ieee c37.92-2005

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IEEE Std C37.92-2005

C37.92TM

IEEE Standard for Analog Inputs toProtective Relays from Electronic Vo l t a g e

and Current Transducers

3 Park Avenue, New York, NY 10016-5997, USA

IEEE Power Engineering Society

Sponsored by thePower System Relaying Committee

20 September 2005

Print: SH95337PDF: SS95337

Licensed to Continuum Plus/Denis KuzminANSI Store order #X290089 Downloaded: 9/26/2006 2:22:47 AM ETSingle user license only. Copying and networking prohibited.

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The Institute of Electrical and Electronics Engineers, Inc.

3 Park Avenue, New York, NY 10016-5997, USA

Copyright 2005 by the Institute of Electrical and Electronics Engineers, Inc.All rights reserved. Published 20September2005. Printed in the United States of America.

IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by the Institute of Electrical and ElectronicsEngineers, Incorporated.

Print: ISBN 0-7381-4697-8 SH95337PDF: ISBN 0-7381-4698-6 SS95337

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the priorwritten permission of the publisher.

IEEE Std C37.92-2005

IEEE Standard for Analog Inputs toProtective Relays from Electronic Voltageand Current Transducers

Sponsor

Power System Relaying Committee

of theIEEE Power Engineering Society

Approved 20 March 2005

IEEE-SA Standards Board

Abstract:Electronic devices that develop or utilize analog signals are not presently covered by

standards. This Standard provides interface connectivity of modern power-system signal transduc-

ers based on electronics, such as magneto-optic current transducers, and electronic relays. The ex-

isting standardized levels from familiar magnetic current and voltage transformers are not readily

generated by new types of electronic signal transducers.Keywords:phase correction, phase error, polarity, sensing system, transient response


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IEEE Standardsdocuments are developed within the IEEE Societies and the Standards Coordinating Committees of the

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Copyright 2005 IEEE. All rights reserved. iii

Introduction

Notice to users

Errata

Errata, if any, for this and all other standards can be accessed at the following URL: http://

standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for

errata periodically.

Interpretations

Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/index.html.

Patents

Attention is called to the possibility that implementation of this standard may require use of subject matter

covered by patent rights. By publication of this standard, no position is taken with respect to the existence or

validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying

patents or patent applications for which a license may be required to implement an IEEE standard or for

conducting inquiries into the legal validity or scope of those patents that are brought to its attention.

This introduction is not part of IEEE Std C37.92-2005, IEEE Standard for Analog Inputs to Protective Relays fromElectronic Voltage and Current Transducers.


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iv Copyright 2005 IEEE. All rights reserved.

Participants

At the time this recommended practice was completed, the Low Energy Analog Signal Inputs to Protective

Relaying Working Group had the following membership:

Eric A. Udren,Chair

P.G. McLaren,Secretary

The following members of the individual balloting committee voted on this standard. Balloters may have

voted for approval, disapproval, or abstention.

Douglas Dawson

PaulDrum

Harley Gilleland

Charles Henville

William Kotheimer

P.J. Lerley

Veselin Skendzic

John Tengdin

William AckermanMark AdamiakSteve AlexandersonMunnu Bajpai

Kenneth BehrendtStuart Bouchey

Robert BrattonGustavo BrunelloJeffrey BurnworthThomas W. Cease

John W. Chadwick, Jr.Simon ChanoDr. Guru Dutt Dhingra

Ratan DasDouglas DawsonPaul Drum

Kenneth FoderoHarley GillelandMietek Glinkowski

Roger HeddingCharles HenvilleEdward Horgan, Jr.

James D. Huddleston, IIIMr. Rene Jonker

William KotheimerDaniel LoveGregory Luri

Jesus MartinezThomas McCaffreyMichael McDonald

Mark McGranaghanPeter McLaren

Dean MillerGary MichelDaleep MohlaBruce MuschlitzJames RuggieriMohindar Sachdev

David SchemppThomas SchossigTony SeegersTarlochan SidhuMark SimonVeselin SkendzicJohn TengdinDemetrios TziouvarasJoe UchiyamaEric A. Udren


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Copyright 2005 IEEE. All rights reserved. v

When the IEEE-SA Standards Board approved this standard on 20 March 2005, it had the following

membership:

Steve M. Mills,Chair

Richard H. Hulett,Vice Chair

Judith Gorman,Secretary

*Members Emeritus

Also included are the following non-voting IEEE-SA Standards Board liaisons:

