Why do you need to connect the neutral and earth cable inside transformer?

Answer 1

The neutral, or 'star point', of a star (or 'wye') connected transformer secondary is grounded (earthed) in order to ensure that the phase voltages are balanced -i.e. each of the line-to-neutral voltages are identical. If the star point was not earthed, and the load currents were unbalanced (due to an unbalanced load, supplied by that transformer), then a situation called a 'floating neutral' would arise, causing the transformer to have different line-to-neutral voltages (both in magnitude and phase).

In addition to this, the presence of harmonic (multiples of the mains' frequency) currents can cause the potential of an unearthed neutral point to 'cycle' or 'oscillate'. This phenomenon of 'oscillating neutral' is avoided by allowing any harmonic currents pass to earth, thereby saving the circuit from voltage unbalances due to these harmonics.

Answer 2

You connect neutral and earth ground together at a distribution transformer because neutral is supposed to be at ground potential. The two are also connected at each distribution panel. Even so, current return is via neutral, not ground - any other path reflects a ground fault, which is an unsafe condition. The purpose in connecting neutral to ground is so that faults in devices that cause hot to ground current paths either trip a GFCI breaker, or they trip an ordinary breaker for the case where fault current is high. Excessive hot to neutral currents also trip breakers - instantaneous for short circuits - and timed for moderate overloads that last beyond the inrush time limit. If you did not connect neutral to ground, and all of the local grounds were also open, then the hot legs could rise to the primary voltage. In a 13.2kv US system, the primary is 7.6kv to ground, star configuration. That amount of voltage inside a building, connected to devices expecting 120, 240, or 480 volts, would be catastophic and extremely hazardous.

Answer 3

The transformer secondary is considered a separately derived system and is treated like an electrical service. The neutral and grounding conductor are bonded at the main service disconnect, so on a separately derived system you do the same. Also in the event this may be on a control transformer, by bonding the neutral to ground, a short to ground on the ungrounded conductor would blow a fuse and prevent electrical shocks and injury.

Answer 4

Protective earth ground and neutral are connected together at a utility distribution transformer, and again at a distribution panel, for two reasons...

The connection creates one half of the circuit that might be completed in the event of a ground fault in the circuit. This protects the circuit, the load, and people by tripping the protective device.

The connection also prevents leakage current in the transformer from raising the potential of the secondary to the potential of the primary. If all of the earth grounds were removed, the voltage in the secondary circuit could become very high. In the US, for a typical primary voltage of 13.2KV and a secondary voltage of 120/240V, the secondary (common mode) voltage could become 7.6KV, a lethal voltage.

Answer 5

Power Transformers are three-phase transformers used in the electricity supply system. If the secondary is star (wye) connected, it is necessary to earth (ground) the star point (neutral point) because an unearthed star point could result in a 'floating neutral', which could cause unbalanced phase voltages. An earthed secondary winding also provides protection to the secondary circuit from an insulation breakdown between the primary and secondary windings.

Answer 6

To protect transformers against ground fault currents

Answer 7

It makes sense to earth one of the supply wires, for safety, and that one is called the neutral.

Answer 8

step up transformer neutral is grounded:

1) To avoid leakage current.(body earthing)

2) Balancing the phase difference( neutral earthing).

Answer 9

In order to maintain balanced phase voltages when the transformer supplies unbalanced loads.