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. Udren Chairman, PSRC WG I5 KEMA T&D Consulting USA Raleigh, NC IEEE PES - Power System Relaying Committee Tampa, FLJanuary 15, 2004

I5 Overview 0104.PPT 2 E.A. Udren 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 todays CTs and VTs including capacitive voltage transformers.

I5 Overview 0104.PPT 3 E.A. Udren 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 todays protective relays, meters, and other IEDs? Forget electromechanical relays…

I5 Overview 0104.PPT 4 E.A. Udren 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

I5 Overview 0104.PPT 5 E.A. Udren 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.

I5 Overview 0104.PPT 6 E.A. Udren 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.

I5 Overview 0104.PPT 7 E.A. Udren Recommended Wiring Practices Shielded twisted pair, #24 or larger. No magnetic- field shielding.

I5 Overview 0104.PPT 8 E.A. Udren 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.

I5 Overview 0104.PPT 9 E.A. Udren Reversible and Non-reversible Polarity Unbalanced or single-ended inputs and outputs, with signal and common terminals, require that commons be tied together to avoid noise problems - non-reversible. Balanced inputs or outputs – full differential electronic signals or isolated transformer winding – reversible to easily change polarity. If either end (source or receiver or both) is balanced, connection is reversible. Manufacturer labels sensor outputs and IED or relay inputs.

I5 Overview 0104.PPT 10 E.A. Udren 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.

I5 Overview 0104.PPT 11 E.A. Udren 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.

I5 Overview 0104.PPT 12 E.A. Udren 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.

I5 Overview 0104.PPT 13 E.A. Udren 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: Todays IEDs can be converted. There must be an analog to digital interface somewhere in the digital system - helpful to apply standard to this.

I5 Overview 0104.PPT 14 E.A. Udren Digital Interfacing Standards IEC and -8 - Instrument Transformers - Electrical Voltage and Current Transducers Direct digital output from an electronic transducer IEC &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.