## 1 Introduction  
Power grids involve the generation, transmission and distribution of electric power. Figure 1 shows an example of the configuration of a power grid. The electrical distribution system delivers electric power through feeders and pole transformers from distribution substations to end users such as houses, office buildings, and factories. Power quality is any power problem manifested in voltage, current, or frequency deviations, that results in failure or malfunctioning of the customer equipment [1]. It is one of the most important issues in electrical distribution systems. Korea Electric Power Corporation (KEPCO) [2] in Korea has monitored basic electrical quantities such as voltage and current from major pole transformers using their temperature. At present and in order to reduce the transmission costs, the information gathered from major pole transformers is transmitted every 30 minutes to a remote monitoring center over the cellular communication network utilized by KEPCO. KEPCO is trying to expand the power quality monitoring system to gather and transmit data towards a monitoring center with low communication costs and at shorter transmission intervals. The system gathers the information about power quality features such as harmonics, voltage sags, and voltage swells. The gathered information is transmitted to accomplish various tasks such as evaluating the current status of power quality, finding places where power quality problems are occurring, and planning measures to improve power quality. Figure 1: An example of a power grid. Various issues such as measurements, controls, databases, and communications must be considered for expanding the power quality monitoring system. We focus on designing an efficient communication network in the electrical distribution system (EDS). Wireless multi-hop communication paradigm is often employed to construct EDS to reduce the deployment and management costs. While wireless sensor networks (WSNs) have been widely studied in the abstract [3-9], to our best knowledge not many studies have focused on building EDS. A WSN is a system composed of numerous computing and sensing devices distributed within an environment to be monitored. In the past decades, WSNs have been applied to autonomous use of computing, sensing, and wireless communication devices for both scientific and commercial purposes [10,11]. For example, ZebraNet [12] has been developed for wildlife tracking. CitySense [13] has been devised to report weather and air pollution. The Sensormap portal [14] has been developed for genetic monitoring services. Research attention has been increasingly paid to designing customized systems like the above to satisfy application-dependent service requirements. Our goal was to design a reliable WSN for a power quality monitoring system to provide high data delivery quality with the least cost of communication system installation and maintenance. EDS delivers power quality information generated by pole transformers to remote monitoring centers in residential divisions. Usually, the power quality information is periodically measured, gathered, and transmitted to the monitoring centers. Once power quality measured at a pole transformer is out of a normal range, an alarm message with detailed contents is promptly sent in the event based manner. To improve the performance in terms of the data delivery rate and the distribution delay of the power quality data, we customize our communication protocol for the distribution pattern of the power quality information. We also examine the performance tradeoffs between the data delivery rate and the distribution delay of the power quality data. The remainder of this paper is structured as follows. Section 2 describes our system architecture and design goals. Section 3 explains our communication infrastructure and discusses the proposed tree-based data forwarding protocol. Following this, we verify the designed system by NS-2 simulations in Section 4.