Page 708 - Industrial Power Engineering and Applications Handbook

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eventuality. Refer to Tables 11.6, 14.1, 32.1(A), 13.2 the machine from heavy fault currents and prevent the
and 13.3 for more details. system from tripping on a ground fault. Such a system is
more prevalent in overhead transmission or long-
distribution networks to save the whole system from an
20.9 Application of different types outage on a ground fault.
of grounding methods (for HT, The more recommended practice is to ground the neutral
HV and EHV* systems) solidly or through an impedance, commensurate with the
requirements of the protective scheme and the fault current
limited to a desired level. The terminal equipment and the
In the preceding paragraphs we have analysed the windings of all the machines may now be designed for a
behaviour and characteristics of a system when grounded voltage corresponding to the relevant GFF.
or left isolated. The ground fault factor (GFF) plays a
very significant role in the selection of insulation level
(BIL) and its coordination with the different equipment 20.9.1 Artificial neutral grounding of a
connected on the system. The application of a particular three-phase three-wire system
method of grounding would thus depend upon
In the previous section we have discussed the theory of
providing a ground fault protection when a neutral was
The ground protection scheme envisaged to decide on already available on the system. This could be utilized
the magnitude of the ground fault current for a solid or an impedance grounding to achieve the
Criticality of the supply system, e.g. whether an imme- required level of fault current on a ground fault, and
diate trip on fault is permissible meet the requirement of the protection scheme or the
Insulation level of the main equipment connected in stability of the system. Here we discuss circuits which
the system. do not have a neutral as a matter of system design, such
as for the purpose of transmission and long HT distribution,
The grounding of a generator, for instance, which where the power is transmitted on a delta circuit to
may be designed for 6.6, 11, 15 or 21 kV, and all other economize on the initial cost, such as from the generator
equipment connected on this system may be solidly to the generator transformer (Figure 13.15) where the
grounded to have the least GFF and hence, add no extra generator is star and the transformer can be YlA, say,
cost to the machine for a higher level of insulation or a 15.75/400 kV. In such a case, when grounding is required
larger size. On the other hand, at such voltages the ground on the delta side of the transformer, this is possible by
fault current may also not be excessive to be of concern creating an artificial neutral point. The basic need for
to cause an extra burden to the windings of the machine such a provision, where a neutral does not exist, may be
or in the selection of protective devices. It is, however, necessary, primarily to achieve the following:
noted that a few application engineers may prefer an
isolated neutral system at certain installations where 1 To reduce the high-voltage transient oscillations in an
continuity of supply is mandatory, even on a ground isolated neutral system and to prevent the voltage of
fault, until an alternative arrangement is made. Examples the healthy phases from rising beyond their line to
are auxiliary drives in a power generating unit or essential neutral voltage, as far as possible. Prolonged existence
drives in a process plant. At such installations, it is of overvoltages may have a tendency to cause a short-
imperative to ensure that the generator and all the circuit from line to ground, even in the healthy phases.
equipment connected in the system are designed for the 2 A ground fault protection scheme that is easy to handle,
higher GFF and a larger size or greater cost of the machines
are immaterial. But the occurrence of another ground clear the fault quickly and prevent it from spreading.
fault before clearing the first will lead to fatality and 3 To eliminate prolonged arcing grounds as a matter of
cause total damage of the faulty equipment. The ground safety to human lives. A live conductor falling on
current can now find its way through the earlier ground ground will remain live if not grounded and cause an
fault and cannot be prevented, as there is no protection arcing through ground leakage capacitances. It may
generate excessive heat and become a hazard to life
available. Isolated systems are therefore generally not
and property.
recommended. Instead, for such require-ments, a 4 On higher voltage systems, due to ground leakage
resonance grounding system may be adopted, limiting
the ground fault current to a desired low value to protect capacitances, the voltage of the two healthy phases
may increase to twice the voltage, similar to double
charging, when switching a capacitor unit (Section
23.5.1).
*I We use HT for all voltage systems above 1.1 kV unless a 5 Electrostatic induction may take place on overhead
comparative reference is necessary. power-carrying systems, through charged clouds, dust,
2 To identify the windings of a transformer, we use HV for the rain, fog and sleet and due to changes in the altitudes
higher voltage and LV for the lower voltage side. of lines. If these induced charges are not freed through
3 When we refer to a transmission system, we classify the different grounding, they will continue to rise gradually and
maximum voltage systems as follows:
HT - up to 66 kV accumulate on the system. This is called floating
HV - above 66 to 245 kV and potential and may result in a breakdown of the system
EHV - above 245 kV. insulation or the terminal equipment.