Research on Remote Maintenance Technology of Relay Protection in Smart Substation

Abstract

According to the work content of relay protection outage maintenance, a remote maintenance scheme covering all work items of relay protection routine maintenance is proposed; Combined with the status monitoring information and remote test results of secondary equipment in intelligent substation during normal operation, the status of the relay protection equipment is evaluated to realize the remote maintenance of the relay protection and automatically generate the status evaluation report, so as to improve the operation and maintenance efficiency; The results of engineering practice verify the feasibility of the scheme.

1 Preface

Relay protection device is an important secondary equipment in substation. The safety and reliability of the device is an important guarantee for the safe and stable operation of the power system. By the end of 2018, the number of 220 kV and above system protection equipment within the regulation scope of State Grid Corporation of China had reached 182172, an increase of 10044 over 2017, an increase of 5.84% [1]. In order to ensure the normal operation of many secondary equipment, a large number of operation and maintenance personnel are required. After the substation is unattended and operated remotely, the substation maintenance personnel need to go back and forth to the substation for many times in order to analyze, locate and deal with the substation defects, and the defect treatment efficiency is low.

Microcomputer based relay protection has self checking function, and the secondary equipment of smart substation realizes information exchange between devices through network. With the popularization and application of the smart substation secondary equipment status monitoring system, the relay protection operation status is observable and measurable in the life cycle. At the same time, combined with the remote automatic test technology, it can realize the status evaluation of relay protection equipment and the status maintenance of equipment, which can significantly improve the maintenance and operation efficiency. Exploring remote maintenance and operation of smart substation secondary equipment is of great significance to improve operation and maintenance efficiency and save labor cost [2].

Automatic test technology of relay protection logic has been studied and applied in recent years [3,4,5]. Based on the test requirements of relay protection devices, reference [3] proposed one-key test system for relay protection equipment in intelligent substations, which integrates the test systems from both software and hardware to reduce the waste of human and material resources in the test process. Reference [4] proposed an online test scheme based on the characteristics of smart station relay protection. The test scheme completes the function test process of relay protection by adding a test host on the running substation network and using the existing MU (merging unit) and intelligent terminal. The automatic testing scheme greatly improves the efficiency of testing.

This paper presents a technical scheme for remote maintenance of relay protection in smart substation to realize remote operation and maintenance of protection equipment. Relying on the operation evaluation of the relay protection equipment during the normal operation of the on-line monitoring system of the secondary equipment in the smart substation, combined with the remote automatic test conclusion of the protection device during the outage maintenance of the primary equipment, the current state of the relay protection is evaluated to realize the remote maintenance of the protection equipment.

2 Secondary Equipment Condition Monitoring

The relay protection and automatic device in smart substation realize the digital acquisition of AC and input, and the digitalization of information exchange. The definitions of these digital information are included in the SCD (substation configuration description) and the digitized secondary circuit is “invisible and untouchable”. In order to facilitate operation and maintenance, the smart substation is equipped with a secondary equipment status monitoring system [6,7,8,9] to realize the intuitive visualization of secondary equipment information. The status display of secondary equipment is generally divided into three levels: Panoramic information of substation equipment operating conditions, information of single equipment operating conditions and physical circuit information of secondary equipment. The equipment operation condition information visually shows the operation status of the secondary equipment in the whole station. The three-level graphics enable the operation and maintenance personnel to intuitively grasp the one-to-one correspondence between the secondary virtual circuit and the actual physical optical fiber link, and the real-time status of the secondary equipment and its connection.

The secondary status monitoring sub station is configured with process layer acquisition equipment, which supports the monitoring and diagnosis of MU and GOOSE (generic object oriented substation event) data during operation. Through the monitoring and analysis of MU and Goose data, the status evaluation is realized.

2.1 MU Data Monitoring and Evaluation

MU collects the current signal of current transformer and voltage signal of voltage transformer, combines the current and voltage signals synchronously, and sends them to intelligent devices such as protection, measurement and control, watt hour meter, etc. in the form of digital signals. Therefore, the quality of MU is directly related to the reliability of relay protection. Once the MU fails, it will lead to incorrect action of multiple sets of protection.

