Distribution network line protection in the presence of distributed generation

Abstract

The evolution of the distribution network from a passive grid with unidirectional power flows to, in the presence of distributed generation (DGs), an active grid with bidirectional power flows can lead to some technical challenges in its operation as well as some opportunities for greater control and grid support. This dissertation studies the impact of high levels of penetration of DGs into the power system on the operation of distribution network line protection. The contribution of DGs during faults results in varying short circuit current levels that are hard to predict. This complicates the design of distribution network line protection. If the DGs are located in between the protection device and the fault, the device sees a lower current than it would see in the absence of the DGs. Conversely, if they are connected upstream of the device, the device sees a higher current than before. This means that the reliability, selectivity and speed of the protection devices can be negatively or positively affected. This dissertation analyzes what can be expected from the DGs and how the protection devices themselves can be enhanced in order to avoid these potential problems. In the planning stage, it is possible to control the outputs of DGs during faults in a way that enhances the operation of the protection devices instead of hampering it. This can be done by enforcing regulations through distribution network grid codes. There are two main grid code requirements that directly impact the fault current levels in the network: fault ride through requirements that specify how long and for what voltages the DGs need to remain connected and dynamic voltage support curves that regulate their reactive current output during faults. From these requirements, three parameters are of particular interest: the voltage threshold above which the DGs need to remain connected, the maximum current that they should be capable of producing and the maximum reactive current that they are required to produce below a certain voltage. Using these three parameters, it is possible to control the fault current levels in the network and consequently increase the maximum amount of DGs that can be connected, without endangering the operation of the protection system. In the operating stage, it is possible to enhance the protection devices themselves so that they can deal with the varying fault current levels. Here, adaptive protection is considered. By gathering information about the changes in the network, including the status of switches and DGs and changing the settings of the protection relays accordingly, the reliability, selectivity and speed of the protection system can be maintained while increasing its complexity and cost. To get the information needed from the network, a modified state estimation is proposed. The distribution network is characterized by low observability due to the low number of measurements available. For this reason, load estimates and zero injection buses are added as measurements. To account for the additional uncertainty introduced by the presence of DGs, information about the DGs and their controls are used to add additional measurements that support the state estimation. When calculating the fault current levels in the network to choose the protection relay settings, it is important to correctly account for the fault contributions of inverter based DGs that represent a significant proportion of DGs connected to the distribution network. Unlike synchronous and asynchronous generators, inverter based DGs have a controlled current output during faults. By using an iterative process where the outputs of inverter based DGs are changed based on the calculated voltage at their terminals, a more accurate calculation of the short circuit current levels can be made. Employing these methods in the planning and operating stages, it will be possible to increase the amount of DGs that can be connected to the distribution network, while avoiding cascading faults, slower operation times and the unnecessary loss of load associated with protection mal-operation. The methods should be applied in reasonable steps and whenever needed in combination with each other to ensure the practicality of their implementation and to avoid unnecessary costs.