Why is the secondary side of the current transformer only allowed to be grounded at one point?

1. Because the grounding grid of a substation is not the actual equipotential surface, potential differences will appear at different points. When a large ground current is injected into the grid, there may be a large potential difference between the points. If an electrically connected loop is grounded at different points in the substation at the same time, the potential difference on the ground grid will enter the connected loop, sometimes causing undue shunting. In some cases, the voltage that does not exist in the primary system may be introduced into the detection circuit of the relay protection, making the measured voltage data incorrect and the waveform distorted, resulting in incorrect actions of the impedance element and the directional element.
2. In the secondary circuit of the current transformer, if there are grounding points on both sides of the relay current coil, on the one hand, the parallel circuit formed by the two grounding points and the ground will short-circuit the current coil, so that the current passing through the current coil greatly reduced. In addition, in the event of a ground fault, the power frequency ground potential difference between the two ground points will generate a very large additional current in the current coil. The combined effect of these two reasons will cause a great difference between the current passing through the relay coil and the secondary fault current of the current transformer, resulting in abnormal response of the relay.