Testing of transformer differential characteristics can be challenging. Three‐phase testing, while straight forward from a magnitude and phase relationship point of view, requires 6 high‐amperage test currents.For delta ‐ grd wye winding transformers, traditional single‐phase testing requires non‐obvious multipliers and does not simulate real‐world fault conditions. This paper discusses an alternate method of single‐phase testing that simulates real‐world single‐phase loading conditions without the need for phase shifts or current multipliers. This alternate method verifies the differential relay settings are appropriate for the installed transformer by using the installation drawings and transformer nameplate diagram to determine the test connections. Application examples of installations at Minnesota Power substations are used to demonstrate the method. more detail please go link to: https://www.relay-test.com/view/549.html

(1) The time constant of the 500KV power system increases. The 22KV system time constant is generally less than 60MS, and the 500KV system time constant is between 80MS-200MS, the system time constant increases, resulting in a longer decay time of the non-periodic component of the short-circuit current, and a longer transient duration of the short-circuit current. (2) As the system capacity increases, the amplitude of the short-circuit current also increases. (3) Due to the requirements of system stability, the action time of the main protection of the 500KV system is generally around 20MS, and the total removal time of the fault is less than 100MS. The main protection of the system operates during the transient process of the fault. Due to a short circuit in the power system, when the transient current flows through the current transformer, a transient process is generated in the transformer. If no measures are taken, the core of the current transformer will soon become saturated. Especially on a circuit equipped with reclosing, when the transient process caused by the first fault has not been attenuated, the transient process of another short circuit is superimposed. Due to the existence of the residual magnetism of the current transformer, it may cause The iron core saturates faster. As a result, the accuracy of the current information of the current transformer is destroyed, causing incorrect operation of the relay protection.

The multiple of the rated symmetrical short-circuit current is for the transient protection CT The ratio of rated primary short-circuit current to rated primary current, Kssc Rated primary short-circuit current, symmetrical root-mean-square value of primary short-circuit current Can refer to standard DL/T725

The accurate limit coefficient is for the protection level current transformer, generally 5, 10, 15, 20, 30. It refers to the primary current multiple when the composite error does not exceed the specified value. This multiple is called the accuracy limit coefficient.