I. Transformer Advantages of Ungrounded Neutral Point Operation
(1) Low interference to communication and signal systems
When the three phases are basically symmetrically operated, the electromagnetic field generated by the power lines to the surrounding space is not large, and will not cause interference to communication and signal systems. Similarly, because the single-phase grounding current of the circuit with the transformer neutral point ungrounded is small, the weak current interference to adjacent communication lines and signal systems is also small. For small systems with ungrounded neutral points in rural power grids, single-phase grounding arcs can automatically extinguish.
(2) Higher power supply reliability
When a single-phase grounding fault occurs in the power grid, the three-phase line voltage and phase current change very little. Since no short-circuit loop is formed, the single-phase grounding current has little impact on user power supply. However, the fault must be quickly cleared within a short time (generally 2 hours) to prevent the fault from expanding. Since tripping does not occur in a short time, the power supply reliability is higher.
II. Transformer Disadvantages of Ungrounded Neutral Point
(1) High insulation level requirements
During single-phase grounding, the voltage to ground of non-faulty phases increases by √3 times. Therefore, the insulation level of equipment in such a power grid is high and the cost is large.
(2) Risk of arc grounding overvoltage
When the single-phase grounding current is small, the arc will self-extinguish when the current passes through zero, and the fault disappears; when the single-phase grounding current is greater than 30 A, a stable arc is generated, forming a continuous arc grounding, which will damage equipment and lead to two-phase or even three-phase short circuits; when the grounding current is greater than 10 A but less than 30 A, an unstable intermittent arc may be generated, followed by arc overvoltage, with an amplitude up to 2.5 to 3 times the phase voltage, enough to endanger the insulation of the entire power grid. Installing an arc suppression coil at the transformer neutral point to compensate the inductive current with the capacitive current will limit the grounding current, even to nearly zero, thereby eliminating the arc at the grounding point and the harm it causes.
(3) Difficulty in selecting grounding relay protection
Therefore, it is relatively difficult to achieve sensitive and selective protection, especially in power grids grounded through arc suppression coils.
Due to asymmetric parameters of the power grid, neutral point displacement occurs, often causing ferroresonance overvoltage, which frequently blows the high-voltage fuses of electromagnetic voltage transformers, or causes the transformer itself to burn out. Measures to limit and eliminate ferroresonance overvoltage:
1. Select voltage transformers with better excitation characteristics or switch to capacitive voltage transformers.
2. Install damping resistors in the open delta winding of electromagnetic voltage transformers to eliminate various resonance phenomena.
3. Adopt temporary switching measures, such as putting the arc suppression coil into operation, temporarily grounding the transformer neutral point, and putting some predetermined lines or equipment into operation.
