a method to increase the integration capacity of dg on weak distribution network

1Sami Repo Finland Session 4 – Block 4

Barcelona 12-15 May 2003

A method to increase the integration capacity of DG on

weak distribution network

Sami Repo

Tampere University of Technology

FINLAND



Problem of weak distribution network

• Weak medium voltage (MV) network– length of 20 kV feeders may be 100 km– demand in sparsely populated areas is very low

=> impedance of feeders may be remarkably high

• Integration of DG is limited by voltage rise– problems may occur during low demand periods– acute in coastal and skerries areas due to wind

power installations



Proposed method to increase the integration capacity of DG units

1. Meshed operation of distribution network– MV networks are constructed meshed for reasons

of reliability, but operated radially

2. Control of DG unit active and reactive power– based on terminal voltage at the case studied – consumption of reactive power is used to limit the

voltage rise problem when necessary– the production of active power must also be

limited, if this is not enough



Meshed operation of MV network

• will divide power flows to the feeders more evenly than radial operation

• losses will be minimised

• voltage rise due to DG can be limited

• may allow more enhanced control and operation principles than today

• should be taken into account in most potential areas of DG in long-term planning studies



Protection of meshed MV network

• When a fault occurs the task is to indicate and locate the faulty ring (see session 3 paper 25)

• Ring is opened using fast switch-disconnectors

• Conventional feeder protection may operate

• The amount of load interrupted due to the fault is the same as in a radially operated network

• Extra delay must be allowed for in the protection



Studied distribution network



Study case data

Wind power units:• connected 22 km away

from HV/MV substation

• 4*750 kW• frequency converters• power factor control

between 0.92 - 1.00 ind. or cap.

Network data:• Upper limit of MV

voltage is 21 kV• Maximum production

capacity is 1.1 MW

Loads:• real hourly load curves

and customer data (annual energy and customer type)



Network transfer capability

Load ofKasnäsfeeder [kW]

Maximumproduction [kW]

Radial Ring

Reactive power ofgenerator [kVar]

Radial Ring

364 1120 1900 -480 -810

572 1300 2100 -553 -895

825 1550 2500 -660 -1065

1174 2000 2900 -852 -1150



Energy production capability- initial data

0 5 10 150

100

200

300

400

500

Wind speed [m/s]

Fre

quen

cy

0 500 1000 1500 2000 2500 30000

200

400

600

800

1000

Power output [kW]

Fre

quen

cy

0 1000 2000 3000 4000 5000 6000 7000 8000300

400

500

600

700

800

900

1000

1100

1200

Time [h]

Load

[kW

]

Load duration curve 1Load duration curve 2Load duration curve 3

Minimum load = 364 kW

Maximum load = 1174 kW



Energy production capability- test results

1528 1556 1591

56

2

58

5

61

4

36

,6

47

,8

60

,8

0

200

400

600

800

1000

1200

1400

1600

1 2 3

Load duration curves

Ele

ctri

city

no

t p

rod

uce

d [

MW

h]

3 MW

2.25 MW

1.5 MW

321

356

389

11,5 22,9

34,2

0 0 00

50

100

150

200

250

300

350

400

1 2 3

Load duration curves

Ele

ctric

ity n

ot p

rodu

ced

[MW

h]

3 MW

2.25 MW

1.5 MW

Radial Ring



Conclusions

• Integration capacity of DG on weak MV network may be increased by proposed method– no centralised control system or operator

interaction is needed to operate the network

– it may be concluded by DG unit terminal voltage

• Requirements of the method are possibility– to operate network as a ring with fast switch-

disconnector

– to control active and reactive power of DG unit



Practical issues

• Possibility to control the integration capacity should be considered in interconnection contract

• Proposed method is quite different from the "minimum load maximum production" planning principle because the integration capacity is flexible and dependent on network conditions

• Further research is needed to clarify impacts on power quality and to extend studies to more complex systems



Thank you!

Please visit

www.tut.fi/~repo/pub.html

to have a full version of the paper (several lines are overlapped with a figure at CD).

a method to increase the integration capacity of dg on weak distribution network

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