Effect of grid disturbances on fault-ride-through behaviour of MV-connected DG-units, in especially CHP-plants

In the near future a significant amount of the consumed electrical energy will be generated by distributed generation (DG). Because of the small size these units are normally connected to the local distribution grid. Connection of DG changes the operation of the distribution grid. In order to minimize the effect of DG during grid disturbances some standards oblige immediate disconnection of DG-units. With an increasing number of DG-units this strategy can lead to a disconnection of a large amount of DG-units and jeopardize the security of supply. To prevent large unbalances between generation and load because of disconnection of DG-units some grid operators have defined fault-ride-through requirements. These fault-ride-through requirements are especially applied for large wind parks which are connected to the transmission grid. In this paper the effect of disturbances in the transmission grid on the fault-ride-through behaviour of medium voltage connected DG-units is discussed. The examined grid is an existing transmission and distribution grid. To the distribution grid a large amount of combined heat and power (CHP) plants are connected. The CHP-plants are equipped with an under-voltage protection which switches-off the CHP-plant at a voltage level (dip) of 0,8 p.u.~100 ms. In the paper it is determined what amount of CHP-plants is disconnected due to transmission grid disturbances. It can be concluded that multi-phase faults as well as single-phase-to-ground faults result in a disconnection of a significant amount of CHP-plants. Voltage dips in the transmission grid below 0,6 p.u. lead to a disconnection of all CHP- plants, between 0,6 and 0,7 p.u. lead to a partly disconnection of CHP-plants and between 0,7 and 0,8 p.u. to no disconnection at all. It is demonstrated that with a setting of 0,8 p.u.~200 ms most CHP- plants stay connected and will not loose stability. Dynamic simulations have shown that the CHP-plants start to consume reactive power directly after f- ault clearing. This is due to armature reaction of the synchronous generator. The amount of consumed reactive power is related to the fault duration. The voltage recovery in medium voltage grids is delayed due to this reactive power consumption. This can lead to a disconnection of CHP-plants even when the fault is cleared within the clearing time of the under-voltage protection. The delay in voltage recovery has to be taken into account when fault-ride-through requirements for transmission grid faults are defined.