The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
The effects of autotransformer action and variations in the effective impedance of the winding with fault position prevent this, making the amount of winding beyond the terminals which is protected very small.
The value of the system is confined to the feeder, which, as stated above, receives high-speed protection throughout.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
The effects of autotransformer action and variations in the effective impedance of the winding with fault position prevent this, making the amount of winding beyond the terminals which is protected very small.
The value of the system is confined to the feeder, which, as stated above, receives high-speed protection throughout.
A distance scheme is not, for all practical purposes, affected by varying fault levels on the and is therefore the best scheme to apply if the fault level may vary widely.
In cases where the fault level is reasonably constant, similar protection can be obtained using high set instantaneous overcurrent relays.
where:
Referring to Figure 2, the required setting to ensure that the relay will not operate for a fully offset fault is given by:
Where IF2 is the fault current under maximum source conditions, that is, when is minimum, and the f covers possible errors in the system impedance details used for calculation of , together with relay and CT errors.
Let the setting ratio resulting from setting be:
Therefore,
Hence,
where:
It can be seen that for a given transformer size, the most sensitive protection for the line will be obtained .
The instantaneous protection is usually applied with a having a lower current setting. In this way, instantaneous protection is provided for the feeder, with the time-delayed element covering faults on the transformer.
When the power can flow in the transformer-feeder in either direction, overcurrent relays will be required at both ends.
In the case of (figure 3), it is essential that the overcurrent relays on the low voltage side be directional, operating only for fault current fed into the transformer-feeder.
If non-unit, non-directional relays are applied to parallel feeders having a single generating source, any faults that might occur on any one line will, regardless of the relay settings used, isolate both lines and completely disconnect the power supply.
With this type of system configuration, it is necessary to apply directional relays at the receiving end and to grade them with the non-directional relays at the sending end, to ensure correct discriminative operation of the relays during line faults.
is normally provided. When the high voltage winding is delta connected, a relay in the residual circuit of the line current transformers gives earth fault protection which is fundamentally limited to the feeder and the associated delta-connected transformer winding.
The latter is unable to transmit any zero sequence current to a through earth fault.
External earth faults cause the transformer to deliver zero sequence current only, which will circulate in the closed delta connection of the secondary windings of the three auxiliary current transformers. No output is available to relay B.
As the earthing of the neutral at a receiving point is likely to be solid and the earth fault current will therefore be comparable with the phase fault current, high settings are not a serious limitation.
Earth fault protection of the low voltage winding will be provided by a restricted earth fault system using either three or four current transformers, according to whether the winding is delta or star-connected
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
The effects of autotransformer action and variations in the effective impedance of the winding with fault position prevent this, making the amount of winding beyond the terminals which is protected very small.
The value of the system is confined to the feeder, which, as stated above, receives high-speed protection throughout.
A transformer-feeder include a transformer directly connected to a transmission line circuit without the intermediation of a HV switchgear. The saving in HV switchgear so achieved is unfortunately lowered by increased complication in the necessary protection.
The primary requirement is intertripping, since the feeder protection remote from the transformer will not respond to the low current fault conditions that can be detected by restricted earth fault and Buchholz protections.
Either unrestricted or restricted protection can be applied.
Moreover, the transformer-feeder can be protected as a single zone or be provided with separate protections for the feeder and the transformer. Examples are shown in Figure 1.
In the latter case, the separate protections can both be unit type systems.
An adequate alternative is the combination of unit transformer protection with an unrestricted system of feeder protection, plus an intertripping feature.
The following sections describe how non-unit schemes are applied to protect transformer-feeders against various types of fault.
High-speed protection against phase and earth faults can be provided by .
The transformer constitutes an appreciable lumped impedance. It is therefore possible to set a distance relay zone to cover the whole feeder and reach part way into the transformer impedance.
The effects of autotransformer action and variations in the effective impedance of the winding with fault position prevent this, making the amount of winding beyond the terminals which is protected very small.
The value of the system is confined to the feeder, which, as stated above, receives high-speed protection throughout.
A distance scheme is not, for all practical purposes, affected by varying fault levels on the and is therefore the best scheme to apply if the fault level may vary widely.
In cases where the fault level is reasonably constant, similar protection can be obtained using high set instantaneous overcurrent relays.
where:
Referring to Figure 2, the required setting to ensure that the relay will not operate for a fully offset fault is given by:
Where IF2 is the fault current under maximum source conditions, that is, when is minimum, and the f covers possible errors in the system impedance details used for calculation of , together with relay and CT errors.
Let the setting ratio resulting from setting be:
Therefore,
Hence,
where:
It can be seen that for a given transformer size, the most sensitive protection for the line will be obtained .
The instantaneous protection is usually applied with a having a lower current setting. In this way, instantaneous protection is provided for the feeder, with the time-delayed element covering faults on the transformer.
When the power can flow in the transformer-feeder in either direction, overcurrent relays will be required at both ends.
In the case of (figure 3), it is essential that the overcurrent relays on the low voltage side be directional, operating only for fault current fed into the transformer-feeder.
If non-unit, non-directional relays are applied to parallel feeders having a single generating source, any faults that might occur on any one line will, regardless of the relay settings used, isolate both lines and completely disconnect the power supply.
With this type of system configuration, it is necessary to apply directional relays at the receiving end and to grade them with the non-directional relays at the sending end, to ensure correct discriminative operation of the relays during line faults.
is normally provided. When the high voltage winding is delta connected, a relay in the residual circuit of the line current transformers gives earth fault protection which is fundamentally limited to the feeder and the associated delta-connected transformer winding.
The latter is unable to transmit any zero sequence current to a through earth fault.
External earth faults cause the transformer to deliver zero sequence current only, which will circulate in the closed delta connection of the secondary windings of the three auxiliary current transformers. No output is available to relay B.
As the earthing of the neutral at a receiving point is likely to be solid and the earth fault current will therefore be comparable with the phase fault current, high settings are not a serious limitation.
Earth fault protection of the low voltage winding will be provided by a restricted earth fault system using either three or four current transformers, according to whether the winding is delta or star-connected
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