Europe: Impact of Dispersed and Renewable Generation On Power System Structure
Europe: Impact of Dispersed and Renewable Generation On Power System Structure
Europe: Impact of Dispersed and Renewable Generation On Power System Structure
291
X8
Europe: Impact of Dispersed and Renewable
Generation on Power System Structure
8.1 Introduction
In Europe the dependency on imported primary energy is increasing annually. As a
countermeasure against this growing dependency, national programs inside the European
Community are directed at increasing the share of renewable energy sources and the
efficiency of power generation by cogeneration of heat and power (CHP). Targets have been
set by the European Commission for each country to gain a sustainable electricity supply in
the future.
Generally, the share of renewable energy sources has to be increased by 2010 from 14% to
22% and the share of CHP has to be doubled from 9% to 18%.
Today approximately 50 GW of wind power are operated in Europe, and about 50 % of it is
located in Germany. Assuming that wind power production will grow primarily in the form
of large wind farms feeding into the transmission grids with an additional 35 GW installed
power by 2010, the dispersed generation based on CHP and small renewable sources shall
achieve an additional growth to meet the mentioned goals.
The output of most of the renewable energy sources depends on meteorological conditions
and the CHP output is driven by the demand for heat. The question arises, how can the
power system be operated with such a large share of mostly non-dispatched power sources?
How can the reserve power be limited, which is required for compensation of power
fluctuations and ensuring a safe network operation?
Thus, it has become clear that advanced planning and energy management approaches have
to be introduced to ensure that the existing high level of power quality will exist in the
future as well.
In this context, the power system of the future might consist of a number of self-balancing
distribution network areas. In each of these areas a significant share of the power demand
will be covered by renewable and CHP generation. However, the power balance of these
areas should be planable and dispatch able in such a way that the import or export of power
from or into the higher-level network has to follow a schedule, which can be predicted with
a high level of accuracy in advance.
As the result of this future set-up, the distribution networks will become active and have to
provide contributions to such system services like active power balancing, reactive power
control, islanded operation and black-start capability. These services have to be coordinated
with the transmission system operators where the responsibility for system stability will be
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allocated in the future as well. On the other hand, large-scale integration of wind power at
the transmission level combined with an international area for trading energy will lead to
higher utilization of the transmission grids. Consequently, the transmission capability has to
be strengthened and short-term congestions have to be managed in an efficient and
innovative way.
8.1.1 New Challenges
Each of these trends creates new challenges for power system operation on all of its levels
and requires the introduction of advanced and economic solutions concerning:
Supervisory control for congestion management
Real-time security assessment
Coordinated centralized and decentralized energy management including the unit
commitment based on predictions of fluctuating power sources, demand side and
storage management
Coordinated trade of energy and transmission capacity.
The new tasks require a significant growth of information exchange. Communication
networks using the existing infrastructure with different communication technologies like
radio channels, power line carrier, fiber optics or traditional telecommunication cables will
be the means of exchange. International communication standards shall be applied to
simplify the engineering and operation of these new types of communication networks.
Under these mentioned circumstances the interplay of transmission and distribution will
reach a new quality.
8.2 Distributed Generation: Challenges and Possible Solutions
Distributed generation (DG), for the moment loosely defined as small-scale electricity
generation, is a fairly new concept in electric energy markets, but the idea behind it is not
new at all. In the early days of electricity generation, distributed generation was the rule, not
the exception. The first power plants only supplied electric energy to customers connected
to the microgrid in their vicinity. The first grids were DC based, and therefore, the supply
voltage was limited, as was the distance covered between generator and consumer.
Balancing supply and demand was partially done using local storage, i.e. batteries, directly
coupled to the DC grid. Today, along with small-scale generation, local storage is also
returning to the scene.
Later, technological evolutions, such as transformers, led to the emergence of AC grids,
allowing for electric energy to be transported over longer distances, and economies of scale
in electricity generation led to an increase in the power output of the generation units. All
this resulted in increased convenience and lower per-unit costs. Large-scale interconnected
electricity systems were constructed, consisting of meshed transmission and radially
operated distribution grids, supplied by large central generation plants. Balancing supply
and demand was done by the averaging effect of the combination of large amounts of
instantaneously varying loads. The security of supply was guaranteed by the built-in
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redundancy. In fact, this interconnected high-voltage system made the economy of scale in
generation possible, with the present 1.5 GW nuclear power plants as a final stage in the
development. Storage is still present, with the best-known technology being pumped hydro
plants.
In the last decade, technological innovations and a changing economic and regulatory
environment resulted in a renewed interest for DG. This is confirmed by the IEA [1]. This
chapter presents the technical challenges and possible solutions when large amounts of
distributed generation are introduced.
8.2.1 Drivers for DG
The IEA identifies five major factors that contribute to the renewed interest in DG. These
five factors can be grouped under two major driving forces, i.e. electricity market
liberalization and environmental concerns. The developments in small-scale generation
technologies have been around for a long time, but were as such not capable of pushing the
economy of scale out of the system. Although it is sometimes indicated, it may be doubted
that DG is capable of postponing, and is certainly not capable of avoiding, the development
of new transmission lines, as, at the minimum, the grid has to be available as backup supply.
8.2.1.1 Liberalization of electricity markets
There is an increased interest from electricity suppliers in DG, because they see it as a tool
that can help them fill in niches in the market, in which customers look for the best-suited
electricity service. DG allows players in the electricity sector to respond in a flexible way to
changing market conditions. In liberalized markets, it is important to adapt to the changing
economic environment in the most flexible way. DG technologies in many cases provide
flexibility because of their small sizes and assumed short construction lead times compared
to most types of larger central power plants. However, the lead-time reduction is not always
that evident. For instance, public resistance to wind energy and use of landfill gasses may be
very high.
