The following points highlight the four main methods used for grounding the neutral point of 3-phase electrical system. The methods are: 1. Reactance Grounding 2. Arc Suppression Coil Grounding (Or Resonant Grounding) 3. Voltage Transformer Grounding 4. Grounding Transformer.
Method # 1. Reactance Grounding:
Reactance grounding means grounding through impedance which is highly inductive.
For circuits between 3.3 kV and 33 kV the earth fault currents are likely to be excessive, if solid grounding is employed. Either resistance or reactance is connected in neutral to ground connection. There is no rule as regards which grounding should be used—resistance or reactance.
If resistance is used fault current is limited and system reactance provides the necessary phase opposition between capacitive ground current and fault current. The reactance grounding provides additional reactance which provides a lagging current that nullifies the capacitive ground current.
ADVERTISEMENTS:
As the value of reactance X connected in neutral to ground connection is increased, the ground faults current decreases and neutral displacement increases. If X is very small, the system behaves as an effectively grounded system. If X is very large, the system behaves as an isolated system.
Reactance grounding lies between effective or solid grounding and resonant grounding. The value of reactance required is to keep currents within safe limits. The transient voltages resulting from arcing increases as the reactance are increased. Similarly during switching operation higher values of reactance are expected to cause higher values of surge voltages.
A reactance grounded system ensures satisfactory relaying, partial grading of apparatus insulation, and reduced interference to neighbouring communication circuits as compared with that in effectively ground systems.
Other features of reactance grounding are:
ADVERTISEMENTS:
1. Ground fault current is reduced but is much larger than capacitive ground fault current.
2. The voltages across healthy phases are between 80 to 100 per cent of line-to-line voltage.
3. Arcing grounds are avoided.
4. Transient ground faults are converted into controlled current faults.
ADVERTISEMENTS:
Reactance grounding may be used for grounding the neutral of circuits where high charging currents are involved such as transmission lines, underground cables, synchronous motors,
synchronous capacitors, etc. For networks where capacitance is relatively low, resistance grounding is preferred.
Method # 2. Arc Suppression Coil Grounding (Or Resonant Grounding):
In the arc suppression coil method of grounding, the arcing-ground danger has been eliminated and the system is approximated to the isolated neutral system, in which one or two healthy phases continue to supply power and complete shutdown on the system is avoided till the fault was located and isolated.
It operates on the principle that if an inductance of appropriate value is connected in parallel with the capacitance, the fault current can be reduced considerably or even it can be neutralized. Thus the magnitude of inductance of the coil depends upon the capacitive currents flowing into the ground capacitances.
An arc suppression coil, also known as Peterson coil or Ground fault neutralizer, is an iron core tapped reactor connected in neutral to ground connection. The reactor is provided with tappings so that it can be tuned to the system capacitance. The function of arc suppression coil is to make the arcing ground faults self-extinguishing and, in case of sustained faults, to reduce ground fault current to a comparatively low value so that the system can be kept in operation with one line grounded.
On occurrence of a ground fault (say on phase B), a lagging reactive current flows from the faulted phase to the ground and returns to the system through the inductive coil. Simultaneously, capacitive currents also flow from healthy phases to ground. The lagging fault current IF and leading capacitive current IC (phasor sum of ICR and ICY) are almost in phase opposition.
By a proper selection of the value of inductance L of the arc suppression coil the two currents can be made almost equal so that there is no current through the ground fault and so there will be no arc. The combination of neutral reactance L and line capacitance C acts as a parallel resonant circuit. The circuit is shown in Fig. 12.5 (a).
If VP is the line-to-neutral voltage,
ICR = ICY = √3 VP ω C … (12.4)
ADVERTISEMENTS:
Capacitive current, IC = ICR + ICY = √3 × √3 VP ω C = 3VP ω C … (12.5)
For balance condition, IL = IC
or VP/ωL = 3 VP ω C
or L = 1/3ω2C … (12.6)
Phasor diagram is shown in Fig. 12.5 (b).
This method of neutral grounding is usually confined to medium voltage overhead transmission lines which are connected to the system generating source through power transformers.
The reasons for this limitation are:
(i) The higher insulation level is required on apparatus associated with arc suppression coil grounded system and it is comparatively difficult to provide on generators as compared on transformers. In addition, adequate winding protection cannot so readily be achieved for generators as for power transformers under arc suppression coil operating conditions.
(ii) Overhead transmission lines are normally subject to transient ground faults due to lightning etc., of short duration.
It has been found that arc suppression coil grounding reduces the line outages from ground faults to 20 or 30 per cent of those obtainable with other types of grounding. As such this method of grounding is advantageously employed on radial lines as this avoids the construction of duplicate circuits for maintaining service continuity and thus there is overall economy.
Ground fault neutralizers should not be used where – (i) fully graded insulation transformers are employed as the neutrals of such transformers are not sufficiently well insulated (ii) auto- transformers having a ratio greater than 0.95 to 1 are used.
