
We will now assume that wind power, P
W
, is connected to the power system in Figure
3.9. As the wind power production P
W
will vary, current I
2
varies. Hence current I
1
will
vary, which will cause a voltage drop in Z
1
. Furthermore, U
1
will vary and possibly also
the voltage close to the customer connection point, volta ge U
3
. The impact of wind
power variations on the voltage variations in U
3
depend mainly on the size of impedance
Z
1
. On the one hand, if Z
1
is large , there will be a strong connection between wind power
variations and voltage variations in U
3
. On the other hand, if Z
1
is very small, voltage
U
3
will be more or less independent of wind power variations. In reality, only consumers
that are rather close to wind farms may be affected by variations in wind power
production. To avoid problems for consumers close to wind turbines, Measures (a)–(c)
can be used as well as the following measure:
.
Measure (d): use local control of voltage U
2
at the wind farm. Depending on the wind
turbine technology, the voltage control of U
2
can be performed by the wind farm (see
also Chapters 12 and 19).
3.7.3.2 Wind power requirement 1: voltage level at the connection point of the wind farm
The voltage U
2
will also depend on P
W
, P
D
and the size of Z
1
and Z
2
. The difference to
the discussion in Sub-sect ion 3.7.3.1, on CR1 is that the size of Z
2
is important instead
of the size of Z
3
. This is mainly of interest when wind power is located at a larger
distance from the consumers. In this situation, Measures (a), (b) and (d) explained above
are of a certain inter est as well as the following:
.
Measure (e) use a controllable transformer close to U
2
. A controllable transformer is
normally slower than application of Measure (d), though.
3.7.3.3 Customer requirement 2: power availability on demand
Again, we will first assume that there is no wind power connected to the power system
shown Figure 3.9. Hence, when consumers increase their consumption, the power will be
delivered directly from the conventional power plants [see Equation (3.16)].
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Conventional power plants generally use synchronous generators, which can be
modelled as shown in Figure 3.10. The figur e includes the generator and the steam or
hydro turbine. The turbine rotates and drives the rotor of the generator. P
T
is the power
delivered by the turbine and P
S
is the power delivered from kinetic energy stored in the
rotating mass consisting of turbine, shaft and rotor. During normal operation P
S
is 0.
P
G
is the electric power delivered to the power system.
If the load P
D
increases, the power generation, P
G
, will directly increa se. However,
the initial increase in power production is not due to an increase in the power produc-
tion in the steam or hydro turbine. The increase of P
G
will originate from the stored
kinetic energy, P
S
. Since the kinetic energy is used, the turbine–shaft–generator rota-
tional system will slow down. Now, as the rotor speed of a synchronous generator is
strongly coupled to the power system frequency, a decrease in rotor speed will result in a
decrease in electric frequency. Therefore, a load increase will lead to a decrease in
electric frequency.
In order to limit the decrease in power system frequency, some power plants are
equipped with a so-called primary control system (see also Appendix A). This system
measures the power system frequency and adjusts the power production of the power
plant (i.e. P
T
) when the frequency changes. The reaction time of primary control units
depends on the power plants (e.g. how fast the production of steam can be increased).
Usually, primary control units can increase their production by a few percent of rated
capacity within 30 seconds to 1 minute. Secondary control (i.e. power plants with a
slower response), will take over the capacity tasks of the primary control 10 to 30
minutes later and will thereby free up capacity that is used for primary control (see also
Chapters 9 and 18).
The requirement of load balancing (i.e. so that consumers’ demand for power can be
met) means that:
.
a power system must have sufficient primary and secondary control capacity available
in order to be able to respond to changes in demand;
.
these power plants must always have sufficient reserve margins to increase the power
production to the level required for always meeting the system demand (i.e. the
aggregated consumer demand).
If wind power is added to such a power system there will be an additional fluctuating
source in the power system. With increasing wind power penetration, the requirements
regarding power system balancing may increase (the details depend on the system design
and load characteristics; see also Chapter 8). However, the primary and secondary
control system will operate in exactly the same way as described above. Also, if wind
power production decreases this has exactly the same effect on the system as an increase
in demand [see Equation (3.16)].
