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17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
A METHOD TO INCREASE THE INTEGRATION CAPACITY OF DISTRIBUTED GENERATION
ON WEAK DISTRIBUTION NETWORKS
Sami REPO, Ari NIKANDER, Hannu LAAKSONEN, Pertti JÄRVENTAUSTA
Tampere University of Technology - Finland
sami.repo@tut.fi
ABSTRACT
The paper investigates the effects and potential of distributed
generation (DG) on the operation of weak distribution
networks. It focuses on the development of a new method to
increase the integration capacity of DG in an existing
network. The paper considers voltage issues and the load
transfer capability of distribution network including
windmills. The proposed method is based on ring operation
of the distribution network and control of windmill active and
reactive power. The applicability of the proposed method is
tested with load-flow simulations on a real life distribution
network and planned windmills. The studies have proved the
capability of the proposed method to increase the integration
capacity of DG units without major network investments.
1. INTRODUCTION
The fast propagation of DG at distribution network will have
a major influence on distribution networks. This implies many
new factors which are not usually considered. The experience
of distribution network companies of these topics is so far
slight. However, these topics will become a part of normal
operation, not just special cases. This will necessitate the
existing networks and operation methods in order to consider
DG in the distribution network.
The voltage drop of a medium voltage network has been one
of the main planning issues in rural distribution networks in
Finland [1]. There are networks where the length of 20 kV
feeders is as much as 100 kilometers and the demand in
sparsely populated areas is very low. The impedance of these
feeders is remarkably high. If a large DG unit is installed far
from a primary substation, voltage rise problems may occur
during low demand periods [2]. The voltage rise problem is
acute in coastal and skerries areas due to wind power
installations. The integration capacity of DG units on weak
distribution network is limited by the voltage rise problem.
The propagation of DG may be seen either as a change of
load curves when production replaces a part of the load or as
an addition of production units to the distribution network.
Some changes in the structure of distribution network are
needed due to DG. The structure and operating principle of
the distribution network will move towards a distribution
system (active distribution network) with several active
components along the network. Meshed operation of
distribution network may allow more enhanced control and
operation principles than today. These issues at least should
be taken into account in most potential areas of DG in
long-term planning studies [2].
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Session 4 Paper No 60
2. PROPOSED METHOD
Medium voltage networks are normally operated radially
although they are constructed meshed for reasons of
reliability. Typically a rural overhead medium voltage
network includes remotely controlled disconnectors at
important nodes where radial feeders may be connected.
The proposed method is based on meshed operation of
distribution network and control of DG unit active and
reactive power. The meshed operation of the distribution
network will divide power flows to the feeders more evenly
than radial operation, losses will be minimised and voltage
rise due to DG can be limited.
The integration capacity of a DG unit may also be increased
by controlling the active and reactive power of the DG unit.
At least for the case studied the control of the DG unit may be
based on terminal voltage. The consumption of reactive
power is used to limit the voltage rise problem when
necessary. If this is not enough, the production of active
power must also be limited. This possibility to control the
integration capacity should be considered in the
interconnection contract. The control of DG unit may benefit
both parties when network investments are avoided. The
proposed method is quite different from the “minimum load
maximum production” planning principle because the
integration capacity is flexible and dependent on network
conditions.
In ring operation the protection of feeders may be based on
existing protection devices, if extra delay is allowed for in the
protection [3]. The proposed protection solution is
straightforward and economically attractive. When a fault
occurs the task is to indicate and locate the faulty ring. When
the ring is opened the conventional feeder protection may
operate. The amount of load interrupted due to the fault is the
same as in a radially operated network. The proposed solution
requires some changes in the feeder terminals software and
replacement of disconnectors with fast switch-disconnectors
at remotely controlled nodes.
3. TEST RESULTS
3.1 Test system
The method was tested with a case study on a distribution
network of Fortum Sähkönsiirto Oy in south-west Finland
including windmills in archipelago skerries. The distribution
network in this area is relatively weak, because the length of
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medium voltage lines (20 kV) is long and the amount of load
is small. There are also considerable restrictions for network
expansions due to environmental reasons. The calculations
are based on real hourly load curves and network data.
