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With the development of intelligent substations, the performance and advantages of intelligent terminals and merging unit equipment have been widely verified, but some problems have also been exposed, such as: unreasonable configuration of merging units reduces the reliability of protection equipment operation, sampling delay increases the action time of the entire protection group, etc. A technical scheme of “conventional sampling+GOOSE tripping” for protection devices in 220 kV and above intelligent substations has been proposed, which also has reference significance for the construction of 110 kV intelligent substations.
1 General Design Overview
In the general design of modular construction drawings for State Grid intelligent substations, 110 kV substations are often used as terminal load stations, with a main transformer scale of 2 units and a typical scheme of single bus segmented (single bus three segmented) or inner bridge (expanded inner bridge) for main wiring. This article takes the 110-A3-3 scheme of expanding the main connection form of the inner bridge as an example to analyze the configuration of the busbar merging unit.
1.1 General Merge Unit Configuration Scheme
In the general design scheme, voltage and current sampling adopts the method of “conventional transformer+merging unit”. Each busbar PT interval is equipped with one intelligent terminal and one merging unit; Two integrated intelligent terminal devices for merging units are installed at intervals between lines and internal bridges; One intelligent integrated device is installed at intervals on the high-voltage side of the main transformer; The intelligent cabinet of the main transformer is equipped with one intelligent terminal and two merging units. On the low-voltage side of the main transformer, each main incoming switchgear is equipped with two integrated intelligent devices. The low-voltage equipment has not changed in the scheme optimization, so no statistics will be made.
1.2 Voltage sampling logic relationship
According to the technical guidelines for intelligent substations, the line, inner bridge, main transformer protection device, SV, and GOOSE information follow the principle of “direct acquisition and direct jump”, and are transmitted point-to-point through optical cables. According to the final scale configuration of the general design scheme, the voltage sampling logic relationship of the protection equipment is shown in Figure 1.
The solid line represents cable wiring, while the dotted line represents optical cable or tail cable wiring. The voltage sampling of the high-voltage side interval merging unit of the line and the main transformer is transmitted through the 9-2 protocol cascade from the busbar merging unit, and can be forwarded to the corresponding line protection, electricity meter, main transformer overload and other devices for use. The voltage sampling of the main transformer protection, backup automatic switching, and low cycle load shedding devices is directly transmitted point-to-point from the busbar merging unit.
Problem analysis
In smart substations, although the reliability of the configuration method of “conventional transformer+merging unit” has been greatly improved compared to the early “electronic transformer+merging unit” mode, the overall reliability of the protection and measurement system has been reduced due to the increased software and hardware modules of the merging unit compared to traditional protection devices, and the increased risk of faults. In addition, the poor operating environment of on-site layout has led to a decrease in the overall reliability of the protection and measurement system. In the general design scheme of State Grid Intelligent Substation, the 110 kV substation is only equipped with 2 sets of busbar merging units according to the final scale, and each set of devices can simultaneously connect to 3 sets of busbar voltage. In this wiring scheme, when one set of merging units fails, it will cause an alarm of the 1/2 main transformer protection device in the station, as well as an alarm or lockout of backup automatic switching, measurement and control, metering and other devices, with a wide range of impact. Considering the risk of faults in busbar merging units, State Grid Corporation of China released the “Standardized Design Specification for Analog Input Merging Units and Intelligent Terminals in Intelligent Substations” in 2016, proposing a scheme of configuring one merging unit per busbar segment for single busbar three segmented connection in 110 kV substations, as shown in Figure 1. There are still several issues in this plan: (1) When two main transformers are constructed in the first phase and expanded in the final phase, the merging units of each busbar configuration are simultaneously connected to three sets of busbar voltage, which requires corresponding busbar equipment to be powered off, increasing construction risks and technical difficulties. (2) The failure of a single busbar merging unit will cause at least 2 sets of main transformer protection devices to alarm, as well as other parts of protection measurement and control, automatic devices, electricity meters and other equipment to alarm or lock, and the impact range is still relatively large. (3) The sampling voltage from the busbar merging unit to the main transformer protection device and the cascade voltage to other interval merging units require a large number of optical cables (tail cables) to be connected.