Synchronous
generator
Steam
turbine
Shaft
P
T
+ P
S
= P
G
Figure 3.10 The Power balance in a conventional power plant
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The consequence is that there will be more variations that have to be balanced by
primary and secondary control. Experience from Europe shows that even very high
wind power penetration levels (up to 20 %) may not require additional primary control
capacity as long as the installed wind power capacity is geographically distributed over a
wide area (see also Chapter 8). The smoothing effect related to geographical distribution
will result in low short-term variations in wind power production. However, owing to
current limitations in wind speed forecast technologies, any mismatch between fore-
casted wind power production and actual wind power production has eventually to be
handled by the secondary control capacity (on wind power forecasts, see Chapter 17; on
the experience in Denmark, see Chapter 10; for additional discussion, see Chapters 8, 9
and 18). The requirements for secondary control capacity are therefore significantly
influenced by the wind power penetration level.
The additional system requirements for keeping the system balanced at all times
depends very much on the individual system; that is, it depends on the load characteristics,
the flexibility of the existing conventional power plants and the wind power penetration as
well as the geographic distribution of the wind farms. The cost of meeting the increased
requirements depends on the type of power plant (i.e. P
G
), the size of interconnections to
neighbouring systems and, of course, on the additional requirements.
3.7.3.4 Wind power requirement 2: network availability on demand
As already mentioned, wind farms want to sell into the power system whenever wind
power production is possible. Depending on the power system design and the wind
power penetration this may cause transmission congestions as well as stability issues
(i.e. volta ge stability). For a more detailed discussion of these issues we refer the
reader to Chapter 20 as well as to Part 4 of this book, in particular to the introduc-
tion of Part 4, Chapter 24.
3.7.3.5 Customer requirement 3: economical power supply
Again, we will first assume there is no wind power connected to the power system shown
in Figure 3.9. In general, power system design must analyse the costs and benefits of a
certain reliability level. No power system will be built with a 100 % reliability, as the
costs will be in no relation to the benefits. Also, the cost of increasing reliability is very
high if one already has a high reliability. There are two issues that have to be taken into
account.
First, the power system must have a sufficient amount of power capacity, P
G
, to meet
maximum demand, P
D
þ P
L
. We assume that we have a reliability of 99.9999 % [i.e.
during 1 hour per year (the expected mean value) there is not enough installed capacity
to meet the load]. If we want to increase reli ability, we have to build a new plant that is
used only during one hour per year. In this case it is sometimes considered economically
more efficient to disconnect consumers (paid voluntary disconnections or forced dis-
connections) than to build a new power plant that is used only 1 hour per year.
For the dimensioning of a power system, the so called N 1 criterion is usually
applied. The N1 criterion means that an outage in the largest power plant should not
cause the disconnection of any consumer. An outage in a large plant causes a decrease in
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frequency and the activation of the primary control system, which increases power
production in some other units in order to compensate this outage (see sub-section
3.7.3.3). Because of the N 1 criterion, it is not acceptable to disconnect consumers
[P
D
remains the same in Equation (3.16)], which means system reserve margins must be
at least of the same size as the largest unit in the system.
Second, there must be sufficient grid capacity to transmit the power from the gen-
erators to the consumers. The consumers are actually distributed in the grid and there
has to be sufficient capacity to supply the maximum load of each individual consumer.
However, no component in a power system has a reliability of 100 %. Therefore,
redundancy within the power system is required (e.g. redundant transmission lines
and backup power plants). For defining how large the redundancy should be the costs
of keeping the redundancy have to be compared with the costs associated with discon-
necting consumers as a result of insufficient redundancy.
The impact of wind power on system reliability is discussed in the following sub-
section, on WR3.
3.7.3.6 Wind power requirement 3: power system reliability
The introduction of wind power modifies the economic trade-off between reliability
costs and consumer costs for insufficient reliability. Some of the trade-offs apply only to
wind farms (i.e. they affect income for wind farm owners, but not consumers) and some
affect the whole system.
