Advantages of using Neutral Grounding Transformer?

[ALTAF]

I want to know the advantages of neutral grounding transformer,used in 3.3kv power generation & all generators (total 5) 3.2MW each star point not grounded?

[Kysu Hai Tung]

Hi all

 

I know that Grounding transformer is used to derive a neutral on an ungrounded three phase system.

But i don't know about below problems:

1. How to select the vector group of grounding transformer (zigzag or YD)

2. How to calculate the rating of grounding transformer

3. How to connect the grounding transformer to the system

Can anyone help me????

 

Thanks

[RalphChristie]

ALTAF:

 

I can think of no reason why the NECs (zig-zag trsfs) are not grounded. At least one of them have to be grounded. A possible reason can be that the don't use the zig-zag winding as a earthing trsf, but rather use it as a local transformer (trsf to supply substation building with power) Sometimes you get a combined zig-zag / local supply trsf. But still, if it is not a ungrounded system, at least one trsf have to be grounded to allow zero-sequence current to return to the source. If it is a ungrounded system, the zig-zag windings are a waste.

 

Kysu Hai Tung:

 

1. You can use both. The primary advantage of the zig-zag winding is that it results in a much smaller transformer for the same fault current capbility. But in reality, almost no one builds these any longer. It will be a special "one of" order and you will have trouble getting quotes.

 

2.The first step is to decide how much ground fault current you want to allow.

 

For a solidly-grounded system, the grounding transformer impedance largely determines the ground fault current. You will need to specify the grounding transformer impedance (normally in ohms) as well as how long the transformer must be able to handle this current (normally 10 seconds).

 

If you want to resistance-ground the system, then you will need to specify the maximum fault current along with the duration. The trend today is to go with resistance earthed systems. (earthing trsf + resistors)

 

The primary determination of transformer size will be the short-circuit current it has to produce and the duration. You do not need to specify a kVA rating. Transformer manufacturer needs to know:

 

Operating voltage / BIL

Desired impedance in ohms

Max. short circuit current

Duration of Short circuit current

Max. continuous current

 

 

3. Connect phases of zig-zag bank to delta system and either solidly ground the groundpoint or take it through a resistor and then to ground.

 

Connect star winding of Yd-trsf to delta system and take starpoint either solidly to ground or through a resistor and then to ground. Delta winding can be left floating.

 

 

This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964:

A zig-zag grounding bank is typically used to provide a source of fault current so a phase to ground fault on a delta system can be detected and interrupted. The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant "k" which is determined by the length of time required for the relay to open the breaker.

 

On a 12kV system the phase to neutral voltage is 7.2kV (12/1.73)

Suppose short-circuit current to be 3000A

 

(7.2)(3000)(K) = kVA

 

k is determined from the following table:

time k

10 sec 0.064

1 min 0.104

2 min 0.139

3 min 0.170

4 min 0.196

5 min 0.220

 

Hope above information is helpful

 

Regards

Ralph

[ysaac]

This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964:

A zig-zag grounding bank is typically used to provide a source of fault current so a phase to ground fault on a delta system can be detected and interrupted. The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant "k" which is determined by the length of time required for the relay to open the breaker.

 

On a 12kV system the phase to neutral voltage is 7.2kV (12/1.73)

Suppose short-circuit current to be 3000A

 

(7.2)(3000)(K) = kVA

 

k is determined from the following table:

time k

10 sec 0.064

1 min 0.104

2 min 0.139

3 min 0.170

4 min 0.196

5 min 0.220

 

Hope above information is helpful

 

Regards

Ralph

 

Ralph,

 

As I could see you may have access to the book "Electrical Transmission and Distribution Reference Book". If it is true, could you help me to have the full table you put in the text above? In fact, I want to learn how to size a neutral grouding transformer to generators, I kown this book have it but I do really not have access on this book. if you desire you can send me a mail: andersonyb@gmail.com

 

Thank you and my best regards.

[ysaac]

This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964:

A zig-zag grounding bank is typically used to provide a source of fault current so a phase to ground fault on a delta system can be detected and interrupted. The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant "k" which is determined by the length of time required for the relay to open the breaker.

 

On a 12kV system the phase to neutral voltage is 7.2kV (12/1.73)

Suppose short-circuit current to be 3000A

 

(7.2)(3000)(K) = kVA

 

k is determined from the following table:

time k

10 sec 0.064

1 min 0.104

2 min 0.139

3 min 0.170

4 min 0.196

5 min 0.220

 

Hope above information is helpful

 

Regards

Ralph

 

Ralph,

 

As I could see you may have access to the book "Electrical Transmission and Distribution Reference Book". If it is true, could you help me to have the full table you put in the text above? In fact, I want to learn how to size a neutral grouding transformer to generators, I kown this book have it but I do really not have access on this book. if you desire you can send me a mail: andersonyb@gmail.com

 

Thank you and my best regards.

[driwatson]

I want to know the advantages of neutral grounding transformer,used in 3.3kv power generation & all generators (total 5) 3.2MW each star point not grounded?

 

 

Type of neutral earthing depends upon the protection relays you wish to use to protect generator outgoung feeders and network in general. To generate enough current for protective relay to operate, zig-zag (broken star) transformers are often used. When so, the secondary of such transformer is used to supply power to auxiliary services (0.4 kV). Care shall be taken that such transformer is designed with delta winding not connected to anything (internal winding) to minimise the risk of protective relay nonselective tripping.

 

If you run more than one generator on the same bus using dedicated earthing transformer has some advantages. If you run single generator on busbar, Neutral Earthing Resistor connected to generator's star point may be simpler to use. Please note that in such case, you will need to provide from somewhere 0.4 kV supply for plant auxiliary services.

 

Before you go any further I strongly suggest you engage professionals who do this for living. BTW it is not me, I am engaged in a similar subject bur for distribution networks

[Bysikmode]

A great exchange of information.