The network studied (Figure 1) consists of one HV/MV
substation, which feeds two MV feeders (Kasnäs and
Byholmen). The other short MV feeders of the substation
were not included in the network model used. Calculations
were carried out with an equivalent network, where some of
the branch conductors of the MV network are omitted. In the
PowerWorld [4] case the constant current loads were located
in the middle of a certain part of the feeder. These loads were
summed up from that particular part of the feeder (based on
different customer groups). For the estimation of load demand
customers’ annual unit consumptions and customer group
based load curves are needed. The complete load model
includes mean values and variations of power demand for
every hour of the year. The models are presented as 365*24
tables, known also as topographies. The load curve models
for 46 different customer groups published by Association of
Finnish Electric Utilities are the most extensive collection of
load models collected in Finland [5].The voltage of the lowervoltage side of the HV/MV transformer (110/21 kV, 16
MVA) was maintained at 20.4 kV with on-line tap-changer.
The Kasnäs feeder is 71 km long and based on information
from the network information system its minimum load was
364 kW (Sunday 18 August 03:00 a.m., simultaneously the
load of the Byholmen feeder was 350 kW) and maximum load
was 1174 kW (Monday 18 February 12:00 p.m., the load of
the Byholmen feeder was simultaneously 800 kW). Wind
turbines (4*750 kW) will be constructed on the island of
Högsåra and they are to be connected to the Kasnäs feeder 22
Kasnäs
0,065 MW
0,019 MVR
4
3
2
1
20,41 kV 20/110 kV 110 kV
0,9810
20,60 kV
23
20
5
22
21,55 kV 6
18
21,41 kV
0,096 MW
0,028 MVR
21,50 kV
15
21,54 kV
20,37 kV
20,38 kV
20,35 kV
26
28
0,030 MW
0,009 MVR
0,074 MW
0,021 MVR24
20,36 kV
12
0,027 MW
0,007 MVR
7
11
0,013 MW
0,004 MVR
0,160 MW
0,047 MVR
21,42 kV 16
21,68 kV
0,040 MW
0,012 MVR
25
21,26 kV
-2,03 MW
0,06 MVR
Byholmen
20,38 kV
19
21,76 kV
20,95 kV
Slack bus
110,08 kV110,0 kV
0,051 MW
0,015 MVR
21,11 kV
Wind mill
8
20,39 kV
0,063 MW
0,018 MVR
10
20,79 kV
21
3,00 MW 9
-1,28 MVR
Barcelona, 12-15 May 2003
20,33 kV
20,31 kV
0,032 MW
17
0,009 MVR
20,31 kV
0,006 MW
0,002 MVR
13
20,32 kV
14
0,071 MW
0,020 MVR27
20,31 kV
20,31 kV
Figure 1. Study case “minimum load of Kasnäs feeder and maximum production of wind mill”.
Table 1. Network transfer capabilities.
Load of KasnäsMaximum production [kW]
feeder [kW]
Radial
Ring
364
1120
1900
473
1250
2000
572
1300
2100
690
1350
2300
825
1550
2500
992
1750
2700
1174
2000
2900
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Minimum voltage [kV]
Radial
Ring
20,30
20,40
20,23
20,39
20,19
20,40
20,19
20,40
20,05
20,40
19,97
20,40
19,95
20,40
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Reactive power of generator [kVar]
Radial
Ring
-480
-810
-530
-852
-553
-895
-575
-980
-660
-1065
-745
-1150
-852
-1150
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km away from the HV/MV substation. The windmills will be
equipped with frequency converters, which allows power
factor control between 0.92 – 1.00 inductive or capacitive.
The Byholmen feeder is 53 km long and its calculated
minimum load was 300 kW and maximum load was 1490
kW.
Figure 1 represents the “minimum load of the Kasnäs feeder
and maximum production of the windmill” in the study case.
The upper limit of MV voltage is 21 kV, which is exceeded
in several locations although the generator is consuming
reactive power at full capability. The maximum production
capacity is 1.1 MW according to network transfer capability.
The reactive power consumed at the generator is coming from
a 44 km long sea cable between nodes 18-7-6 and a few other
shorter cables.
3.2 Ring operation
The ring operation of the feeders at Kasnäs and Byholmen
may be realised with an automatic switch-disconnector.
Currently node 12 includes a remotely controlled
disconnector, which does not have capability to switch fault
currents. However, the ring operation of these feeders has a
remarkable advantage compared to radial operation of
feeders. The network transfer capability for the windmill
studied may be increased from 1.1 MW to 1.9 MW by
operating the network as a ring in the minimum load case
(Table 1). The network transfer capability increases as a
function of the Kasnäs-feeder load. The network transfer
capability during maximum load condition is 2.0 MW and 2.9
MW for radial and ring operated networks respectively.