The coils of the ground fault neutralizers are ten-minute time-rated on system where permanent ground faults can be located and removed promptly by ground relays or other suitable means. Otherwise, continuous time- rated neutralizers are used on all other systems.
A short-time rated coil is equipped with an automatic circuit breaker, which by action of a relay shorts the arc-suppression coil after a certain time lag and connects the neutral directly to the ground, as shown in Fig. 12.6. Circuit breaker is normally open but is closed by the trip coil when the relay operates after a predetermined time. With this the fault current is bypassed through the resistor branch.
A short-time rated coil allows the clearing of intermittent faults, without any interruption of supply. Prolonged faults are removed by isolating the faulty section by the protective relay. A continuously rated coil allows a fault to remain in the system till it is located and removed.
An arc-suppression coil is also provided with an auxiliary winding for the energization of a relay to operate the short-circuiting device of a short-time rated coil.
Special features of arc-suppression coil grounding are given below:
1. During a single line-to-ground fault, the voltages across healthy phases rise to line-to-line value.
2. Continuity of supply can be maintained for long periods with one line grounded till the faulty section is isolated.
3. During sustained ground fault, the current is a small fraction of capacitive ground fault current. Burning and heating effects are reduced to minimum and damage to the equipment is limited.
4. Transient ground faults are suppressed. More than two-third of such faults are cleared without necessitating the operation of the circuit breaker.
5. Voltage gradient on ground surface in the vicinity of fault is greatly reduced and so the danger to the life in the proximity of fault.
6. Inductive interference to neighbouring communication circuits is lesser in magnitude but duration may be long.
7. Relaying needs special provision and is usually restricted to indication.
8. During normal operation, the arc suppression coil has little effect on balance of system voltages to ground and losses occurring in it are negligible.
Method # 3. Voltage Transformer Grounding:
In this system of neutral grounding the primary of a voltage transformer is connected between the neutral and the ground, as shown in Fig. 12.7. A low resistor and a relay combination are connected across the secondary of the voltage transformer. An earth fault on the primary side will produce a voltage across the relay that will cause the protective device to operate.
This system of neutral earthing has got the following advantages:
1. The transient over-voltages owing to switching and arcing grounds are reduced because voltage transformer provides extremely high reactance to the earth path.
2. Arcing grounds are eliminated.
3. In this system of grounding, the neutral is grounded through a single phase voltage transformer and so the system operates virtually like an isolated neutral system.
The major drawback of this system is that the grounded neutral acts as a reflection point for the travelling waves passing through the machine winding and in order to avoid high voltage build-up connection of a surge diverter between the machine neutral point and ground, as shown in Fig. 12.7 (a), becomes imperative.
The voltage transformer acts as a fault indicator, the extent of the fault can be determined by measuring the voltage on its secondary.
The use of this system of neutral grounding is normally confined to the generator equipment which is directly connected to step-up transformers. The generating networks of such equipment are physically isolated from the main distribution network. This is because interconnecting cables between the generator and transformer windings are usually of very short length, the electrostatic capacity of the circuit will be negligible.
Method # 4. Grounding Transformer:
The neutral or star point is usually available at every voltage level from generator or transformer neutral. However, if no such point is available due to delta connections or if neutral point is desired on bus-bars, a zig-zag transformer is most commonly used. Such a transformer has no secondary.
It is a core type transformer having three limbs built up in the same fashion, as illustrated in Fig. 12.8. The connections are also shown in Fig. 12.9. It is seen that the currents in the two halves of the winding on each limb are in opposite directions. As a result there will not be any undesirable harmonic prevailing in the circuit and so stresses on the insulation of the transformer are considerably reduced.
The fault current is quite high as the impedance of earthing transformer is quite low. The resistance or resistances are inserted either in the neutral circuit or in the windings of earthing transformer, as illustrated in Figs. 12.9 and 12.10 respectively in order to limit the magnitude of the fault current.
Under normal condition, only iron losses will be continuously occurring but at the time of fault copper losses will also occur in addition, because of heavy fault current, in the earthing transformer. The duration of occurrence of copper losses is quite short, being the duration of fault, which is usually of 30-60 second duration.
As compared to the power transformer the rating of a zig-zag transformer is quite different. A grounding transformer is usually specified by the single phase fault current that it handles. The two most common intervals specified for the fault current are 30 and 60 s. A 60 s grounding transformer will be more expensive as compared to a 30 one.
If a zig-zag transformer is not available, a star-delta transformer can be used without loading the delta side, as shown in Fig. 12.11. It is a step-down transformer. The star-connected primaries are connected to the bus-bars and its neutral is grounded.
The secondaries are delta-connected and generally do not supply any load but provide a closed path for triple harmonic currents to circulate in them. Under normal operation the current in the grounding transformer is only its own exciting current. However, in the event of a single line-to-ground fault condition, large current may flow. So it should be of sufficient rating to handle the fault current.