1． Digital substations　

　　A digital substation means a substation whose information acquisition, transmission, processing, and output process has been completely digitalized. Its basic characteristics are equipment intelligentization, communication networking, and operation and management automatization. Specifically, digital communication is realized on both the primary electric equipment and the secondary electronic devices in the substation; there are uniform data models and data communication platforms in the whole substation; and on the basis of these platforms, mutual operation can be realized between smart devices.

　　In principle, substation automatic systems have been widely used in the substations in China. Basically, the tertiary and secondary equipment for the conventional substation automatic systems such as the monitoring devices, telemechanical devices, protective relaying equipment, and automatic safety devices all use digital technologies. Based on the present technologies, a substation will develop into a full digital substation after using smart switches, electronic instrument transformers, and primary equipment on-line detecting devices.

2． Development and application of digital substations

　　With the continuous development of computer technologies and electric technologies, the real-time and reliable network communication technologies satisfying the requirements of protective functions and the electronic technologies adapting to instrument transformers, switches, and severe environment have basically become mature. It has become a hot spot of the development of new substation technologies to realize information exchange digitalization at the bay level, equipment digitalization at the process level, and information exchange digitalization between the bay level and the process level. A digital substation means a substation whose information acquisition, transmission, processing, and output process has been completely digitalized. Its basis characteristics are equipment intelligentization, communication networking, and operation and management automatization.

2.1 Reference of HGIS digital projects



2.2 Construction of HGIS digital substations

　　Regarding the construction of HGIS digital substations, HGIS has the following features that other similar products (such as GIS) do not have:

　　1) CTs are installed at the bottoms of HGIS bushings ---- installed at zero potential points. Rogowski coils can be used to fully realize outputting of digital signals, and current acquisition and conversion do not need high voltage insulation.

　　2) PTs are inside bushings and adopt the principle of capacitive voltage division.----SF6 gas insulated. Used for acquisition and transmission of feeble voltage signals.

　　3) Features: safe, economical, reliable, and delicate.

2.2.1 Application, research, and development of electronic VTs and CTs

Main technical indices:
Technical parameters of LDH-110P electronic CTs


2.2.2 Operation monitoring systems using optical fiber cables
Smart control panel for HGIS


Optical fiber cables are used for both the process and the control ---- the control cables are omitted, and the power cables are reserved.




3． Main features of digital substations

　　Primary equipment intelligentization
　　Secondary equipment networking
　　Automatization of operation management system


4． Advantages of digital substations

　　4.1 The various functions of a substation can share the same information platform, thus avoiding repetition of equipment.
　　4.2 Facilitating the newly added functions and expansion scale of substations
　　4.3 Communication networks replace complicated control cables.
　　4.4 Improving measuring accuracy
　　4.5 Improving the reliability of signal transmission
　　4.6 Using electronic instrument transformers to solve the inherent problems with traditional instrument transformers
　　4.7 Avoiding the problems caused by cables such as electromagnetic compatibility, transmission over-voltage, and two-point earthing
　　4.8 Solving the problems with the mutual operation between equipment
　　4.9 Further increasing the automatization and management level


5． Special smart primary equipment for the HGIS of digital substations

　　5.1 Smart cabinets
　　　　Because there is no domestic smart primary equipment, imported smart primary equipment is very expensive. Equipment intelligentization at the process level can be realized by installing smart control units in the wiring terminal boxes of traditional primary equipment.
　　　　Smart cabinets are used together with HGIS HV combined switchgear, and they are comprehensive control cabinets integrating micro-computer measuring and controlling and HGIS operational status monitoring.
A smart cabinet mainly includes operation units and smart units, covering local and remote operation, and switch position status and life on-line monitoring.
　　　　Operating units include operation boxes, mainly realizing the local operations of CBs, DSs, and ESs and satisfying the blocking relationship between CBs and DSs/ESs. They can realize perfect “5-prevention”blocking functions.

　　5.2 Smart units (merging unit DSI series)

　　5.2.1Functions

　　　　　The main functions realized at present are as follows: acquisition of the remote signalling quantities such as the open and closed position of CBs, DSs, and ESs and gas pressure alarm blocking as well as the remote operation of CBs, DSs, and ESs.
　　　　　The on-line monitoring of the basic operational status of HGIS mainly includes:
　　　　　　　① Statistics and calculation of CB electric life;
　　　　　　　② Operational status and tear and wear of CB’s spring charging mechanism;
　　　　　　　③ Monitoring of the opening and closing coils of CB;
　　　　　　　④ SF6 gas pressure monitoring;
　　　　　　　⑤ Monitoring of the operational status of the electrically operated operating mechanisms of DSs and ESs.
　　　　　The electronic components of smart primary equipment work on the site. And the HV parts even have to stand abominable temperature, vibration, and electromagnetic environments. This is a big challenge for the stability and service lives of electronic components. Therefore, we have contribute a lot of energy to the structure, ventilation, heat dissipation, and manufacturing technology of the cabinet bodies, so as to further improve the cabinets.