Satish K. Aggarwal,NRC Representative

Richard DeBlasio,DOE RepresentativeAlan Cookson,NIST Representative

Michael D. FisherIEEE Standards Project Editor

Mark D. BowmanDennis B. BrophyJoseph BruderRichard CoxBob DavisJulian Forster*Joanna N. GueninMark S. HalpinRaymond Hapeman

William B. Hopf

Lowell G. JohnsonHerman Koch

Joseph L. Koepfinger*

David J. Law

Daleep C. Mohla

Paul Nikolich

T. W. Olsen

Glenn Parsons

Ronald C. PetersenGary S. Robinson

Frank Stone

Malcolm V. Thaden

Richard L. Townsend

Joe D. Watson

Howard L. Wolfman


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vi Copyright 2005 IEEE. All rights reserved.

Contents

1. Overview.............................................................................................................................................. 1

1.1 Scope............................................................................................................................................ 1

1.2 Purpose......................................................................................................................................... 1

2. Normative references........................................................................................................................... 2

3. Definitions ........................................................................................................................................... 3

4. General requirements........................................................................................................................... 3

4.1 Terminations ................................................................................................................................ 3

4.2 Signal isolation from ground ....................................................................................................... 3

4.3 Polarity marking and reversibility ............................................................................................... 3

4.4 Auxiliary outputs from sensing systems...................................................................................... 4

4.5 Electrical environment withstand capability................................................................................ 4

5. Electrical requirements ........................................................................................................................ 5

5.1 Signal specifications .................................................................................................................... 5

5.2 Phase correction value ................................................................................................................. 7

5.3 Output burden capability ............................................................................................................. 7

5.4 Common-mode rejection ............................................................................................................. 7

5.5 Output dc offset ........................................................................................................................... 7

5.6 Bandwidth and transient response ............................................................................................... 7

5.7 Squelching on error detection ...................................................................................................... 8

5.8 Signal description for valid-data signal ....................................................................................... 8

6. Intermediate devices ............................................................................................................................ 8

6.1 Purpose......................................................................................................................................... 8

6.2 Performance requirements ........................................................................................................... 8

6.3 Other requirements ...................................................................................................................... 9

7. Interconnection wiring practices.......................................................................................................... 9

Annex A (informative)................................................................................................................................... 14


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Copyright 2005 IEEE. All rights reserved. 1

IEEE Standard for Analog Inputs toProtective Relays from ElectronicVoltage and Current Transducers

1. Overview

1.1 Scope

This standard defines the interface between voltage or current transducer systems or sensing systems with

analog electronic outputs, and suitably designed protective relays or other substation measuring equipment.

These transducer systems reproduce the power system waveforms as scaled values.

This standard also defines requirements for optional intermediate summing or ratio-adjusting amplifiers

required to add or subtract the outputs of more than one sensing system for measurement by a single relay or

measuring device.

1.2 Purpose

The standardized measurement signal between the transducer system and the relay systems is characterized

as an analog electrical signal of 11.3 V peak, at a maximum power of 3.2 mW.

A prime example of a sensing system with analog electronic output is an optical voltage or current sensing

system with an optical-to-electronic interface. Figure 1shows the typical configuration of system elements

for an optical current sensing system in a high-voltage station. In this case the optical sensing systems are

located on the bus at high potential. In other cases the sensing systems may be embedded inside power

apparatus or insulators. The optical signal is transmitted through fiber-optic cables to the ground level before

being converted to electrical signals scaled and formatted for use by protective relays and other intelligentelectronic devices (IEDs). The optical-to-electrical conversion module is usually located in the control

house, but may also be located near IEDs in the switchyard. This standard specifies the electrical signals

between the optical-to-electrical conversion module and the relays or other IEDs using these signals.

The interaction between the optical sensing system and the conversion module is a proprietary scheme of a

particular manufacturer's sensing design, not subject to standardization. It is the output of the conversion

module, and therefore, the input of relays and other measuring functions, that is to be standardized here for

interoperability. The marked section of Figure 1shows the location of the interface defined in this standard.


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IEEEStd C37.92-2005 IEEE STANDARD FOR ANALOG INPUTS TO PROTECTIVE RELAYS

2 Copyright 2005 IEEE. All rights reserved.

Figure 1Optical current sensing system with standardized analog interface

2. Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenceddocument (including any amendments or corrigenda) applies.

