Type tests are required to prove that a relay meets the published specification and complies with all relevant standards. Since the principal function of a protection relay is to operate correctly under abnormal power conditions, it is essential that the performance be assessed under such conditions. Comprehensive type tests simulating the operational conditions are therefore conducted at the manufacturer’s works during the development and certification of the equipment.
The standards that cover most aspects of relay performance are IEC 60255 and IEEE C37.90. However compliance may also involve consideration of the requirements of IEC 61000, 60068 and 60529, while products intended for use in the EU also have to comply with the requirements of Directives. Since type testing of a digital or numerical relay involves testing of software as well as hardware, the type testing process is very complicated and more involved than a static or electromechanical relay.
Functional Test on Relay
The functional tests consist of applying the appropriate inputs to the relay under test and measuring the performance to determine if it meets the specification. They are usually carried out under controlled environmental conditions. The testing may be extensive, even where only a simple relay function is being tested., as can be realised by considering the simple overcurrent relay element.
To determine compliance with the specification, the tests listed in below are required to be carried out. This is a time consuming task, involving many engineers and technicians. Hence it is expensive.
Sr No | Test Description |
1 | Three phase non-directional pick up and drop off accuracy over complete current setting range for both stages |
2 | Three phase directional pick up and drop off accuracy over complete RCA setting range in the forward direction, current angle sweep |
3 | Three phase directional pick up and drop off accuracy over complete RCA setting range in the reverse direction, current angle sweep |
4 | Three phase directional pick up and drop off accuracy over complete RCA setting range in the forward direction, voltage angle sweep |
5 | Three phase directional pick up and drop off accuracy over complete RCA setting range in the reverse direction, voltage angle sweep |
6 | Three phase polarising voltage threshold test |
7 | Accuracy of DT timer over complete setting range |
8 | Accuracy of IDMT curves over claimed accuracy range |
9 | Accuracy of IDMT TMS/TD |
10 | Effect of changing fault current on IDMT operating times |
11 | Minimum Pick-Up of Starts and Trips for IDMT curves |
12 | Accuracy of reset timers |
13 | Effect of any blocking signals, opto inputs, VTS, Autoreclose |
14 | Voltage polarisation memory |
When a modern numerical relay with many functions is considered, each of which has to be type-tested, the functional type-testing involved is a major issue. In the case of a recent relay development project, it was calculated that if one person had to do all the work, it would take 4 years to write the functional type-test specifications, 30 years to perform the tests and several years to write the test reports that result. Automated techniques/ equipment are clearly required.
Rating Test on Relay
Rating type tests are conducted to ensure that components are used within their specified ratings and that there are no fire or electric shock hazards under a normal load or fault condition of the power system. This is in addition to checking that the product complies with its technical specification. The following are amongst the rating type tests conducted on protection relays, the specified parameters are normally to IEC 60255-1.
Thermal Withstand Test on Relay
The thermal withstand of VTs, CTs and output contact circuits is determined to ensure compliance with the specified continuous and short-term overload conditions. In addition to functional verification, the pass criterion is that there is no detrimental effect on the relay assembly, or circuit components, when the product is subjected to overload conditions that may be expected in service. Thermal withstand is assessed over a time period of 1s for CTs and 10s for VTs.
Relay Burden
The burdens of the auxiliary supply, optically isolated inputs, VTs and CTs are measured to check that the product complies with its specification. The burden of products with a high number of input/output circuits is application specific i.e. it increases according to the number of optically isolated input and output contact ports which are energised under normal power system load conditions. It is usually envisaged that not more than 50% of such ports will be energised concurrently in any application.
Relay Inputs and Output contact
Relay inputs are tested over the specified ranges. Inputs include those for auxiliary voltage, VT, CT, frequency, optically isolated digital inputs and communication circuits.
Protection relay output contacts are type tested to ensure that they comply with the product specification. Particular withstand and endurance type tests have to be carried out using d.c., since the normal supply is via a station battery.
Insulation Resistance
The insulation resistance test is carried out according to IEC 60255-27, i.e. 500V d.c.+10% or -10%, for a minimum of 5 seconds. This is carried out between all circuits and case earth fault, between all independent circuits and across normally open contacts. The acceptance criterion for a product in new condition is a minimum of 100M-ohm. After a damp heat test the pass criterion is a minimum of 10M-ohm.