　　5.2.2 Communication

　　　　　Selecting IEC 61850 substation communication networks and system standards

　　　　　This standard includes the communication requirements, information models, communication protocols, engineering management, and uniformity detection, supports the communication between the substation control level, bay level, and process level, fully satisfying the requirements of digital substations.

　　5.2.3 Merging unit
　　　　　The merging unit sends synchronous signals to ECTs and EPTs, and the A/D convertors are activated to carry out sampling. According to a certain protocol, ECTs and EPTs, through optical fiber cables, transmit the conversion results to the merging unit. After the FPGA finishes decoding, the merging unit obtains the sampling values. The sampling values are transmitted to the main control CPU of the merging unit. Then, after framing and packaging, they are sent out in the standard message format of IEC 6850-9-1.
　　　　　Analog input signals need to be changed into sampling values through A/D conversion. Each time, the A/D conversion will be triggered by synchronous signals.
　　　　　The figure below is the structure diagram of a typical merging unit provided by IEC 61850-9-1. In the figure, there are 7 current quantities and 5 voltage quantities. Clock input is a synchronous clock input, and Digital output is a protocol message output of IEC 61850-9-1 or IEC 61850-9-2.




　　Note: SC of EVTa is the secondary convertor of the electronic VT of phase A (refer to 60044-7), and SC of ECTa is the secondary convertor of the electronic CT of phase A (refer to 60044-8).

　　To facilitate data management and transmission, our company produces two types of merging units:
　　　　● DSI 5281 smart universal data acquisition merging unit (MU)
　　　　● DSI 5282 smart voltage quantity acquisition merging unit (MU)

　　5.2.3.1 DSI 5281 smart universal data acquisition merging unit (MU)
　　　　The synchronous mode of the merging unit is a self-adapting synchronous mode. It can be configured into either an internal synchronous mode or an external synchronous mode. When synchronous signals are lost, the device will automatically change from the external synchronous mode to the internal synchronous mode.

　　　　The number of sampling points can be configured.

　　　　Parameters of the device:
　　　　　　● Input power supply voltage: AC 220 V or DC 220 V
　　　　　　● 2-channel FX 100 optical fibre Ethernet
　　　　　　● 12-channel ECT or EVT optical fibre synchronous sampling output
　　　　　　● 12-channel ECT or EVT optical fibre sampling value digital input
　　　　　　● 2-channel GPS synchronous input or sampling synchronous input
　　　　　　● Number of sampling points: 20 to 500; they can be set.
　　　　　　● 24 V DC power supply output
　　　　　　● Sampling value transmission protocols: IEC 61850-9-1 and IEC61850-9-2

　　Schematic diagram of the device



　　5.2.4 Schemes available for selection

　　5.2.4.1 Protection configuration and network construction scheme
　　　　　　Several bay-level network construction schemes recommended by IEC 61850-9-2

　　Scheme 1
　　It describes a kind of bus structure: Each bay has its own bus segments. To allow the protective equipment and controlgear to acquire the data from other bus segments, we need to set up an independent bus within across whole substation. This bus uses an exchanger or a router to connect to the bus segments of each bay, thus transmitting the required data streams.
　　Scheme 2
　　It describes a kind of bus structure that is similar to that of Scheme 1. However, in this scheme, the bus segments of each bay cover several bays. The data streams required by multi-segment buses are to be transmitted through an exchanger or a router. The example shows that the data of the bus PTs are being used by the direction earthing fault relays of all the bays.
　　Scheme 3
　　It describes a kind of single communication bus across the whole substation. All the equipment is connected to this bus. This scheme requires very high bus efficiency, but the router is omitted.
　　Scheme 4
　　It describes a kind of function-oriented bus structure. In this case, the bus segments are to be set up according to the protected areas. Though a router is needed, the setup of bus segments can minimize the exchange capacity of data between bus segments.
　　The process bus should be considered to be a whole substation with stipulated interfaces. Therefore, it may include such properties as reliability and certain functions reflecting the features of the interfaces. These properties will, together with other entities, be included in the design and integration of the substation, for example, in consideration of reliability and performance. The process bus may also include some parts that are different due to different manufacturers and applications.


　　5.2.4.2 System bay-level protection configuration and network construction scheme
　　The power supply incoming lines and the transformer bays of this system adopt the above recommended Scheme 4, and the low voltage side adopts Scheme 2, but only one bus segment is used.

　　Power supply incoming line


　　电压合并单元：Voltage merging unit
　　电流合并单元：Current merging unit
　　交换机：Exchanger
　　进线保护：Incoming line protection
　　电压合并：Voltage merging
　　电流合并：Current merging
　　进线保护：Incoming line protection
　　备自投：Backup automatic switch-on
　　智能终端：Smart terminal

　　In consideration of the independence and importance of the operation of power supply incoming lines and to guarantee reliability, it is better to carry out network construction by taking each incoming line as an independent bay and connect the incoming lines to their own exchangers. The sectionalizing switch and the backup power supply automatic switch-on device belong to the network segment