Ungrounded Transformer

Hello Dorsey

The problem would occur if somewhere after the transformer, there was a phase fault to ground.

While no substantial current would flow, except enough caused by secondary winding capacitance to grounded case of the transformer, out through the ground/conducting pipes, fencing etc to the fault, the problem becomes severe if a second ground fault developed on a different phase in a second location.

At that time, the phase to phase voltage would be between the two separate phase fault grounded points, and would be extremely dangerous to persons and equipment.

The usual method is to connect to ground the star point of all power transformer secondary windings, have Earth Fault Sensing Relays at that transformer Substation, and have Earth Fault Discrimination Relays on the main switchboard of each connected customer, to disconnect the supply at the faulty Installation, or Earth Leakage Circuit Breakers at various Sub-switchboards, to disconnect the faulty equipment.

Kind Regards....

Hi Sparkstation,

Thanks for the answer. May I ask, what if an overcurrent relay was installed on the secondry side just before the load, can that help in reducing the damage to lives and property. All the motors supplied by the transformer are all connected to an earth bus.

Regards

Hello again, Dorsey

Normally each individual motor starter should include an overcurrent relay/sensor, which disconnects the motor before burnout occurs.

That overcurrent sensor is additional to the fuses (probably HRC fuse is used is up to 1kV motor supply) or circuit breaker if over 1kV motor supply.

Any Overcurrent relay installed as you suggest, would not assist with safety for persons, because the extremely small current (30mA can kill) which causes personal electrocution, is extremely small when compared with large motor currents, and the potential Voltage rise during an electrical fault.

Normally Earth-Leakage equipment would be used in a breaking-down sawmill, where the logs arrive often dirty and/or wet from being washed.

That Earth-Leakage equipment is often included as part of each modern electronic variable frequency or "soft" starter for each individual motor over 1kW.

Likewise as the breaking down mill for the timber drying kilns, which have a hot, damp atmosphere.

A planing department of the sawmill which sizes to fine measurements and smooth planes that timber, is always dealing with air or kiln-dried timber, which has a much lower moisture content. Thus earth Leakage equipment is not normally regarded as required for planing mill equipment.

It is important with every large industrial installation, sawmills are included, that careful planning is done before construction commences, so that expensive additional equipment is not required later.

Most such Main Switchboards are made on a modular basis, and extra modules may be ordered from the original Main Switchboard Maker, and added later, to suit expansion of the entire works.

Trust that assists you.

Kind Regards....

Hi Sparkstation,

Thanks again. If I understand you correctly. The 500KVA, Delta/Star transformer could be installed without the neutral point been connected to ground so long as the motors DOL starters are equiped with an RCCB/ELCB. What if a Phase to earth fault occure before the DOL ( In the busbar ) would it do any significant damage to the transformer? Thanks.

Hello again Dorsey

The neutral of the Power Transformer secondary is ALWAYS grounded via a NGR = Neutral ground resistor.

That enables proper Earth Fault Discrimination Relays to work correctly, on the small voltage rise across the NGR when a ground fault occurs.

That takes care of the problem, when the protection equipment at the Transformer, Substation, Main Switchboard, and connected starters etc are carefully chosen and setting done so that Cascade Faults do not occur.

A Cascade Fault is when a failure is part of a motor (for example) triggers the chain of protection, all the way back to the transformer, or even to the area distribution network, or even worse, drop off-line a power station, or even worse still takes out substations and power stations over a wide area.

There have been major Cascade Faults in many locations and several memorable Cascade Faults have occurred in the Northeast US.

Causes are such a simple thing as a dirty relay contact.

To read about several different Cascade Faults in Northeast US, please refer Google: Results 1 - 10 of about 21,700 for northeast US electrical outage fault. (0.17 seconds)

The Northeast US has been particularly vulnerable to Cascade Faults, because of the many interdependent Power stations, Substations, Networks, and Supply/Distribution Authorities, all of whom historically have set protection/Discrimination relays to suit just their own equipment, not taking into account the Supply/Distribution Network as a whole.

The equipment further down the chain = motors etc in the sawmill, each have their ELCB equipment etc, set so just the motor drops out if it is faulty, without the fault spreading further back up the Supply chain.

As I said earlier, most modern electronic soft starters (SS) or Variable Frequency Drives (VFD), include the Earth fault as part of the design, because at that point it is cheap and easy to include that protection.

Kind Regards....