IEEE Std 525, IEEE Guide for the Design and Installation of Cable Systems in Substations.1

IEEE Std 1050, IEEE Guide for Instrumentation and Control Equipment Grounding in Generating

Stations.

IEEE Std C37.90, IEEE Standard for Relays and Relay Systems Associated with Electric Power

Apparatus.

IEEE Std C37.90.1, IEEE Standard Surge Withstand Capability (SWC) Tests for Relay and Relay

Systems Associated with Electric Power Apparatus.

IEEE Std C37.90.2, IEEE Standard for Withstand Capability of Relay Systems to Radiated Electromag-

netic Interference from Transceivers.

IEEE Std C57.13, IEEE Standard Requirements for Instrument Transformers.

1IEEE publications are available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854,USA (http://standards.ieee.org/).

Control House

RelayOtherMeas.Device

Relay

StandardizedAnalog

Interface

Optics/ElectronicsModule

Optical CurrentSensing Element

High VoltageInsulation

High VoltageBus

Optical Fiberin Conduit


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IEEEFROM ELECTRONIC VOLTAGE AND CURRENTTRANSDUCERS Std C37.92-2005


3. Definitions

For the purposes of this standard, the following terms and definitions apply. The Authoritative Dictionary of

IEEE Standards, Seventh Edition, should be referenced for terms not defined in this clause.

3.1 one per unit (abbreviated 1 p.u.): The measurement value or measuring system output that corresponds

to rated primary rms value of voltage or current in the circuit being measured.

3.2 relay input:The analog electronic input of any protective relay, meter, measurement or control device,

or intelligent electronic device that is compliant with this standard.

3.3 sensing system:the electronic sensing device, system, optical-to-electrical interface, or analog signal

source that conveys values of power system voltage or current, and whose output is compliant with this

standard.

4. General requirements

4.1 Terminations

The sensing system output, and the relay input, shall be provided with widely available standard connectors

capable of meeting the surge and high potential withstand requirements of 4.4. The connectors shall be

designed for easy field wiring and termination. Screw terminals are a well-suited option. Each input or out-

put comprises a pair of signal terminals defined and marked as explained in 4.3. The equipment supplier

shall provide additional ungrounded terminals or means for interconnection of shields as described in 7.

4.2 Signal isolation from ground

Both terminals of the sensing system output, and any relay input, shall be insulated from safety or case

ground for dc or power-frequency signals. A capacitive path is permitted between either terminal andground, not to exceed 0.01 F.

4.3 Polarity marking and reversibility

Interfaces shall have polarity marking consistent with that of conventional cts and vts. See IEEE Std

C57.13.2

For unbalanced sensing system outputs, the active output terminal shall be equivalent to the marked polarity

or X1 secondary terminal of a conventional instrument transformer.

When a power-system primary current is represented by a voltage output from the sensing system output, a

positive voltage as measured on the polarity-marked terminal with nonpolarity reference shall correspond tocurrent flow into the polarity-marked primary current terminal.

Furthermore, each sensing system and each relay shall be labeled by the manufacturer as having reversible

or nonreversible polarity.

Reversible polarity refers to a fully isolated or balanced input or output, allowing connection in

either polarity according to power system application needs.

2Information on references can be found in Clause 2.


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Nonreversible polarity refers to a single-ended or unbalanced input or output, such that only active-

terminal to active-terminal and signal-common to signal-common connections are allowed.

In general, a single sensing system output signal fans out to a number of relays or signal-using devices.

When interconnecting, the following considerations apply:

If one or more inputs of a number of relays have nonreversible polarity, the user may not be able to

achieve desired polarity of connections for all devices even if the source device has reversible polar-ity. Note: internal or software settings of a particular relay may be available for compensation of

input polarity.

If the input pair for each of a number of relays has reversible polarity, then each can be connected

with polarity as required, even though the output from the source is nonreversible.

This emphasizes the flexibility inherent in reversible-polarity inputs for the relays or other devices that use

the analog electronic sensing system outputs.

Balanced or reversible output terminals shall be symmetrically referenced to ground.

4.4 Auxiliary outputs from sensing systems

4.4.1 Sensing system trouble signal

This optional signal, intended for alarming, shall represent any malfunction or degradation requiring mainte-

nance attention or repair. Auxiliary power supply failure shall result in a trouble signal.

