AckermannTh. (ed) Wind Power in Power Systems
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In comparison, in 2002, the maximum hourly consumption reached 2700 MW in the
Elkraft area and 3700 MW in the Eltra area. As can be seen, the ratio between installed
wind power and consumption is about three times higher in Western Denmark than in
Eastern Denmark. In fact, the Eltra area has the largest amou nt of installed wind power
in the world relative to the size of the power system.
As a result, prim arily the Western Danish power system has to face the large technical
challenges resulting from managing and operating a power system with a significant
wind power penetration. This chapter therefore focuses on discussing wind power in the
Eltra area. Table 10.1 shows power load and power production capacities in the Eltra
area, and Table 10.2 shows the allocation of production in 2002.
The transmission grid in the Eltra area includes 712 km of 400 kV lines and 1739 km
of 150 kV lines. The Eltra area is connected to Norway and Sweden by high -voltage
direct-current (HVDC) links and to Germany by alte rnating current (AC) links (400 kV,
220 kV, 150 kV and 60 kV). The connection to the neighbouring countries has a total
capacity of about 3000 MW (see Figure 10.3).
The local combined heat and power (CHP) plants (about 700 units) and the wind
turbines are located in the distribution grid. The installed distributed generation (DG)
capacity thus amounts to about 50 % (see Figure 10.4).
Table 10.1 Power load and production capacities, end of
year 2002
Power (MW)
Local combined heat and power 1596
Wind power 2315
Primary thermal (extraction) units 3107
Peak load 3685
Typical offpeak load, summer 1400
Typical offpeak load, winter 1900
Minimum load 1189
Table 10.2 Distribution of production in the Eltra area, 2002
Energy (GWh) Percentage of
consumption
Primary (extraction) units 12 928 —
Local combined heat and power 6 723 32.2
Wind power 3 825 18.3
Total production 23 476 —
Foreign exchange 2 619 —
Consumption 20 858 —
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Sweden
5
8
0
M
W
1
0
0
0
M
W
HORNS REV
Germany
LÆSØ
0
950 MW
580 MW
1000 MW
1200 MW
800 MW
Norway
100 km
610 MW
HVDC
HVDC
A
B
400 kV
220 kV
150 kV
NVV
VHA
ADL
FER
IDU
TJE
LYK
SVS
LAG
KAS
ENS
FVO
FGD
MKS
MAL
TRI
Figure 10.3 Eltra’s transmission grid and capacities of foreign transmission links to Norway,
Sweden and Germany
International
International
International
400 kV (Eltra)
60 kV
10–20 kV (distribution undertaking)
Low voltage
4 primary power stations (1500 MW)
4 primary power stations (1600 MW)
Offshore wind turbines (160 MW)
17 local plants (600 MW)
50 wind turbines (30 MW)
711 local plants (1000 MW)
4938 wind turbines (2125 MW)
150 kV (transmission company)
Figure 10.4 Production capacities at each voltage level within the Eltra area
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A variety of DG types are connected to the Eltra system. Some examples are:
.
wind turbines, ranging from 11 kW to 2 MW;
.
gas-fired CHP units, ranging from 7 kW to 99 MW;
.
straw-fired CHP units, ranging from 2 MW to 19 MW;
.
coal-fired units, ranging from 18 MW to 44 MW;
.
waste-fired units, ranging from 90 kW to 26 MW.
The production system in Denmark has changed radically, as shown in Figure 10.5. The
traditional system was based on power stations situated in coastal areas with cooling-water
and coal-handling facilities. Now, the system has a great number of local CHP units and
wind power plants.
10.2 Operational Issues
Figure 10.2 shows the penetration of wind capacity in the Eltra area. Until the 1990s,
wind energy did not pose any major operational problem (i.e. it did not affect the
balancing task). Already in the mid-1980s, though, the European Commission invited
a number of member countries to participate in a study to investigate the effect of 5 %,
10 % and 15 % wind penetration on power system operation. The study for the Western
part of Denmark (EC, 1986) was based on the assumption that the de fined geographical
area should always be able to balance demand and generation and not depend on
neighbouring power systems. The capacity mix for the scenario with wind therefore
included gas turbines for situations with no wind.
