Chapter 7
Communication of the microgrid
Abstract
Communication of the microgrid, introduces special requirements, design principles, and schemes of the microgrid communication system.
Keywords
bidirectional communication
plug and play
optical fiber communication
Plc
PTN
functional model
communication design of the local control layer
maximum power point tracking (MPPT)
communication design of the centralized control layer
communication design of the distribution network dispatch layer
The microgrid is operated, controlled, and managed in a different way from conventional grids mainly in that it relies more on information collection and transmission. The response characteristics of microgrid equipment pose higher requirements on timeliness and reliability of the communication system. The communication system is vital for managing and controlling the microgrid.
A microgrid effectively combines flow of power, service, and information. Real-time information collection, timely and reliable data transmission, and efficient processing and intelligent analysis of multilevel data can meet requirements for timeliness, accuracy, and comprehensiveness of data.
7.1. Special requirements on communication
Different from common communication systems, the communication system of a microgrid is expected to meet a wide range of requirements for the bandwidth, timeliness, reliability, and security. The requirements on the communication system are special in the following four aspects.
7.1.1. High integration
The high integration of the communication system is expressed in technologies and services. The communication system integrates computer network technology, control technology, sensing, and metering technology, and can connect the microgrid with various power communication networks (switched telephone network, power data network, relay protection network, teleconference network, enterprise intranet, and security system), thus realizing seamless communication from generation to consumption, allowing for access of various types of generators and ESs, simplifying the integration process, and making possible the “plug and play” of the microgrid service and application and automatic control of the microgrid.
7.1.2. High reliability
Unlike a robust macrogrid, a microgrid is relatively weak. This requires the microgrid to have fast recovery and automatic control, which therefore calls for a highly reliable communication system. When a fault occurs on the microgrid, the communication system should quickly isolate the fault, switch loads to reliable power sources, and collect key data of the faulty section to reduce the downtime after a severe fault.
7.1.3. Commonly recognized standards
To achieve bidirectional, real-time, and efficient communication, the communication system must be established based on open and common standards, which may support high-speed and accurate communication between sensors, advanced electronics, and applications.
7.1.4. Higher cost-effectiveness
The communication system facilitates the operation of a microgrid in such a way that through predication, it prevents the occurrence of events adversely affecting the reliability of the grid to avoid cost rise due to poor power quality, and the microgrid-based automatic communication surveillance function also significantly reduces costs for personnel monitoring and equipment maintenance.
7.2. Design principles of the communication system
A microgrid shall be provided with a high-speed, bidirectional, broadband, and automatic communication system to allow for flexible access of multiple services and “plug and play” communication. As such, the following principles should be observed in designing the communication system of a microgrid.
7.2.1. Unified planning and design
The communication system is not only the basis for control and operation of the microgrid, but also a prerequisite to make its commercialization possible. Therefore, the communication system should be open, scalable, secure, open, and coordinated with the services and commonly recognized communication standards.
7.2.2. Secure, reliable, and open
Given the great variety and frequent interaction of users of a microgrid, the design of the communication system shall not only meet the requirement for openness, but also ensure the security of major equipment and users’ privacy.
7.2.3. Scalability
The increasing penetration of DGs and loads and amount of data place a higher requirement on the bandwidth and reliability of the transmission network. This should be fully considered in designing the communication system by providing redundancy for network extension and maintenance and upgrade.
7.3. Communication system of the microgrid
7.3.1. Communication technologies
At present, quite a lot of communication technologies are available and they can be roughly classified into wired communication and wireless communication. Wired communication includes optical fiber communication and power line communication (PLC), while wireless communication includes spread spectrum communication, WLAN (IEEE 802.11), WWAN (IEEE 802.20), GPRS/CDMA, 3G/4G, satellite communication, microwave, shortwave/ultrashortwave, and space optical communication. The following briefly introduces some of these technologies.
7.3.1.1. Optical fiber communication
Optical fiber communication, including traditional communication, Ethernet communication, and passive optical network communication, is advantageous in high communication capacity, low loss, long transmission distance, high resistance to electromagnetic interference, and high transmission quality and speed.
Ethernet passive optical network (EPON) is a point-to-multipoint optical fiber transmission and access technology adopting the broadcast mode in the downlink and time division multiple access (TDMA) mode in the uplink. It can be flexibly configured in a tree, star, or bus topology, and at the branching points, except for an optical splitter, no node equipment is required, contributing to saving of optical fiber resources, sharing of bandwidth, saving of investment in computer room, high equipment security, easy networking, and low networking cost. EPON is best suited for point-to-multipoint communication, in which a passive optical splitter alone can achieve distribution of optical power.