The voltage profile of the radial network is very different at
the Kasnäs- and Byholmen feeders. The voltage of the Kasnäs
feeder is very high in all cases and at the same time the
voltage of the Byholmen feeder is determined by the loading
of the feeder. The control of voltage at the primary substation
must be a compromise between these two competing tasks.
However, the ring operation of the network may be used to
balance the voltage profile at these feeders. The lowest
voltage appears at the primary substation in all cases, which
indicates a possibility for reduction of voltage profile and
enhancement of network transfer capability.
Barcelona, 12-15 May 2003
in a similar way.
3.3 Consideration of energy production capability
The energy production capability of the windmill and
distribution network studied is considered in both operation
modes and compared, this enabling the technical and
economic considerations to be combined.
The wind is assumed to be stochastic in this study. The
distribution of wind speed is modelled with two normal
distributions, which have expectation values 5.5 and 8 m/s,
and standard deviations 2 and 1.5 m/s. These parameters are
based on approximations of monthly wind statistics from
similar locations [6]. Two distributions are used to model
annual variation of wind speed. The output power of the
windmill is determined by the wind speed – power curve.
The annual energy produced in standard atmospheric
conditions would be 7868 MWh (capacity factor 0.3) when
the availability of units is 100 %. The distribution of wind
speed and power output of windmills is shown in Figure 2.
The network transfer capability may be determined in this
case by the Kasnäs feeder load in planning (Table 1) and by
windmill terminal voltage in operation. In order to calculate
the amount of electricity not produced due to output power
limitations a certain load duration curve (Figure 3) must be
assumed. The output power of the windmills (Figure 2) is
compared to the network transfer capability (Table 1) in
different load conditions taken from the load duration curve.
The amount of electricity not produced is the difference
between the output and the capability. If the real load and
wind curves are known, more precise results may be
calculated than with the load duration curve and stochastic
wind.
500
400
Frequency
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300
200
100
0
0
5
10
15
Wind speed [m/s]
1000
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Session 4 Paper No 60
800
Frequency
The generator consumes reactive power at full capacity
(0.92ind.) in all cases. Although the losses of generation units
will be increased when the generator is consuming reactive
power, the consumption of reactive power is advantageous for
the network transfer capability. The control of reactive power
may be based on windmill terminal voltage, because the
highest voltage always appears at the windmill terminal when
it is operating. If the terminal voltage exceeds the maximum
limit, the consumption of reactive power is increased. If this
is not enough, i.e. if the voltage still exceeds the maximum
limit, then the active power must be limited. In this way the
integration capacity may be higher than the capacity based
only in worst case conditions. The control of voltage and
reactive power may be realised at the minimum voltage limit
600
400
200
0
0
500
1000
1500
2000
Power output [kW]
2500
3000
Figure 2. Distribution of annual wind speed and power
output for all units.
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Barcelona, 12-15 May 2003
389
400
356
Load duration curve 1
Load duration curve 2
Load duration curve 3
Load [kW]
900
800
700
600
500
400
300
Minimum load = 364 kW
250
3 MW
200
2.25 MW
1.5 MW
150
100
50
0
1
0
1000
2000
3000
4000
5000
Time [h]
6000
7000
8000
Figure 3. Load duration curves.
The amounts of electricity not produced are presented in
Figures 4 and 5 in radial and ring operation modes for three
different load duration curves and integration capacities. The
distribution network operation mode has a strong influence on
the amount of electricity not produced. The influence of load
duration curve, however, is minor. The difference in the
amount of electricity not produced between operation modes
with 3 MW integration capacity is about 1200 MWh or
27400 € (using year 2001 monthly average of Nordpool
elspot-price, 22.83 €/MWh). The difference in the amount of
electricity not produced between 3 and 2.25 MW integration
capacities is about 1000 and 300 MWh with radial and ring
operation respectively. It is also interesting to note that the
amount of electricity not produced is extremely low when the
integration capacity is 1.5 MW in radial operation. When this
is compared to the network transfer capability (1120 kW)
determined by the “minimum load maximum production”
planning principle, there seems to be a great opportunity to
increase wind power production by changing the distribution
network planning principle.
1600
1591
1556
1528
1400
1200
1000
2.25 MW
614
585
600
3 MW
562
800
1.5 MW
47,8
200
60,8
400
36,6
Electricity not produced [MWh]
321
300
0
1
2
3
Load duration curves
Figure 4. Electricity not produced in radial network for three
different load duration curves and integration capacities.