Primary production
in the mid-1980s
Local production today
Primary units
Other units
Wind turbines
Primary units
Other units
Wind turbines
Figure 10.5 Basic changes in the Danish production system: from primary to local production
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In the mid-1980s, there were no signs of a Scandinavian-wide electricity market or a
European internal electricity market, even though the EU referendum had just been
accepted.
Managing an integrated system with substantial wind energy production and, at the
same time, taking into account the behaviour of the European wind regime requires a
concept that will guarantee the balance of demand and supply in the case of unforesee-
able wind speed variations.
The studies in the 1980s assumed that demand and supply would have to be balanced
within the Danish system. Today, the TSOs in Europe cooperate in different ways (e.g.
on real-time markets) in order to balance production and consumption in real time.
Historically, Nordel and UCTE have evolved differently regarding the balancing of
production and demand.
The synchronous Nordic system (Norway, Sweden, Finland and the eastern part of
Denmark) applies the following measures:
.
In the case of disturbances, half of the primary regulation is to be activated within 5 s
and the rest within 30 s. The primary regulation covers the loss of 1100 MW of
production and a self-regulation of 100 MW of the load (i.e. the volume must be
1000 MW).
.
Frequency regulation reserves of 6000 MW/Hz.
.
Manually activated regulating capacity, up or down, within 10 to 15 min.
This balancing philosophy has proved efficient for many years. Recently, however,
market activities with hourly schedules have demonstrated poorer frequency qualities
at hourly changes, and the TSOs are trying to compensate. An obvious solution would
be to switch the schedules to time steps of 15 min instead of one hour, as on the
continent.
In the UCTE area (see www.ucte.org), the balancing philosophy is based on the
following:
.
Primary regulation in the case of disturbances is to be a ctivated within 30 s. The
primary regulation covers the loss of 3000 MW of production and a certain degree of
self-regulation of the load. Based on the UCTE rules, the primary control capacity
within the Eltra area is 35 MW.
.
Frequency regulation reserves of 8000 to 15 000 MW/Hz.
.
Network controllers covering a certain geographical area: the network controller
integrates the total import–export to or from a particular area and controls the
schedules agreed between the different TSOs.
.
Manually activated regulating capacity up or down in severe situations.
Eltra uses the following facilities:
.
Day-ahead market at Nord Pool for balancing wind power production based on the
day-ahead forecast.
.
Regulating power market (the real-time market) for balancing within the hour prior
to operation. In the Eltra area, the secondary control capacity is currently 550 MW
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up, and 300 MW down. The manual secondary control consists of 300 MW long-term
contracts plus access to the regulating power markets in Sweden and Norway. In
general, the cheapest alternative for manual secondary control is used. However, in
the case of network congestion, the 300 MW long-term contracts must be used. From
2004 onwards, primary and secondary control will be purchased via a monthly tender
procedure instead of long-term contracts.
.
Fast-acting production technologies for fine adjusting the balance.
.
Network controller that activates the HVDC interconnection to Norway and will
replace a more traditional network controller that controls one or several power
stations.
Balancing wind production is like driving a big lorry in different situations:
.
In the case of stable wind conditions, it is like driving on the motorway at moderate
speed – no problem.
.
When front s pass, it is like driving in the countryside, on small winding roads – or
even under foggy conditions where you do not know if the road turns to the right or to
the left, but you still have to keep up the speed, you cannot stop and from time to time
you touch the ditch. This is similar to situations where the manual handling of the
regulating power goes wrong.
Within the framework of the European internal electricity market, renewable energy
and CHP production can be given a high priority. In the Danish system, end-consumers
currently have simply to accept their pro rata shares of priority production. This rule is
expected to end in 2004 and legislation is in preparation. Until then, Eltra as the TSO is
in charge of balancing the priority production.
