Introduction

In future, the layout of RE sources in the electricity network will be a mixture of both concentrated and distributed. The choice will depend on many factors such as geographic location, load diversity, network configuration, population density etc. Loads in LV network are normally residential and commercial which constitute a greater portion of most electricity systems [1]. For example, in the US, domestic loads consume nearly 40 percent of national electricity generation [2]. In France, buildings consumes nearly 45% of primary energy [1]. Generally, it is desirable to have electric energy generation close to the loads. This approach can help in avoiding long distance transmission of electric energy that can be associated with cost and energy losses [3], improving the security of supply and reducing the electricity bill for customers [4]. Consequently, it makes sense to encourage customers in residential/commercial buildings to install RE sources. This way, the customers can generate electricity for their own consumption and export the surplus power to the grid. As a result, it is anticipated that future LV networks with residential and commercial buildings will see a widespread use of small scale RE sources like PV and wind turbine. The high penetration of RE sources may cause power quality issues such as voltage rise [5-11], voltage unbalancing [12], line overloading [5, 13-18], flicker and harmonics [19]. Voltage rise is normally the main limiting factor in increasing RE generation in LV networks [1]. This problem is likely to happen during the periods when load is low and RE generation is high, causing reverse power flow and therefore voltage rise [20]. Overvoltage is not permitted as it may cause damaging effect on customer electric devices and sensitive electronic equipment [21]. Therefore, loss of RE supply may become inevitable if the voltage violates stationary limits.

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Reasearchers proposes the following strategies to deal with the voltage rise issue.

1- Network upgrading, such as increasing conductor size. This approach needs new expensive investment by utilities for upgrading the networks [22-24].

2- Changing network static set points. Based on the randomness of RE generator output power, the main limitation of this approach is that the static set points would have to change frequently which is not practical [23, 25].

3- Utilization of Distributed Energy Resources (DERs) in customer side such as RE generator and controllable devices for voltage support. It has been estimated that nearly 33% of customer's demand including, dishwasher, washing machine, cooling load, storage units and electric vehicle, can be considered as controllable loads [26]. Therefore, in period of high generation, e.g., mid day for residential area with solar PVs, these resources can be utilized to deal with voltage rise problem.

The third approach is the most promising. However, the main challenge is to determine a coordination strategy for these resources. There are three types of coordination strategies that can be implemented. The first strategy is a centralized control in which the central controller coordinates various resources in the LV network. This approach, even though most efficient, requires online information of the network resources, large memory space for data storing and high data sorting and calculation speed. In addition, it requires fast and reliable communication links. The second strategy is a localized control which is based on local measurements only, such as the one proposed in [20] and [27]. This control approach is robust in the sense that only local measurements are utilized. However, it cannot effectively utilize all the available resources. The third approach involves distributed control. This approach can have the effectiveness of centralized approach while avoiding its complexity. However, the robustness of this approach still depends on the communication links.

Many large-scale projects are currently being conducted around the world on smart meters to make the network more intelligent. Italy and Sweden have nearly 100% installations by the end of 2011 [28]. The primary role of smart meters are to generate customer consumption data for utilities [28]. In addition, the meter can give the customers the ability to use its data and control their load, local generation and battery storage through their Energy Management System (EMS) [29]. The smart meters also have the ability to communicate through communication infrastructure. In this paper, it is proposed that a new functionality can be added to the smart meters in which they can communicate with each other. This functionality

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can give the flexibility to customers in a local area to use their EMS and collaborate with each other for different operational goals including voltage support, which is the focus of this paper.

Based on the suggested functionality, this paper proposes a new coordination arrangement for LV network which can deal with voltage rise problems due to increased penetration of RE sources. PV is considered as the customer RE source. Two operating modes have been defined for a LV network, normal operating mode and voltage control mode. In the normal operating mode, customers continue to operate based on their own objectives. However, in the voltage control mode, two control strategies, localized and distributed, will be used to avoid permissible voltage limit violation. Localized control strategy responds fast and keeps the voltage in permissible range by determining the share of PV inverters active and reactive power for voltage reduction. However, to make the voltage reduction more efficient, a distributed control strategy based on consensus algorithm will be initiated as a supplementary control to find the contribution of customers in voltage control mode using controllable devices.

The rest of the paper is organized as follows: in Section 5.3, the problem of voltage rise is defined and the effective parameters for voltage support are introduced. Section 5.4 provides the details of the proposed approach. This method is applied to a typical LV network in section 5.5 and the results are analyzed. Finally, the paper concludes in Section 5.6.