Distribution grid operation including distributed generation : impact on grid protection and the consequences of fault ride-through behavior

Today’s power systems are undergoing major changes. These changes are driven by both economical and environmental issues such as liberalization of energy markets, unbundling of utilities and an increase use of renewable energy sources. Efforts are made to stimulate development of sustainable energy sources, such as wind turbines and photovoltaic-systems. Also the combined heat and power production is encouraged. This has lead to an increasing penetration of small generating units in medium voltage (MV) grids. Small generation units, also distributed generation (DG), can have a significant impact on, amongst others, power flow, voltage profile, power quality and grid protection. Because of this increasing penetration level the effect of these units cannot be neglected anymore. In the Netherlands horticultural areas are developed in designated areas. In these areas greenhouses are built and in each greenhouse a combined heat and power (CHP) plant is installed. These developments lead to compact distribution grids with a significant number of small generators. For these types of distribution grids three problems have been considered in this thesis: effect of CHP-plants on distribution grid protection, effect of fault ride-through criteria on the dynamic behavior of the distribution grid, effect of grid protection and fault ride-through criteria on the stability of the CHP-plants. The protective system of current distribution grids is simple, cheap and effective and mainly consists of overcurrent relays. In chapter 4 it is demonstrated that the synchronous generator contributes to the fault current in such a way that the total fault current is increasing while the grid contribution decreases. Important parameters which determine the effect of the synchronous generator on the grid contribution to the fault current are the total feeder impedance, the size and the location of the generator. Besides that, the local short-circuit power is also of importance. The protection problems which can occur due to the integration of CHP-plants can be categorized into two categories, namely fault detection problems and selectivity problems. An example of a fault detection problem is blinding of protection which can cause delayed or no fault clearing at all. False tripping is a protection problem which belongs to the category of selectivity problems and occurs when a healthy feeder is switched off due to the contribution of the CHP-plants to a fault at an adjacent feeder. Both problems can manifest themselves for either temporary or permanent faults. For Dutch distribution grids it is shown that false tripping is quite possible, especially for faults near the substation. However, all CHP-plants are cleared by the undervoltage protection before the feeder protection clears the fault hence in practice false tripping is not likely to occur. It was also demonstrated that blinding of protection does not happen at all. In general it can be stated that blinding of protection is not expected in Dutch distribution grids including CHP-plants. A powerful measure to prevent interference of CHP-plants with the traditional protective system is immediate disconnection of the CHP-plants in case of a fault or large voltage dip. For distribution grids with a large number of CHP-plants this can lead to a disconnection of a significant part of these units due to disturbances in the transmission, sub-transmission and distribution grids. In chapter 5 it is demonstrated that the traditional distribution grid protection is too slow prevent disconnection of the CHP-plants. Moreover, the stability limits of the CHP-plants are exceeded when these are kept connected during and after a disturbance in the distribution grid. Therefore a modification of the protective system and a change in distribution grid operation is proposed. The protective system is enhanced with communication channels and the principle of upstream blocking is applied. Also the distribution grid operation has changed from radial operation to loop operation. It is shown that it is possible to clear the fault without disconnection of the CHP-plants and violation of the stability margins. However, for distribution grid faults the stability margin of the CHP-plants is small and the fault clearing has to be sufficient fast. Therefore, to obtain correct relay settings relay timing components as well as the action time of the circuit breaker have to be taken into account. Because of the growing number of DG, grid operators have defined fault ridethrough criteria in order to prevent disconnection of a large amount of DG during transmission grid disturbances. Chapter 6 gives an inventory of what disturbance in the transmission grid leads to a disconnection of CHP-plants in the distribution grid. All fault types in the transmission grid leads to a dip in the distribution grid and a possible disconnection of CHP-plants. It can be concluded that disconnection of the CHP-plants can be prevented easily without violating the stability margins with a proper setting of the undervoltage protection. However, for sub-transmission and distribution grid faults more elaborate settings of the undervoltage protection are needed. In this chapter the German fault ride-through criteria are taken as a reference and it is demonstrated that sub-transmission and distribution grid faults can be survived without the disconnection of CHP-plants. Because distribution grid faults cause deep dips the stability margin of the CHP-plants is low and fast fault clearing is a necessary condition to prevent disconnection and instable operation of the CHP-plants. This is a challenging task even for the modified protective system. For Dutch distribution grids including CHP-plants it has been shown that disconnection of all CHP-plants due to a disturbance leads to a large reversal of active power although the accompanying steady-state voltage deviation is limited. This voltage deviation can easily be corrected with the tap changer of the transformers. Keeping the CHP-plants connected after a disturbance results in a large active power swing which is also noticeable in the sub-transmission and distribution grid voltage. Another consequence of keeping the CHP-plants connected to the grid is the delayed voltage recovery due to the reactive power consumption for a small period of time. This delayed voltage recovery leads to a violation of the fault ride-through criteria and a disconnection of the CHP-plants anyhow, even when the voltage dip is survived. For the improvement of the voltage recovery in chapter 7 it is proposed to integrate a STATCOM as a source of reactive power in the local distribution grid. It has been shown that the STATCOM has a positive effect on the voltage recovery and as a result of this improved voltage recovery the disconnection of the CHP-plants is prevented. Besides that, the improved voltage recovery also limits the maximum amplitude of the rotor angle swing which for the CHP-plants results in reaching the steady state faster.