Many DG technologies are flexible in several respects: operation, size and expandability.
Making use of DG allows a flexible reaction to electricity price evolutions. DG then serves as
a hedge against these price fluctuations. Apparently, this is the major driver for the US
demand for DG, i.e. using DG for continuous or peaking use (peak shaving). The energy
efficiency is sometimes very debatable. In Europe, market demand for DG is, for the
moment, driven by heating applications (through CHP), the introduction of renewable
energies and potential efficiency improvements.
The second major driver of US demand for DG is quality of supply or reliability
considerations. Reliability problems refer to sustained interruptions, being voltage drops to
near zero (usually called outages). The liberalization of energy markets makes customers
more aware of the value of a reliable electricity supply. In many European countries, the
reliability level has been very high, although blackouts have occurred in recent years.
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Customers do not really care about supply interruptions, as they do not feel it as a great risk.
However, this may change in liberalized markets. A high reliability level implies high
investment and maintenance costs for the network and generation infrastructure. Because of
the incentives for cost-effectiveness that come from the introduction of competition in
generation and actions from regulators aiming at short-term tariff reductions for network
companies, it might be that reliability levels decrease. However, having a reliable power
supply is very important for society as a whole, and industry specifically (chemicals,
petroleum, refining, paper, metal, telecommunications, ). Companies may find the grid
reliability to be of an insufficient level and decide to invest in DG units in order to increase
overall reliability of supply to the desired level.
Apart from voltage drops to near zero (reliability problems), one can also have smaller
voltage deviations. The latter deviations are aspects of power quality. Power quality refers
to the degree to which power characteristics align with the ideal sinusoidal voltage and
current waveform, with current and voltage in balance [2]. Thus, strictly speaking, power
quality encompasses reliability.
Insufficient power quality can be caused by failures and switching operations in the grid,
mainly resulting in voltage dips, interruptions, and transients and by network disturbances
from loads yielding flicker (fast voltage variations), harmonics, and phase imbalance. The
nature of these disturbances is related to the short-circuit capacity, being a measure for the
internal impedance in the grid, depending on its internal configuration (e.g. length of the
lines, short-circuit capacity of generators and transformers) [3].
DG could partially serve as a substitute for investments in transmission and distribution
capacity (demand for DG from T&D companies) or as a bypass for transmission and
distribution costs (demand for DG from electricity customers). This is only possible to the
extent that alternative primary fuels are locally available in sufficient quantities. For
example, increased use of DG could result in new congestion problems in other networks,
such as the natural gas distribution network.
Finally, DG can also contribute in the provision of ancillary services, including those
necessary to maintain a sustained and stable grid operation of the customers. This may be
the capability of the grid operator to generate active power on demand, for instance to
stabilize a dropping frequency due to a sudden under capacity in generation or excess
demand, or reactive power to support the voltage.
8.2.1.2 Environmental Concerns
At present, environmental policies are probably the major driving force for the demand for
DG in Europe. Environmental regulations force players in the electricity market to look for
cleaner energy solutions. Here, DG can also play a role, as it allows optimizing energy
consumption of firms that have a large and constant demand for heat. Furthermore, most
government policies aiming to promote the use of renewables also results in an increased
impact of DG technologies, as renewables, except for large hydro and wind parks (certainly
off-shore), have a decentralized nature.
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In this way, conventional protection selectivity can be restored, guaranteeing person and
equipment safety. In the future, when more DG is used, this requirement would reduce
expected benefits of DG. To make optimal use of DG, unnecessary disconnection of DG
should be avoided. Generators should be able to ride through minor disturbances [6].
DG flows can reduce the effectiveness of protection equipment. Customers wanting to
operate in islanding mode during an outage must take into account important technical
(e.g. the capability to provide their own ancillary services) and safety considerations, such
that no power is supplied to the grid during the time of the outage. Once the distribution
grid is back in operation, the DG unit must be resynchronized with the grid voltage.
8.2.3 Voltage Quality and DG
Imbalances between demand and supply of electricity cause the system frequency to deviate
from its rated 50/60 Hz value. These deviations should be kept within very narrow margins,
since the proper functioning of many industrial and household applications depends on it.
In economic terms, system frequency can be considered as a public good. As a consequence,
the transmission grid operator is appointed to take care of the system frequency as well as of
other services with a public good character that need to be provided.
The installation and connection of DG units are also likely to affect the system frequency.
These units will free ride on the efforts of the transmission grid operator or the regulatory
body to maintain system frequency. They will probably have to increase their efforts and
have an impact on plants efficiency and emissions. Therefore, the connection of an
increasing number of DG units should be carefully evaluated and planned upfront.
The relation between DG and power quality is an ambiguous one. On the one hand, many
authors stress the beneficial effects of DG for power quality problems [1], including the
potential positive effects of DG for voltage support and power factor corrections [4].
On the other hand, large-scale introduction of decentralized power generating units may
lead to instability of the voltage profile: due to the bi-directional power flows and the
complicated reactive power equilibrium arising when insufficient control is introduced, the
voltage throughout the grid may fluctuate. Eventually an islanding situation may occur in
which a local generator keeps a part of a disconnected grid energized leading to dangerous
situations for the repair personnel coming in.
