Eric A. Udren Chairman, PSRC WG I5 KEMA T&D Consulting USA Raleigh, NC

I5 Overview 0104.PPT 1 E.A. Udren

IEEE C37.92 – Standard for Low-Level Analog Interface Between Electronic

Sensors and Protective Relays

Eric A. UdrenChairman, PSRC WG I5

KEMA T&D Consulting USA

Raleigh, NC

IEEE PES - Power System Relaying Committee

Tampa, FL January 15, 2004


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

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

•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 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

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 specsEquipment must have CMRR of greater than 86

dB to meet this specification.Differences less than 50 V to avoid damage.


Recommended Wiring Practices

Shielded twisted pair, #24 or larger.

No magnetic-field shielding.


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

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 RequirementsSimplified 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

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 InterfacingWhy 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 StandardsIEC 60044-7 and -8 - Instrument Transformers - Electrical Voltage and Current

TransducersDirect 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 Bus9-1 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.

Eric A. Udren Chairman, PSRC WG I5 KEMA T&D Consulting USA Raleigh, NC

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