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

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

Energy production capability - initial data

Energy production capability - test results 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

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