Auxiliary Supplies of Relay
Digital and numerical protection relays normally require an auxiliary supply to provide power to the on-board microprocessor circuitry and the interfacing opto-isolated input circuits and output relays.
The auxiliary supply can be either a.c. or d.c., supplied from a number of sources or safe supplies – i.e. batteries, UPSs, etc., all of which may be subject to voltage dips, short interruptions and voltage variations. Relays are designed to ensure that operation is maintained and no damage occurs during a disturbance of the auxiliary supply.
Tests are carried out for both a.c. and d.c. auxiliary supplies and include mains variation both above and below the nominal rating, supply interruptions derived by open circuit and short circuit, supply dips as a percentage of the nominal supply, repetitive starts. The duration of the interruptions and supply dips range from 2ms to 60s intervals.
A short supply interruption or dip up to 20ms, possibly longer, should not cause any malfunction of the relay. Malfunctions include the operation of output relays and watchdog contacts, the reset of microprocessors, alarm or trip indication, acceptance of corrupted data over the communication link and the corruption of stored data or settings.
For a longer supply interruption, or dip in excess of 50ms, the relay self recovers without the loss of any function, data, settings or corruption of data. No operator intervention is required to restore operation after an interruption or dip in the supply.
In addition to the above, the relay is subjected a number of repetitive starts or a sequence of supply interruptions. Again the relay is tested to ensure that no damage or data corruption has occurred during the repetitive tests.
Specific tests carried out on d.c. auxiliary supplies include reverse polarity, a.c. waveform superimposed on the d.c. supply and the effect of a rising and decaying auxiliary voltage. All tests are carried out at various levels of loading of the relay auxiliary supply.
Routine Factory Production Tests
These are conducted to prove that relays are free from defects during manufacture. Testing will take place at several stages during manufacture, to ensure problems are discovered at the earliest possible time and hence minimise remedial work. The extent of testing will be determined by the complexity of the relay and past manufacturing experience.
Commissioning Tests
These tests are designed to prove that a particular protection scheme has been installed correctly prior to setting to work. All aspects of the scheme are thoroughly checked, from installation of the correct equipment through wiring checks and operation checks of the individual items of equipment, finishing with testing of the complete scheme.
Periodic Maintenance Checks
These are required to identify equipment failures and degradation in service, so that corrective action can be taken. Because a protection scheme only operates under fault conditions, defects may not be revealed for a significant period of time, until a fault occurs. Regular testing assists in detecting faults that would otherwise remain undetected until a fault occurs.
Frequently Asked Question (FAQ)
Why Protective relay testing required?
For accurate operation of Protective relay and to protect power system equipment from major fault. Different types of testing carried out on protective relay. It identifies any potential problems or defects in the relay or its associated components, such as wiring, sensors, or communication channels. Fourth, it provides documentation and records for maintenance, troubleshooting, and compliance purposes.
What is benefits of Relay testing?
If designed correctly can reduce human error. It Can complete repetitive, boring tasks in a very short amount of time allowing the end user to focus more of their time in the areas that need attention.
What is malfunction of Relay operation?
Mal-functioning of relays can lead to false tripping of circuit breakers which can cause cascading failures. Therefore it is important to verify the settings of protection relays before they are commissioned in the real power system to avoid any unfortunate incident.
What is Real time simulators for Relay testing?
Real-time simulators (RTS) are currently being used to simulate large power systems and to analyze their behavior in both steady state and faulted states. These RTSs are equipped with Analog and Digital I/Os and can be interfaced with the real devices. Such kind of approach is called RT HIL simulation.
How many types of Relay testing?
Type test, Installation Test, Maintenance test, Repair test, Inspection and burnishing of contacts (old electromechanical style of relays),Automatic self-test of relay, Adjustments checked, Breakers tripped by manual contact closing, Screws checked for tightness, Covers cleaned.
What is relay accuracy?
Relay accuracy is a measure of how well a protective relay responds to a given input signal and produces a desired output action. It depends on several factors, such as the type of relay, the setting values, the operating conditions, the calibration procedures, and the testing equipment. Relay accuracy can be expressed in terms of percentage error, tolerance, or deviation from the nominal or expected value. For example, a current relay with a 5% accuracy means that it will operate within 5% of the specified current value.
Pingback: Types of tests in power system according to IEEE Standard - Electricalsphere