• (1) Monitoring of data messages

  MU message is transmitted by IEC61850-9-2 protocol. The message contains the quality factor of the data. The length of the quality factor Q is 4 bytes, including 16 quality bits, including validity, overflow, failure and bad reference. By detecting MU messages, the status of MU devices can be known in real time. Diagnose MU status by analyzing the data status of the same MU equipment in an area.

• (2) Homologous data comparison

  By comparing the collected values of the same electrical quantity, the correctness of the collection of electrical quantity can be monitored. The methods of homologous data comparison are various.

  220 kV system, dual configuration of relay protection, comparison methods include: comparing the sampling value of the same electrical quantity of two sets of protection configured in dual configuration, and comparing the sampling of bay protection (line or transformer Bay) and bus differential protection. For 110 kV system with single bus connection, the difference between line and bus protection acquisition can be compared.

  In addition to the direct comparison of unified electrical quantities, some anomalies can also be identified through mutual comparison. For example, there are the following relationships between three-phase current and zero sequence current, three-phase voltage and zero sequence voltage:

  $$ \begin{gathered} 3i_0 = i_a + i_b + i_c \hfill \\ 3u_0 = u_a + u_b + u_c \hfill \\ \end{gathered} $$

• (3) Transient data monitoring

  Transient data monitoring judges the validity of sampled data by the harmonic content and peak error of waveform. When a lateral fault occurs in the power system, the current contains a decaying DC component, which affects the CT transmission. By comparing the transient current error and harmonic content during fault, whether the current transmission is abnormal is judged.

2.2 GOOSE Data Monitoring and Evaluation

GOOSE communication service mapping uses a special retransmission scheme to obtain the appropriate level of reliability. Each message in the retransmission sequence has an allowable lifetime parameter, which is used to inform the receiver of the maximum time to wait for the next retransmission. If no new message is received within this time interval, the receiver will consider the association lost. The event transmission time is shown in Fig. 1.

GOOSE data monitoring includes: heartbeat message monitoring during normal operation, transmission mechanism monitoring during event generation and analysis of displacement records.

• (1) Heartbeat message monitoring

  Heartbeat messages are sent at fixed time intervals. When the device is abnormal, the sending time interval is disordered or not sent for a long time.

Fig. 1.

Transmission time for events

• (2) Sending mechanism when an event occurs

  In case of power grid fault relay protection action or one equipment operation, IED (intelligent electronic device) equipment generates events and generates GOOSE message for sending. The goose sending mechanism complies with the provisions shown in Fig. 1. GOOSE monitoring includes the number of frames, retransmission time, and the validity of the message.

• (3) Displacement record

  Switch position, pressing plate status and tripping/closing information, which are generated during operation, are stored in the database for equipment operation status evaluation.

2.3 Other Monitoring Information

In addition to the status monitoring function, the secondary equipment status monitoring system also contains some advanced functions to serve the operation, maintenance and repair of the secondary equipment in the substation, including: automatic patrol function, fixed value management function, monitoring and early warning, fault location, protection action analysis, online control of SCD configuration and error prevention [10, 11].

The operation and maintenance sub station inspects the operation information of the secondary equipment of the substation at a set time point, including: communication records of the whole station within a certain time interval, alarm records, displacement records, action records, status information (temperature, power supply voltage and optical power), protection on/off, differential current of the main transformer and the bus, etc. These operation information are stored in the database for query, statistics and analysis.

The fixed value management function records the fixed value and the change process of the fixed value in the whole life cycle of the protection equipment.

SCD configuration management and control can ensure that the process layer configuration of relay protection equipment is consistent with the SCD file, and alarm will be given in case of error correction.

The status information monitoring and early warning function monitors the change process of device parameters, including power supply voltage, optical power and device temperature.

3 Technical Framework of Remote Maintenance

The architecture of remote maintenance is shown in Fig. 2.

Fig. 2.

Architecture of remote maintenance

Remote maintenance for secondary equipment in smart substation is realized through secondary operation and maintenance sub station. The secondary operation and maintenance sub station has the secondary equipment status monitoring function and remote test function.

