Types of Relay Tests
The goal of protective relay testing is to maximize the availability of the protection and minimize the risk of undesired operation. Therefore, we must define adequate testing and monitoring methods with appropriate intervals to ensure availability and security are maximized. An electromechanical relay can fail without any external indication. Typically, the only way to detect a failure in an electromechanical relay is through routine maintenance or an undesired operation (i.e., a nuisance-trip or failure to trip). A modern digital relay performs self-diagnostics on key elements to ensure reliable operation. As a minimum, digital relay self-tests include tests of memory chips, a/d con- verter, power supply, and microprocessor. However, a digital relay failure may result in an undesired operation if the self-test routines do not detect the failure in time. Most failures are significant enough to either generate a self-test failure or cause the user to recognize the problem during routine operation.
When a utility engineer selects a new relay design, it is essential to perform tests of the selected relay to ensure correct operation for the intended application. These tests are referred to as type tests and are usually implemented on a single representative relay from the manufacturer. During type tests, utility staff is introduced to new relay models and functions. If there are specific appli- cation questions, utility staff discusses these questions with the relay manufac- turer until there is a clear understanding of all the protective functions. Type tests include detailed tests of the relay characteristics such as mho circle plots, time–overcurrent curve plots, relay element accuracy, etc. The main objec- tive of the type test is verification of the relay algorithms and characteristics.
Commissioning and start-up tests
Utilities typically require tests of each relay prior to placing relays in service.
These tests are referred to as commissioning or installation tests. Once the utility accepts the results of the digital relay type tests, the requirement for commissioning testing is reduced. The operating characteristics of microprocessor-based relays are consistent. This allows us to rely on the type tests for detailed characteristic tests and focus the commissioning tests on simple tests of the relay hardware and implementation of the settings in accordance with the coordination study for the facility or substation.
Relay commissioning tests may be limited to include tests for calibration for implementing the new settings, input/output functionality, simple element accuracy tests, etc. Commissioning tests should also verify the effectiveness of calculated relay element and logic settings. Greater reliance on the type tests for the detailed relay characteristic tests is well justified because those characteristics are fixed in the relay algorithms.
The goal of routine maintenance is to verify that the protective relay will not operate unnecessarily and will operate when required. How can routine testing find problems in protective relays? In order to find problems that might be present, it is helpful to examine the type of problems that can occur in both classes of relays. Then, examine the types of tests being performed to see if they are exercising the relays in meaningful ways. Routine maintenance is necessary for the electromechanical relays since these relays are susceptible to nvironmental contamination and drift over time. These relays should be inspected, cleaned, and calibrated every year or every two years to ensure that they are functioning correctly. The maintenance frequency may be adjusted based on the problems found during the first few maintenance cycles.
Digital relays do not require any adjustment or calibration. The manufacturer performs all calibration before the product is delivered, field calibration or adjustment is not required. Digital relays usually include automatic self-test functions. These self-tests verify correct operation of critical relay components. If a self-test detects an abnormal condition, the relay can close an output contact, send a message, or provide some other indication of the failure. The digital relay disables trip and control functions on detection of certain self-test failures. On self-test failure, the relay should be removed from service and returned to the manufacturer for repair.
Electromechanical relays require periodic inspection, calibration, and adjustment. These adjustments may simply be adjustment of the spring tension or as complex as replacing coil, resistors, or capacitors. Electromechanical relays have been in use for many years and users have developed instructions for troubleshooting techniques for these relays. Another good source of information is the manufacturer ’s documentation for a particular device.