This output shall be available as a form C contact, dry, as specified by the sensing system manufacturer. The

relay coil shall be energized for normal correct operating conditions, to provide alarming for loss of

auxiliary supply as well as for sensing system malfunction.

4.4.2 Data valid signal

This required signal shall reflect the results of any internal self-monitoring checks of the sensing system

electronics that indicate that a problem has occurred in the output analog signal that could lead to undesired

operation of connected relays.It is also used to indicate a startup or shutdown condition during which the

sensing system output could be subject to serious errors or transients. Connected relays may use this signal

to block tripping.

The signal may be provided in either or both of the following forms:

A form A contact, dry, as specified by the sensing system manufacturer. The relay coil shall be

energized for normal correct operating conditions, to provide alarming or protection blocking for

invalid output signal. The contact shall be suitable for tripping duty according to IEEE Std

C37.90. Delay from triggering event to output blocking shall not exceed 12 ms.

As a TTL-level (0 V or 5 V) logic signal with response of 1 ms or faster. See 5.8. A logic level oftrue (5 V) shall indicate valid data.

4.5 Electrical environment withstand capability

The following type tests shall be applied to sensing system outputs, compatible analog electronic relay

inputs, sensing system trouble and valid-data outputs, valid-data inputs of relays, and intermediate devices

described in Clause 6.This is in addition to other electrical environment tests of the relay or sensing system

electronics required by relevant standards.


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4.5.1 Dielectric test

This test should be performed in accordance with dielectric test methods described in IEEE Std C37.90. The

test voltage is applied only in the common mode-between each pair of input or output terminals, and safety

or case ground. Signal circuits of 50 V or below are subjected to a lower value of dielectric test voltage as

listed in IEEE Std C37.90.

4.5.2 Surge withstand capability tests

Any device connected to the interface shall withstand the oscillatory surge withstand test and the fast

transient surge withstand test defined in IEEE Std C37.90.1. These test signals are applied as specified in

that standard for communications circuits.

Relays shall be connected to sensing systems and be energized as specified in IEEE Std C37.90.1. Relays

shall not yield trip outputs. Sensing systems and relays shall sustain no damage or change of calibration. The

sensing system shall not produce any spurious output that causes operation of a relay whose immunity has

been previously demonstrated with no sensing system connected. The sensing system shall not produce false

transitions of the sensing system trouble or valid-data signals, if provided.

4.5.3 Test for withstand capability of relay systems to radiated electromagnetic interferencefrom transceivers

The sensing system and compatible relays shall withstand the radiated electromagnetic interference test

defined in IEEE Std C37.90.2.

Relays shall be connected to sensing systems and be energized as specified in IEEE Std C37.90.2. Relays

shall not yield trip outputs. Sensing systems and relays shall sustain no damage or change of calibration. The

sensing system shall not produce any spurious output that causes operation of a relay whose immunity has

been previously demonstrated with no sensing system connected. The test shall not produce false transitions

of the sensing system trouble or valid-data outputs, if provided.

5. Electrical requirements

5.1 Signal specifications

5.1.1 Signal description for current sensing systems

Dynamic range: 0.05 to 40 times rated current

Nominal (Inor 1 p.u.) output level: 200 mV rms

Maximum instantaneous value: 0.200 * 40 * 1.414 = 11.3 V peak


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Magnitude and phase accuracymaximum variation from true scaled primary signal value at 50 Hz or 60 Hz:

Total harmonic distortion shall be equal to or less than magnitude error.

Signal to noise ratio shall be equal to or greater than 54 dB for signals greater than 0.1 p.u. The measurementis to be performed using a power-frequency signal and a noise measurement bandwidth of at least 120 Hz.

A current sensing system may provide an optional output whose nominal output level is 2 V rms at 1 p.u.,

with a maximum output of 4 p.u. This is intended for informational metering applications for which the

accuracies given above are acceptable. For revenue metering applications, the sensor manufacturer shall

separately state compliance with relevant accuracy standards such as IEEE Std C57.13 or its subparts.

5.1.2 Signal description for voltage sensing systems

Dynamic range: 0.05 to 2.0 times rated voltage

Nominal (Vn or 1 p.u.) output level: 4 V rms

Maximum output: 4.0 * 2.0 * 1.414 = 11.3 V peak

Magnitude and phase accuracymaximum variation from true scaled primary signal value at 50 Hz or 60 Hz:

Total harmonic distortion shall be equal to or less than magnitude error.

