Improving Relaying for Islanding Detection

摘要

Nowadays, the electric power system is going under great changes. Traditionally, an electric power system has consisted of large power plants which generate electric energy through a primary energy source. This energy is delivered to customers by means of a unidirectional vertical power flow. Improvements made on the collection of primary energy sources, power electronics, control systems and the reduction of the cost of the components involved, have undergone a transformation in the way the electrical power system is conceived. Therefore, the electrical power system is no longer a vertical system in which energy flows from generation to consumption, but turns into a more multi-directional horizontal system where generation and consumption are located anywhere.This change in the notion of the electric power grid has caused the emergence of new problems and challenges. One of these is the islanding incidents detection [1].The problem of islanding happens when a part of the distribution network is disconnected from the rest of the system but the generators located in the isolated area keep feeding that part of the network. This situation involves several problems, regarding the equipment and personnel safety, so, it is common practice to disconnect all distributed generators to minimize the risks associated to the islanding situation.The methods to detect an islanding situation can be classified into active, passive and communication aided methods. This paper deals with two of those methods. On the one hand, a new algorithm for one of the most popular passive methods used to determine whether the islanding has happened or not, the so called vector shift, vector jump or vector surge relay is presented. On the other hand, a new algorithm to detect islands based on synchrophasors (communications aided method) is also presented.The principle of operation of the vector shift relay is quite simple. During normal operation, the terminal voltage of an embedded synchronous generator lags the synchronous electromotive force by an angle “δ”. If the grid supply is suddenly disconnected from the section of the network partially supplied by the embedded generator, the load on the generator increases or decreases and this causes a shift in the rotor displacement angle. The terminal voltage jumps to a new value and the phase position changes.The new vector shift algorithm takes into account that the real voltage shift can happen in a wave zerocrossingor that it can be split into two different half cycles. It also measures the true vector shiftmagnitude in the three phases, allowing future changes of settings in order to adequate them to the realrelay working situation, but the main goal of this paper is the presentation of a new anti-islandingalgorithm based on synchrophasors.Synchrophasors allow us to compare magnitudes of the power system in real time for monitoringapplications or doing some non-time critical protection functions. Installing relays equipped with thistechnology in different points of the electrical system, allows the detection of islanding situationswithout perturbing the quality of supply.The method presented in the paper uses all the information available from the PMUs for the islandingdetection simultaneously, i.e., it uses the information about the phase angle, the frequency and theROCOF as a whole to determine if the islanding has happened. The algorithm has been tested(simulations) with all types of DG and the results are very promising. The principle of operation isquite simple and it detects islands with speed and security.The need of two types of relaying is due to the fact that the islanding process is different for thedifferent types of DG. The vector-shift relay is dedicated mainly to synchronous DGs, while thesynchrophasors based method is specially valid for inverter-based DGs.The study was carried out as a part of the PROINVER project which is financed by the SpanishMinistry of Science and Innovation (INNPACTO program).