DL/T-995 specification for inspection of relay protection and power grid safety automatic devices stipulates that newly installed equipment shall be put into operation after passing the acceptance inspection. The first full inspection must be conducted within one year after the device is put into operation. After the second full inspection of the device, if it is found that the device is in poor operation or has exposed defects that need to be supervised, it may be considered to appropriately shorten some inspection cycles and select inspection items purposefully and emphatically. Periodic inspection can be divided into three forms: full inspection, partial inspection and circuit breaker tripping and closing test with devices. All inspection contents of relay protection include the following aspects:

• (1) Correctness of electrical quantity collection;

• (2) Correctness of input circuit;

• (3) Correctness of protection logic;

• (4) Correctness of action circuit;

• (5) Functions of the communication loop.

The remote maintenance of relay protection is realized by two functional modules: secondary equipment status monitoring and remote test. Among them, the secondary status monitoring verifies the correctness of the input and output circuits, and the remote test verifies the correctness of the protection logic functions.

4 Relay Protection Remote Maintenance

The remote maintenance for relay protection is realized through three steps: state evaluation during normal operation, remote function test and comprehensive evaluation.

4.1 Evaluation of Normal Operation Status

The evaluation contents of relay protection equipment during normal operation include: electrical quantity collection, input quantity collection, action circuit and communication circuit. The basis for evaluation is the equipment operation data. The secondary equipment status monitoring records the operation status of relay protection in real time, and stores the data in the database for query and analysis.

The electrical quantity collected by relay protection includes current, voltage and DC quantity.

Traditional electric quantity collection and verification is realized by relay protection tester. Manually set the tester to output AC quantity to the tested equipment, observe whether the output quantity of the instrument is consistent with the collected quantity of the device, and calculate whether the measurement deviation is within the allowable range of the specification.

The electrical quantity collection of the smart substation can be monitored, analyzed and diagnosed in real time. When the sampling of the device is abnormal, the device alarms itself or through the measurement and control equipment. For the collection and verification of the electrical quantity of the protection equipment of the smart substation, fault maintenance or even condition maintenance can be adopted. The equipment shall be overhauled in case of abnormal sampling or possible abnormal sampling.

The input value collected by relay protection includes pressing plate signal, circuit breaker position signal and some interlocking signals during operation.

For different input signals, different processing is required to realize their state evaluation.

• Pressing plate signal: during normal operation, the pressing plate signal of the protection device shall not change. When the platen signal changes, the platen state of the protection device changes and the corresponding goose message is triggered. The secondary status monitoring module collects the plate displacement information in real time. When the plate exits abnormally, the monitoring module reports an abnormal alarm. During normal operation, when the pressing plate is switched on or off, it needs to be switched on or off manually; When the pressing plate is switched on or off manually, it is only necessary to manually check whether the device correctly collects the status of the pressing plate;

• Circuit breaker position signal: circuit breaker position signal is very important. Circuit breaker position is required for some protection (such as post acceleration protection) and reclosing functions. During normal operation, the circuit breaker position signal does not change; When the power grid fails, the relay protection acts and the switch trips, the position change process of the circuit breaker shall be consistent with the relay protection action process; When the primary switch is opened or closed due to maintenance, the circuit breaker position signal shall correctly reflect the action process of the switch. Through these processes, the correctness of the action circuit is diagnosed.

• Interlocking signal: interlocking signal includes startup failure, reclosing, etc. When the protection action triggers the interlocking signal, check the correctness of the interlocking signal. The interlocking signal has been tested when it is put into operation. During operation, it is controlled by SCD and checked by CRC code of process layer protection to ensure that the configuration file is not changed; Even if no interlocking signal is generated within a period of time, the correctness of interlocking signal can be ensured.

The communication status of relay protection equipment is evaluated in real time during operation.

4.2 Remote Test

The remote test module automatically tests the tested equipment by remotely controlling the excitation quantity output of the excitation quantity generator. The remote test module in the secondary operation and maintenance sub station realizes the test function of relay protection. The architecture of remote test is shown in Fig. 3.

