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Understanding the Difference and Connection between Relay Protection 74 and Differential Protection 87: Principles and Testing Methods using Relay Testers

Title: "Understanding the Difference and Connection between Relay Protection 74 and Differential Protection 87: Principles and Testing Methods using Relay Testers"


Relay protection plays a crucial role in power systems to ensure efficient and safe operation. Two commonly used types of relay protection are 74 protection and 87 differential protection. While both serve the purpose of protecting power systems against faults, there are fundamental differences in their principles and testing methods. In this article, we will explore these differences and connections from both theoretical and practical viewpoints, and explain how relay testers can help maintain and test these protections.


Firstly, let's look at the principle behind these protections. The 74 protection is a directional overcurrent protection that monitors the phase current of the protected equipment, and trips the circuit breaker when the current exceeds a certain value and flows in the opposite direction as the design current flow. On the other hand, differential protection detects the difference between currents of two sets of windings of a transformer or other equipment, and trips the breaker when the difference exceeds a certain threshold. This principle is based on the Kirchhoff's Current Law, and provides more accurate and selective protection for internal faults.

To ensure the reliable operation of these protections, it's crucial to perform regular testing and maintenance. One effective method is to use relay testers, which can simulate fault conditions and verify the performance of the relays. For 74 protection testing, the tester can inject a current that exceeds the threshold of the protection to see if it trips as expected. This test can be performed in both forward and reverse directions to ensure the directional feature is working. For differential protection testing, the tester can simulate a fault by creating a small current unbalance between the two sets of windings. The tester will then verify if the protection measures the current difference and trips the breaker accurately.


In conclusion, the 74 protection and 87 differential protection serve different purposes and have distinct principles. However, they are both essential components of effective relay protection in power systems. Regular testing and maintenance are crucial to ensure their reliable operation, and relay testers provide a useful tool for this purpose. By understanding the difference and connection between these protections, as well as the testing methods using relay testers, power system operators and engineers can ensure the safety and efficiency of the power system.

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