Transformer Delta secondary connection and grounding 2

[Mr zezo]

In medium voltage distribution networks there is a 3 wires coming from substation transformer with Delta secondary connection, and there's a grounding transformer which is used to create a neutral point which we connect it to ground to create a ground path to detect ground faults,now before we connect that neutral point to ground,we have ungrounded system and there is no meaning of phase to ground voltage, is it right? As if I measured voltage between any line and ground there will be no reading and if any one touch any line while touching ground he will not get a shock as if there isn't any other fault,is it right? But after grounding the neutral point , the. The voltage between any line and ground will be 11/sqr(3)=6.4kv , is it right? Now ,
First question can we use 6.6 kv insulators instead of 11kv insulators while neutral point is grounded?
2) the medium voltage indicator, how it works? Does it depends on the ground as reference meaning when choosing a voltage rating should I choose 6.4 kv indicator or what? And does it work for ungrounded system? In the first case when the neutral point isn't grounded voltage indicator will indicate avoltage or not?

 

[7anoter4]

In a case when a phase falls on the ground the ground potential rises up to this phase potential and the voltage difference on the other insulators will be 11 kV [up to ground fault clearing]

 

[beyond86]

no meaning of phase to ground voltage, is it right

No, if an earth fault occurs in your network, isolation of cables between conducted wire and armor is put in condition when phase voltage increase to line voltage. In this case, isolation of cable may not withstand high voltage for long period.

As if I measured voltage between any line and ground there will be no reading and if any one touch any line while touching ground he will not get a shock as if there isn't any other fault,is it right?

No, if you touch one phase you will get a shock from capacitance current which flows from phase to ground through your body. Look at the attached file.

First question can we use 6.6 kv insulators instead of 11kv insulators while neutral point is grounded?

Which type of neutral grounding are you going to use? High impedance or low or solid? If not solid, you can't use insulators with a nominal voltage 6.6 kV because, in cases of earth faults, phase voltage will increase to line voltage for a short period until protective devices clear fault.

the medium voltage indicator, how it works? Does it depends on the ground as reference meaning when choosing a voltage rating should I choose 6.4 kv indicator or what? And does it work for ungrounded system? In the first case when the neutral point isn't grounded voltage indicator will indicate avoltage or not?

It doesn't matter for a medium voltage indicator is grounded neutral or not. It measures voltage between phase and virtual neutral point which is created when you connect an indicator to all three phases.

 

[Mr zezo]

Thanks alot for your answer beyond86, ok first for ungrounded Delta how can these capacitances affectes the system meaning how can you calculate it like if I measure the voltage between any line and ground what will be the reading, and how much current should pass through body if any one touched it?
2) the voltage indicator (voltage detector) point, what do you mean by (It measures voltage between phase and virtual neutral point which is created when you connect an indicator to all three phases.) , Voltage indicator just make an alarm if there is a voltage at the line which you are testing , only one line is being tested each time there is no connection between the other lines when using it, and it doesn't used to measure
any thing it just get you an alarm if there is a voltage or not before working for you safety purpose ,and there is 2 types of it contact one which must me connected to the line and non contact one .
So what's the working principle of these voltage detectors, and is there any relationship between the system being grounded or not b,
Thanks alot again for your help .

 

[beyond86]

ok first for ungrounded Delta how can these capacitances affectes the system meaning how can you calculate

It depends on the length of cables in your network. The length is longer the capacitance current is higher.

and how much current should pass through body if any one touched it?

Again, it depends on the capacitance of your network. Also the resistance of body can impact to the current level. To calculate this current you can use software like Matlab or the old method with formulas.

the voltage indicator (voltage detector) point, what do you mean by (It measures voltage between phase and virtual neutral point which is created when you connect an indicator to all three phases.) , Voltage indicator just make an alarm if there is a voltage at the line which you are testing , only one line is being tested each time there is no connection between the other lines when using it, and it doesn't used to measure

I thought about stationary voltage indicator which is usually located in switchboard and connected to all three phases.

So what's the working principle of these voltage detectors

This voltage detector reacts to capacitance current which flows from phase to ground. There is usually enough a small level of capacitance current to detect voltage in a line. If express the principle with a simple description, you just connect a capacitor between phase and ground and current which flow to this capacitor ignites your lamp indicator and shows you that a line is alive.

 

[beyond86]

Also look at this link:

https://www.eeeguide.com/ungrounded-neutral-system/

Hope it helps you to understand the nature of capacitance current in ungrounded systems.

 

[Mr zezo]

Thanks beyond86, so how can I choose a voltage indicator to my system which is 11kv coming from Delta secondary transformer using grounding transformer , so should I choose it according to phase voltage which is 6.6kv , is it right? And what about if the system is not using grounding transformer (ungrounded system) , what will be the rating of the voltage indicator?

 

[FacEngrPE]

This tech note "PERCENT INSULATION LEVEL" attached states

For medium voltage cables, insulation levels greater than 100% are intended to provide an additional safety factor for the un-faulted phase conductors in the event of a fault on one phase in a three phase system.

Your voltage sensor should tolerate these conditions also, cable is more forgiving of over voltage events than voltage sensors selected with little operating margin.

[ul]
[li]If a fault exists for less than 1 minute, then the 100% insulation level is suitable.[/li]
[li]If the fault exists for more than 1 minute, but less than 1 hour, then the 133% insulation level is required[/li]
[li]A 173% insulation level is used when a phase to ground fault, depending on the system, appears as full phase-to-phase voltage across the insulation for more than 1 hour.[/li]
[/ul]