In a Solidly grounded system, the neutral is directly connected to the earth without any intentional resistance or reactance in between. Compared to the other grounding systems, the transient overvoltages are held to a minimum when the system is effectively grounded. In an effectively grounded system, ground fault currents will be considerably higher, making ground relaying both sensitive and selective. The downside of this system is that if the fault is not cleared promptly, it will cause considerable damage to the system.
A closer look at a Solidly grounded system
Even though there is no intentional resistance or reactance between the neutral and the ground, the source impedance and the impedance of connection to the ground have to be considered while evaluating the system. Ground fault current values are very critical when determining the effectiveness of a grounded system. An effectively grounded system will have a line-to-ground
short-circuit current of at least 60% of the three-phase, short-circuit value.
A solidly grounded system or a transformer or an alternator does not provide an ideal zero impedance circuit connection to the Earth. Also, protection from transient overvoltages may not be achieved, if the system zero-sequence circuit’s reactance is too high compared to the system’s positive-sequence reactance. Similarly, desired suppression of overvoltages in unfaulted phases may not be achieved if unintentional resistance is too high.
The majority of the generators utilized in these systems have a zero-sequence impedance that is significantly lower than their positive-sequence impedance. A delta-wye transformer’s positive-sequence impedance won’t be greater than its zero-sequence impedance. However, there are some circumstances in which relatively large zero-sequence impedance can happen.
In a power system fed by several generators and transformers, if the neutral of one of them is grounded, the zero sequence impedance of the grounded source may exceed the effective positive sequence impedance of the other sources in the system.
Where is solid grounding recommended?
- In a low voltage system where ground faults cannot be sensed if resistance grounding is done.
- In a low voltage system where immediate isolation of faulted phase can be tolerated.
- When it comes to medium- or high-voltage applications, the possible hazards of arc flash and potential gradients to workers outweigh the need for a higher magnitude of ground-fault current to be able to perform selective ground-fault detection on lengthy distribution feeders.
Advantages of an effectively grounded system
- The ground faults can be easily detected and isolated by circuit protection devices.
- Due to high fault currents, short circuit protection devices such as fuses or circuit breakers are sufficient to isolate the ground faults.
- Selective fault detection and isolation are possible.
- No transient overvoltages.
Disadvantages of an effectively grounded system
- When used in medium or high voltage systems, the low ground impedance results in extremely high fault currents which may exceed the short circuit rating of the system itself.
- High fault currents can deteriorate the duty ratings of equipment connected to the system.
- Even though ground faults may not cause serious damage to low voltage systems, faults happening with motors and transformers can extensively damage the magnetic parts of the equipment.
Due to these reasons, the use of solid neutral grounding is limited to systems of lower voltage (380 V/480 V) that are often used on consumer premises. For preventing damage to crucial equipment components in all the other situations, some kind of grounding impedance is always applied.
For the use of single-phase loads (Phase-neutral connections), it is mandatory that the neutral must be solidly grounded. Otherwise the neutral will float with respect to the ground which may cause voltage imbalance and instability within the system.
Read More: Types of grounding