In the future, a number of market players are expected to emerge in the context of
balancing priority production. Their approach will nevertheless be similar to the way
Eltra works today. Therefore, it is interesting to give an example of how Eltra handled
the situation up to 2004.
Before explaining the example, we have to describe the Nordic market model.
10.2.1 The Nordic market model for electricity trading
When the Nordic electricity market was set up, the Nordic parliaments of all the
countries involved (Sweden, Norway, Finland and Denmark) introduced similar legisla-
tion. In each country, a regulator was given an important role in implementing the
market opening. All the Nordic countries chose an approach with a TSO. The owner-
ship of the TSOs varies, though, ranging from state ownership via mixed private and
state shareholders to cooperatives (i.e. consumer-owned organisations).
Several fields of knowledge and experience have contributed to the development of
the Nordic electricity market. A market mechanism has been developed. Power and
energy balances still have to be maintained. Striving for compet ition has to be balanced
with maintaining the stability of the system. There must be the appropriate infrastruc-
ture to develop the market.
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The opening of all networks to all market players and the introduction of a point tariff
system were important parts of the market opening. There are several marketplaces.
Bilateral trade and the power exchange coexist to their mutual benefit. Brokers operate
separate marketplaces, and there is a large number of players in the retail market.
A number of service providers have emerged, such as analysis service providers,
information service providers, portfolio managers and balance responsible parties.
For further information, see also Tonjer et al. (2000), Granli et al. (2002) and
Rønningsbakk et al. (2001).
10.2.1.1 The Nordic electricity market
The common Nordic electricity market comprises the four countries Denmark, Finland,
Norway and Sweden. Iceland is also part of the Nordic community (e.g. it is in the
Nordel organisation) but it is not interconnected with the other countries. Table 10.3
shows key figures in the Nordic electricity system in 2001. The peak load of the
interconnected system was slightly less than 70 000 MW and, for the same area, the
total consumption was 393 TWh.
The population of the common trade area totals 24.0 million people, with 5.4 million
in Denmark, 5.2 million in Finland, 4.5 million in Norway and 8.9 million in
Sweden.
In 2001 the day-ahead power exchange (Elspot at Nord Pool) reached 111.2 TWh.
The same year, the financial trade (Eltermin and Eloption) at Nord Pool total led
909.8 TWh. Finally, 1748 TWh of bilateral contracts were cleared. The Elspot
trade amou nted to 28.3 % [(111:2/393) 100] of the total consumption in the market
area.
Table 10.3 Key figures in the Nordic electricity system, 2001
Denmark Finland Iceland Norway Sweden Nordel
Installed capacity (MW) 12 480 16 827 1 427 27 893 31 721 90 348
Consumption (GWh):
generation 36 009 71 645 8 028 121 872 157 803 395 357
imports 8 603 12 790 — 10 753 11 167 43 313
exports 9 180 2 831 — 7 161 18 458 37 630
total
a
35 432 81 604 8 028 125 464 150 512 401 040
Breakdown of electricity
generation (%):
hydro power 0 19 82 99 50 55
nuclear power — 31 — — 44 23
other thermal power 88 50 0 1 6 20
other renewable power 12 0 18 0 0 2
— No nuclear power production.
a
generation plus imports minus exports.
Note: 0 indicates less than 0.5 %.
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The border between Eltra and Germany is managed in a two-step procedure (for
more information on the web, see http://www. eltra.dk or http://www.eon-netz.com):
1. capacity auctioning (yearly, monthly and daily);
2. capacity used to transfer energy across the border based on hourly schedules.
10.2.2 Different markets
The Nordic electricity market consists of several markets: (1) the physical day-ahead
(Elspot) trade; (2) the hour-ahead (Elbas) trade and (3) the real-time market. In the
following, we will describe the characteristic features of these markets.