Signal to noise ratio shall be equal to or greater than 70 dB for signals greater than 0.85 p.u. The

measurement is to be performed using a power-frequency signal and a noise measurement bandwidth of at

least 120 Hz.

Table 1Signal description for current sensing systems

Current range Magnitude Phase

0.05 p.u. to 0.1 p.u. 1.0% 1.0

0.10 p.u. to 1.0 p.u. 0.6% 0.5

1.0 p.u. to 5.0 p.u. 1.0% 1.0

5.0 p.u. to 40 p.u. 10.0% 10.0

Table 2Signal description for voltage sensing systems

Voltage range Magnitude Phase

0.05 p.u. to 0.85 p.u. 1.0% 1.0

0.85 p.u. to 1.15 p.u. 0.3% 0.51.15 p.u. to 2.0 p.u. 1.0% 1.0


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5.7 Squelching on error detection

The output from a sensing system interface shall be clamped to zero at the moment of internal detection of a

malfunction that could cause serious errors or false trips. This includes sensing system auxiliary-supply

energization or shutdown transients. Time from detection of a problem to squelching shall be less than 0.2

ms.

Typically, the squelching is driven by the same error detection functions as the valid-data output described

in 4.4.2.

5.8 Signal description for valid-data signal

The optional valid-data signal of 4.4.2 shall be a TTL-level (0 V or 5 V) signal, insulated from safety ground

and suitable for transmission using the same wiring methods as for the analog sensing system signals. See

Clause 7.A logical true signal of 3.0 V to 5.5 V indicates valid output from the sensing system. A logical

false signal of 0 V to 0.5 V shall indicate invalid output. The output shall be able to maintain voltage within

specifications with a load resistance of 200or more. Delay from triggering event to output change should

not exceed 1 ms.

Receiving circuits in relays should be isolated from safety ground, and have an input impedance of greater

than 2000. Only signals of greater than 2.5 V are accepted as logical true.

6. Intermediate devices

6.1 Purpose

Intermediate devices may be used to create the sum or difference of separate sensing system outputs. They

may also be used to isolate the inputs of different relays or measuring devices connected to a single output of

a sensing system. The intermediate devices may have unity gain, or may include scaling of individual inputsto change the effective ratio of the sensing system.

Intermediate devices may also be used to combine the outputs of conventional instrument transformers with

electronic sensing system outputs. The performance requirements defined in this clause apply only to inter-

mediate devices with analog electronic outputs.

6.2 Performance requirements

Accuracy, bandwidth, and noise performance of intermediate devices shall be much better than that of the

sensing systems themselves. Specific requirements are as follows:


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Performance requirements shall apply to amplifier gains of unity. The manufacturer shall state performance

of nonunity gain amplifiers.

6.3 Other requirements

Intermediate devices shall conform to all other relevant requirements of Clause 4and Clause 5 and not

superseded in 6.2. They shall comply with specifications over the range of operating and nonoperating

conditions specified in IEEE Std C37.90.

7. Interconnection wiring practices

Figure 2, Figure 3, and Figure 4 show connection examples for single and multiple sources and loads. They

are provided to illustrate suitable interconnections for distances of less than 50 m between sensing system

and the most remote relay input. The shielded twisted-pair conductors are typically run within the controlhouse, where the ground potential differences among connected systems is less than 20 V during faults.

Wire gauge of 24 or larger is acceptable. If multiple pairs are contained within a single shield, the differen-

tial mode crosstalk between pairs should exceed 70 dB.

Table 3Performance requirements

THD Less than 0.1% of 1 p.u. current from 1 Hzto 20 kHz

Gain error Less than 0.1% of 1 p.u. current between45 Hz and 75 Hz

Phase error Less than 0.1 between 45 Hz and 75 Hz

Frequency response Stated by the manufacturer; flat at least towithin +0 dB and -1 dB between 15 Hz

and 10 kHz

SNR Better than 80 dB at 1 p.u. current orvoltage, with a noise measurement

bandwidth of at least 120 Hz


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Figure 2One sensing system and one relay input

Figure 3One sensing system with multiple relay inputs

Optional capacitiveshield grounding atsource end for high-frequency EMIreduction.

