High Voltage Generation and Distribution
4.7 Resistance Earthed Systems
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.
Chapter 4
Ground Current
Transformer
Neutral Grounding Resistor Neutral
If
If
Figure 4.19 Connection of the NER
It also allows a sufficient flow of fault current that can help to detect and operate earth protective relays to clear the fault.
Although it is possible to limit fault currents with high resistance neutral grounding resistors (or neutral earthing resistors), the earth fault current can be reduced. Because of this fact, protection devices may not sense the fault. Therefore, it is common to limit single-phase fault currents with low impedance neutral grounding resistors, to approximately the rated current of the transformer and / or the generator.
Generator windings Generator windings
Low impedance grounding High impedance grounding
Resistor or reactor Resistor
Figure 4.20 Neutral Grounding Methods
Resistance Grounding Systems limit the phase-to-ground fault currents. The reasons for limiting the phase-to-ground fault current by resistance grounding are:
1. To reduce the arc blast or flash hazard.
High Voltage Generation and Distribution
3. To secure control of the transient over-voltages while at the same time.
4. To improve the detection of the earth fault in a power system.
As an example, the value of the NER to limit the earth fault current to the full load rating for a 2 MVA, 0.8 PF, 6.5 kV 3-Phase alternator,
T e a e a full load current = 2,000,000 = 222 A 1.732x 6500 x .8
Under earth fault conditions, the phase voltage = 6500 / 1.732 = 3757 V will drive through the NER.
So, the value o e NER = 3757 / 222 = 17 4.7.1 Advantages
1. Enables high impedance fault detection in systems with a weak capacitive connection to the earth.
2. Some phase-to-earth faults are self-cleared.
3. The neutral point resistance can be chosen to limit the possible over-voltage transients to 2.5 times the fundamental frequency maximum voltage.
4. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase- to-ground arcing current conditions only.
5. It will eliminate the mechanical damage and may limit thermal damage to a shorted transformer and rotating machinery windings.
6. Prevents operation of over-current devices until the fault can be located (when only one phase fault occurs with respect to the ground).
4.7.2 Disadvantages
1. Generates extensive earth fault currents when it is combined with a strong or moderate capacitive connection to the earth.
Chapter 4
Resistance Grounding Systems thereby have many advantages over solidly grounded systems including arc-flash hazard reduction, thereby limiting mechanical and thermal damage associated with faults, and controlling transient over voltages.
4.7.3 Marine and Offshore NERs
A 7.5 A 10 second / 1.5A continuously rated NER utilising a VT and CT for use within the protection system of a 6.6 kV / 11 kV generator supply, for marine and offshore applications utilises coiled coil resistors mounted between insulation boards. The surface coating on this design is suitable for marine and offshore environments and is painted stainless steel. Other materials, such as unpainted 304 or 316 stainless steels can easily be used.
Figure 4.21 A Marine NER Box
Figure 4.22
A Marine Neutral Earthing Transformer Box
4.7.4 Monitoring of the NER
The value of the NER should neither be very low nor very high. If the value of the earthing resistance R is very low, the earth fault current will be large and the system becomes like the solid grounding system. On the other hand, if the earthing resistance R is very high, the system conditions become like an ungrounded system.
High Voltage Generation and Distribution
0 V 0 V
NER short-circuited NER open-circuited
0 V 0 V
NER short-circuited NER open-circuited
a) Desired fault current cannot flow.
b) Ungrounded system.
a) Undesired fault current can flow.
b) Solidly grounded system.
Figure 4.23 Faults in the NER 4.7.5 NER Monitoring Relay
The NGR-Monitoring and Protection Relay is utilised and made specifically for a failsafe condition.
It has a highly insulated sensor. This sensor is connected to the neutral terminal of the generator or transformer. The sensor has also the capacity to protect the relay from any sudden transient peak voltage.
This relay ensures the fail-safe function of a restricted neutral grounding system
Chapter 4
Supply Gnd
Sensor Reset
Power Fault
1 2
3 4 5
Sensor
NER L1
L2 L3
Earth CB Trip
NER Monitoring
Relay
Gen Neutral
Figure 4.24 NER Monitoring Relay
It continuously monitors through its sensor, the continuity of the NER (or variation in the resistance value of the NER) as well as the continuity of neutral grounding connections. It trips the circuit breakers as and when any open circuit (or variation in resistance value) occurs in the NER, in the neutral grounding connections or in the relay ground and sensor connections.
This relay provides earth fault protection also to save the NER. The relay senses the earth fault current by its sensor. When the fault current exceeds the limit, the relay operates to trip the circuit breaker. The relay by its own earth-fault feature protects the NER independently.
4.7.6 Isolation of the NER
As per the IEC standards, a disconnecting means shall be fitted in each generator neutral ground resistor connection to permit complete isolation of the generator for maintenance purposes.
Neutral Earthing Resistors (NERs) can be fitted with manual or motorised off-load isolators, either singly or in interlocked pairs if the purpose is to switch between resistance earthing and solid earthing
High Voltage Generation and Distribution
Figure 4.25 Isolation of the NER