THE B.T.H. AUTOMATIC SECTION SWITCH
There were many pieces of anciliary electrical apparatus, one of which
was the "Automatic section switch". Below is reprinted the B.T.H.
technical data for this interesting device......
B.T.H. AUTOMATIC SECTIONALING SWITCH TYPE ASC. 500
REFER DRGS. YD. 33331 AND YD. 33332
A device that
will prove of considerable value to Transport Organisations facing the question
of the equalisation of load distribution on feeders for Traction Systems, has
been developed by the B.T.H. Co, of Rugby.
The object of
the Switch is to enable more lightly loaded sections to assist adjacent
sections more heavily laden. This is of the greatest service at tines of
localised peak loads such as football matches, closing tine of works etc.
It also ensures
a ready path for regeneration currents to assist nearby loads.
On Trolley Bus
systems the negative return circuit bas a higher resistance than on a Tramway
System as the track is not available as a return conductor. It is necessary
therefore to use every means available for lessening voltage drop on the
overhead line, whether this be due to drop on the positive or negative side.
Since the speed
of a traction motor is approximately proportional to the voltage available at
its terminals, any drop in line volts reduces the speed of the service.
Hitherto, heavy
demand for current in certain areas had been associated with lowered voltage,
reduced speed of equipment, and possibly irregular operation of the control.
Efforts to meet
this difficulty have been made by bridging the section insulators, on the
overhead, by hand operated switches in section pillars.
This has meant
that the effect of a fault has been far-reaching, and has resulted in the
shutting down of a considerable area until the trouble is cleared, or the
section isolated by hand.
The B.T.H.
Automatic Sectionalising Switch has been developed to meet this particular
difficulty.
In effect, it
permits of the section insulators being bridged by means of a switch which is
normally closed, thus enabling the feeders, distributors, and the overhead to
be operated in parallel.
The result is
the reduction of feeder losses, the keeping up of line volts, the maintenance
of schedule speed, and reliability of control operation.
At the same
time, should a fault develop, the defective section is immediately isolated
from the rest of the system.
The Automatic
Sectionalising Switches open automatically together with the individual feeder
breaker feeding that section.
This means
immediate localisation of the faulty area and the maintenance of service on the
remainder of the system. Immediately the fault has been cleared and the section
breaker closed, then the Automatic Sectionalising Switches reclose
automatically.
Consider for a
moment the delay associated with the bridging of section insulators by hand
operated switches in section pillars.
On the
occurrence of a fault, a linesman has to be despatched to open section pillars
and. clear faulty section by hand.
On clearing the
line, these section pillars have to be revisited and the hand operated switches
reclosed, if this is omitted all the troubles of delayed schedule and other
complaints associated with lowered voltage are reported.
These delays and
troubles may be removed by installing the B.T.H. Automatic Sectionalising
Switches. These devices open on overload and will not reclose until fault
cleared, when they immediately reclose, and this without any attention other
than the reclosing of the feeder breaker at the substation controlling the
faulty section.
The device,
which may be arranged for pole mounting, or erection in a suitable type of
section pillar, is self-contained and does not depend on pilot wires or other
form of remote control.
It consists
primarily of a contactor, the operating coil of which is under control of a
relay having a small time lag.
A simple
overload relay is arranged to trip open the contactor in the event of trouble
on the overhead.
Once opened, the
switch will not reclose until potential has been re-established across the
section insulator, which is effected by reclosing the section breaker at the
substation.
A control switch
is incorporated to render the device inoperative if desirable.
To sum up, the
installation of these devices -
1) Isolates
faulty sections automatically.
2) Makes better
use of existing feeders.
3) Maintains
speed schedule.
4) Secures greater
continuity of supply.
5) Improves bus
lighting by keeping up volts.
6) Reduces power
distribution losses.
Systems using
regenerative braking will appreciate the fact that continuity of the overhead
will lessen pressure rises and allow of trolley buses in adjacent sections
absorbing the regenerated load, thus levelling out voltage peaks, and
increasing the efficiency of the system.
The following
technical description of the operation of the device should be read in
conjunction with Drawings YD.33331 and YD.33332.
The above
drawings show the internal connections as arranged for Double Pole and Single
Pole operation respectively, the latter being used where the negative return
circuit is earthed, the Single Pole operation being described below.
"F" is
a relay of the balanced armature type having two operating coils, each
respectively energised from opposite sides of the section insulator separating
adjacent substation areas.Any excessive difference of potential between the two
coils in either direction, opens the relay contacts.
Approximately
equal potentials on both coils allow the relay contacts to close thus
completing the operating circuit for the time delay interlock "E".
The closing of
"E" completes the circuit for the operating coil of contactor
"A" which is the contactor bridging the section insulator.The action
of contactor "A" closing, closes an interlock making a retaining
circuit for the contactor, and simultaneously other interlocks open the circuit
of the time delay interlock and open the circuits of both coils of the balanced
relay. Thus it will be noted that once the contactor is closed all circuits are
opened except that for the contactor operating coil, which is thus held
positively closed by the line potential.
An overload coil
"C" in the contactor circuit opens the operating circuit for
contactor "A". Thus on a fault occurring in either section, or should
any excessive interchange of current take place between one substation end the
adjacent one, the contactor "A" -drops open, thus automatically
isolating the two sections.When the fault or overload has been removed and
potential restored to both sections, then the sequence of operations already
described recloses the contactor.It will be noted that under normal operation
the contactor is always closed, thus levelling peak loads, providing increased
area for absorbing regenerated currents, preventing voltage peaks, and reducing
line losses.The relay is adjustable to permit of reclosing of contactor at a
per-determined difference of volts on both sides of the section insulator.If
one substation is shut down, the adjacent one carries on feeding both sections
provided no excessive overload occurs.If it is desired to deaden one section,
open both feeder breakers and reclose one; the time delay interlock ensuring
that the relay has definitely operated before the contactor operating coil can
be energised.A manually operated control switch isolates the operating
mechanism for inspection purposes.It is recommended that the device be housed
in a feeder pillar of standard design, although pole mounting can be provided.
Where current is charged on the maximum demand system per substation,
considerable economies are capable of being effected. A special layout of the
device could be arranged to meet space available in standard feeder pillar on
receipt by us of drawing showing dimensions of maximum panel size capable of
being accommodated in the pillar.
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