The remote test module in the secondary operation and maintenance sub station realizes the logic test of relay protection.

Fig. 3.

Architecture diagram of remote test

The secondary operation and maintenance substation has SCD file, setting value information of protection device, soft pressing plate information, etc.; There are various protection test cases in the remote test module; Combining the data of the two, the remote test of the device is completed.

The remote test process is as follows:

• (1) Preparation before test: the equipment is powered off for maintenance, and the switch is in the tripping position. The operation and maintenance personnel connect the excitation output network port and goose port of the mobile tester in the station to the network port corresponding to the relay protection device; At the same time, connect the tester to the station control layer network;

• (2) Enter the remote test sub interface and select the corresponding protection device from the IED list provided in the SCD file. The test module determines the type of test equipment according to iedname and calls the corresponding test option table. Manually trigger logic test of protection device;

• (3) The test module shall be tested item by item according to the test table configured in the maintenance test specification;

• (4) Through the station control layer network, the test module collects the digital information of the tested equipment and judges the correctness of the protection logic action;

• (5) All logic tests are completed and test conclusions are formed.

  After the test, a test report shall be formed. The first part of the test report is the overall conclusion of the test, as shown in Table 1.

Table 1. Line protection test report

The test summary table contains all test items and conclusions of individual test items. Detailed test methods and execution process view the detailed test process documents according to the number information provided in the general table.

4.3 Comprehensive Evaluation

The relay protection status evaluation is based on the status evaluation during normal operation and the remote test result evaluation, and the evaluation report is generated. The contents of the evaluation report refer to the report form of conventional manual maintenance, including:

• (1) Basic information: device model, software version, report formation date, etc.;

• (2) Input evaluation results;

• (3) Electrical quantity sampling evaluation results;

• (4) Protection logic test evaluation;

• (5) Wave recording report;

• (6) Fixed value during device recovery;

• (7) Other information.

4.4 Design Method of Evaluation Report

The comprehensive maintenance evaluation report is output in word file format, which is convenient for users to edit and modify on the basis of the automatically generated evaluation report document. The automatic generation of the report is realized by the joint programming of c++ and python. C++ is used to calculate and process the secondary state data and automatic test process; The output of word file report is realized by using Python's own library file.

The implementation process of the evaluation report is shown in Fig. 4.

Fig. 4.

Flow chart of comprehensive evaluation

• (1) Select the evaluation device and start the evaluation;

• (2) Determine the starting time of the evaluation, retrieve the secondary status monitoring data, and evaluate the input quantity, AC quantity, communication status and loop status. Extract the action process evaluation information for the equipment with protection action process evaluation;

• (3) Call the remote protection logic test results;

• (4) Synthesize the evaluation results and output the device evaluation results in word version.

5 Engineering Application

In order to verify the feasibility of the scheme, a verification system was built in the maintenance area of Zhejiang Jinhua power supply company. The configuration of the system is shown in Fig. 5.

Fig. 5.

Verification system diagram

The main operation and maintenance station in the work area has been connected to the secondary operation and maintenance sub stations of multiple substations and one secondary operation and maintenance sub station for test and training in the training room. Combined with the operation evaluation data of the operation station and the remote test module, a comprehensive evaluation report on the maintenance of the device is formed.

The evaluation items of line protection include AC flow, switching value, setting value, alarm, communication status, automatic test report, etc. Comprehensive evaluation information to form the final evaluation of the equipment.

6 Conclusion

This paper puts forward a technical scheme of relay protection remote maintenance and verifies the feasibility of the project scheme. The secondary equipment status monitoring module of the secondary operation and maintenance sub station monitors the operation information of the relay protection equipment in real time, forms the status evaluation during the normal operation of the equipment, and judges the correctness of the input and output circuits of the equipment; The remote automatic test module completes the remote test of the protection equipment logic when the equipment is powered off for maintenance; Combined with operation evaluation and remote test conclusion, the remote maintenance of relay protection equipment is realized. The verification system shows that the scheme is feasible.