10.2.2.1 Elspot market
The ba sic philosophy of the Nordic day-ahead market (Elspot market) is as follows:
.
The power exchange Nord Pool is the common market place for physical Elspot
trading in Denmark, Finland, Norway and Sweden.
.
The transmission grid is the phy sical marketplace – the marketplace has a number of
players with equal access to the transmission system based on point tariffs.
.
The Nordic marketplace is one integrated market.
.
The TSO decides the available transfer capacity (ATC) between the potential price
areas.
.
The ATC is completely supplied to the market (Nord Pool). There are some inter-
connections between the Nordic marketplace and surrounding systems and markets,
which are handled only by the owners of these interconnections. According to a new
directive, the ‘use-it-or-lose-it’ principle has to be implemented for these interconnec-
tions.
.
Congestion between Elspot areas is handled by market splitting.
.
The Nordic system is operated by five TSOs whose aim it is to act, as far as possible,
as one TSO.
.
In the afternoon before the day of operation, hourly production schedules are set up
with the basic intention of balancing the system as a result of the Elspot trading.
.
Elbas trade and balancing activities start in the evening before the day of operation.
Table 10.4 shows the activities to be completed during the day before the 24-hour period
of operation.
10.2.2.2 Elbas market
The Elbas market is a physical market for power trading in hourly contracts, for
delivery the same day and the followin g day. It allows trading in Finland and Sweden
for those hours when Elspot trade has been accepted, covering individual hours up to
one hour before delivery. After clearing the Elspot market, the ATC between Finland
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and Sweden for the next day is announced and the market players can start submitting
their bids.
There is an ongoing discussion on whether to extend the Elbas market or not.
10.2.2.3 Real-time market
The Nordel Operations Committee has developed a new concept for cooperation
between the Nordic countries regarding the balance regulation. The goal has been to
create a common regulating power market for all countries. As part of the control
cooperation, the part of the system with the lowest control costs will be used for
regulation purposes. The rules and prices for the players in the different parts of the
system will be as harmonised as possible.
First, the new concept will be implemented in the synchronised system (on the Nordel
System, see http://www.nordel.org). Second, Jutland (Western Denmark), which
belongs to another control area, wi ll also be part of the cooperation.
10.2.2.4 Main principles
Each TSO receives regulating power bids within its system area and submits the bids to
NOIS (Nordic Operational Information System), which is a web-based information
system for the exchange of operational information between TSOs. In NOIS, a merit
order list of all regulating power bids is composed to form a ‘stairc ase’ that is visible to
all TSOs.
The balance regulation of the synchronised system is frequency-controlled. Control
measures are generally activated according to the order of operational costs, which can
be derived from the common list of the regulating power bids. At the end of each hour,
the common regulating power price is determined in accordance with the marginal price
Table 10.4 Daily routine
Time Routine
08:00–09:30 Capacity for market use is calculated based on the ‘use-it-or-lose-it’
principle across the Danish–German border
10:00–10:30 TSOs inform Nord Pool spot market about the ATC between
potential price areas within the Nord Pool market area
12:00 Deadline for bids and offers to Nord Pool Spot
13:00–14:00 Nord Pool Spot clears the market
14:00–19:00 Generators make final production plans
15:00–19:00 Production plans are submitted to the TSOs
16:00–19:00 Final production schedules are determined by the TSOs
16:00–19:30 Bids are made to the regulating power market (there may be changed
up to half an hour before the hour of operation)
20:00–24:00 Load forecasts are performed by TSOs
Note: ATC ¼ available transfer capacity; TSO ¼ transmission system operator.
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of operation. This price is used as the reference price. When settling the balance or
imbalance, it is included in the calculation of the settlement prices for all the subareas.
The subareas continue to use different models for balance settlement, though (one-price
or two-price models, marginal prices or middl e prices).
In the case of network congestion on the interconnections and within Norway, the
regulating power market is divided into different price areas. Bids for regu lating power
that are ‘locked in’ are excluded from the ‘staircase’. This way, there will be different
regulating power prices in different subareas.