*Sensing System

Relay or IED 3

10 nF*

* *

Relay or IED 1 Relay or IED 2

*

10 nF*

Relay or IED

*Sensing System

Optional capacitive shieldgrounding at source end forhigh-frequency EMI reduction.*


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are generally helpful for short wiring runs, but have been known to produce unpredictable high-fre-

quency shielding results for longer wiring runs.

For connections involving switchyard-mounted equipment, where these benign conditions may not apply,

the user is responsible for engineering more elaborate schemes of shielding, shield grounding, and device

isolation. See IEEE Std 525. An additional robust outer shield is needed, grounded at both ends to conduct

current that counters and shields low-level measurement signals from magnetic and electromagnetic fields atpower frequencies. The source electronic device may need to be insulated from ground.


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A.3 Response to power-system transients

Transient or step response is quite separate from bandwidth, although it is intimately linked with the specific

high-frequency filtering or rolloff characteristics of the electronics in the sensing system. Faults and switch-

ing will yield outputs that exhibit overshoot or undershoot, and possibly damped high-frequency oscillatory

behavior.

The user should check on the response of relays to these distortions. The overshoot or undershoot may lead

to reach errors of high-speed relays.

Furthermore, in wideband high-speed differential schemes, differences in transient response of sensing sys-

tems of different generations or vendors may lead to false differential values and reduction of security mar-

gin or even false tripping.

If the sensing system bandwidth and distortion frequencies are three or more times the antialiasing filter cut-

off of a connected microprocessor relay, the problems may not exist.

Note that 5.6 includes a specification for the step response of a voltage sensing system.

A.4 Power-frequency phase delay

The time lag from primary power-system measurement value to the delivery of that value by the sensing sys-

tem to connected systems may be short compared to measurement window times and seemingly unimpor-

tant. However, it could become a serious problem in any relay or measurement system that is comparing two

values from different sensing system designs. The current-differential comparison is a good example-high-

speed schemes are sensitive to differences in phase delay between two sensing systems. Distance and direc-

tional relays, and particularly revenue meters, may suffer even more-they precisely compare the time rela-

tionship of voltages to currents. The voltage sensing systems use totally different measurement methods as

compared to current sensing systems, with no assurance of comparable delays of the primary waveforms.

Clause 5.2 describes the option of a vendor-supplied phase correction value.

A.5 Output capability

The drive current capability of voltage-mode outputs should be able to deal with all of the connected loads

as a parallel group. The addition of more loads may yield accuracy out of limits, depending on source

impedance, but the results still may be acceptable in many applications. This is parallel to the effects of bur-

dens on conventional CTs and VTs.

A.6 Malfunctions and alarms

Designers should evaluate the impact of failure modes-notably failure of electronic components; and impact

of site-vulnerability events such as fiber disturbance, or cuts or breaks. It will not be possible to avoid prob-

lems for all such events, but some can be helped or extra precautions can be taken.

In connection with this, the designer can help by providing the detection capabilities and fast response time

of self-monitoring systems that mute or squelch the output and block external connected devices. Note that

the muting of the output may interact with relays-differential schemes may false-trip unless the valid-data


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signal can be used to block tripping. Loss of voltage to distance relays will cause false tripping or invoke

loss-of-potential logic (if used) with very restricted protection abilities.

The ability of the sensing system to self-diagnose minor problems and raise a nonurgent alarm without

squelching or blocking gives the maintenance crew an opportunity to solve the problem before it becomes

serious. A data communications port that can report a specific diagnosis via modem or WAN increases the

chance that the repair crew arrives with the right parts and equipment.

A.7 Calibration

The user should learn from the supplier of the sensing system about the methods by which the overall pri-

mary-to-user-output calibration of the system is established and maintained. In particular, ensure that the

connected IEDs have any features that might be required to deal with calibration procedures. The sensing

system supplier should address what happens to calibration when the primary sensor is left in place while a

failed conversion electronics module is replaced.

A.8 Digital interfaces

This standard covers only low-level analog interfaces, including those embedded within larger systems hav-

ing digital data interfaces elsewhere, when interoperability at the analog interface is of importance to manu-

facturers and users. Digital interfaces require the specification of sampling processes and rates, and the

multiple layers of the data communications protocol for exchange between the sensing system and the relay.

Digital data interfaces for power system data are covered in IEC 61850-9-1, IEC 61850-9-2, IEC 60044-7,

and IEC 60044-8.

ieee c37.92-2005

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