Remote Maintenance Testing in Digital Substations

Author: Alexander Apostolov, USA

Replacing the hard-wired interfaces with IEC 61850 based communications interfaces allows remote access to the substation for remote testing. The replacement of part or all the hardwired interfaces with communication links requires the development and implementation of methods and tools that maintain the same level of security during the testing process, while at the same time take advantage of all the benefits that IEC 61850 provides. All of the discussions in the article will be based on the following definitions, which are based on the research of many different definitions available on the Internet:

  • Effectiveness - the degree to which objectives are achieved, without consideration of the resources being used.
  • Efficiency - the extent to which a resource is used in order to effectively achieve an objective.

Maintenance Testing
One of the key requirements for correct maintenance testing is the reason for the test. Maintenance testing in general is testing which is performed to diagnose and identify equipment problems or confirm that different actions taken to change settings, upgrade or repair the protection device or another component of the fault clearing system have been effective. The tests to be included in the maintenance test will depend on which of the listed above measures have been implemented.
Maintenance testing can also be related to changes in firmware or addition of new equipment to the substation. In this case it will include installation and commissioning testing, so it is not considered as a topic of this article.

Maintenance Testing in Case of Incorrect Protection System Operation: Problems of the different elements of the fault clearing system can be of two main types – if the system does not operate when it has to and if it operates when it should not. These two types of problems are usually detected when the system is in service and an event occurs. The operation needs to be analyzed in order to determine the reason and take some corrective action to prevent future incorrect operation of the system.

Failure to Operate: The main role of a protection relay is to detect when a fault occurs in the electric power system and to take the necessary actions to clear the fault by disconnecting the faulty equipment from the rest of the system. In some cases, such as transmission line or distribution feeder faults of temporary nature the protection system may also attempt to restore the pre-fault system topology using autoreclosing functions. Failure to operate under fault conditions may have severe impact on the stability of the electric power system due to the increased duration of the fault caused by the operation of backup protection functions and the switching-off of healthy system components.

Undesired Operation:  As many system disturbances and blackouts have shown, one of their main causes have been operations of the protection system under non-fault conditions. These failures also need to be prevented since they may also have a negative impact on the stability of the electric power system and result in deterioration of the conditions and a wide area disturbance.

Maintenance Testing Requirements in Case of Incorrect Operation: The maintenance testing in case of incorrect operation are of two types:

  • Tests used to determine the reason for the operation
  • Tests used to confirm that a required corrective action has been successfully implemented
  • Determining the reason for the incorrect operation is typically done using as a first step replay of waveform records available from the relay itself or from other recording equipment at the substation. The second method is preferred for several reasons:
  • The record in the failed relay may be affected by the failure of the device itself or a component of the fault clearing system – for example instrument transformers or the wiring between them and the relay
  • The sampling rate of the recording by the relay may be too low which will not correctly represent the abnormal system condition

In some cases, comparison of the recording from the relay that operated incorrectly and the record from another device can indicate the reason for the operation and which component of the system has failed.
After the reason for the incorrect operation has been determined, a corrective action is required, followed by maintenance testing to ensure that the measure has been successful. The maintenance tests in this case can be based on replay of the same files used to determine the cause of the incorrect operation, or some other tests to verify changes in settings or programmable scheme logic using network system simulation.

Remote Testing Requirements and Benefits: IEC 61850 based digital substation allow a significant improvement in the efficiency of maintenance testing. This is the result of the availability of testing related features defined in the standard which allow the isolation of the test object and testing system from the rest of the live substation without the need for physical switching or connections of equipment in the live substation.
One of the benefits of digital substations is that all devices (PAC IEDs, substation computers and test devices) are connected to the substation communications network. If there are testing tools that are connected to the network in the substation on a permanent basis, it becomes possible to perform the tests from a remote location. This can be useful in many cases:

  • Long distance between the substation and the base of the test team
  • Difficult terrain with bad roads
  • Difficult weather conditions
  • Requirements for reduction of outage time because of maintenance

The remote testing improves the efficiency by eliminating the need to travel to the substation to perform the testing. This leads to the significant reduction in the time spent by the testing team in relation to a specific maintenance test.
Additional savings in time are the result of eliminating the need for connecting the test equipment to the test object. The ability to isolate only a function element that is being tested improves the efficiency of operation of the electric power system by eliminating the need for an outage during the testing.  To be able to perform remote testing the system needs to meet the following requirements:
Analog and digital interfaces between the process and the protection, automation and control system are communications based (IEC 61850 SVs and GOOSE)

  • Support of virtual isolation of test objects
  • Ability to test a subset of the protection function elements
  • Remote secured access to the substation’s test system
  • Isolation between the test computer and the substation station and process bus
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