The regulating TSO that performs the control action receives compensation for this
work. The price of balancing power between the subareas is settled at the common
regulating power price or the middle price, if these are different.
In future, Norwegian Statnett and Swedish Svenska Kraftna
¨
t will share the main
responsibility for frequency control in the Nordel area, in a similar way as today, and
will initiate the control procedures in the synchronised system. Western Denmark will be
basically managed as today, with a planned supportive power exchange. In that way,
this region will participate in the regulating power market. Eltra provides frequency
control within the UCTE system and consequently manages the balance control within
the Eltra area. The model has been in force since 1 September 2002.
10.2.3 Interaction between technical rules and the market
The procedure for determining the trading capacities is the key to the interaction
between market and system operation. A typical course of events may be as follows:
.
forecast expected utilisation of the interconnection s between Nordel and neighbour-
ing areas;
.
forecast load, wind production and other prod uction plans;
.
incorporate maintenance plans for production and transmission assets;
.
review limiting thermal v alues based on the n 1 criterion (n 1=safe operation even
if one component is missing);
.
review limiting dynamic values based on the forced outage of the largest unit or
dimensioning faults;
.
review voltage stability;
.
make calculations for certain points of time in the forthcoming 24-hour period;
.
determine permissible exchange capacities ba sed on the 24-hour calculations.
Thus, the TSO can ensure security of supply from a technical point of view. In practice,
this means that the TSO’s operational planning must ensure that:
.
it is possible to transport reserves through the system, in the case of forced outages or
other events that require a fast activation of production in order to avoid system
collapse – the reserves in question are instantaneous reserves that are activated
automatically within seconds;
.
the balance can be restored in the individual areas after major forced outages, in order
to avoid thermal overloa d or instability – the individual areas must have reserve
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capacity at their disposal, and activation of this capacity may take place automatically
or manually, according to instructions or otherwise;
.
temporary overloading of single components can be contained within their short time
ratings.
The transport capacity made available to the market players has to be maximised while
also taking into account system reliability aspects.
In Western Denmark, owing to internal congestion, the market capacity depends on
the amount of forecasted wind production. A new 400 kV line from mid-Jutland to the
north will substantially increase market capacity.
10.2.4 Example of how Eltra handles the balance task
The TSO’s major task is to secure a balance between production and demand. This can
be done:
.
in the range of seconds via automation;
.
in the range of minutes via automation and manually (telegraph);
.
on an hourly basis via trade in balancing power ;
.
on a daily basis via Nord Pool Elspot.
Normally, the balancing task is related to:
.
variations in demand;
.
tripping of power plants;
.
random variations in power output from production units .
In view of the increasing share of wind production, the fluctuating output from wind
production units creates a major challenge. Figure 10.6 shows a typical aggregated
output from all wind turbines in the area on a 15-minute basis.
The power output from wind production fluctuates as a result of the inherent power
characteristics and variations of wind speed. Figure 10.7 shows a general power curve
for a single turbine. If the wind speed fluctuates between 5 m/s and 15 m/s, this fluctu-
ation will result in a variation of the output from zero to maximum installed capacity.
Wind speed forecasts with reference to physical locations of installed wind capacity
are essential. Typically, meteorological institutes focus on forecasts of extreme values
and not on variations in the range between 5 m/s and 15 m/s. Considerable efforts have
to be made to improve the forecasts (see also Chapter 17).
In order to be able to use the present day-ahead market structure, given the oper-
ational requirements of the Elspot market, wind forecasts must define an hourly mean
wind speed up to 40 h ahead with reference to a geographical location. Weather-related
output from local CHP units must also be evaluated when balancing priority produc-
tion.
The factual output from a commercial wind turbine is more or less linear in the range
between 5 m/s and 13 m/s. Above 13 m/s, production levels out, and at about 20 m/s or
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