High Voltage Generation and Distribution
4.6 The Neutral Point of a Supply System
Chapter 4
4.5.3.1 The Effects of Diode Failure
1. If an open circuit occurs in any diode, then the remaining healthy diodes would continue to supply the main field.
2. But when the AVR controls the exciter field, the current would be automatically boosted
a a e e e a a e e e d de a e a bab
undetected; but this will gradually overheat the exciter. But when load of the generator is increased more than 50 %, there will be a drop in the voltage and the AVR cannot compensate it any further.
3. If the diode short circuits, it is far more serious as it leads to a short-circuited exciter;
rapid overheating of the exciter will occur.
Figure 4.14 – R a Di de i he A e a R
Varistor / Silistor metal oxide semiconductors are placed in the field coil in order to protect the diodes against high voltage surges that occur mainly due the sudden change in the excitation of the field coil. Under normal operating conditions, the Silistor has a very high resistance which is used for surge absorption. However, when the voltage across the Silistor exceeds its rated value, its effective resistance decreases strongly and bypasses the surge voltage generated by the inductance effect of the field coil; in this manner, the diode is protected.
High Voltage Generation and Distribution
Ph1 Ph2
Ph3 N
Figure 4.15 The Neutral Point
Shipboard main LV systems at 440 V are normally provided with a neutral insulated system. On the other hand, HV systems are usually provided with neutral earthed system via a neutral earthing resistor.
In a HV system, certain essential loads can be supplied by a transformer with its secondary insulated to ensure no earth fault current flows in the equipment. This maintains the continuity of service.
Maritime regulations require that the hazardous areas of a tanker namely the cargo area, the pump room should have a neutral insulated system to prevent any stray earth current from flowing in the hull and causing an explosion hazard.
However, an exception is included in case if tanker has a 3.3 KV system, the earthed system is permitted provided that the earthed system does not extend forward of engine room bulkhead and into the hazardous area.
4.6.1 IEC Regulations Related to Shipboard Neutral Systems
Distribution systems shall be three-phase, 3-wire and may be operated with the neutral either insulated or grounded.
Where the system neutral is insulated, the dielectric strength of all electrical equipment is to be sufficient to withstand any possible transient over-voltage with respect to the ground.
Such a system is called a neutral insulated or neutral isolated system. Where the system neutral is grounded, the connection to the ground shall be made through a resistor which can
Chapter 4
If an earth fault occurs in an earthed distribution system, it would be equivalent to a short- circuit fault across the load via the ship s hull. The resulting large earth faults current will immediately trip the circuit breaker in the line; thus, the equipment is isolated from the supply and rendered safe. This may result in hazardous situations, if the equipment is classed as essential for e.g., the steering gear. Thus the earthed distribution system requires only one earth fault on the line conductor to cause an earth fault current to flow and the system to trip.
If the earth fault occurs in insulated neutral distribution system , it will not cause any equipment to go out of operation and thus maintains the continuity of operation of the equipment. Thus, a single earth fault will not complete a circuit for the fault current to flow. If a second earth fault occurs, only then the two earth faults together, would be equivalent to a c c a ( a ) e in a large current flowing and would operate the protection devices, cause disconnection of perhaps essential services thereby creating a risk to the safety of the ship.
An insulated neutral distribution system requires two earth faults on two different lines to cause an earth fault current to flow. Thus, an insulated neutral system, is, therefore, more effective than an earthed system in maintaining continuity of supply to equipment. It also prevents circulating currents in the hull which in the case of hazardous areas, can be disastrous.
On occurrence of an insulation fault or a phase being accidentally earthed, the values taken by the fault currents, touch voltages and overvoltage are closely related to the type of neutral earthing connection. An earthed neutral system helps to limit overvoltage; however, it generates very high fault currents. On the other hand, an isolated or unearthed neutral limits fault currents to very low values but encourages the occurrence of high overvoltage. In any installation, service continuity in the presence of an insulation fault also depends on the earthing system. An unearthed neutral allows continuity of service in medium voltage, if the security of persons is respected.
On the other hand, an earthed neutral, or low impedance-earthed neutral, requires tripping to take place on occurrence of the first insulation fault. The extent of the damage to some equipment, such as motors and generators having an internal insulation fault, also depends on the earthing system.
High Voltage Generation and Distribution
Equipment Grounding Conductor Ground Bus
Grounding Electrode
To Loads
Loads Source
Grounding Impedance
Other Grounds Permitted on Equipment Grounding Conductor
Figure 4.16 Low Impedance Grounding 4.6.2 Importance of Neutral Grounding
There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. This protective measure offers many advantages over an ungrounded system, like,
1. Reduced magnitude of transient over voltages 2. Simplified ground fault location
3. Improved system and equipment fault protection 4. Reduced maintenance time and expense
5. Greater safety for personnel 6. Improved lightning protection 7. Reduction in frequency of faults.
Chapter 4
4.6.4 Ungrounded / Neutral Insulated Systems
In an ungrounded system, there is no internal connection between the conductors and ea . H e e , a a e , a ca ac e c e be ee e e c d c
a d e ad ace ded ace . C e e , e ded e a ca ac e ded e b e e d b ed ca ac a ce.
Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral point for the system; Thus, the phase conductors are stressed at only the line-to-neutral voltage above the ground potential.
N
R Y B
CR
CY
CB
Figure 4.17 An Ungrounded Capacitive System
But problems can arise in ground fault conditions. A ground fault on one line results in a line-to-line voltage appearing throughout the system.
N
R Y B
F IC
IR
IY CR
CY
Figure 4.18 A Ground Fault in an Ungrounded Capacitive System
Thus, a voltage 1.73 times the normal voltage is present on all insulation points in the system. This situation can often cause failures in older motors and transformers, due to insulation breakdown; every phase has inherent distributed capacitance with respect to the earth.
High Voltage Generation and Distribution
If an earth fault occurs on phase B, the distributed capacitance discharges through the fault. The capacitance again gets charged and gets discharged. Because of this, sever voltage oscillation occurs in healthy phases. These voltage oscillations cause stress on the insulation of the connected equipment.
4.6.4.1 Advantages
After the first ground fault, assuming it remains as a single fault, the circuit may continue to operate until a convenient shut down for maintenance can be scheduled.
4.6.4.2 Disadvantages
1. The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times the normal value) to appear from the line to the ground during normal switching of a circuit with a line-to ground fault (short). These over voltages may cause insulation failures at points other than the original fault.
2. A second fault on another phase may occur before the first fault can be cleared. This can result in very high line-to-line fault currents, equipment damage and disruption of both circuits.
3. The cost of equipment damage may be high.
4. Complicates the location of fault(s), involving a tedious process of trial and error by first isolating the correct feeder, then the branch and finally the equipment at the fault. The result is unnecessarily lengthy and results in expensive downtime.