Others also stress the potential negative externalities on power quality, caused by the
installation of DG capacity. According to [7], the impact on the local voltage level of DG
connected to the distribution grid can be significant. The same reaction was noted through
the CIRED questionnaire [8], where, next to the general impact on power quality, a rise in
the voltage level in radial distribution systems is mentioned as one of the main technical
connection issues of DG. The IEA [1] also mentions voltage control as an issue when DG is
connected to the distribution grid. This does not need to be a problem when the grid
operator faces difficulties with low voltages, since in that case the DG unit can contribute to
the voltage support. But in other situations it can result in additional problems.
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Small and medium-sized DG units often use asynchronous generators that are not capable
of providing reactive power. Several options are available to solve this problem. On the
other hand, DG-units with a power electronic interface are sometimes capable of delivering
reactive power.
Some DG technologies (PV, fuel cells) produce direct current. Thus, these units must be
connected to the grid via a DCAC interface, which may contribute to higher harmonics.
Special technologies are also required for systems producing a variable frequency AC
voltage. Such power electronic interfaces have the disadvantage that they have virtually no
inertia, which can be regarded as a small energy buffer capable to match fast changes in the
power balance. Similar problems arise with variable wind speed machines [7].
8.2.4 Practical Distribution Network
An existing Belgian medium voltage distribution system segment has been used to study
the power quality and voltage stability with different DG units (Figure 8.2). The system
includes one transformer of 14 MVA, 70/10 kV and four cable feeders. The primary winding
of the transformer is connected to the transmission grid and can be considered as an infinite
node. Normal operation of the distribution system is in radial mode and the connections at
node 111 with feeders 2, 3 and 4 are normally open.
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Figure 8.4 illustrates the voltage at node 406 with different power generation levels and
power factors. Compared to the case where DG only injects active power or operates at the
unity power factor, synchronous generators raise the voltage of the system faster due to
reactive support. For induction generators, the voltage rise is slower and at a certain level of
power generation, the voltage starts to decrease. This is due to the fact that induction
generators need reactive power, yielding in a reduction of the voltage rise.
Through this study, it can be seen that the impact of induction generators is less than that of
synchronous ones in terms of voltage rise (Figure 8.5). If an over voltage occurs with a
synchronous generator, it has to operate under-excited and to absorb reactive power instead
of injecting it.
1.08
1.06
1.04
U (pu)
1.02
1.00
0.98
0.96
Syn 6MW
Syn 3MW
Ind 3 MW
Ind 6 MW
Base case
0.94
0.92
0.90
1
401
402
403
404
Node
405
406
407
408
Syn 0.95
Syn 0.98
PF = 1
1.20
U (pu)
1.15
Ind 0.95
Ind 0.90
Ind 0.85
1.10
1.05
1.00
0.95
0.90
0
DG power (MW)
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1.25
8 MW
7 MW
6 MW
5 MW
4 MW
3 MW
2 MW
1 MW
0 MW
1.15
U (pu)
299
1.05
0.95
0.85
Syn 0.95
Syn 0.98
PF = 1
Ind 0.95
Ind 0.90
Ind 0.85
Power Factor
Fig. 8.5. Voltage at Node 406 with Different Power Generation Levels
In order to see the voltage fluctuation problem with DG, a photovoltaic (PV) system is used.
The reactive power is produced by a capacitor of the inverters grid filter and is almost
constant. The PV system is treated as a PQ node with negative active power. The PV power
is calculated from 5-s average irradiance data measured during one year in Leuven,
Belgium. In this study, a PV array with 50 kW rated peak power is connected at node 304.
Figure 8.6 shows the one-hour power output of the PV system at noon of a slightly cloudy
summer day. In order to isolate the voltage fluctuation impact of PV from short-time load
variation at individual nodes, the loads are assumed constant during the calculation. The
total load in the system is 4.4 MW, 1.9 MVAr. In Figure 8.6, the voltage fluctuations
correspond to the variations of injected active power of the PV system. At times when
clouds cover the sun, the power generated can quickly drop by 60%, causing sudden
variations in node voltages in the range of 0.1%. The installed capacity of PV in this study is
rather low compared to the capacity of the distribution system and the loads, so the value of
voltage fluctuation is limited. However, with a high connection density or the connection of
a large PV system, the voltage fluctuation problem might become more severe.
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0.045
P (MW)
0.040
Q (MVar)
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
10
15
20
10
15
20
25
30
35
40
45
50
55
60
25
30
35
40
45
50
55
60
Time (minute)
1.0774
1.0773
U (pu)
1.0772
1.0771
1.0770
1.0769
1.0768
1.0767
1.0766
Time (minute)
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in the system and lasts for several seconds (Figure 8.9). It is due to an initial magnetizing
inrush transient and power transfer to bring the generator to its operating speed [9]. This
results in a major problem for sensitive loads connected near the DG. If the distribution
system is equipped with an under-voltage relay and the DG unit has islanding protection,
the voltage dip may lead to an action of the protection relay resulting in an outage of the
system. A soft-start circuit is required for large connected induction DG.
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for the other load characteristics. Compared to induction DG, the synchronous generator has
a larger impact on the voltage stability because of its capability of reactive power injection.
On the other hand, the influence of induction DG on voltage stability is not so different from
the base case (without DG).
Fig. 8.10. Static Voltage Stability at Node 111 with a Synchronous Generator
Fig. 8.11. Static Voltage Stability at Node 111 with an Induction Generator
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the future distribution networks will also have to contribute to the system services in
coordination with the transmission system. The idea of virtual power plants (VPP) will
become reality where a number of dispersed and renewable generation units (partially with
intermittent power output), storage units and controllable loads will be clustered and
managed in such a way that the power exchange with the outer world can be scheduled and
dispatched with a high level of accuracy. The decentralized energy management inside
VPPs requires communication facilities that are mostly not applied in todays practice of the
distribution system operation.
