The Analysis of Communication Architecture and Control Mode of Wide Area Power Systems Control

Transcription

1 The Analysis of Communication Architecture and Control Mode of Wide Area Power Systems Control Tong Xiaoyang, Liao Guodong, Wang Xiaoru, Zhong Shan School of Electrical Engineering, Southwest Jiaotong University, Chengdu , China Abstract Power systems get more unstable and insecure with more transmission congestion and smaller generation reserves. There is prospect of wide area control schemes meeting the challenge instead of traditional local ones. Communication network technologies, which have made a great stride during last decade, will play an important role in the new schemes. Control systems based on Wide Area Networks (WAN) are studied in this paper. Multi-agent concept is introduced to improve the control performance. A novel wide-area control system that utilizes Phasor Measurement Unit (PMU), wide-area communication and multi-agent technology is presented. The distributed system architecture and multi-agent working mechanism are discussed in order to use phasor data and to realize optimal power system stability control. Keywords: Power system; Wide Area Control; Multi- ; WAN; PMU 1. Introduction Blackouts such as that occurred in North American on August 14, 2004 indicate that better coordinated protection and stability control are critical in modern power system, where power nets interconnection and power system deregulation are prominent [1]. In China, there are six big power subnets of North, Northeast, Northwest, Center, East, and South. After the Center Subnet had been connected to East Subnet in 2003, all power subnets have been connected into one big whole system. Some faults or disturbances occurred in one subnet may influence the stability of other subnets. The great gap between power supply and power consumption, especially in North and East China, makes the power system to run in the edge of insecurity. The big power transmission from west to east subnet to alleviate the stress makes the system more vulnerable to instability. Therefore, in the new environment, new methods are needed to meet this challenge [2]. The lately proposed wide area control schemes are considered superior to traditional local ones in that wide area information will be used to make a better or even optimal control strategy [3-5]. Control action of power system primary protection is required to be within 20ms. Delay time of transient stability control ranges from 150ms to 2s [5]. These ask for a real time and reliable communication system [6]. It is necessary to communicate among the control centers, substations and power plants. The data such as structure parameters of system X, operation variables Y and fault variables V are needed to get control action U after a decision process [7]. U is the function of vector variables of X, Y, V, represented as U = f (X, Y, V). X includes the structure parameters of each power unit such as the generators, transformers, transmission lines and their connecting of. Y consists of the operation variables of voltage, current and power, while V expresses the fault or disturbance. Y is also related with the dispatch schedule and real-time information from power markets. U is the command sent to control the power system such as generation rejection and load shedding. When power systems run normally, X seldom changes and Y change slowly, ranging from a few seconds to hours. But V rapidly changes, ranging from a few microseconds to seconds. Therefore the data are divided into two categories of slowly changed data and rapidly changed data. The synchronized phasor measurements techniques offer a unique capability for tracking system dynamic phenomena in real-time, and also offer possibility of enhancing monitoring, protection and control of electric power networks [10]. Phasor Measurement Unit (PMU) that uses GPS can gain power angle with time tag. The time tag offers same coordinate to compare each other and the relative /05/$ IEEE. 59

