7PG21 Solkor Rf Applications Guide
differential trip. This achieves intertripping of the remote end circuit breaker without the requirement of a
dedicated additional communications channel or intertrip receive equipment and associated trip relay at the
remote end. Additional equipment (TEC/B34) is required to disconnect the local end relay and apply suitable
delays to achieve the best attainable compromise of remote end operation time when the injection signal is
compared to the full range of possible measured remote end current.
Injection Intertripping is generally difficult to apply successfully in conjunction with Overcurrent Guard relays since
the remote Guard relay will block operations resulting from intertrip injection if the remote end CT current is below
the Guard setting.
Inputs to the Injection intertripping are supplied for pulsed and sustained intertrip. A 2 second self resetting
intertrip pulse is usually specified to operate the remote relay. A sustained intertrip is often specified to cause a
persistent trip at the remote end which will stop auto-reclose and prevent manual close at the remote end for a
fault which is identified as permanent at the local end.
When sustained intertripping is applied, the inverter output can be reduced to a level which will hold the remote
Solkor in the operated condition, after the initial higher level has been used to ensure high speed operation. This
Two Stage Intertripping is achieved by switching an external resistor into the inverter circuit after a 2 second time
delay following an intertrip initiation. For this purpose a TEC relay with an additional 2s time delayed element
providing a normally closed output contact is available. The connection arrangement is shown in the Diagrams
section of this manual.
9
Capacitive Charging Currents
Significant electrical capacitance exists between HV primary conductors and the adjacent earth such that a
capacitive charging current will exist with any energised line. The level of current is dependent on the system
voltage, the feeder length and the construction including materials and proximity of earthed conductors. The
highest levels are found in separate phase, individually screened and armoured conductors with lowest levels
found on overhead line feeders. These currents are generally supplied from one end only as balanced 3 phase
and as such constitute a differential current to the relays but is usually significantly lower than relay 3P setting.
During out of zone earth faults however, the voltage on the faulted phase may be significantly depressed such
that the charging current is reduced. The Solkor summation transformer will measure charging current on two
phases only and interpret this as a residual differential current for which relay settings are significantly lower than
for 3P balanced differential current. This issue is compounded in systems which are not solidly earthed because
the unfaulted phase voltage may increase, leading to increased charging current on these phases, during an
earth fault. The transient switching of charging current limits the maximum charging current to 1/3 of the most
sensitive earth fault setting for solidly earthed systems or 1/9 of the most sensitive earth fault setting for
resistance earthed systems.
On higher voltage systems, where separate single phase cables are more commonly used and feeders are
generally longer it is common to find phase segregated Solkor Rf systems where 3 separate Solkor relays are
fitted at each end, each connected to a separate pairs of pilots with one phase of the system CT connected to
each relay. This avoids the problem of summation of charging currents.
10 N/N1 Setting
The N1 tap can be used to increase the relay sensitivity to earth faults by lowering settings for these faults without
affecting the phase fault settings. This may be particularly desirable for the 15kV scheme where all settings are
naturally raised by the increase in energy required to drive the additional isolation transformers. It must be noted
that the use of the N1 tap will increase the burden on the CT and therefore should only be used if the CT knee
point voltage Vk easily exceeds the minimum requirements stated below, which is often the case with modern
CTs. Prior to the introduction of cold rolled iron in CT design, the CT magnetising current effects could cancel out
any reduction in setting by increasing the excitation currents required at the higher level of relay burden. Care
should be taken when applying the N1 tap to older designs of CT with limited Vk.
The Primary in Zone Capacitance may also limit the use of the N1 tap as loss of charging current may lead to
mal-operation at the lower earth fault setting as described above
.
11 In-Zone Tapped Load
The relay is able to tolerate a limited amount of tapped off balanced load within the zone of protection based on
the relatively insensitive level of fault setting for balanced 3P differential current. The typical setting is 72% for 3P
faults or differential load current. To allow for switching transients of the tapped load a factor of 3 is advisable. The
steady state feeder charging current and CT inaccuracy will also erode the stability margin resulting in a
maximum bleed off of 10-20% of rated load current. Zero sequence infeed during out of zone earth faults from
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Chapter 3 Page 12 of 15
