1. Sami Repo Tampere University of Technology FINLAND
A method to increase the integration capacity of DG on weak distribution network
Sami Repo
Tampere University of Technology
FINLAND

2. 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

3. 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

4. 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

5. 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

6. Studied distribution network

7. Study case data Wind power units:
connected 22 km away from HV/MV substation
4*750 kW
frequency converters
power factor control between 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)

8. Network transfer capability

9. Energy production capability - initial data

10. Energy production capability - test results
Radial
Ring

11. 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

12. 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

13. 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).