Optimization plan
According to the final scale of the entire station, one conventional voltage parallel device will be installed in the 110 kV I and III bus voltage transformer PT control cabinets to achieve voltage parallel function between adjacent busbars. Two bus merging units will be installed in the II bus PT control cabinet to collect the voltage of three bus sections. Two integrated intelligent devices are installed on the line, inner bridge, and high-voltage side interval of the main transformer. The merging unit configuration of the intelligent cabinet of the main transformer body is cancelled, and the neutral point zero sequence and gap current are connected to the high-voltage side interval merging unit of the main transformer through cables. The logic relationship of voltage sampling for process level equipment is appropriately simplified.
The bus voltage is connected to the line and the high-voltage side of the main transformer through cables, and the cascading circuit from the bus merging unit is cancelled. The main transformer protection voltage sampling is directly collected from the high-voltage side of the main transformer point to point. The sampling of public equipment such as backup automatic switching and low cycle load shedding can be directly collected from the merging unit of the II bus PT control cabinet. To reduce the impact range of faults in the merging unit, two bus merging units are set up, and the fiber optic interface is reasonably allocated to other public equipment for use.
Compared with the general design scheme, the advantages of this scheme are: (1) enhanced reliability of the interval layer protection measurement and control system. The bus voltage is connected to the line and the main transformer interval merging unit through cables, which increases the reliability of the circuit and can avoid the adverse effects of sampling and conversion caused by the bus merging unit, as well as device failures on the protection and measurement device of this interval. When a single merging unit fails within the interval, it will only cause an alarm for the single set of main transformer protection measurement and control device, and the range of fault impact will be reduced by half. (2) Simplify the busbar merging unit optical cable wiring. Compared with Figure 1, Figure 2 shows that the configuration of protection and automatic devices remains unchanged. The number of process layer equipment (including 2 conventional parallel devices) is reduced by 2, and the number of bus merging unit optical ports occupied is reduced from 22 to 7. (3) The construction is convenient and the power outage range is limited when expanding the III busbar. Only the II busbar needs to cooperate with the power outage, without modifying the I busbar equipment. (4) Reduce equipment procurement costs. The improved plan reduces the number of process layer equipment by 2 units, and the price of 2 conventional voltage parallel devices is significantly lower than that of the merged unit equipment, resulting in an overall cost reduction. When the 110 kV equipment energy meter is placed in the intelligent control room, analog input meters can be used, which is much cheaper than digital input meters and further reduces the cost of engineering equipment.
Shortcomings include: (1) An increase in the number of equipment installed in the busbar control cabinet. For the A3-3 scheme, due to the merging of the layout of the high-voltage side interval of the main transformer and the PT interval of the busbar, the busbar control cabinet needs to install both the main transformer interval merging unit and the busbar merging unit, as well as intelligent terminal equipment. The original universal design scheme required the installation of three devices in each busbar control cabinet; The optimization plan requires the installation of 4 devices in the I and III bus control cabinets, and 5 devices in the II bus control cabinet. During the construction drawing design, it is possible to communicate with the GIS equipment manufacturer to increase the cabinet size or install one protective screen cabinet inside the GIS room. (2) After the voltage circuit adopts cable connection to the line interval intelligent unit, additional cabinet wiring circuits and voltage air switches and other accessories will be added, but the quantity is not large and the circuit wiring is simple, which has little impact on the construction process.
The 110 kV intelligent substation is equipped with a conventional voltage parallel device, and the voltage sampling of the line and main transformer protection is connected to the corresponding interval merging unit through cables. This has obvious advantages in improving the overall reliability of the protection and measurement system and reducing the impact range of single merging unit faults on the protection device. At the same time, two busbar merging units are set up to collect the voltage of the 110 kV busbar separately, for use by other public equipment such as backup automatic switching, which can meet the technical characteristics of digital collection and data sharing in intelligent substations.