The efficiency of future communication networks at the distribution level requires some
basic principles.
In contrast to the existing practice, where power generation is located on a rather
concentrated area and, therefore, information and data is transferred on local networks or
field busses, the supervisory control and dispatching of dispersed generation will be spread
over a wide area. For economical reasons the pre-existing infrastructure has to be used; that
also means the utilization of different communication channels like radio, fiber optics,
power line carrier and telecommunication cables will be applied within one network as long
as they are available in the environment.
The communication over the different physical layers has to be compliant to a common
standard regarding data modeling and communication services. The main requirements for
such a standard are:
plug and play ability,
possibilities for mapping to different physical layers,
expandability of the data models and introduction of new models in accordance with
the new and enhanced communication tasks.
Thus, if the communication network for dispatching the VPP covers a whole distribution
network additional system services can be provided by the same network. Therefore,
communication tasks for distribution networks of the future include:
the contribution to the active power balancing through dispatch of power generation,
storage and controllable loads in the framework of a VPP,
the transfer of metered values as a support for the decentralized energy management
and for billing,
the provision of further system services like congestion management, reactive power
and voltage control, fault location, network recovery after faults, islanded operation,
black start capability etc.
The application of these ideas is investigated in the framework of the German Network for
Energy and Communication, a project sponsored by the German Ministry for Education
and Research.
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Households
Business
Farms
10 MWhth/
1 MWth
M
4x300 kW
G3
2x600 kWel/
1200 kWth
G
100x4 kW
G0
M
50x8 kW
G1
200 kWel/
500 kWth
Legende
Boiler/
Storage
200x2 kW
H0 22x2 kW 15x4 kW
Bio-CHP
200x2 kW
H0
24x2 kW
200x2 kW
H0
20x4 kW
200x2 kW
H0 14x2 kW 15x4 kW
M
5x20 kW
L0
60 kWel/
120 kWth
Solar cell
Fuel cell
+ -
Battery
Load
2x4 kW
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5s
2s
20 s
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The content and the classes of information exchange have to be defined for each active
component of the network - loads, generators, storage units, substation equipment. The
amount of data for communication varies by type. For example, only the metered value
will be communicated every 15 minutes for non-controllable loads or photovoltaic
units. On the other hand, the larger CHP plants provide 6 alarms, 24 event messages, 12
measured and 2 metered values, 6 controls, 2 target values as well as target profiles for
active and reactive power.
The volume of data transfer has to be defined in accordance with operational needs for
worst case and normal scenarios. In the normal case the metered values of all
components will be transferred in a 15 minutes interval. One time per day the target
profiles of the generation units above 100 kW will be communicated. Furthermore, 40
target values, 20 event messages, and 10 controls will be communicated. In the worstcase scenario (e.g. voltage dip) each component will send a report with alarms and
measured values, and this has to be performed within 5 s.
The selection of the communication protocol defines the data volume for each data
class. Chapter IV discusses special features of available IEC standards, in particular the
application of IEC 61850.
The selection of communication channels is based on their availability, a cost
comparison of different alternatives and the baud rates providing the performance in
worst case and normal scenarios.
The experience gained in initial pilot projects with VPPs [10], [11] underlined the need to
apply communication protocols based on common standards for all channels used.
Otherwise the engineering expenses will grow and the operation of the communication
network will become inconvenient.
8.3.3 Communication Standards
The first international standards for digital communication in power systems were
developed in the 1990s. These standards were limited regarding their plug and play ability.
Figure 8.13 gives an overview of the IEC standards for supervisory control in electric
networks.
Only the latest standard IEC 61850 for communication in substations (published as standard
in 2004) responds to the requirements of chapter II, topic 2.
The plug and play - ability is reached by the detailed object modeling based on logical
nodes (objects like circuit breaker or transformer etc.) and data (information like status
ON or Buchholz alarm etc.) with the supplement of different attributes (like time stamps,
validity information etc.) [12].
The mapping to different application layers was foreseen in the reference model of the
standard in accordance with Figure 8.14.
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Control
Center 1
IEC 608706-TASE.2
Network
Control
Center 2
103
Plug and play for limited
protection data
IEC 6087060870-5-101/104,
-6-TASE.2
Station
IEC 61850
101,104, TASE.2
TASE.2
IEC 608705-103
Bay
61850
Plug and play through
object modeling
Extendability through
building bricks
Process
Application
(ACSI, Abstract Communication
Service Interface)
SCSM 1
SCSM 2
SCSM n
AL 1
AL 2
AL n
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As a result of these features the standard IEC 61850 is suitable to serve as a general standard
for all communication tasks in power systems. Therefore, the basic rules and models of IEC
61850 are inherited in the following subsequent standards:
IEC 61400-25 for communication of wind power plants [13],
IEC 62350 for communication of dispersed generation [14].
As a goal of the new standards it was declared that all existing services and models of IEC
61850 would be taken over, as defined and only necessary extensions will be added.
In accordance with Figure 8.12 there will be a need to communicate information from wind
power plants, other D&RES and substation equipment over a common communication
network. Consequently, the consistency of the data models used is mandatory.
The relevant IEC working groups of TC 57 (62350) and TC 88 (61400-25) are requested to
ensure the consistency of all subsequent standards with IEC 61850. Otherwise there will be
no acceptance of the new standards from both the power automation industry and the
utilities.
