Transducers
measure power system parameters by sampling instrument transformer secondaries.
They provide a scaled, low-energy signal that represents a power system
quantity that the SA interface controller can easily accept.
This shield
should be grounded where it leaves a station control house to enter a
switchyard and where it reenters another control house. These grounds are
terminated to the station ground mass, and not to the SA analog grounds bus.
Transducers
also isolate and buffer the SA interface controller from the power system and
substation environments. Transducer outputs are dc voltages or currents in the
range of a few tens of volts or milliamperes. Transducers measuring power
system electrical quantities are designed to be compatible with instrument transformer
outputs.
Potential
inputs are based around 120 or 115 Vac, and current inputs accept 0 to 5 A.
Many transducers can operate at levels above their normal ranges with little
degradation in accuracy provided their output limits are not exceeded.
Transducer
input circuits share the same instrument transformers as the station metering
and protection systems; thus, they must conform to the same wiring standards as
any switchboard component.
Wiring
standards for current and potential circuits vary between utilities, but
generally 600-V-class wiring is required, and no. 12 AWG or larger wire is
used. Special termination standards also apply in many utilities.
Test
switches for “in-service” testing of transducers are often provided to make it
possible to test transducers without shutting down the monitored equipment.
Transducers may also require an external power source to operate.
When this is
the case, the reliability of this source is crucial in maintaining data flow. Transducer
outputs are voltage or current sources specified to supply a rated voltage or
current into a specific load. For example, full output may correspond to 10 V
at up to 1.0 mA or 1.0 mA into 10 kΩ, up to 10 V maximum.
Some
over-range capability is provided in transducers so long as the maximum current
or voltage capability is not exceeded. The over-range can vary from 20 to 100%,
depending on the transducer.
However,
accuracy is usually not specified for the over-range area. Transducer outputs
are usually wired with shielded, twisted-pair cable to minimize stray signal
pickup. In practice, no. 18 AWG conductors or smaller are satisfactory, but
individual utility practices differ.
It is common
to allow transducer output circuits to remain isolated from ground to reduce
the susceptibility to transient damage, although some SA controller suppliers
provide a common ground for all analogs, often to accommodate electronic
multiplexers.
Some
transducers may also have a ground reference associated with their outputs.
Double grounds, where transducer and controller both have ground references,
can cause major reliability problems.
Practices
also differ somewhat on shield grounding, with some shields grounded at both
ends, but it is also common practice to ground shields at the SA controller end
only. When these signals must cross a switchyard, however, it is a good
practice to not only provide the shielded twisted pairs, but it also to provide
a heavy-gauge overall cable shield.
No comments:
Post a Comment