Non Grounding System

Use an isolating transformer.

I worked for a DC elevated railroad for many years. Case grounds to our structure (negative return, 1 mile to substation) were absolutely necessary. Our transient to utility ground reach as much as 60 volts, and could exceed 10,000 Amps. Messengers get fried very rapidly. The floating neutral presented no problems except with some polarized single insulated tools. You knew real quick, often with smoke. Isolation transformers still could not cross grounds. our DC wouldn't cross transformers, so overall, the utilities were safe from us.

RichH

thanks, actually am thinking I don't know. Am mechanical engineer so electrical is always difficult to understand.

Think of it in hydraulic terms , pressure is volts , flow is current , give it some thought and even the values , if in PSI and GPM are reasonably equatable for safety/risk hazards.

Even in control the what ifs are similar and assertable in nature. I majored in both mechanical and electrical disciplines and find it helps to to relate the two in my present position as a controls engineer.

Jon

?? What is the rated voltage of the system??

Maybe the frequency (if AC = alternating current) could be revealed?

You need to check it out with NEC = National Electrical Code if in North America.

it's a 10.kv alternator, the client asks us not to connect neutral grounding resister and keep it non-grounded.

Grounded or non-grounded, most electronic devices will still operate properly. Since you do not have a neutral line, you will have to ground the cases of your components for safety purposes otherwise they could potentially float at half your line voltage which is still enough to get shocked.

If you have components that internally connect the neutral line to ground, you will have to install an insolation transformer in front of it otherwise all floating equipment on the line will be grounded through that one device.

Since both lines will be hot, all breakers and switches should be double-pole otherwise you run the risk of having your equipment be energized even though it is turned off.

The typical non-grounded system, the neutral from the transformer is grounded with the transformer, often with the messenger. The service electrical system thus has a separately grounded neutral, and maintains it's 120v/0v (outside ground) totally iso;ated from the structure ground.

The hazard is the case (structure) ground will drift against your isolated ground. As I said, with a train passing near the station our structure ground could reach 60 v to the utility neutral. With a possible 10,000 amps. Even the telephone lines needed a case ground and a second utility ground for the operation of their coin mechanism. Otherwise, they couldn't figure out why the coin mech didn't work. The first time that mech grounded from the coin drop, it grounded to a DC voltage well beyond what the ground leg was supposed to carry. If the break was outside the phone, you could get a fist full of coins by momentarily grounding the case when no train was around.

Stable 120/208 neutrals, if separate AC, voltage to structure ground will vary, but neutral to outside ground (isolated) should be stable while isolated ground to structure or line to structure could run all over the charts.

RichH

thanks, Rich, thks for reply which is my another question, in our project the structure grounding (lightening protection) and AC grounding are connected to the same net, the customer also connects the DC grounding (for control panel with PLC) to the same net, will that be ok?

My previous question is on the NGR Panel (Neutral Grounding Resistence) which connects the alternator neutral to the AC ground via a grounding switch, we believe the NGR panel provide better protection for the alternator, however, the customer insists a non-grounding neutral system, which means the neutral will be floating, just like to know if the alternator neutral is floating, what kind of potential damage will be and what types of protection that we may need to consider?

JoeZ

Salute!

You did not answered if any of answers/tips got you a solution.

Your question does not have info: Voltage (V), frequency (DC or 50 or 60 or 400 Hz).

I see excellent information about DC power to trains, some for US residential system 120-0-120 (240V, 60 Hz) systems, etc. From the power system "point-of-view" its protection means are also very important. A proper complex answer is not so easy.

Are you still waiting?

yes, sir, we have 15 10.5kv gensets connecting to a 220kv grid via step-up transformer, we want these 10.5kv gensets connected to a NGR Panel (Neutral Grounding Resistence) Panel, however, the client asked not to ground the neutral, so this 10.5KV alternator neutral is floating now... our concern is whether or not it will have potential damage to the alternator, and what kind of protection we shall consider.

Joe

Yes, you are right to ask questions in this matter.

Even your alternators (60 or 50 Hz, does not matter) will be connected to the 220 kV grid it is very likely that any stronger lightning around the overhead lines (220 kV side) could induct a very high voltage on the 10.5 kV side and damage alternator(s) windings. All three phases of generator terminals must have surge single-phase protections. They could be in a separate cell in the switching board/panel. Their common point must be, of course, grounded.

Overcurrent protection in "traditional configuration" should have CT (Current transformers) on the back.rear side of three phase windings of the generators and if differential protection is installed with zero second time delay it should protect against overcurrent spikes too. There are very good protection schematics referred by generator manufacturers. I understand that monitoring of faults-to-ground are installed too so no significant unpredictable damages can happen.

I do not make any suggestion to step-up transformers protection. It is most of the time in the grid technical management responsibility. You may just get approval from them according to the coordination of electrical protection systems installed on your side (alternators) vs. their protections.

thks. we do have the DCT (Differential CT) and CT for SR489 Multi-functional Relay, they are for alternator protection... also since the client asked for non-grounding system (no neutral grounding resister), we are going to install a ligntening arrester panel to capture lightening surge... can you comment on this system whether it has sufficient generator protection?

One more thing, what we don't have in our system is the utility failure relay, which originally we requested the 220kv sub station to provide, however, they are reluctent. So currently in our system, we don't have a vector df/dt dv/dt relay for utility failure protection... can you comment on this whether or not we have to install a vector df/dt dv/dt relay? whether our current SR489 multi-function relay can protect in case of utility failure?

Joe

Put in this way: It is always good to have redundancy protection unless it will create "bad side effects (pharmaceutical expression)". This question MUST be directed to the manufacturer + installer(s) as the institutions that give you warranty (in legal sense).

As many already stated: Non-grounded system is OK, grounding is necessary to safety (metallic covers, frames etc.), complex lightening protection systems, shielding electronics, low voltage loads (on the secondary side of isolation transformers e.g. 10500V (isl.) / 100 or 120 or 220 or 240 V single phase w/neutral grounded. Be sure the grounding resistance is low enough- in case of a main computer it should be below one Ohm. With this low resistance it is OK to connect lightening grounding system to the rest of "earth point" system.

If Isolation x-fer is 3-phase use secondary Zig-zag connection with N (neutral -zero) grounded.

thks all for the good comments and suggestions.

i feel more comfortable now for the grounding system the customer choose.

You may also want to consider the effects the non-grounded system may have on any VFD's or power factor correction systems that you may have. But this can be resolved by the isolation transformer solution as mentioned before.

JimB

That makes it an "IT" system. Google it.