8.3.4 Design of the Communication Network
IEC 61850 was analyzed regarding the size of telegrams for each data class. The results in
Table 8.1 present the worst case, which means the maximum possible number of bytes. In
practice the services of IEC 61850 create reports within a given time interval in which all
changed information is to be embedded. Therefore, the net bytes will be much lower then
stated. However, these figures build a good base for the communication network design.
The design task consists of the distribution of the communication clients over the possible
communication channels with minimum expenses and under the condition that the baud
rates of the selected channels ensure the required performance in worst case and normal
scenarios. A possible design of the communication network which meets the performance
requirements and combines different physical channels is shown in Figure 8.15. The large
CHP- plants of the industrial network play a significant role in the power balance of the
distribution network and impact the energy tariff of the industrial plant. They are connected
by a dedicated ISDN line that was available. The other generation and storage units in the
shopping and business area as well as the access to weather forecast data (for load and
renewable generation prediction) need only a dial up line. The wind power plant is
connected via a radio channel with the aim to combine this kind of communication with the
others.
The main load of communication is assigned to the Distribution Line Carrier (DLC), which
can reach baud rates higher than 300 kBd [15]. Over this channel the dispersed generation
units in the household and rural networks communicate, the metered values of all loads are
reported, the control commands for demand side management are sent out and the
equipment in the substations is incorporated to provide a new class of distribution system
management. For this network the installation of new communication lines was avoided.
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Data
class
Raw data
array
Overhead
Layer 7
(MMS)
Overhead
Other layers
Overall
Status
inform.
11
161
64
236
Control
14
1245
384
1643
Measured
value
15
161
64
240
Metered
value
15
161
64
240
Array (96
metered
values)
1440
1320
128
2888
15
693
192
900
480
388
128
996
Target value
Schedule (96
target
values)
Substations
VPP and
distribution
management
Boiler/ Storage
Battery,
G0
Bio CHP,
+ G
FC, G1
Mod.
Mod.
Mod.
Mod.
Mod.
Mod.
Weather
forecast
Metered values
all loads
Solar cells
G1, H0-1,3,4,L0
FC, H0-1,2,4
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Controllable loads
G3, G0,G1
311
1
4
2.1
2.2
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150 kV AC-lines to Germany. To the north, it is connected to the Nordel synchronous area
via HVDC links to Norway (1,000 MW) and Sweden (600 MW). The eastern Danish system
is operated at 400 kV and 132 kV, respectively, as a meshed transmission system with AC
connection to Sweden and HVDC connection to Germany.
Figure 8.17 and Figure 8.18 give the key figures of the Danish power system. The primary
power plants are thermal units, fired by coal or gas. A significant part of todays installed
capacity in the Danish system are decentralized units, such as wind turbines and combined
heat and power (CHP) units, mostly connected to the distribution grid. This combination
has resulted in a change of the classical hierarchical load flow structure - former passive
networks have become active networks due to the changed load flow direction, especially
on windy days.
In the western system the offshore wind farm Horns Rev A (HRA) with a rated power of 160
MW is connected to the 150 kV transmission system. The construction of the second offshore
wind farm, Horns Rev B (HRB), with a rated power of 215 MW should be finished by the
year 2009 [16].
In the eastern system another new big offshore wind plant with a rated power of 215 MW is
planned to be operating in 2009 -2010.
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Development of technical specifications for the grid connection of wind turbines that
are based on prior experience; e.g. requirements like fault-ride-through capability of
large offshore wind farms [17].
Constant improvement of wind power forecasts.
Long- and short-term balance for the Danish power system.
Responsibility for voltage stability and power quality.
Wind turbine modeling as part of the Danish power system model.
Preparation of the system for the implementation of more wind power.
8.4.2 Wind Energy
Large offshore turbines usually are located close to each other and show significant
correlation between their output powers. Experience from the operation of Horns Rev A
(HRA) shows that power fluctuations within 10-min intervals can be remarkable high due to
the concentration of wind power in a small area of about 20 km2 [16]. The power gradients
may reach values of 15 MW/min for this 160 MW wind farm resulting in changes of
generated power from none up to the rated power within 10 to 15 minutes. Without control
such power fluctuations may be introduced into the transmission system and even
distributed to the neighboring transmission systems.
A control system has been developed which reduces this effect [16]. This is achieved by
applying power gradient limits of the wind farm and by using secondary control of the
primary power plants and, additionally, using fast power control of HVCD. The main target
of keeping the power balance is to adjust power generation including power import and
power consumption, including power export, as well as keeping the power exchange
between western Denmark and the UCTE synchronous area at the planned level.
The high share of wind power within the system results sometimes in extreme requirements
for system operation due to the power fluctuations mentioned above.
An impressing example is the hurricane on the 8th of January 2005 that crossed the whole
area of Denmark resulting in a disconnection of nearly the total wind production (Figure
8.19). In this case, the system operator had to handle a record high imbalance between
schedule and production of more than 1,700 MW. Until now a sufficient amount of
regulating power has been available in the western Danish power system to compensate for
the intense power fluctuations from HRA by applying the load-frequency controller (LFC)
accessing the secondary control on the central power plants.
The second offshore wind farm HRB will be located very close to the existing wind farm
HRA. An analysis showed that it might be critical to compensate for the additional power
fluctuations using only the domestic regulating power 16]. A part of the power fluctuations
will be reduced by the offshore wind farms` control themselves. In the analysis HRB was
obliged to comply with the power gradient limit of +5 MW/min. Additionally, the use of
the fast power control of the HVDC- connections will be necessary to keep the power
balance in the western Danish power system. This requires applying the capacity of some of
the HVDC- links to the regulating power to compensate for the wind farms` fast power
fluctuations.