An important reliability issue is related to the capacity margi n in a given power
system (i.e. there must be enough capacity available in a power system to cover the
peak load). If we assume a certain power system, there is always a probability that the
available power plants are not sufficient to cover the peak load. If wind power is now
added to a power system, reliability will increase as there is a certain probability that
there will be a certain amount of wind power production available during a peak load
situation, which will decrease the risk of capacity deficit. Adding more wind power
capacity to a power system may also allow a decrease in the installed capacity of other
power plants in the syst em without reducing the system reliability. This is treated in
more detail in Chapter 9.
In addition depending on the power system design and the wind power penetration,
power system reliability might be affected by the introduction of wind power because of
the influence it has on stability in the case of a fault in the power system. For a more
detailed discussion of these issues we refer the reader to Part D of this book, in
particular to the introduction of Part D, Chapter 24, but also to Chapters 10, 11 and 20.
It should also be considered that, in contrast to systems with only varying load, active
power balancing in a system with both wind power and load varying may require more
balancing equipment to keep a certain system reliability level, since the total variation
may increase. However, the cost–benefit analysis should consider that, for instance,
the largest possible decrease in wind power (which requires an increased pr oduction
from other power plants) can coincide with high wind power production. In such a
situation, other power plants may have previously reduced their power production as a
result of increasing wind power production. Hence, these (conventional) power plants
may be able to increase production if wind power production decreases. But also in this
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case the trade-off between the consumer benefit of high reliability and system costs for
back-up and reserves have to be taken into account. An important issue here is, for
instance, how fast aggregated wind power production can decrease during times when
aggregated load levels typically increase very fast.
If we consider now the reliability issues related to the installation of a certain wind
farm, we have to discuss the dimensioning of the transmission system between the wind
farm and the rest of the grid. We can start with the assumption that good wind resources
are located in remote areas, at a larger distance from the rest of the power system (i.e.
the cost of Z
2
-lines will be rather high), hence a back-up transmission system between
the wind farm and the main grid will be rather expensive compared with the economic
benefits a back-up transmission system may provide (for a more discussion on back-up
transmission systems for offshore wind farms, see Chapter 22) Hence the lack of a
redundant transmission line may have a negative impact on the technical availability of
wind farms. In this case, the comprom ise must consider the costs of a redundant
transmission system and the lost income for wind farm owners during times when the
transmission system is interrupted.
Furthermore, the required power quality level (U
2
) at the connection point of the
wind farm might be of importance. If a very stable voltage level is required, the grid,
represented by Z
2
, has to incorporate some voltage regulating equipment. An alterna-
tive is that the wind turbines that are used have a reduced sensitivity to voltage
variations and/or have a voltage control capability.
It must also be noted that the current variations from the wind farms will affect the
voltage drop over Z
1
. The most extreme cases occur during times wi th maximum wind
power production (P
W
) and minimum consumption (P
D
), and zero wind power produc-
tion (P
W
) and maximum consumption (P
D
). Thi s implies that U
1
will show increased
variation with increasing installed wind power, which may require additional voltage
control equipment. However, it might be important to consider the probability of those
extreme cases. The probability of full production in all wind farms at the same time that
the load is at its minimum, for inst ance, might be low, so that additional voltage control
equipment will not be requir ed. The question is, then, for how severe situations the
voltage control equipment should be designed. It is not economically relevant for all
points within a network to keep a voltage within certain limits 100 % of the time.
3.8 Conclusions
This chapter has provided an overview of the challenges regarding the integration of
wind power into power systems. The overall aim of power system operation, independ-
ent of wind power penetration levels, is to supply an acceptable voltage to consumers
and continuously to balance production and consumption. Furthermore, the power
system should have an acceptable reliability level, independent of wind power penetra-
tion. Therefore this chapter has discussed the impact of wind power on voltage control
and overall system balance.
The chapter has included basic electric power system theory and the basics of how
wind power production behaves in order to arrive at a better understanding of integra-
tion issues. The appendix gives a mechanical equivalent to a power system with wind
power in order to illustrate how a power system is operated.
