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IEEE C37.92 – Standard for Low-Level Analog Interface Between Electronic Sensors and Protective Relays PowerPoint Presentation
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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. Our topic of interest – new sensors.

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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

our topic of interest new sensors
Our topic of interest – new sensors
  • New transducers for sensing power system primary voltages and currents:
  • Magneto-optic current transducers (MOCT).
  • Electro-optic voltage transducers (EOVT).
  • Rogowski coils (linear couplers; air core CTs).
  • Special compact low-burden, high ratio, high-accuracy CTs with electronic amplification.
  • Capacitive or resistive dividers with electronic buffer amplifiers.
  • Replacements for today’s CTs and VTs including capacitive voltage transformers.
new sensors
New Sensors
  • Advantages:
  • 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…
the application of the interface
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
key electrical specifications
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.
interconnection assumptions
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.
recommended wiring practices
Recommended Wiring Practices
  • Shielded twisted pair, #24 or larger.
  • No magnetic-field shielding.
recommended wiring practices1
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.
intermediate summing amplifier
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.
summary of installation requirements
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.
issues in annex to c37 92
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.
digital versus analog interfacing
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.
digital interfacing standards
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.