IEEE C37.92 – Standard for Low-Level Analog Interface Between Electronic Sensors and Protective Relays

1. 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

2. 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.

3. 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…

4. 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

5. 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.

6. 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.

7. Recommended Wiring Practices • Shielded twisted pair, #24 or larger. • No magnetic-field shielding.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.