A method to increase the integration capacity of DG on weak distribution network

1. 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. 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) Study case data

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

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