There are two ways in which three-phase system can be operated and they are: 1. Isolated Neutral or Ungrounded Systems 2. Grounded Neutral Systems.
Way # 1. Isolated Neutral or Ungrounded Systems:
The main feature of an ungrounded system is its ability, in some cases, to clear earth faults without interruption. The self-clearing feature disappears when the length of the line becomes excessive.
A simple three phase system with isolated neutral is shown in Fig. 12.1 (a). The line conductors have capacitances between one another and to earth, the former being delta- connected, while the later are star-connected, as shown in the Fig. The effect of line capacitances on the grounding characteristic of the system is little and, therefore, can be neglected.
First of all consider a 3-phase line (perfectly transposed) having same capacitances to ground. In such a line the charging currents for each line to earth capacitor lead the phase voltage by 90° and are equal. The magnitude of these currents is given by VP/XC where VP is the phase voltage and XC is the reactance due to the capacitance of the line to ground.
ADVERTISEMENTS:
The charging currents ICR, ICY and ICB are balanced and their resultant is zero and no current flows to the earth.
Under balanced conditions, the potential of neutral is held at ground due to the presence of the shunt capacitances of the system, shown in Fig. 12.1 (a). Under balanced condition the phasor diagram is shown in Fig. 12.1 (b).
Now consider a phase to earth fault in line Y say at point F. Under these circumstances, the faulty line takes up the earth potential while the potentials of the remaining two healthy lines R and B rise from phase values to line value. The capacitance currents become unbalanced and fault current IF flows through the faulty line, into the fault and returns to the system via earth and through the earth capacitances CR and CB.
ADVERTISEMENTS:
Thus fault current IF has two components ICR and ICB which flow through capacitances CR and CB respectively under the potential differences of VRY and VBY respectively. These currents lead their respective voltages by 90° and their phasor sum is equal to fault current IF.
∵ Phase voltages are equal, say, equal to VP and capacitances from line to ground are also equal, say, equal to XC.
ADVERTISEMENTS:
Since the fault current, IF is equal to phasor sum of ICR and ICB, therefore, fault current,
Phasor diagram for fault on phase Y is shown in Fig. 12.2 (b).
The following conclusions can be drawn from the above:
ADVERTISEMENTS:
(i) There is no zero sequence current.
(ii) There will be little interference with communication lines because of the absence of zero sequence currents.
(iii) In case of one phase becoming earthed the voltages of the remaining two healthy phases to earth rise from their normal phase-to-neural voltage to fall line value (√3 times their normal value). This causes stress on the insulation of all the machines and equipment connected to the system. The voltage rise of phase above earth is sustained and thereby insulation failure is likely to occur on connected machines, though fault current in arcing ground may be negligible.
ADVERTISEMENTS:
(iv) The capacitance currents in the two healthy phases increase to √3 times their normal values.
(v) The capacitance current in the faulty phase becomes 3 times the normal value.
(vi) For operating the protective devices, it is essential that the magnitude of the current supplied should be adequate to operate them. But in case of an earth fault on an isolated neutral system, the fault current may be too small to actuate the protective devices. Thus in ungrounded systems the earth fault relaying becomes complicated.
(vii) The overvoltage’s due to induced static charges are not discharged to ground in isolated neutral systems. The voltage due to lightning surges do not find path to earth.
(viii) The danger to equipment on the occurrence of line-to-line ground fault is appreciable and danger to life in the proximity of fault is often prolonged.
(ix) A capacitive fault current IF flows into the earth. Such a current if exceeds 4-5 amperes, is sufficient to maintain an arc in the ionized path of the fault, even though the medium causing the fault has cleared itself.
The persistency of the arc due to the flow of capacitance current gives rise to a condition known as “arcing ground” in which cyclic charging and discharging of the system capacity through the fault results in high frequency oscillations being superimposed on the whole system and a build-up of very high voltages can occur. This results in phase voltage to rise to 5 to 6 times of normal voltage. The build-up of high voltages may result in insulation breakdown.
(x) At the time of an earth fault in an isolated neutral system, the capacitive currents in each phase will become unbalanced as they are contributing towards the fault current. Due to unbalanced capacitive currents it is not possible to install discriminative type fault indicator.
However, neutral displacement indicator can be connected across such system, but this will give only warning that there is a ground fault in one phase without indicating the actual location.
For the reasons mentioned above it is not desirable to operate a high voltage 3-phase system of considerable capacitance with an isolated neutral.
Hence the above discussion brings us to a conception of earthed neutrals, which has got the following advantages:
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 equipment, 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.
Effectively Grounded System:
The term ‘effectively grounded’ is now used in place of old term ‘solidly grounded’ for the reason of definition.
According to IEEE definition, a system or a portion of a system can be said to be effectively grounded when for all points on the system or specified portion thereof, the ratio of zero sequence reactance to positive sequence reactance is not greater than three (X0 /X1 ≯ 3) and the ratio of zero sequence resistance to positive sequence resistance is not greater than one (R0 / R1 ≯ 1) for any condition of operation and for any amount of generator capacity.
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.
Solid or Effective Grounding:
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, as shown in Fig. 12.3 (a)].
When there is a ground fault over any phase, the phase to earth voltage of the grounded phase will become zero, but the voltage to earth of the remaining two healthy phases will be the normal phase voltages as in this case neutral point will not shift. Under an L-G fault on phase B, as shown in Fig. 12.3 (b), the neutral and terminal B are at earth potential. The phasor diagram for such a condition is shown in Fig. 12.3 (c).
The reversed phasor is shown at VB. Capacitive current ICR leads VNR by 90° and ICY leads VNY by 90°. The resultant capacitive current IC will be phasor sum of ICR and ICY. It should be noted that in this system, in addition to capacitive currents the supply source also supplies the fault current IF. This fault current will go to the fault point F through the faulty phase and then return back to supply source through the earth and neutral connection.
The fault current IF lags behind the faulty phase voltage by approximately 90° since the circuit is predominantly inductive (due to transformers, machines and line inductances). As illustrated in Fig. 12.3 (c), the fault current IF will be in phase opposition to capacitive current IC. Due to this effect, the capacitive current IC will be fully neutralized by the large fault current.
The main features of effectively grounded neutral systems are given below:
1. Since fault current eliminates the effect of the capacitive currents, chances of occurrence of arcing grounds and over-voltages are eliminated up to a greater extent.
2. The flow of heavy fault current permits the use of discriminative protective gear.
3. Ground fault relaying is simple and satisfactory.
4. Since in this system the voltage of the healthy phases in case of a line-to-ground fault does not exceed 80% of the line-to-line voltage and is much less as compared to other types of systems, the equipment for all voltage classes are less expensive. An 84% lightning arrester instead of 105% can be used. On system operating at 115 kV and above additional savings are possible because of the transformers with the insulation graded towards the neutral are less costly.
5. The ground fault current is large. Its maximum values sometimes exceeds even the 3-phase short-circuit current.
6. Since the ground fault current is large, danger to personnel in the vicinity of fault is high.
7. Even transient ground faults may get converted into short-circuit.
8. The heavy ground fault current may cause considerable damage.
9. Because of large ground fault current, the interference due to electromagnetic induction with neighbouring communication circuits may be high.
10. Because of large fault current, the current handling in the circuit breakers is a difficult proposition. This problem is, however, overcome by employing high rupturing and high speed circuit breakers and fast operating protective relays.
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.