2 angle is the key factor for judging the stability of electric power systems. It is PMU that make wide area control possible. In a PMU-based monitoring system [11], IP+ATM+SDH communication structure is adopted. In another voltage stability control system [12], two-layer architecture is proposed with System Protection Center (SPC) in control center and PMUs at substations. The study on new communication network technologies such as SDH, WDM and MPLS for power system wide area control is presented in Section 2. The analysis of the communication and control performance in a multi-agent compute environment is given in Section 3. Section 4 gives a sample that combines wide-area communication, multi-agent technology and PMU. The influence factors of communication speed, capacity and multi-agent structures are discussed. The control characteristics are summarized in conclusions. 2. Wide Area Communication Network Technologies 2.1. The Current IP over Optical Network Topologies Wave Division Multiplexing (WDM) technology in the optical network multiplexes electrical signals into certain wavelength, which improves the network capacity greatly and gets rid of the limitation of electrical process speed. The most capacity of current WDM system reaches 400 Gbit/s (lucent corporation) and, it has attained to 2.6 Tbit/s in laboratory. The current IP over optical communication network based on the multi-layer architecture [6] is shown in Fig1 a) b), The WDM deployment uses SONET/SDH (Synchronous Optical Network / Synchronous Digital Hierarchy) as a sub-layer to interface to the higher layer of the protocol stacks. These different protocol stacks provide different functionality in aspects of bandwidth overhead, scalability of transmission rate, traffic engineering and QoS (Quality of Service) that encompasses available bandwidth, latency and jitter. The Asynchronous Transfer Mode (ATM) layer performs segmentation and reassembly of data packet, classifies service and establishes connections from source to destination; The SONET/SDH layer mainly interfaces to optical domain, adopts highly reliable ring-based topologies, and performs mapping of Time- Division Multiplexing (TDM) time slot from the digital hierarchical levels and defines strict jitter bounds [8]. Multi-protocol label switching (MPLS) is the convergence of the conventional Internet s routing protocol and connection-oriented forwarding techniques [10], and has been held up as the solution to IP QoS, gigabit forwarding, network scaling, and traffic engineering. In fact, MPLS is not only an application technique, but also a network technique of cushion layer between network layer and data link layer. As shown in the Fig.1a), b), IP layer for interfacing to ATM or SONET/SDH layer uses MPLS cushion layer. IP/ATM/SONET IP over SONET IP Over WDM Layering Layering Layering IP/MPLS ATM SONET/SDH Optical WDM IP/MPLS SONET/SDH Optical WDM IP/ MPLS Optical WDM a) b) c) Fig.1. The IP over optical network topology 2.2. New communication network for power system control The current communication network does have some problems, such as latency, complexity, high running cost, and function over-lap etc. Facing these problems, IP over WDM topology has been proposed for transmitting IP packets transparently in the optical domain. The IP over WDM network technology simplifies the wide network communication architecture; MPLS is being chosen to control both IP and WDM layers. Its topology is shown in Fig.1 c). In this way, MPLS cushion layer solely does the traffic engineering function of ATM; the transport capabilities of SONET/SDH are being absorbed into the optical layer. Therefore, the absence of some specific layers makes the IP packet transmit transparently. The latency and the cost of operation fall down compared to those encountered in the current network communication system. Fig.2 shows a simple example of IP-Over-WDM network. A1, A2, A3 and A4 are the access network (e.g. 1Gbps or 10Gbps Ethernet network) attached to MPLS backbone. LSR (Label Switching Router) is a router, which supports MPLS and co-locates OXC (Optical Cross-Connect) providing wavelength capabilities, the same is with LER (Label Edge Router). In the entrance LER, the incoming packet from the 60

3 source access network is classified and labeled based on the destination and required QoS, and switched into the egress LER, then in the egress LER, it is given ride of the label and forwarded into the destination through the destination access network. LER as well as LSR has an IP control-plane to establish routing path LSP (Label Switched Path) in MPLS domains for the IP packet between the entrance and egress LER. At the physical layer of IP-Over-WDM network, LSP in the MPLS domain is mapped into optical connection established by all-optical concatenations of WDM channels called lightpaths. OXCs are connected each other through optical fibers. A1 A3 as power system communication backbone of Metropolitan Area Network (MAN), which attaches to the communication network backbone of power system. Fig.3 shows the communication architecture design scheme for Chengdu Electric Power Bureau, which employs 1Gbps Ethernet for MAN and conventional Ethernets for LANs in substations. 1Gbps Ethernet is the MAN for power system, which converges information from LANs in substations and attaches to the communication backbone for power system. The next generation power communication network is Integrated Service Digital Network (ISDN), which transmits voice, real-time data, video and information. So IP over WDM can be a main selection of the next generation power system communication backbone, and the 1Gbps or 10Gbps Ethernet can be used as the network technique for attaching to power system communication backbone. A2 MPLS backbone A4 3. The Analysis of the Communication Architectures Based on Multi-agent LER LSR OCX Fig.2. A simple example of IP-Over-WDM network Nowadays, the conventional Ethernet (10 Mbps and 100 Mbps) techniques are very mature and simple, and have been put forward as main selection of electrical substation automation communication network. Additionally, the researchers have proposed that the 1Gbps and 10Gbps Ethernet techniques have the merits of high performance, high efficiency, low cost etc., have good compatibilities with conventional Ethernet techniques, and can be constructed on the network techniques as SDH, WDM and optical fibers etc. Hence, 1Gbps and 10Gbps Ethernet can be applied Communication Backbone 3.1. Hierarchical Centralized Control Mode The conventional control mode in China is hierarchically centralized, shown in Fig.4. C P S 1Gbps Ethernet MAN LANs in Substations CC-Control N-National C t CC, S-Subnet CC P-Province CC, C-City CC SS-Substation PP-Power Plant Data and order flow Fig.4. Hierarchical Centralized Control mode Fig.3. Communication Architecture Design Scheme for Chengdu Electric Power Bureau There are four layers of CC respectively as National CC, Subnet CC, province CC and City CC. Generally, structure parameters X and running variables Y are 61