EPON has such advantages as simple equipment, low installation and maintenance cost, low investment, flexible networking in tree topology, star topology, bus topology, hybrid topology, redundant topology, and ease of configuration. It can be used indoors or outdoors.
7.3.1.2. PLC
PLC is a promising broadband access technology. In this transmission mode, multimedia service signals, including high-speed data, voice, and video, are transmitted over low-voltage power lines.
PLC has such advantages as the power line is under the full jurisdiction of the power sector and easily managed; it can connect to any measurement and control point; signals can be transmitted through power lines, obviating the need to erect special lines; and no permission from the Federal Communications Commission is necessary.
It also has some disadvantages, such as low transmission speed; sensitivity to disturbance, nonlinear distortion, and cross-modulation between channels; large size, and high price of capacitors and inductors used in the PLC system.
7.3.1.3. PTN
Packet transport network (PTN) is a new-generation transport network oriented to packet data services. It is a layer configured between the service layer with Ethernet as the external manifestation and optical fiber transmission layer such as wavelength division multiplexing. It is intended to meet the sudden increase and statistical multiplexing transport requirements of IP services, and with packet services as the core, supports transport of multiple services. This technology not only contributes to a lower overall cost, but also inherits the advantages of synchronous digital hierarchy (SDH). Its functions are totally customized based on IP service transport requirements. It has the following features:
1. A connection-based technology, meeting requirements for carrier-class services. With high quality of service, identification and differentiated management of users, and prioritizing of services and bandwidth management, this kind of flexible connection management provides more management modes, more user access options, and higher cost efficiency of statistical multiplexing than traditional circuit-based connection.
2. Higher operation administration and maintenance (OAM) capability. All PTN technologies emphasize end-to-end OAM capability. Specifically, in addition to the connection management and loopback means required for traditional packet transport devices, PTN technologies also enhance performance management to meet requirements for carrier-class services, such as packet loss rate and time delay measurement. Another significant feature is tandem connection monitoring.
3. Rapid network protection. PTN provides linear protection switching and ring network protection, and in particular, point-to-point channel protection switching within 50 ms.
The PTN technology that integrates the native time division multiplex (native TDM) and packet service is the best solution to data transmission. It not only meets the high requirements for timeliness of relay protection and telecontrol, but also supports the evolution of the operation management platform to packet services. All in all, PTN can ensure reliable communication service for the microgrid.
7.3.2. Structure
The communication system of a microgrid is used throughout power generation, transmission, and distribution, and can be divided into two parts based on the scope of application: (1) the monitoring communication network for the microgrid and (2) the communication network between the control center of the microgrid and that of the distribution network.
7.3.2.1. Microgrid monitoring communication network
Figure 7.1 shows the architecture of the network. With advanced communication technologies, the network works for power dispatch, on-line real-time monitoring of power equipment, management of site operation videos, energy information collection, and anti-theft of outdoor facilities. Communication is mainly through electrical optical fiber network, PTN, and WLAN.
Figure 7.1 Architecture of the microgrid monitoring communication network.
7.3.2.2 Communication network between the control center of microgrid and that of the distribution network
The network is generally set up by referring to the architecture of the communication network of a smart distribution network, in which the microgrid is regarded as an active, controlled client.
7.3.3. Design
In designing the communication system of a microgrid, the communication technologies should be reasonably selected and mixed by fully considering such factors as their respective advantages and applications, costs, and ambient environment, so that the advantages of various technologies can be fully utilized. The communication technologies are selected mainly based on the type of data to be transmitted, geographical distribution of communication nodes, and scale of the microgrid.
7.3.3.1. Functional model of the communication system
The communication system of a microgrid is intended for bidirectional, timely and reliable transmission of control, monitoring, and user data. It is a comprehensive platform integrating communication, information, and control technologies. Figure 7.2 shows the communication flowchart of a microgrid.
Figure 7.2 Communication flowchart of microgrid.
SDH, synchronous digital hierarchy; MSTP, multiservice transport platform.
SDH, synchronous digital hierarchy; MSTP, multiservice transport platform.
The control information of generation, transmission, transformation, distribution, and dispatch of the microgrid is transmitted through the dispatch data network, while the control information of protection, safety, and stability, which has a stringent requirement for time delay, is transmitted through special lines.
User information is communicated with technologies that suit the user network access characteristics of the power system and meet interaction requirements.
