Earthig for 270V DC 2

[AusLee]

Hi,

I have an application where a static converter takes at the input 230V AC 3 phase 50Hz and produces at the output 270 V DC. The converter does not have an isolating transformer. What I know that the 270V DC will be cabled into an equipment that has its body earthed.

My question is: can you please point out some code requirements with regards to the earthing and protection of this small DC section?

Thanks!

 

[ScottyUK]

Without an isolating transformer the positive and negative poles will connect, less a forward volt-drop, to the three AC line voltages in sequence. The sequence depends on which diode or thyristor is conducting. You can not connect either pole to earth unless you have a dedicated floating source.

 

[AusLee]

Thanks Scotty, I think we'll have an RCD and not earth any pole, but would you mind please clarifying a bit more?

It seems from your answer that the only time the (+) or (-) can be earthed is when power is sourced from a PV system, as this is the only truly isolated system, so it can be earthed (referenced to the ground) and its metal parts bonded.

An installation which uses batteries for DC supply cannot earth any of the DC poles because the battery is connected to a charger and the charger's (+) and (-) are connected to the AC system via the thyristors. Therefore only bonding will be installed. But telecom systems usually do have the (+) grounded, so what is the special construction in their chargers that permits this and that we will need to have in our converter if (for any reason) we are going to earth say the (-) pole?

 

[davidbeach]

If the AC system is ungrounded then the DC system can be grounded. You'll find an isolation transformer on the telecom 48VDC system. You'll also find an isolation transformer on the chargers for truly floating DC systems, such as the 125 VDC station batteries, which have a stable voltage to ground from both positive and negative.

 

[PHovnanian]

You will have to look at your DC load equipment and find out what its input voltage requirements are. Grounded case does not always mean either DC (+ or -) inputs need to be grounded. However there may be some maximum voltage to ground specs on these inputs.

A quick analysis of a 6 pole rectifier (simplest case of a static converter) fed by a 230V AC line to neutral Wye source with the neutral point grounded produces a +/- 270 Vdc output. Grounding any other point of the circuit will result in the same differential voltage between + and -, but superimposed upon a sinusoidal voltage of up to 400 Vac. Although the DC load may operate with both + and - inputs floating with respect to ground, this additional ripple will result in much higher peak voltages and stress between components to the case ground. The grounded AC neutral point with ungrounded DC inputs is the least stress case if the load can tolerate floating DC.

Code wise (I'm in the USA), this is analogous to a two wire line to line circuit. The source (AC) is grounded and fixes the circuit voltages to ground. In addition, the AC neutral ground will provide a return path for both AC and DC ground faults, so overcurrent protection will operate.