Keywords: relay protection training, relay protection experiments
Relay protection plays roles of protection, measurement, and control in power systems, mainly applied in projects such as power plants, substations, and distribution stations.
It has a wide scope of use and many application sites. Therefore, it is a very important secondary electrical device in the power system. Before equipment goes into operation, products need commissioning and testing. Only after meeting project requirements can they be put into normal service.
During my studies and work I was fortunate to participate in complete system training on relay protection. Today I briefly organize my experiences for reference.
Protection Relay Training — theory part
Understand basic knowledge
It is necessary to understand the industry’s basic knowledge in advance and learn the principles of protection operation.
For example, common protections include three-stage overcurrent protection, differential protection, overload protection, overvoltage protection, loss-of-excitation protection, etc. You need to understand the protection logic and principles of these common protection functions.
Only by understanding these can we lay the foundation for later product familiarity.
Know the products
There are many types of relay protection products. Different protection objects and different voltage levels have different protection products.
For example, for transformers, common products include transformer differential protection, transformer backup protection, transformer non-electrical-quantity protection, etc.
For generator sets, common ones include generator differential protection, generator backup protection, and generator rotor-grounding protection. Therefore, different products are applied in different places and protect different objects, so we need to have certain knowledge and understanding of the products and refer to industry technical standards.
It is also worth noting that products from different manufacturers operate somewhat differently, so you must strictly follow the manufacturer’s manual and drawings.
Learn to read power schematic diagrams
It is very necessary to learn to read secondary electrical schematic diagrams, because secondary control diagrams are not as intuitive as primary diagrams. You must understand the principles, recognize product symbols, and understand numbering to read them. By reading the schematics, you can better prepare for subsequent experiments.
Learn to use relay protection test sets
Relay protection experiment regulations require that experimental equipment must meet standards and regulations, operate normally, have corresponding calibration/verification certificates with valid periods, and receive regular maintenance.
Example:
In the current protection experiment of relay protection, taking a 10 kV line relay protection device as an example, the electrical schematic clearly marks the wiring relationships of the current transformer (CT), relay, trip circuit, and signal circuit.
In practice, if wiring is not done according to the schematic and the CT secondary winding polarity is mistakenly reversed, the protection device will sample current in the opposite direction and show the experimental anomaly of “device refuses to operate during a fault.”
his situation is caused by not performing CT polarity testing. Under normal circumstances, current enters the like-named terminal and exits the unlike-named terminal. After checking against the schematic and correcting the CT polarity wiring, repeating the experiment can achieve accurate operation of the protection device.
protection relay testing training— experimental part
The experimental part of relay protection is very important. Experiments must be conducted according to specifications; only then will the product meet commissioning conditions. If incorrect operations occur during experiments, such as non-standard procedures or improper operations, the product may be directly damaged, or the product may fail to meet operational requirements during service, creating safety hazards for future equipment operation.
Relay protection accuracy experiment For relay protection
the first task is to perform accuracy experiments, i.e., what we commonly call relay calibration. The accuracy experiment directly reflects the product’s sampling accuracy, mainly including voltage and current. By testing the device with a relay protection test set, the values can be directly and intuitively read from the relay.
The injected quantities from the relay protection tester must be within the allowable error range of the values displayed by the device to be considered qualified. Sometimes, if the displayed values deviate significantly, you can adjust and calibrate the device itself to correct the accuracy.
Relay protection function experiments
This part mainly covers the protection functions of the relay protection, using experimental equipment to debug the protection relay.
To complete this functional experiment, a reliable relay protection test set must be used. The relay protection tester must inject quantities to the electromechanical protections to complete the test.Steps are as follows:
First understand which protection function tests must be done for this product.
Often relay protection products contain many functions; testing every single one would not only waste time, but some protection functions will not be commissioned in actual operation. Therefore, it is unnecessary to test everything.
Follow relay protection training and operate in accordance with the relay protection product instructions.
Before doing the experiment, the product parameters need to be set. This includes enabling/disabling protection functions, setting protection thresholds, CT ratio, PT ratio, etc. After the protection device parameter settings are completed, the protection experiments can be performed.
Refer to the protection operation logic diagram in the product manual
The relay protection manual will describe the protection function operation logic and corresponding descriptions. Conducting experiments according to the logic diagram can ensure reliable device operation.
The above are my personal views and are for reference only. However, in the process of learning about and understanding relay protection, I personally feel that transformer differential protection experiments are relatively difficult.
First, you must learn to calculate the setting values for transformer differential protection. A common method is as follows:
Transformer Differential Protection Setting Calculation
Rated Current Calculation:
CT Secondary Connection Branch Current:
Second Harmonic Current:
Minimum Pickup Current of Biased Differential Protection:
Differential Operating Current:
Calculate the secondary rated value using transformer capacity and CT ratio, then enter it into the protection device to complete a series of setting values. Then use a relay protection test set to perform experiments, simulate internal transformer faults, and obtain experimental data.
I hope my personal experience is helpful to you.