Information services of the management, operation, maintenance, and sales departments and other administrative departments are carried over the integrated data network, and with the development of the microgrid, voices and other private line services will also be carried over the integrated data network.
7.3.3.2. Typical solution to communication of an insular microgrid
Figure 7.3 shows the structure of a typical insular microgrid. The communication system can be divided into three layers by function: (1) local control layer, (2) central control layer, and (3) distribution network dispatch layer. The local control layer, consisting of various data collection devices directly connected with the measured objects, transfers the measured data to and receives orders from the central control layer; the central control layer receives measured node information from the local control layer, gives control orders to the local control layer, and receives instructions from the distribution network dispatch layer and then transfers the information to the local control layer; the distribution network dispatch layer receives data from the central control layer, analyzes the data, and makes decisions on control of the microgrid.
1. Communication design of the local control layer: The microgrid is a system combining loads and power sources. The local control layer should have such functions as collection of microgrid data, local protection and control of equipment, high-precision and quick collection of measured data of feeders, protection of feeders against failure, monitoring of inverters for grid connection of DGs, maximum power point tracking, grid dispatch mode, control of automatic and seamless transfer between P/Q mode and U/f mode of power converter system, and hybrid access of ESs (lithium iron phosphate battery, lead acid battery) and supercapacitor. These functions necessitate support of a strong communication system. Various parts of the microgrid should have a unique communication architecture that corresponds to their respective application and security demands.
For a small microgrid with a few nodes and a simple network structure, the communication protocols of the collection devices (e.g., RS-485, real-time industrial Ethernet) may be directly used; for a large microgrid with many nodes and a complicated network structure, optical fiber transmission may be adopted.
2. Communication design of the central control layer: The central control layer mainly serves dynamic power disturbance control and central fault protection, including central differentiated protection of the microgrid, control of transition between grid-connected mode and islanded mode, quick dynamic stability control following a grid disturbance, support of quick communication protocol of IEC 61850 process layer, and clock synchronization with SNTP, IEEE 1588, or IRIG-B time code. It collects measured data from the local control layer and decides which information needs to be transmitted to the microgrid control center (MGCC), receives instructions from the MGCC, and transmits them to the local control layer. This requires a high data transmission capability and quality; therefore, long-distance and large-capacity communication technologies should be used. The PTN and IEC 61850-9-2 + GOOSE protocol can achieve flexible access, aggregation, and transmission of data from the local control layer.
a. PTN: The PTN technology, with an IP core, can effectively fulfill the convergence and transmission of a larger amount of small-granule data services, and can best address transmission of the large amount and suddenly rising data services at the central control layer. In addition, PTN inherits the high protection and OAM capabilities of transmission equipment, and can provide system-level protection and monitoring management for data transmission.
When using the PTN technology in a microgrid, a GE (for a small microgrid) or 10 GE (for a large microgrid) ring network is usually set up. The introduction of the PTN technology at the central control layer is mainly because of its advantages in flexibility, convergence of services on two layers, and statistical multiplexing of service transmission and access, to provide reliable, real-time, and flexible communication.
b. IEC 61850-9-2 + GOOSE communication protocol: GOOSE is a real-time communication protocol that can realize tripping and closing of switchgear and operation control of protection equipment. It replaces the traditional hard wiring between intelligent electronic devices (IEDs), improving the speed, efficiency, and reliability of communication between logic nodes. It realizes true point-to-point communication. With this protocol, any IED can connect to another through Ethernet, receive data for the subscription server, and provide data for the push server. Network connection reduces equipment maintenance costs.
3. Communication design of the distribution network dispatch layer: The distribution network dispatch layer is expected for economic and optimal dispatch of the microgrid, forecast of outputs of DGs and loads, smooth control of power outputs of DGs, quick transfer control of master power source, quick energy balance control, and economic and optimal operation of the grid. This necessitates support of quick, real-time, and reliable communication technologies. To enhance the security, efficiency, and intelligence of the communication system of the microgrid, adapt it to a smart and distributed microgrid in the future, and meet demands of new services, an effective solution is to combine large-capacity and long-distance OTN and ASON technology for communication of the MGCC to realize seamless connection with the conventional distribution network and intelligent management of the microgrid.
Figure 7.3 Structure of an insular microgrid.
MMS, manufacturing message specification; OTN, optical transport network; SNTP, simple network time protocol; ASON, automatic switched optical network.
MMS, manufacturing message specification; OTN, optical transport network; SNTP, simple network time protocol; ASON, automatic switched optical network.
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