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Grounding question No.11 - Power transformer grounding
What is the advantage and disadvantage of combination of power transformers MV/LV grounding systems? Whether can we use the cable armor as grounding conductor in power system?
Author : SABA - From: IRAN
 
#1
Tue, March 10, 2009 - 14:09
Section 250-118 of NEC: 1999 permits the use of various grounding conductors including tubular conduits used as cable raceway, cable armor and cable trays. Where grounding conductors are employed, it must be ensured that they are sized to withstand ground fault currents of value and duration appropriate to the circuit under consideration.
If the conductor sizing is inadequate, the following may result:
-Damage to the insulation of the ground conductor: In case the conductor is bare, insulation damage may occur in the phase conductor with which it is in contact.
- Fusing of the ground conductor in extreme cases: Though this will result in the interruption of the fault current, it will cause the potential of the enclosure of the faulted equipment to rise beyond acceptable limits.
The maximum temperature of the conductor during a fault can be calculated by the formula:



For the purpose of reviewing the selection, the following value of Tm can be used:
Copper fusing temperature 800 ºC.
Maximum permissible temperature of insulation:
For thermo-plastic insulation 150 ºC
For XLPE insulation 250 ºC.
Current and time setting of interrupting device should not cause unacceptable temperature rise of the grounding conductor. If, however, this exceeds the limits of temperature, the following are the options:
Increase the size of the grounding conductor
Adding special ground fault equipment to the system to sense even low value of earth fault currents and trip the circuit faster.

Industrial power systems typically use the scheme shown in Figure . It may be noted that the transformers are cable fed and they do not incorporate surge arrestors. The transformer LV neutral is grounded directly to an earth electrode and at the same time connected to the plant grounding network. The tank is connected to the ground network using at least two earth leads. The LV system neutral and the ground network of the plant to which the safety ground of all equipment is connected are thus bonded all through with metallic connections and faults to enclosures.



On the other hand, the situation in a remotely installed pole-mounted transformer fed by an overhead line and again feeding isolated consumers through LV overhead lines is somewhat different. It is noteworthy that the installation recommends the LV neutral to be grounded at a point well away from the transformer (usually at the first LV pole). The tank earthing is mainly to take care of faults between the MV system and the tank. The MV windings are protected by surge arrestors mounted on the body of the transformer and the LV neutral is connected to the transformer tank through a neutral surge arrestor. Though the MV surges will be transmitted into the LV system by the coupling effect, the high surge impedance of the LV lines will prevent the surges being propagated into the LV system.




Also the surges being of very short duration will not pose a safety hazard. This design takes into account the following factors:
Need to prevent the LV neutral from assuming dangerous touch potential when an MV to tank fault occurs
Need for limiting the voltage across the neutral arrestor
Need for detecting MV to tank faults by MV earth fault protection.

We will now discuss in some detail the reasons for the grounding philosophy adopted for pole-mounted MV/LV transformer substations illustrated in Figure

LV grounding

The purpose of LV system grounding is as follows:
To maintain the LV neutral potential to as close to earth potential as possible thereby prospective touch voltages in all the grounded metal parts of equipment
To provide a low-impedance return path for any LV ground faults
To ensure operation of MV protection in the event of an inter-winding fault (MV and LV) within the transformer.

MV grounding

The surge arrestors of MV lines are connected to the transformer tank, which in turn is grounded through the MV, ground electrode. This limits the voltage between the tank and the lines to the voltage drop across the arrestors in the event of a surge. In case the arrestors are connected by a separate lead to the ground, the voltage drop across the resistor would also additionally appear between lines and tank and cause insulation failure.


Combined MV/LV grounding
Though it is theoretically possible to have a combined ground at the transformer for both MV and LV, such a practice may lead to unsafe conditions in the event of an MV to LV fault. Figure shows the reason.



The total impedance for a fault between HV and MV winding (neglecting the line impedance and the leakage impedance of the transformer windings) is the substation ground mat resistance of 10 Ω, the NGR (neutral grounding resistance) value and the MV/LV combined ground electrode resistance assumed as 1 Ω.
For a 22-kV system (with line to ground voltage of 12 700 V) the current flow is:



35 Ω being the NGR value for a 22-kV system.

This gives a figure of 276 A. This current will cause the potential of 276 V to appear on the transformer tank and through the neutral lead to the enclosures of all equipment connected in the LV system with respect to true earth potential (this is because in TN-C-S type of systems, which we saw in the last chapter, the neutral and equipment ground are one and the same). This value is unacceptably high. In actual practice, the line and transformer impedances come into play and the value will therefore get restricted to safe values. Use of combined MV and LV grounding is therefore possible only if the ground resistance can be maintained below 1 Ω.

Separate MV/LV grounding
MV electrode
In view of the difficulty of maintaining a very low combined ground resistance arrived at above, the code allows the use of a separate ground for the LV neutral away from the transformer. The only point of connection between the LV system and the transformer tank is the LV neutral surge arrestor whose grounding lead is connected to the transformer tank (refer Figure).



The problem with this connection is that a fault within the transformer (MV winding to core fault) resulting in rise of voltage can cause a high enough voltage to ground causing the neutral surge arrestor to fail and communicate the high voltage into the LV system.
Assuming a maximum LV voltage of 5000 V for withstand of neural surge arrestors, the voltage rise across the MV ground electrode resistance should not be greater than this value. For a 22-kV system (with line to ground voltage of 12 700 V) the ground electrode voltage can be calculated using the potential division principle as follows.



where Rm  is the resistance of MV ground electrode. It can be calculated that Rm can have a value of 29 Ω to be able to limit the voltage.
For 11-kV system, a value of 100 Ω is permissible. The limit for the electrode resistance should also consider the ground fault current so that the MV ground fault relay can operate reliably to isolate the fault.
A value of 30 Ω is taken as the limit for ground electrode systems for all MV systems. Standard configurations are available in the code for 30 Ω electrodes and can be used in the design.


LV electrode
The LV electrode resistance should be normally expected to permit sufficient fault currents for detection. Since with the LV line to neutral voltage of 240 V, the resistance limit works out to 2.4 Ω if a ground fault current of 100 A is to be obtained. However, with the TN-C-S type of system, all equipment enclosures are directly connected to the neutral at the service inlet itself and thus the current flow does not involve the ground path at all.
So, the limit of LV grounding resistance is decided by the criteria of obtaining sufficient fault current when there is an MV to LV fault without involving the tank or core (refer Figure).



Assuming an MV earth fault protection setting of 40 A, the ground loop resistance can be arrived at 318 Ω (12 700/40) for 22-kV system. The permissible ground electrode resistance works out to 273 Ω (after taking off the values of NGR and substation ground resistance).
If we consider a safety factor of 400%, the maximum value of LV ground resistance can be taken as 68 Ω. The safety factor will ensure that the seasonal changes of soil resistivity will have no adverse effect on protection operation. Standard configurations are available in the code for 70 Ω electrodes and can be used in the design.
 
Author : Hamid - From: IRAN
 
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