The grounding requirement of DP Current Transformer must be considered according to its voltage level, installation scenario and system characteristics. The specifications are as follows:
I. Core Grounding Principles
Second Single-Point Grounding
High voltage current transformer (10kV and above): The secondary polarless terminal (S2) must be grounded at a single point to prevent the primary voltage from entering the secondary circuit in the event of failure and to ensure the safety of persons and equipment.
Low voltage current transformer (400V and below): grounding is generally not required. However, if the system has special requirements (such as lightning protection, electromagnetic compatibility, etc.), a single point grounding in accordance with high voltage standards can be used.
Difference between independent current transformer and shared current transformer
Independent transformer: When there is no electrical connection to the secondary circuit of other transformers, the ground shall be made at a point in the switchboard (distribution equipment area).
Shared transformers: Secondary windings are allowed to be grounded at only one point, either on the relevant protective panel or in the control room to avoid current shunting or voltage sampling errors caused by multiple grounding points. ii. Specific Application Scenarios
II.High-voltage systems (e.g., 10kV distribution networks)
Differential protection: When using a three-phase all-star connection, the secondary circuit must be grounded at a point in the protective plate to ensure accurate transmission of fault current.
Relay protection: When two transformers are connected in V-shape, the single point grounding must be maintained to prevent false protection trips, even though the sensitivity is low.
Low-voltage systems (e.g., 400V industrial power distribution)
Single-phase meter range expansion: When connecting an external low-voltage current transformer, the S2 terminal shall not be grounded without removing the internal connector of the meter. If the connector is removed, S2 must be grounded to ensure safe power supply to the voltage coil.
Three-phase four-wire circuit: When using a two-phase incomplete star-shaped connection, the combined current of A and C forms an inverse B phase current. Therefore, the secondary circuit must be grounded at a point in the switchboard.
III. Special Environment and Equipment Requirements
Electromagnetic Interference Environment
The shielding of the shielding cable must be grounded at both ends of the the switchyard and control room. In the control room, the shield must be connected to the grounding busbar inside the panel cabinet, and in the distribution box, the shield must be connected to the junction box to reduce high-frequency interference. Optical cables should be used instead of traditional cables to avoid electromagnetic coupling when the distance between the protective device and the communication device transmitting the digital signal exceeds 50 metres.
Lightning and Overvoltage Protection
Line line outlet lightning arrester grounding resistance ≤ 4 omega to ensure current transformer secondary voltage match grounding potential.
Grounding resistance of independent lightning rod can be relaxed to ≤ 30 omega, but must be isolated from current transformer grounding.