4 transmitted hierarchically from the lower layers to the higher one while control orders U are dispatched from up to down. There are few data exchanged between the CCs in the same layer. Supposed that there are 20 substation and power plants and 64k bps bandwidth is provided for each of them, 1280k bps bandwidth or higher has be required to support the communication in CC with CDT (Cyclic Digital Transmission) mode. Therefore, the communication bottleneck makes the rapidly varying fault or disturbance variables cannot be transmitted in real time so that real time wide area control cannot be realized in this hierarchical centralized mode Regional Control Mode Regional control mode is often used where it is imperative for some hinge substations or plants to take emergency controls. The quantity of transmitted data gets much smaller than that in Fig.4. Rapidly varying fault or disturbance information within this section is sent to hinge substations or plants while control orders are sent to those plants and substations where actions will be taken. Multi-agent technology can be utilized to improve the performance of regional control. There is currently no any communication and collaboration among these hinge substations or plants. technology will facilitate the collaboration, which is significant to a globally optimal control performance. An agent-based regional control mode is shown in Fig.5, in which dotted lines are added to a conventional structure. s are the software of control equipment at various substations and plants. There are HSA-hinge substation agents, SA-substation agents, HPPA-hinge power plant agents and PPA-power plant agents. CC here mainly monitors the running of system or reset the control parameters. s communicate with other agent via WAN to obtain wide-area information. The HSA or HPPA can be worked as a core agent. It collects data from all other agents within its section, collaborate with other section core agent, make a decision and send control order. Each agent can also make the decision individually. A mechanism of negotiation and cooperation has to be set up in this way. The agent searches for information for its control decision and cooperate with other agents by the help of negotiate agent Decentralized Control mode In a decentralized control system, there is no control center like one in the hierarchically centralized system or superior in the regional control system. All control units are equal. This decentralized mode is proper for the wide area protection system, which is a kind of specific control system and where the property of autonomy is very significant for each relay. Traditionally, there is no any communication both among relays and between relays and control center in a protection system. With the development of LAN in substations and plants, relays begin to communication with control center for engineers to reset the relay and obtain relay sampling data remotely. A wide area power system using agent technology will improve on the traditional isolated component system. In a wide area decentralized backup protection system, geographically distributed agents are located in every relay [9]. These relay agents are capable of autonomously interacting with each other agents. Each has its own thread of control, but read local sensors and communicate with other agents (via LAN or via WAN) and act in response to that environment. An agent makes decisions without the direct intervention of outside entities. This flexibility and autonomy adds reliability to the protection system because any given agent-based relay can continue to work properly despite failures in other parts of the protection system. 4. Wide-area multi-agent control system B D A E C Section A Section B CC-Center Control SA-Substation HSA-Hinge Substation PPA-Power Plant Fig.5. The regional control of multi-agent A three layers structure for wide-area multi-agent control system is presented. It is composed of System Control Center (SCC) in global control center, Region Control Center (RCC) in regional control center and PMU in power plants or substations. The proposed wide area control system employs IP+MPLS+WDM wide-area communication technology, which guarantees rapid data transmission from power plants or substations to control center or among them. The system is distributed with collaborative multi-agent. 62