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Session 4 Paper No 60
0
34,2
1000
350
11,5
1100
Electricity not produced [MWh]
Maximum load = 1174 kW
22,9
1200
0
2
0
3
Load duration curves
Figure 5. Electricity not produced in ring operation for three
different load duration curves and integration capacities.
4. DISCUSSION
The proposed method has been proved to increase the
integration capacity of windmills on a weak distribution
network. The requirements of the method are a possibility to
operate the network as a ring with fast switch-disconnector
and to control the active and reactive power of the windmill.
No centralised control system or operator interaction is
needed to operate the network in this case. The integration
capacity of windmills may be concluded by the windmill
terminal voltage.
The distribution network planning and calculation of windmill
integration capacity based on local wind statistics gives more
flexibility for planning decisions compared to the “minimum
load maximum production” planning principle. The decision
on integration capacity may also be easily converted into
financial terms. The planning decisions of the distribution
network and wind power production may be based on
calculations of network transfer capabilities and electricity not
produced with different amounts of integration capacities,
capacity factors, wind statistics, load duration curves, etc.
There are, however, a few open questions. One major
question is the influence of the method on power quality.
Although the number of interruptions would be the same for
the whole network, the number and duration of interruptions
seen from the customer side may increase. The reliability of
fast switch-disconnector and protective devices also has a
central role for the safety and reliability of the whole network.
Voltage quality problems such as voltage dips, flicker and
harmonics may also disturb a larger area than previously.
Another question is what happens when the ring of two or
more feeders is switched to radial operation. There exists a
danger of over-voltages at the windmill terminals if the
control of the windmill is not fast enough or the switching
operation is not co-ordinated with windmill operation. At the
same time the voltages of other switched feeders would drop
decisively. In any case there would be large and sudden
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changes in the voltage levels due to switching. This may
stress the network operator during storms due to repeated
switching operations. Thus it may be easier to operate the
network in the radial operation mode during periods of
repeated switching operations. In normal operation the
voltage control is more precise and the voltage level is closer
to nominal with the proposed method than with traditional
radial operation.
The research of the method proposed should also extended to
more complex systems. It is not certain that the method
proposed would work properly in cases with several
windmills at different locations, with different production
types or with more complicated network structure. There is
also a potential to improve the voltage / reactive power
control of the system and enhance the network transfer
capability in this way.
5. CONCLUSIONS
The proposed method based on the ring operation of a
distribution network and control of active and reactive power
of a windmill has been tested with a real life distribution
network and planned windmills. The requirements of the
method are a possibility to operate the network as a ring with
fast switch-disconnector and to control the active and reactive
power of the windmill. No centralised control system or
operator interaction is needed to operate the network in this
case. The integration capacity of windmills may be concluded
by the windmill terminal voltage. The integration capacity and
network transfer capability can be increased by the use of the
proposed method in the weak distribution network studied.
Barcelona, 12-15 May 2003
ACKNOWLEDGEMENTS
The authors are grateful to the Fortum Sähkönsiirto Oy for the
interesting real life research topic, comments and research
co-operation.
REFERENCES
[1] E. Lakervi and E.J. Holmes, 1995, Electricity distribution
network design, IEE Power series 21, London, UK, 325
p.
[2] N. Jenkins, et al., 2000, Embedded generation, IEE Power
and Energy series 31, London, UK, 273 p.
[3] A. Nikarder, et al., 2003, “Utilizing the ring operation
mode of medium voltage distribution network”,
Proceedings of 17th International Conference on
Electricity Distribution, CIRED 2003
[4] PowerWorld simulator User’s guide, version 7.0, 2000
[5] A. Seppälä, 1996, Load research and load estimation in
electricity distribution, VTT Publications 289, Espoo,
Finland, 118 p., (http://www.enease.fi/asepthes.pdf)
[6] T. Laakso and H. Holttinen, Wind energy statistics in
Finland - Monthly statistics,
http://www.vtt.fi/pro/pro4/pro42/tuulitilastot/monthly.htm
The proposed method allows more flexibility in distribution
network and windmill planning than conventional the
“minimum load maximum production” planning principle.
The advantages of ring operation compared to radial
operation may be evaluated by comparing initial data of the
analysis. In the case studied the electricity not produced with
3 MW integrated capacity respectively is 4.5 % and 19.8 %
of annual energy production capability when the network is
operated in ring and radial operation mode.
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