Architecture of integrated wide area protection and control
The proposed integrated wide area or regional protection and control system (IWAPC) is illustrated in Fig. 2. There have been fast developments in both power transmission and distribution networks, e.g., the series compensation in AC lines and high-voltage DC lines in transmission systems, distributed generation and energy storage in distribution systems, etc. These new developments result in far more complicated characteristics than that of conventional systems. Consequently, the existing protection and control system will no longer be effective to cope with the new systems, and this has led to the proposed IWAPC system. As is shown, the IWAPC system consists of different equipment at different layers: from bottom to top, there is the integrated multiple-function intelligent equipment at the local level; the substation communication network and the integrated substation protection and control at the substation level; the wide area communication network, the integrated wide area information platform and the integrated wide area (regional) protection and control at wide area level. The key parts of the system are the high-speed wide area communication network and the real-time synchronisation information platform.
The IWAPC is further extended to dispatching in order to achieve the integration of dispatching automation, protection and control of power grid, and according to the three-level dispatching (country, province, regional) architecture to implement the functions of regional protection, control and dispatching managements.
Multiple functions intelligent equipment at the local level
As shown in Fig.
2
, the Intelligent equipment at local level is an integrated multiple function secondary equipment in the substation, which mainly consists of the MU, intelligent terminal, metrology measurement, PMU & local protection. The equipment is responsible for sampling all real-time data and sending information to the integrated substation P&C and wide area P&C. It also receives and carries out the control commands from the integrated substation P&C and the IWAPC. The equipment can be integrated into primary power apparatuses and achieve local protection for 90 % of its associated line sections. It has a redundant configuration to ensure reliability, together with other integrated functions such as fault recorder, data storage and network analysis, etc.
Integrated substation protection and control at the substation/plant level
The substation P&C integrates functions of line, bus, transformer protections, switch failure; autoreclosure, automatic bus transfer, UFLS, UVLS, overload inter-tripping and substation control function, etc. It utilizes information from the entire substation to achieve substation backup protection and safety automatic control, etc. The CBs are used as units to configure the adaptive backup protection, and current differential protection is used to replace the stage overcurrent protection, breaker failure protection and dead zone protection in the conventional protection system.
Integrated wide area/regional protection and control
The IWAPC specially designed for the protection and control of power network is able to offer fast protection. In addition, they both integrate functions of automatic UFLS and UVLS, voltage and frequency control, oscillation detection and out-of-step separation, etc. In addition, the IWAPC also incorporates the function of transmission cross-section safety P&C. Unlike conventional protection and control, which are separated in both design and operation, the IWAPC integrates protection and control into one optimal combined system, which effectively coordinates the wide area (regional) protection and control, in order to achieve significant improvements in the protection and control of power systems.
Synchronised high speed communication network
One of the most important elements of the IWAPC system is the fast communication network. In this respect, the latest development in communication network, the Packet Transport Network (PTN) may be a better choice to implement such a task. The present power communication network is mainly used in multi-service transport platform based on the Synchronous Digital Hierarchy (SDH). Its advantages lie in its high efficiency for carrying TDM services, low latency, high reliability, with end management capabilities. However, with the new trends in smart grid development, SDH technology gradually revealed its limitations, such as low bearing efficiency and poor flexibility for data services. In contrast, PTN can realise statistical multiplexing and efficient transfer of packet service by using packet-switched core, which can overcome the weaknesses of SDH rigid bandwidth. In addition, it can provide good quality of service, operation, administration and maintenance. Self-healing fibre optical network is employed to connect a number of substations in the region, to ensure full sharing of dynamic and transient information for all electrical measurements, breaker status and protection operations; using high reliability IEEE-1588 technology to ensure the synchronization timing of the sharing data, to prove the data for the integrated wide area protection and control. However, SDH is still an option for the task since it has been widely applied in power network.
Synchronized information platform
Substation is installed with a wide range of electrical equipment with complex designs and is difficult to maintain. With the continuing improvement in power system automation and the intelligence level, the system network has been expanding, along with the huge amount of information in protection and control. As each piece of information is collected and stored by different devices in each separate system, the interoperability of the internal power system data between systems is poor, whereas complex communication protocols tend to create information islands. Consequently, the measurement data and protection control mechanism cannot be shared, which restricts the information integration. The protection and control of smart grid requires dealing with the new situation demands of the application, in order to improve further the information platform capabilities for the future development of key technologies, and to make the information platform system more open.
The real-time synchronized information platform accurately collects wide area information and conducts data mining to investigate the logic relation between the real-time information to increase the sensitivity, reliability and fault tolerance capability. The data received from the platform includes static, dynamic, transient measurements and states of circuit breakers, etc. Valuable information is extracted from the data and allocated to various specially-designed computation algorithms in the platform to perform advanced functions of protection and control for the power network. In the platform, sets of data need to be transferred and their transferring speed depends on the application, e.g., slow speed for contingency analysis, near real time speed for monitoring, real time speed for control, and high speed for wide area protection,; in particular, time synchronization. The information can also include other types of data, such as the oil and ambient temperature of the transformer, wind speed and direction, sun intensity, etc. On the other hand, the information is stored in a hierarchical manner instead of a centralized one, which comprises the hierarchical protection and control system. Equipped with the latest high-speed synchronised communication technology, integrated with the advanced protection techniques and the latest developments in control system, the system offers not only fast protection, but also complete control of entire power network.
The advanced computing technology is introduced to establish a synchronized information platform for wide area protection and control, to build a panoramic operation and maintenance data collection network, providing a standardized interface to the terminal device, to form a resource sharing, flexible and interactive, open and ordered information platform. In summary, advanced computing technologies are used to build a distributed collaborative intelligent information platform, simplifying terminal data collection equipment, and breaking the barriers between protection and control systems at different substations through the specially designed synchronized information platform.
Wide area power cloud
Based on the information platform mentioned above, a distributed cloud system is designed to implement functions at substation and regional levels, such as wide area fault location, fault line selection, power quality monitoring, protection settings, etc. The extended functions also include the equipment monitoring, life cycle and operation management, as shown in Fig. 3.
Currently, many kinds of secondary equipments achieving different functions are installed in each substation, and an increasing number of distributed energy resources of small capacity added to the system greatly increase the number of equipments. To implement these equipments, complex functions in a specially developed distributed “cloud” system will greatly reduce the equipment investment. The cloud at substation level receives the data from process level, and the regional cloud receives the data from the information platform, which includes static, dynamic, transient measurements and states of circuit breakers, extracting valuable information and allocating them to various specially-designed computation algorithms in the platform to perform advanced functions in order to identify the faulted line, the accurate fault location and the contents of harmonics, etc.
The cloud computing platform can make full use of “processing ability of cloud” to reduce the burden of terminal secondary equipment. Based on big data technique, the computing clouds enjoy strong processing power based on demand. There is no need for endless upgrades to improve the processing capacity of the equipment, and there is also no need to update the software to achieve a variety of task processing. There are many more advantages which can be derived from the cloud system, such as the wide area information sharing, standardization of software and algorithm, reduction of equipment investment, substation area occupation and work load for operation and maintenance.