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However, the LFC control is not capable of handling the power regulation that is required
for the case shown in Figure 8.19.
Fig. 8.19. Need for Regulation in Western Danish System during the Passing of a Wind Front
Improving forecasting systems is one of the possibilities to improve the power balance.
Reliable wind forecasts are essential for power system operation in Denmark.
The planned active power from a wind farm is based on wind forecasts that are transferred
to active power forecasts. The first active power forecast is made a day ahead, but can be
updated during the day. The active power produced by wind farms is part of the power
supplied from a group of power plants available to the Power Balance Responsible Player
(PBRP). The PBRP controls the active power from this group of the power plants according
to the latest power forecast in a way that complies with the planned total power production.
Deviations between power forecast and the delivered total active power are injected into the
transmission system and should therefore be minimized.
The aggregated western Danish wind power curve (Figure 8.20) has a very high power
slope, resulting in a deviation of +320 MW for a +1 m/s wind velocity prediction fault
appearing between wind speeds of 5 and 15 m/s. A relieving factor is the regional
distribution of the wind turbines over the whole western Danish area.
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Fig. 8.20. Aggregated Wind Power Production Curve for Western Denmark
The wind forecast models have to be improved in several ways:
Improvement of day-to-day forecasts because the amount of grid incorporated wind
power is significant and still increasing (work in progress).
Improvement of hour-by-hour forecasts: they have to comply with the power balances
and planned operation of the power plants, planned power transits and consumption
(work in progress).
8.4.2.1 MELTRA
In 2002, Energinet.dk funded a research project on ensemble forecasting at University
College Cork (UCC), Ireland. In this context a real-time forecasting system called MELTRA
was designed to meet specifically set requirements in Energinet.dk. It consists of 75
ensemble members and a graphics package for visualization of the forecasts (Figure 8.21).
MELTRA has undergone many changes since its first implementation. The upgraded 2005
system generates 3-day forecasts every hour and consists of around 6000 forecasts per day.
Half of the forecasts are carried out as nested forecasts in higher resolution. The forecasts are
converted into probabilities and, in combination with observations, provide the best
possible forecasts of wind power. The MELTRA ensemble system is run on a 92 processor
Linux cluster, which is believed to be a very cost-effective hardware solution. The resolution
in the meteorological model is 45 km with a finer 5 km nested grid covering Denmark.
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Further, the establishment of an offshore transmission system connecting the large offshore
wind farms with the grids of Norway, Denmark, Germany and Holland may reduce the
impact onto the Danish transmission system.
8.4.3 CHP Units
Since the energy crisis of the 1970s, small-scale CHP power plants have been established to
supply local heating systems of small cities. Simultaneously industrial CHP units have been
installed. This concept has been followed until today resulting in a high share of dispersed
installed capacity, which is not as a matter of course available for power regulation and
thus, does not contribute to system balance.
The distributed CHP-units' range in size is from a few kW up to 100 MW. Most of these
units are gas turbines or gas engines. Traditionally the power production from these units
depends on the heat demand, thus heat and electricity are strongly coupled. To eliminate
this dependence, these units are equipped with heat storage tanks.
Most of the large thermal units are coal-fired CHP units that can extract steam for heat
production. These units have an operating domain between 20 % and full power load
without heat production. However, the operating domain for the power depends on the
heat production - with higher heat production the minimum power load increases and the
maximum power load decreases. According to the power station specifications [19], these
thermal units have a regulating capability of 4 % of full load/minute in the operating
domain from 50-90% and 2 % of full load/minute below 50 % and above 90 % load. Besides
the normal regulating capabilities these units can disconnect the heat production and, for a
short period, utilize the extracted steam for electricity generation.
Increasing security problems have led to a reconsideration of the traditional high degree of
independence between TSOs and DSOs (distribution system operators).
A new control strategy shall include all local grids with DG into new responsibilities, such as
control of reactive power, provision of data for security analyses, supervision of protection
schemes at local CHP plants, updating under-frequency load shedding schemes and new
restoration plans, including controlling dead start of local plants in emergency cases.
The implementation of such new responsibilities will require development of new control,
communication and information systems. During normal operation all functions should be
automatic. For emergency situations restoration plans have to be carefully prepared and
trained. The targets concerning the systems redesign are:
balance between supply and demand shall be ensured by sufficient available domestic
resources
operators need to have access to an improved knowledge of the actual system
conditions, both locally and centrally
efficient system control shall be available, especially during emergencies
Black start capabilities using local generators shall be provided.
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SIVAEL solves the week-plan problem on an hourly basis and finds the optimum load
dispatch with regard to start-stop, overhauls and outages. The optimum load occurs when
the total variable costs are at a minimum.
Figure 8.23 shows the wind energy production, the share that can be sold immediately and
the surplus electricity. It shows that the system can absorb about 30% of the wind power
with no surplus electricity. On the other hand, the surplus grows substantially when the
share of wind power is more than approximately 50%.
Following this idea, there will be two different residual markets: one for demand and one
for overflow. The SIVAEL-Model is calculated for a share of 100 % wind power with a
residual energy consumption of 8 TWh / year and a surplus energy of 8 TWh / year, thus
the resulting residual market has an energy volume of 16 TWh and a capacity differential of
about 9,000 MW. (Comparison: For a pure thermal system the volume of the electric energy
market equals 26 TWh and the demand for capacity about 4,500 MW.)