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Appendix: A Mechanical Equivalent to Power System Operation with
Wind Power
For engineers that are not very familiar with power system analysis it may be sometimes
rather difficult to understand the relevant problems related to the interconnection of
wind power into a power system. Therefore, the following mechanical equivalent to a
power system was developed. The mechanical equivalent consists of a bike with multiple
seats (see Figure 3.11), referred to as ‘long bike’ throughout the following text (this is a
shortened and modified version of So
¨
der, 2002).
When taking the image of the long bike, the active power balance refers to the
operation of the bike at a constant speed. The goal to keep a constant voltage (i.e. to
keep the reactive power balance) corresponds to the challenge to keep the bike in
balance (i.e. to kept the bike on the road without it falling to one side).
Introduction
Assume a very long bike according to Figure 3.11. The bike is made of a nonrigid
material, and the chains between the different chain rings are slightly elastic. When the
bike stands still, all pedals will be at exactly the same position (e.g. at the bottom
position). We can now assume that the bike runs on a flat, straight road and the aim
is to keep the bike at a constant speed and in an upright position. The rolling resistance
and air resistance are neglected.
Figure 3.11 A long bike (Reproduced by permission of the Royal Institute of Technology,
Sweden.)
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Active power balance
On the bike, there are some cyclists (pedalling cyclists are generators, and braking cyclists
are loads) who pedal continuously, but different cyclists apply different force. Since all
chain rings are connected via a chain, the chain rings will rotate with the same speed.
A so-called ‘bike rpm’ (system frequency) related to the speed of the bike is obtained.
Some of the braking cyclists try to stop their pedals from rotating (motor loads). It is now
important to note that the chain rings connected to these pedals also have the same bike
rpm. As air resistance and rolling resistance are not considered, the bike will roll at a
constant speed if the total force of the biking cyclists (total generation) is exactly the same
as the total braking power of all braking cyclists (total load). Otherwise, the speed of the
bike will change (the frequency in the power system will change).
Synchronous machines
Some cyclists, so called ‘synchronous cyclists’, have their pedals connected directly to
the chain rings . This is the most common way of connecting pedals with chain rings for a
‘grown-up’ cyclists (large power plants), but for cyclists who bike only when it is windy
(wind power plants), this connection is very rare. This implies that all synchronous
cyclists will bike at the same speed. It must be noted, though, that the chains between the
cyclists are slightly elastic. This means that there will be an angle difference between the
different cyclists; that is, the pedals will not be at the bottom position at exactly the same
time [the voltage angles, arg(U), in different points in the power system are different]. It
can be noted that some cyclists (who could be hydropower stations) prefer to bike slower.
They have to have a gear (several machine poles) between pedals and sprocket. It becomes
obvious that a sudden change of the force when pedalling causes oscillations (power
system angle dynamics) since there is some inertia in the sprocket connected to the pedals,
and the chains to neighbouring chain rings are elastic.
Asynchronous machines
Some cyclists, usually children, that cannot bike as well as others (generators with low
power production, such as wind power or distributed generation) prefer to use a softer
connection between pedals and sprocket. Instead, they use a belt, which implies that
they, depending on the belt slip, have to bike slightly faster than the bike rpm. These
cyclists are therefore called ‘asynchronous cyclists’. There are also some asynchronous
cyclists who use the belt connection to the sprocket and toe clips. They try to stop the
bike (asynchronous motor loads).
Power electronic interfaces
Another technique is used by ‘variable cyclists’ (power plants with an electronic inter-
face between the generator and the grid). These cyclists use a stepless transmission (e.g. a
wind power plant with a double-feed asynchronous generator) between pedals and a
sprocket instead of a gearbox. This way, the speed of the pedals is more independent of
the bike rpm.
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Frequency control
Some of the (normally synchronous) cyclists continuously look at the speed of the bike
(power plants used for frequency control). If the speed of the bike decreas es, because of
a nonreliable cyclist (forced outage in a power plant) or too many braking cyclists (the
load on the system increases), these ‘speed controllers’ adjust their stroke on the pedals
in order to adjust the speed. They always have to keep a margin (power plants cannot run
at maximum installed capacity) and cannot continuously bike at their maximum capacity.
