There are two ways in which three-phase system can be operated and they are: 1. With isolated or free or ungrounded neutral 2. With grounded neutral.
Way # 1. Ungrounded Neutral System:
The 3-phase 50 Hz ac power systems with neutral earthed (or grounded) at every voltage level are used for generation, transmission, distribution and utilization of electrical power. The neutral or star points of star-connected 3-phase winding of power transformers, generators, motors, grounding transformers are connected to low resistance ground (earth electrode/earth mat). Such a connection is called neutral earthing or neutral grounding.
Before 1950s the power systems were often without neutral grounding. Such systems are called ungrounded systems or insulated systems. The chief argument in favour of such a system is the possibility of maintaining continuity of supply in case one phase gets grounded until it is convenient to disconnect and repair the faulty line.
This argument is particularly true for an overhead transmission system when the ground fault on one line is not likely to develop into a fault between two or more lines. But such systems experience repeated arcing grounds. The earth fault protection of ungrounded systems is difficult. Insulation failure may occur in several equipment and machines over entire voltage level during a single ground fault at remote location.
The ungrounded systems must necessarily have equipment insulation withstand level corresponding to next higher system voltage so that cascade insulation failures can be avoided. Such a system needs a costlier insulation system of next higher voltage level (e.g., 11 kV insulation for 6.6 kV bus-bars and motors, transformers, CTs, PTs etc.).
Ungrounded systems have advantage of negligible ground fault current but drawback of arcing grounds. Modern power systems are with grounded neutrals except some continuous process systems and essential protection/auxiliary supply systems where single phase to ground faults should not trip entire bus supply.
The chief advantage of neutral earthing are:
1. Persistent arcing grounds can be eliminated by employing suitable protective gear. The arcing ground current flowing through the neutral to ground connections is made almost equal and opposite to the capacitive currents from healthy lines to ground. Thus sum of current flowing through neutral to ground connections and capacitive currents is zero and arcing grounds are eliminated. The system is thus not subjected to overvoltage surge due to arcing grounds.
2. In this system the neutral point is not shifted (i.e., stable neutral point).
3. The voltages of healthy phases with respect to ground remain at normal value. They do not increase to √3 time the normal value as in the case of isolated neutral system.
4. Earth faults can be utilized to operate protective relays to isolate the fault in case of grounded neutral system.
5. The induced static charges do not cause any disturbance as they are conducted to ground immediately.
6. There is a possibility of installing discriminative protective gear on such systems.
7. By employing resistance or reactance in ground- connections, the ground fault current can be controlled.
8. Improved service reliability due to limitation of arcing grounds and prevention of unnecessary tripping of circuit breakers.
9. Such system provides greater safety to personnel and equipment. This is because of operation of fuses or relays on ground fault and limitation of voltages.
10. The life of insulation is long as voltage surges caused by arcing grounds are eliminated. Thereby maintenance, repairs and breakdowns are reduced and continuity of supply is improved.
11. Life of equipments, machines, installation is improved due to limitation of voltage. Thus, overall economy is achieved.
Way # 2. Grounded Neutral Systems:
In grounded neutral systems, the neutral point is grounded directly or through resistance, reactance etc., depending on particular requirement.
Neutral grounding can broadly be classified in two categories viz. effective grounding (or solid grounding) and non-effective grounding.
For non-effective grounding of neutral any of the following four methods can be used:
1. Resistance grounding.
2. Reactance grounding.
3. Peterson coil or arc suppression coil grounding.
4. Voltage transformer grounding.
1. Effectively Ground System:
The term ‘effectively grounded’ is now used in place of old term ‘solidly grounded’ for the reason of definition.
The effective grounded systems are less expensive than any other type of grounding for any operating voltage because for such a system in the event of a single line-to-ground fault, the maximum voltage of healthy phase does not exceed 80% of line-to-line voltage while for all other grounded systems the voltage of healthy phases rises to about 100% line-to-line voltage.
In solid grounding, also called the effective grounding, a direct metallic connection is made from the system neutral to one or more earth electrodes [rods, pipes or plates buried or driven into the ground.
In order that the fault current remains within the limits, this system is used on the networks where normal impedance is quite large. Experience shows that combined impedance of the apparatus, circuit and ground return path in systems operating at voltages below 3.3 kV and those operating at voltages exceeding 33 kV is sufficiently large so as to limit the value of fault current to a safe value.
2. Resistance Earthing:
When it becomes necessary to limit the earth fault current, this type of grounding is used. In this method of neutral grounding a resistor is inserted between the neutral and earth to limit the fault current to a safe value. The resistor used for neutral grounding is usually a metallic resistance mounted on insulators in a metallic frame. For voltages below 6.6 kV liquid resistors are also used. Metallic resistors do not alter with time and need little or no maintenance.
They are, however, slightly inductive and this is a drawback with overhead lines exposed to lightning, because travelling waves or impulses are subjected to positive reflection and this may cause undue stress to the insulation of the equipment and result in breakdown. Liquid resistors are free from this drawback and are normally of simple robust construction. They may be mounted outdoor.
The value of resistance used for neutral grounding should be such the ground fault current does not exceed 3-phase short-circuit current. This is necessary so that the power loss in the grounding resistance is not excessive. Neutral grounding resistors are normally designed to carry their rated current for a short period, usually 30 seconds.
Hence, resistance grounding is usually employed for the systems operating on voltage exceeding 3.3 kV but not exceeding 33 kV. For circuits below 3.3 kV (i.e., say 400 V distribution networks), the external resistance in the neutral circuit is unnecessary because the voltage available between phase and ground is only 230 V. The earth resistance of earthing electrode, earth connections etc. is of the order of 1.5 Ω.
The earth current is limited to 230/1.5, i.e., 153 A even if the grounding resistance is not used. For networks above 33 kV the capacitive currents are large enough to neutralize the reactive fault currents, and therefore no external resistance is required in neutral connection to limit the fault current.
3. Reactance Earthing:
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.
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 voltage resulting from arcing increases as the reactance is 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:
i. Ground fault current is reduced but is much larger than capacitive ground fault current.
ii. The voltages across healthy phases are between 80 to 100 per cent of line-to-line voltage.
iii. Arcing grounds are avoided.
iv. Transient ground faults are converted into controlled current faults.
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.
4. 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.
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, birdage 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.