5 4.1 Communication network structure The communication structure of wide-area control system is shown in Fig.6, where 10 Mbps or 100 Mbps Ethernet are adopted in Power Plants or substations, Regional Control Center and System Control Center. These LANs are attached to IP over WDM backbone network via GE (1 Gbps Ethernet) Bridge. Therefore, the wide-area control system could be integrated with automation systems of plants or substations, Energy Management of SCC and Distribution Management System of RCC through an IP over WDM network. 4.2 Multi-agent based wide-area control system Multi-agent architecture of the wide-area control system is shown in Fig.7, which is here focused on the angle stability control. PMUs simultaneously sample and calculate power system variables such as power angle of generators. These PMU data will be transmitted to agent located at SCC, RCC or other power plant and substations. Various control algorithms will determinate the control actions according to the PMU Measurements. For example, the measurements will be used to judge whether the system is transient stable. If instability is predicted, the proper control actions will be taken at substations or plants. Control command could be either sent from SCC or RCC to substations and plants, or given locally. SCC: System Control Center Measurements of variables such as power angles of all generators can be obtained in SCC due to PMUs and wide-area communication. The online security assessment can be achieved by Analyzing with some analysis knowledge Database (Analysis KB) and operation circumstance data from SCADA/EMS. Multi-agent system is oriented-object/task, which is based on data-driven and event-driven. When instability is predicted, an emergent signal is sent to Emergent Control as well as Monitoring Program. Emergent Control searches established control projects in the Operate Database (Operate DB), gives primary Regional Adjusting command to each regional control center. It also receives human intervening from Monitoring Program. RCC: Regional Control Center Regional Control Center makes Regional control Scheme, In order to obtain the optimal regional control scheme If Regional Coordinating receives regional adjusting command enough early; Optimal Control will realize the global optimal control. Otherwise it will execute action based on regional information. Regional Coordinating and Negotiating will coordinate and negotiate with other agents in order to achieve control action as global optimal as possible. Plants and substations Local control agent takes the action such as generation rejection and turbine valve control. System Control Center User Interface Data Base Switcher GE Bridge System Cooperate Control Processor LER LER IP Over WDM LER GE Bridge GE Bridge PMU Switcher Switcher Regional Processor Control Center Power Plant Regional Control Center Fig.6. The communication structure of wide-area control 63

6 Phasor Data DB Analysis KB Operate DB Online Analyzing Emergent Control Communication Monitoring Program SCC Regional Coordinating Cooperative Info. Regional Control Negotiating PMU Data Optimal Control RCC 1 Generator 1 PMU Generator 1 Control Circuit Generator N Control Circuit Local Control Generator Status DB Power Plant 1 Other Power Plants Other RCC Fig.7. Multi-agent architecture of wide-area control system 5. Conclusions 1) IP+MPLS+WDM could be chosen to construct the communication backbone for wide area power system control. 2) The regional control mode, which is a kind of hierarchical decentralized mode, is superior in the multi-agent based wide area power system emergency control. The core agent can make a decision with information within the section and negotiation with other wide section core agent. Also agents make individual decisions and, if a collision occurs, they will negotiate with other agent or seek agents at high layer to make decisions. 3) The decentralized control mode is proper for the wide area real time protection system, where the flexibility and autonomy adds reliability to the protection system. 4) The hierarchical centralized control mode has a communication bottleneck and is not suit for the power system real time wide area control. 5) A distributed wide-area multi-agent control system has the prospect that agents coordinate and negotiate to achieve global optimal control. 6. Acknowledgement This work is supported by the Scientific Research of Southwest Jiaotong University (No: 2003A01). 7. References [1] Yong-hua YIN, Jian-bo GUO, Jian-jun ZHAO, Guangquan BU, Preliminary Analysis Of Large Scale Blackout In Interconnected North America Power Grid On August 14 And Lessons To Be Drawn, Power System Technology (Chinese), vol. 27, no. 10, 2003, pp [2] Cheng-shan Wang, Xu-yang Yu, Distributed Coordinative Emergency Control Based On Multi-agent System. Power System Technology (Chinese), vol. 28, no. 3, 2004, pp [3] IEEE Committee Report, Wide area protection and emergency control,

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