In the future this business area can be cultivated by market players, e.g. by means of
developing new products.
Fig. 8.23. Wind Power Production on an Annual Basis (TWh/year), the Share of Wind
Power that Can Be Sold for the Assumed Consumption (TWh/year) and the Remaining
Surplus
8.4.4.2 Demand response
The increasing share of wind energy has resulted in an increasing need for balance tools,
which also may be located on the demand side. Demand response is defined as a short-term
change in electricity consumption as a reaction to a market price signal [22]. The Nordel
study [23].identifies demand response as both an alternative and a prerequisite for
investments into new production capacity and recommends that all Nordic TSOs prepare
action plans for developing demand response.
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The TSO is responsible for maintaining the instantaneous balance between supply and
demand for each control area. The TSO agrees with the supplier on the amount of power
that has to be available at a certain time. If the reserve is activated it is financially
compensated for according to the suppliers bid. Sometimes energy is very cheap - even free
(Figure 8.24). It would be valuable to use this cheap energy rather than activating reserve
energy that has to be paid for and simultaneously exporting the wind energy.
A further expansion of wind power capacity makes only sense if consumption is increased
accordingly or thermal production can be reduced. Demand response manual reserves can
be activated by suppliers or consumers, whereas up regulation means interrupted
consumption and down regulation means extra consumption. If there is an unbalance in the
system, either the production can be increased or the consumption decreased or vice versa depending on the kind of unbalance. The smallest bid is 10 MW, and the price for being
available as reserve power for the system operator can be between 27,000 EUR/MW/year
and 67,000 EUR/MW/year for up regulation power and up to 20,000 EUR/MW/year for
down regulation power. Thus, not only supply, but also electricity consumption should
follow price signals. The former philosophy of influencing consumer behavior by means of
time-tariffs or campaigns is substituted by new market products, which illustrate the market
value of consumers` reaction and capitalize market gains. The system operator acts as a
catalyst promoting the consumers` price flexibility. By this means utilization of cheap wind
energy instead of valuable coal or oil shall be achieved. During Energinet.dk`s
demonstration projects, for some big customers like such as an iron foundry, it has turned
out to be economically efficient to install a parallel electricity based consumption system
which is used during times of extremely low prices for wind energy.
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In Denmark there is also a large technical potential for increased electricity consumption in
district heating systems to substitute fossil fuels during periods of heavy wind production.
Consequently, the substitution of primary resources is obtained and investments in noneconomic peak load units can be avoided. The respective change of consumer behavior can
be: moving the time of consumption to periods with lower prices; reducing or stopping
consumption during periods when consumer benefit from using electricity does not exceed
the price (possibly by means of substitution to another energy source); or increasing the
consumption during times when the electricity price is lower than the marginal utility and
the price of another energy source, e.g. during times of high wind production. This measure
results in a smaller slope of the demand curve where, due to limited demand response, there
may sometimes be no market clearing point found (Figure 8.25). An action plan has been
made including 22 specific initiatives aiming at the development of demand response in the
electricity market and all Nordic TSOs are cooperating on this topic [24].
Fig. 8.25. Supply and Demand Curve for Different Elasticity Coefficients due to Grade of
Demand Response
In summary, Section 8.4 has highlighted that the Danish system is facing various difficulties
on several levels: Technically, a high share of dispersed generation challenges the
transmission system operator who is responsible for reliability and security of supply and
constantly has to balance supply and demand. This is additionally complicated by high
transits passing through the system. Interconnections to neighboring countries are essential
for the functioning of the system, and a further expansion of the network as well as the
interconnections has to be planned carefully.
Referring to market requirements the Danish transmission system operator, being situated
in two synchronous areas operating with different schedules, has to adapt to both systems
and use the opportunities of the market to improve the national power balance situation by
means of the real time market.
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In Denmark a further wind energy expansion is expected, but it has been decided, that there
will be a maximum limit for the price at which energy can be sold. Consequently, the future
role of small-scale CHP units has to be newly defined aiming at better utilization through
operation on market terms.
Also, the use of electricity is being re-discussed. A demand response project illustrated the
potential of integrating the consumer into the well functioning of the market. For example,
in times of high wind production it can be economically efficient to use electricity for district
heating systems by using heat pumps or heat boilers.
8.5 Further Reading
Further reading on integrating dispersed renewable generation sources into European Grids
is given in References [25].
8.6 Acknowledgement
This Chapter has been prepared by Zbigniew A. Styczynski (Head and Chair of Electric
Power Networks and Renewable Energy Sources, Otto-von-Guericke University,
Magdeburg, Germany and President, Center of Renewable Energy Saxonia Anhalt,
Germany). Contributors include Johan Driesen and Ronnie Belmans (KU Leuven, Leuven,
Belgium), Bernd Michael Buchholz (Director, PTD Services, Power Technologies, Siemens
AG, Erlangen, Germany), Thomas J. Hammons (Chair International Practices for Energy
Developments and Power Generation IEEE, University of Glasgow, UK), and Peter B.
Eriksen, Antje G. Orths and Vladislav Akhmatov (Analysis and Methods, Energinet.dk,
Fjordvejen, Fredericia, Denmark)
8.7 References
[1] IEA, Distributed Generation in Liberalised Electricity Markets, Paris, 128 pages, 2002.
[2] Eto J., Koomey J., Lehman B., Martin N., Scoping Study on Trends in the Economic
Value of Electricity Reliability to the US Economy, LBLN-47911, Berkeley,2001, 134
pages.