If the speed of the bike changes too often or too much, they require other cyclists to
change their pedalling efforts. On some bikes (in deregulated power systems) a special
person, who is not biking and who is in charge of the speed control (independent system
operator), finds out who is prepared to make these changes least expensively (cheapest bid
on balancing market) and asks this cyclist to change the pedal stroke.
Wind power
Some cyclists only bike when it is windy. When the wind speed is low, these cyclists wi ll
pedal with less force. But still the bike rpm is about the same. The consequence is that
the bike runs a bit slower and the bike speed controllers increase their force on the
pedals. This is from the bike’s perspective exactly what happens when some cyclists
increase their braking force. When it is windy some other cyclists can bike less. These
cyclists can drink some water, to build up energy for the time without wind (storage of
water in hydro reservoirs). Remember that it is still the speed controller who makes the
bike run at the same speed (keeps the balance between production and consumption).
Reactive power balance
A problem on the bike is that the seats of some cyclists and braking people are not
positioned at the midpoint of the bike. The consequence is that the bike will fall to the
side (a so-called voltage collapse, which occurs when there are no reactive power sources
to keep an acceptable voltage) if these forces are not balanced. Figure 3.12 shows what
such a four-person bike might look like from above.
P
G
, the biking cyclists, try to accelerate the bike, whereas P
D
, the braking cyclists, try to
decrease the speed. Q
L
are forces that try to overturn the bike to the left (producers of reactive
power), whereas Q
R
try to overturn it to the right (consumers of reactive power). The aim is
to keep the bike in an exact vertical position (voltages ¼ 1perunit¼ nominal voltage), but
a slight angle difference from vertical is still acceptable (a slight voltage deviation).
P
G
P
D
4
3
21
Q
R
Q
L
Figure 3.12 Forces on a long bike: a four-person bike
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Asynchronous machines
The asynchronous cyclists, the ones with the belt, always sit a bit to the right of the bike
midpoint. This implies that this causes a force on the bike (they consume reactive power)
that overturns it to the right if this force is not compensated. It does not matter if they
bike (asynchronous generators) or if they brake (asynchronous motors).
Capacitors
There are some special high-cap acity cyclists, who never bike or brake. They just sit on a
seat, located to the left-hand side of the midpoint of the bike (capacitors producing
reactive power). The best location is to have them close to asynchron ous cyclists in order
to compensate them directly (it is common to have capacitors close to wind power plants
with asynchronous generators). If the high-capacity people are located at another part
of the bike, extra forces will be induced on the bike frame (if there is no local compensa-
tion of the reactive power consumption in wind power asynchronous generators, the
reactive power has to be produced at other locations and transported to the locations
where it is used, which will increase losses in the system, for instance).
Synchronous Machines
The synchronous cyclists have seats that could move from the right-han d side of the
midpoint (reactive power consumption) to the left-hand side (reactive power produc-
tion). By studying the balance of the bike they move from left to right in order to keep
the bike in an upright position (to control reactive power to keep an acceptable voltage).
It can be noted that the possibility of moving from left to right is limited by the force
on the pedals. When they bike a lot they cannot move so much from left to right (the
total current limits reactive power production or consumption at high active power
production).
Power electronic interfaces
The variable cyclists use the stepless transmission. This type of system often lets the person
sit slightly to the right or to the left of the midpoint (with the possibility of controlling the
voltage by modifying the reactive power production or consumption). A common solution
is to let the biker sit on the midpoint of the bike when the cyclist bikes as much as possible
(power factor ¼ 1 at installed capacity in wind power plants with variable speed).
References
[1] Burton, T., Sharpe, D., Jenkins, N., Bossanyi, E. (2001) Wind Energy Handbook, John Wiley & Sons, Ltd/
Inc., Chichester.
[2] Gasch, R., Twele, J. (2002) Wind Power Plants, James & James, London.
[3] Ender, C. (2004) ‘Windenergienutzung in der Bundesrepublik Deutschland – Stand 31.12.2003’ (Wind
Energy Use in Germany – Status 31.12.2003), DEWI Magazin Number 24 (February 2004) pp. 6–18.
50 Wind Power in Power Systems: Introduction