Grid Current to calculate GPR 1

[Kiribanda]

Hello all members,

The following issues are regarding a Ground grid design for a 230 kV outdoor switchyard.

Assume the switchyard feeding an industrial plant consists of a step down transformer of 230 to 13.8 kV. The network has got the following allowable or calculated P-E fault currents.

230 kV side - 10 kA (Feeding from the Utility)

13.8 kV level – 400 A (Low resistance grounding)

480 V LV level – 20 kA (Solid Grounding)

1)If a Ground grid is to be designed for the 230 kV outdoor switchyard, what should be the value of the grid current out of above three, to be taken for calculating the step, touch, GPR voltages etc.?

2)If the maximum earth fault current value on the network is taken as the Grid current regardless of the voltage level, in this example 20 kA, then is it an over designed, expensive ground grid?

3)Is it necessary to design a ground grid by taking the grid current as 20 kA for LV substations even though it is an indoor substation?

4)Are ground grids installed under the concrete floors of an indoor substation too?

Your valuable inputs are welcome!

Regards!

Kiribanda

 

[jghrist]

For calculation of step- and touch-potentials and GPR, the current that counts is the current that flows through the earth. Fault currents on the 13.8 kV and 480 volt systems flow through metallic ground wires back to the source (local transformers), so these do not create earth potentials (except to the extent of the impedance of the metallic return paths). Much of the 230 kV fault current will flow through the earth to get back to the source (remote 230 kV substations). Part of the fault current will flow through the 230 kV shield wires. The portion of 230 kV fault current that flows through the earth is the grid current to use in calculating step- and touch-potentials and GPR.

 

[cuky2000]

1) If a Ground grid is to be designed for the 230 kV outdoor switchyard, what should be the value of the grid current out of above three, to be taken for calculating the step, touch, GPR voltages etc.?
The total current considered in calculating the ground grid should be injected into the ground (split factor). This current is a fraction of the total SC current that could be estimated based on the number of incoming/outgoing feeder and the footage grounding resistance of the towers near the substation. A common engineering practice is to add the SC growth or limit the SC to the interrupting rating of the protective devices.[/color blue]

2) If the maximum earth fault current value on the network is taken as the Grid current regardless of the voltage level, in this example 20 kA, then is it an over designed, expensive ground grid?
a- Overdesing could be avoided if you know in depth how to calculate the require ground grid or used a good grounding software. For many substations, the limiting factor is the compliance with the minimum resistance imposed by the utility company that typically is 1 Ohm for 230 kV substations.
b- It is possible to calculated the maximum fault current from a representative HV, MV & LV electrical model. However, is more practical to use tested and proven programs commercially available in the market. The downside is this suggestion is that a good grounding program could be very expensive. Other alternative is to use specialized consulting.
Beware that the worst SC case for grounding calculation could happen in any place in the system many time located in the transmission line outside the substation.[/color blue]

3) Is it necessary to design a ground grid by taking the grid current as 20 kA for LV substations even though it is an indoor substation? The transformer and other power apparatus should be grounded and the step and touch potentials must be less than the allowable safe values as require for the standards (Consult the IEEE Std 80)[/color blue]

4) Are ground grids installed under the concrete floors of an indoor substation too? Yes. Beware that in concrete the allowable step and touch potential will be reduced significantly since the resistivity of concrete is 50 Ohm-m compared with 3000 Ohm-m for crushed rock in the substation. Take advantage that the concrete slab and grounding the steel rebar to reduce the need for large amount of copper. [/color blue]

 

[jbartos]

Suggestions to Kiribanda (Electrical) Mar 12, 2004 marked ///\\The following issues are regarding a Ground grid design for a 230 kV outdoor switchyard.
Assume the switchyard feeding an industrial plant consists of a step down transformer of 230 to 13.8 kV. The network has got the following allowable or calculated P-E fault currents.
230 kV side - 10 kA (Feeding from the Utility)
13.8 kV level – 400 A (Low resistance grounding)
480 V LV level – 20 kA (Solid Grounding)
1)If a Ground grid is to be designed for the 230 kV outdoor switchyard, what should be the value of the grid current out of above three, to be taken for calculating the step, touch, GPR voltages etc.?
///20kA since the fault current multiplied by resistance or impedance is causing elevated potential levels dangerous to touch and step.\\2)If the maximum earth fault current value on the network is taken as the Grid current regardless of the voltage level, in this example 20 kA, then is it an over designed, expensive ground grid?
///It is necessary to understand that current multiplied by resistance causes voltage, i.e. Ohms Law is needed, E=RI.\\3)Is it necessary to design a ground grid by taking the grid current as 20 kA for LV substations even though it is an indoor substation?
///Yes, since indoor locations do have floors with relatively small resistivities of floor materials, e.g. concrete, false floors, etc.\\\
4)Are ground grids installed under the concrete floors of an indoor substation too?
///Yes.\\\