304 PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 6/2012 Tengku Juhana TENGKU HASHIM 1 , Azah MOHAMED 2 , Hussain SHAREEF 3 Universiti Kebangsaan Malaysia A review on voltage control methods for active distribution networks Abstract. Power distribution systems are in the state of transition from passive to active networks due to the rising penetration level of distributed generators (DGs). One of the technical challenges of active networks is to maintain an acceptable voltage level. This problem has initiated many researchers to control network voltage profile. Several approaches to mitigate the voltage issues include the use of coordinated or centralized and decentralized methods. Both methods have been proven to be able to alleviate the voltage rise issue in distribution systems with DGs. This paper presents a literature review on the various voltage control methods that have been implemented in active distribution networks. Streszczenie. Przy współpracujących sieciach rozproszonych problemem jest utrzymanie napięcia na pożądanym poziomie. W artykule przedstawiono przegląd różnych metod sterowania poziomem napięcia które moga być zastosowane w aktywnych sieciach rozproszonych. (Przegląd metod sterowania poziomem napięcia w aktywnych sieciach rozproszonych) Keywords: Distributed Generator, active network, centralized and decentralized methods, voltage control. Słowa kluczowe: sieci zasilające, sieci rozproszone, kontrola napięcia. 1. Introduction The connection of DGs in distribution networks has created a challenge for distribution network operators (DNOs) to change their usual passive approach to an active system. This is due to the fact that the conventional distribution networks are designed based on the assumption of unidirectional power flow. With the increasing connection of DG, the network has become more dynamic with bidirectional power flow and it known as active distribution networks (ADN). An active distribution network is defined as a distribution network with systems in place to control a combination of distributed energy resources comprising of generators and storage [1]. In [2], ADN is defined as a new system that adopts integration of control and communication technologies such that distribution network operators can manage and accommodate the new distribution network. The working group CIGRE C6.11 on the development and operation of active distribution networks has reported on the strengths and weaknesses of AND [1]. Some of the highlighted strengths are automation and control which will lead to improved network access for load customers. ADN will also provide increased operational reliability in terms of power delivery. However, there are some weaknesses which are associated with ADN such as maintenance issue, present lack of experience, and existing communication infrastructure. Some of the impacts and challenges addressed in the implementation of distribution networks with the presence of DGs include voltage levels and power flow, equipment thermal ratings, fault current levels and also protection issues [3]. With all these issues arising, an active network management (ANM) scheme is essential to provide coordination to power system operation. According to [4], ANM is defined as the use of real-time control and communication systems to provide a means to better integrate renewable distributed generators. With the increasing number of DG penetration, the issue of voltage level in distribution systems has become important. Increasing the number of connected generators will result in voltage rise above its permissible level [5]. The voltage rise effect due to the connection of a DG is illustrated by using a simple circuit shown in Figure 1. In this simple system, the generator, G with generation P G , Q G together with local load, P L , Q L and a reactive compensator, Q C is connected to the distribution system through a weak rural overhead line with impedance Z and a transformer with an on load tap changer (OLTC). Figure 1: A simple system connected with DG to model voltage rise From the figure, the voltage at busbar 2 (V 2 ) can be approximately calculated as follows: (1) V 2 ≈ V 1 + R (P G + P L ) + (Q G +Q L + Q C ) X This equation can be used to qualitatively analyze the relationship between the voltage at busbar 2 and the amount of generation that can be connected, as well as the impact of the alternative control actions to manage voltage rise [6]. The voltage rise is more severe when there is no demand due to the fact that all the local generation is exported back to the primary substation. Basically, there are two types of voltage issues which can be categorized as short term and long term voltage problems in distribution systems [7]. The short term voltage problem is usually caused by voltage sag or dip which is defined as a drop in voltage at a duration between one half-cycle and sixty seconds [8]. It is generally caused by a fault in the power system [8]. In contrast, overvoltage or undervoltage can be considered as a long term voltage problem which can lead to a more serious problem to power systems. The overvoltage problem calls for a management scheme that could alleviate the excessive voltage rise issues. 2. Voltage Control Methods with Distributed Generation Current ANM schemes may be categorized as centralized or also known as coordinated control, semi-coordinated and decentralized control strategies. Centralized or coordinated control strategy provides voltage regulation from the substation to the rest of the network, potentially using a wide range of communication systems to coordinate Qcomp DS OLTC Z= R +jX P L ,Q L Q C Q G P G G 1 2