[3] Renner H., Fickert L., 1999. Costs and responsibility of power quality in the deregulated
electricity market, Graz.
[4] Dondi P., Bayoumi D., Haederli C., Julian D., Suter M., Network integration of
distributed power generation, Journal of Power Sources, 106, 2002, pp.19.
[5] Woyte A., De Brabandere K., Van Dommelen D., Belmans R., Nijs J, International
harmonisation of grid connection guidelines: adequate requirements for the
prevention of unintentional islanding, Progress in Photovoltaics: Research
Applications, 2003, Vol.11, No.6, pp.407-424.
[6] Gatta F.M., Iliceto F., Lauria S. Masato P. Behaviour of dispersed generation in
distribution networks during system disturbances. Measures to prevent
disconnection, Proceedings CIRED 2003, Barcelona, 12-15 May 2003.
[7] Ackermann T., Andersson G., Soder L., Distributed generation: a definition, Electric
Power Systems Research, 57, 2001, 195204.
[8] CIRED, 1999: Dispersed generation, Preliminary report of CIRED working group WG04,
June, p. 9+Appendix (p.30).
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[9] Jenkins N., Allan R., Crossley P., Kirschen D., Strbac G., Embedded Generation, The
Institute of Electrical Engineers, London, 2000
[10] B. Buchholz a.o. Advanced planning and operation of dispersed generation ensuring
power quality, security and efficiency in distribution systems. CIGRE 2004, Paris,
29.August - 3.September 2004
[11] J. Scholtes, C. Schwaegerl. Energy Park KonWerl. Energy management of a
decentralized Supply system. Concept and First results. First international
conference on the integration of Renewable energy sources and Distributed energy
resources. Brussels, 1.-3. December 2004
[12] IEC 61850 Part 1-10. Communication networks and systems in substations
[13] IEC 612400-25-2. Wind Turbines. Communication for monitoring and control of wind
turbines. Part 25-2. Information models. IEC 88/214/CD
[14] IEC 62350. Communication systems for distributed energy resources. IEC 57/750/CD
[15] Bumiller, G., Sauter, T., Pratl, G. Treydl, A. Secure and reliable wide area power line
communication for soft real- time applications within REMPLI. 2005 International
Symposium on Power Line Communications and its Applications, Vancouver,
April 6-8 2005
[16] V. Akhmatov; H. Abildgaard; J. Pedersen; P. B. Eriksen: "Integration of Offshore Wind
Power into the Western Danish Power System" in Proc. 2005 Copenhagen Offshore
Wind International conference and Exhibition, October 2005, Copenhagen,
Denmark.
[17] Specifications TF 3.2.5, "Connection Requirements for Wind Turbines connected to
voltages over 100 kV" (in Danish) Available: http://www.energinet.dk.
[18] P. B. Eriksen; Th. Ackermann; H. Abildgaard et. al.: "System Operation with High
Wind Penetration", IEEE Power and Energy Magazine, vol 3 No. 5, pp 65-74, Nov.
2005.
[19] Power Station Specifications for Plants > 50 MW, Elsam, Denmark, SP92-230j, 16 pages
+ 3 pg annex, August 1998; Kraftvrskspecifikationer for produktionsanlg
mellem 2 og 50 MW: Elsam, Denmark, SP92-017a, 16 sider + 5 sider bilag,
september 1995 (in Danish).
[20] P. Lund, S. Cherian, T. Ackermann: "A Cell Controller for Autonomous Operation of a
60 kV Distribution Area" in Proc. 10th Kasseler Symposium Energie-Systemtechnik
2005, ISET, Kassel. pp. 66-85.
[21] J. Pedersen: "System and Market Changes in a Scenario of Increased Wind Power
Production " in Proc. 2005 Copenhagen Offshore Wind International conference
and Exhibition, October 2005, Copenhagen, Denmark.
[22] K. Behnke, S. Dupont Kristensen: "Nordel - Danish Action Plan for Demand response",
Elkraft/ eltra, Nov. 2004 (intern document)
[23] ["Enhancing Efficient Functioning of the Nordic Electricity market", Nordel, Februar
2005. Available: http://www.Nordel.org.
[24] "Ensuring Balance between Demand and Supply in the Nordic Electricity Market",
Nordel, 2004, Available: http://www.Nordel.org.
[25] T. J. Hammons: Integrating Renewable Energy Sources into European Grids,
International Journal of Electrical Power and Energy Systems, vol. 30, (8), 2008, pp.
462-475.
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ISBN 978-953-307-155-8
Hard cover, 802 pages
Publisher InTech
and the DIC, and Ph.D. degrees from Imperial College, London, UK He is a member of the teaching faculty of
the School of Engineering, University of Glasgow, Scotland, UK. He was Professor of Electrical and Computer
Engineering at McMaster University, Hamilton, Ontario, Canada in 1978-1979. He is the author/co-author of
over 440 scientific articles and papers on electrical power engineering and is Editor of a book on Renewable
Energy that was published by INTECH in December 2009. He has lectured extensively in North America,
Africa, Asia, and both in Eastern and Western Europe.
Dr Hammons is Past Chair of the United Kingdom and Republic of Ireland (UKRI) Section IEEE and Past Chair
of International Practices for Energy Development and Power Generation of IEEE. He is also a Past Chair of
the IEEE PES Task Force on harmonizing power-engineering standards worldwide and Past Permanent
Secretary of the International Universities Power Engineering Conference. He is a Chartered Engineer (CEng)
and a registered European Engineer in the Federation of National Engineering Associations in Europe.
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