Interaction of transmission and distribution systems from voltage control and protection settings point of view

This paper addresses the importance of analyzing the interaction between transmission and distribution systems while taking into consideration the fact that difference between the two systems is decreasing in the last few decades. Till few years ago, electrical distribution systems were designed and operated based on the assumption of centralized generation, with the corollary that the power flow always flows from transmission system to the distribution system and from distribution substation to the end-use customers. Distribution networks (e.g. 110 kV) are now operated often in parallel (fed from several UHV/110 kV transformers) instead of in more common radial mode. In addition, with the increasing penetration of small distributed generators (Renewable sources - RES), the RES power generation could not only offset the load, but could also cause reverse power flow through distribution system. These factors make system operation, control and protection more complex and they should be analyzed carefully. At the point where distribution system is connected to the transmission network, it is essential to maintain the voltage variation within a specified limit for satisfactory operation of connected customers´ equipments. Normally, this goal is achieved by controlling operation of compensating devices, such as On-load tap changing transformers (OLTC), shunt capacitors, series capacitors, shunt reactors, static VAr compensators and Synchronous compensators. Our concern in this paper is OLTSs operation.