Transcript Statistical Performance Measures

IEEE PES - Power System Relaying Committee
Tampa, FL
January 15, 2004
IEEE C37.92 – Standard for Low-Level
Analog Interface Between Electronic
Sensors and Protective Relays
Eric A. Udren
Chairman, PSRC WG I5
KEMA T&D Consulting USA
Raleigh, NC
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Our topic of interest – new sensors
New transducers for sensing power
system primary voltages and currents:
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Magneto-optic current transducers (MOCT).
Electro-optic voltage transducers (EOVT).
Rogowski coils (linear couplers; air core CTs).
Special compact low-burden, high ratio, highaccuracy CTs with electronic amplification.
• Capacitive or resistive dividers with electronic
buffer amplifiers.
Replacements for today’s CTs and VTs
including capacitive voltage transformers.
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New Sensors
Advantages:
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Linear - high accuracy over a wide dynamic range.
Accurate transient response.
Protection and metering in one sensor.
Big space and weight savings.
Integrate with power apparatus.
Some replace analog wires with optical fibers.
One drawback – electronic output only delivers
milliwatts (unless you add a $$$$ power amplifier)
How do we interface low-energy output sensors
to today’s protective relays, meters, and
other IEDs? Forget electromechanical relays…
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The Application of the Interface
•Analog signal bus
•Shielded twisted pair
interconnection
•Distances typically
less than 50 m.
•Op-amp signal levels
less than 12 V peak
•Low current; mW of
energy
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Key Electrical Specifications
 1 p.u. current = 200 mV rms.
 Peak signal =11.3 V (20 p.u. fully offset peak).
 0.6 % current error, 0.3 % voltage error, and 0.5
degrees phase error at normal load.
 Accuracy specified for relaying.
 Optional metering output 1 p.u. = 2 V - points to
C57.13 for accuracy requirements.
 SNR 54 to 70 dB nominal.
 Bandwidth to 5 kHz (many do much better).
 Response below power frequency to reproduce a
20 p.u. fault current having 100 ms full dc offset,
with less than 10% ratio error.
 Control signals from sensing system to relays for
security – valid data, and sensor malfunction.
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Interconnection Assumptions
 Distances less than 50 m.
 All within the control building or other shielded
environment.
 Good grounding bond among interconnected
devices.
 GPR induced ground potential differences less
than 20 V for accuracy specs
 Equipment must have CMRR of greater than 86
dB to meet this specification.
 Differences less than 50 V to avoid damage.
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Recommended Wiring Practices
Shielded twisted
pair, #24 or
larger.
No magneticfield shielding.
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Recommended Wiring Practices
If twisted pairs
share a shield
(e.g. CAT 5
cable), common
mode
interference
among pairs
must be less
than –70 dB.
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Intermediate Summing Amplifier
Summing amp
may be needed for
line or differential
relays.
Accuracy 10 times
better than
sensors.
This is not tough
to do.
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Summary of Installation Requirements
Simplified grounding practices for connections
among equipment in close proximity.
Equipment is all bonded to good ground - minimal
GPR.
Low energy signals - no safety issues in
maintenance.
Low-cost approach for use inside sensing and
measurement systems.
If the sensor electronics are to be placed in
switchyard feeding relays in control house….
Standard gives cautions and advice.
Not what this interface was designed for – do something
else.
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Issues in Annex to C37.92
Low frequency response cutoff – different from
CTs and VTs.
Startup and shutdown transients – when auxiliary
power is applied and removed.
Accurate response to power-system transients –
what do the relays do?.
Power frequency phase delay – correctable for
precision/metering measurements.
Output fan-out capability to multiple IEDs.
Malfunctions and alarms – squelch and sensor
trouble.
Calibration – ask questions about maintenance.
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Digital Versus Analog Interfacing
Why digital interfaces are a big, tough jump:
Every bit must be exactly right for the interface to work at all.
Rip into the guts of existing IED designs, or design all-new
ones.
Sampling rates, antialiasing filters, phase delays, etc. must
align between IED and digitizing sensor designs.
Synchronization of sampling requires design cooperation.
Sharing of transducer signals requires multidrop data buses
with master-slave or peer-to-peer design requirements.
Analog is an important bridge:
Today’s IEDs can be converted.
There must be an analog to digital interface somewhere in
the digital system - helpful to apply standard to this.
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Digital Interfacing Standards
IEC 60044-7 and -8 - Instrument Transformers - Electrical Voltage
and Current Transducers
Direct digital output from an electronic transducer
IEC 61850-9-1&2 Communication Networks and Systems in
Substations - Specific Communication Service Mapping (SCSM)
- Unidirectional Point to Point Link; & Process Bus
9-1 is data stream definition uses Ethernet stack for simple
and focused point to point solution.
9-2 is application and object layer definition for a general
purpose data acquisition bus system for substation
protection, control, and automation.
These projects are merging their approaches.
C37.92 analog and IEC digital interface
standards are complimentary, not
competitive.
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