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Abstract Optical voltage and current sensors offer several

benefits for use in high-voltage substations. Interface to relays

and other secondary devices is one of the key issues when using

optical sensors. Low energy analog interfaces provide an

effective way for connecting optical sensors to relays. In this

paper, several examples and applications of optical sensors

connected to relays and recorders using low and high energy

analog interfaces are discussed. In all cases, the performance of

the entire system has been satisfactory. Lessons learned in these

applications are also discussed.

Index Terms-- current measurement, voltage measurement,

high-voltage techniques, optics, optical current sensor, optical

voltage sensor, transducers, optical fiber devices, power

measurement, low-energy analog interface.

I. INTRODUCTION

SE of optical voltage and current sensors is growing

steadily for high-voltage (HV) and/or high-current (HC)

measurement applications. These sensors offer attractive

features such as safety, exceptional accuracy and linearity,

wide bandwidth, light weight and compact size, flexible

sensitivity, as well as environmental benefits such as

elimination of oil or SF6 insulation from instrumenttransformers. Fiber-optic current sensors can also offer

features such as a flexible window-CT design, and the ability

to measure very high currents easily. Leading substation

engineers and designers are taking advantage of these features

by incorporating and integrating these sensors into substation

metering, monitoring, protection, and control schemes [1]-

[11]. One of the key elements of this integration is the

interface between the optical sensors and the secondary

devices used for these applications. There are several options

available for this interface, including digital schemes, low

energy analog (LEA) interface, and high energy analog

interface (HEA). In this paper, we focus on the key role of the

LEA interface in adoption of optical technology for

measurement of voltage and current in electric power systems.

II. INTERFACE OPTIONS AND STANDARDS

One the most critical factors regulating the adoption of

F. Rahmatian is with NxtPhase T&D Corp., Vancouver, BC V6M 1Z4

Canada (e-mail: frahmatian@nxtphase.com).

non-conventional voltage and current measurement

technology in HV substations is the interface. Most secondary

devices in HV substations, including relays and meters, are

designed to accept high-energy analog (HEA) signals

provided by magnetic current and voltage transformers (CTs

and VTs). For a CT, the output is usually in the form of 1A or

5A rated secondary outputs, with significant energy (burden)

capability. For the VT, the HEA secondary output is typically

rated at a value between 100/3 V and 120 V, again with

significant burden capability. Most modern meters and relays

used in substations are microprocessor based digitalinstruments with interfaces designed to convert these HEA

signals into LEA voltage signals and then into a digital signal

to be used by the microprocessor. These relays and meters,

unlike their electro-mechanical predecessors require very little

energy (very low burdens) from the instrument transformers.

Most modern optical instrument transformers are digital

instruments capable of providing digital output. Ideally, a

digital interface between the optical instrument transformer

and the microprocessor-based relay and meter provides the

most efficient, economical, and accurate interface, optimizing

use of energy and eliminating unnecessary circuitry.

However, in order to facilitate the interoperability of various

devices, this digital interface needs to follow one or a limitednumber of universally acceptable standards. IEC 60044-8

[12], IEC 61850-9-1 [13], and IEC 61850-9-2 [14] provide

first commonly used digital interface standards for non-

conventional instrument transformers. These standards are

relatively new and many manufacturers are in the process of

providing relay and meter prototypes that work with these

standards, while several users are planning on pilot projects

for gaining experience and comfort with these standards. IEC

61850-9-2, together with a UCA guide [15] on a simplified

implementation of it, is quickly becoming the preferred

standard for digital communication between instrument

transformers and secondary devices.

Meanwhile, as the digital interface has been going throughdefinition and standardization over the past few years, the

low-energy analog (LEA) voltage interface has provided a

very practical and efficient vehicle for early use of optical and

other non-conventional instrument transformers. It is

generally simple for electronic instrument transformers to

convert digital data into low-voltage analog data using

commercially available electronic components; the added cost

and power requirements are usually minimal, and the accuracy

Design and Application of Optical Voltage and

Current Sensors for RelayingFarnoosh Rahmatian, Member, IEEE-PES

U

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and performance is only minimally degraded compared to

digital data. Also, on the other side, there is usually an LEA

stage inside electronic relays and meters, allowing simple

modification of those devices (essentially removing input

transformers and adding some surge protection circuitry) to

accept LEA input directly from the voltage or current sensor.

The LEA interface provides several advantages over both

HEA and digital interfaces. Compared to the digital interface,

LEA is simpler to define and test. IEEE C37.92 [16], IEC

60044-7 [17], and 60044-8 [12] provide requirements of the

LEA interface. The requirements given by IEEE and IEC

standards are mostly consistent and defined in a simple way.

The LEA output of an electronic CT or VT is a signal less

than 10 V, with load (burden) being a few kilo ohms or larger.

Compared to a high-energy interface for an electronic

instrument transformer, the LEA option provides several

advantages. First, the HEA output is usually driven from an

LEA output, using additional circuitry to amplify the signal.

Accordingly, the LEA output is a necessary part of an HEA

output. Next, the addition of these amplifiers increases the

cost and reduces the performance of the system (albeit still

satisfactory). Both bandwidth and accuracy performance of

the system is better if the power amplifiers (HEA) are

eliminated. Finally, reliability and power requirements of the

electronics associated with instrument transformers are also

better if the amplifiers are eliminated. Most applications of

optical instrument transformers for protection applications to

date use the LEA interface between the sensor and the relay.

Several examples are provided in this paper.

The HEA interface effectively presents the most expensive

and least efficient option for connecting an optical instrument

transformer to a relay or meter; nevertheless, because of the

infrastructure in place and availability of secondary devices

with standard HEA input, it provides another practical option,

particularly for metering applications. Since HEA output is

driven from LEA, via an amplifier, every use of an HEA

interface for optical sensors is also a use of the LEA interface.

The cost, performance, space, and energy requirements of

these amplifiers is such that they are still a practical solution

for metering applications, but a lot more difficult for

protection applications. As such most energy metering

installations for optical CTs and VTs use the HEA interface.

In this paper, we briefly reference a few examples of these

systems, which all use the standard LEA output to drive the

HEA signal.

For practical and economic reasons, the HEA outputs used

for optical sensors usually correspond to relatively low burden

levels as compared to those of magnetic instrument

transformers. The signal levels are the same typical values

(1/5 A and 69/120 V), but the burden capability is chosen to

work with modern electronic meters and relays which

represent very little burden on the instrument transformer.

Accordingly, the draft version of IEEE P1601 [18], being

prepared as an IEEE standard for optical instrument

transformers, and new Canadian standards C60044-7 [19] and

C60044-8 [20] for instrument transformers have introduced

lower burden classes to be used for HEA interface of optical

(and other non-conventional) instrument transformers. IEEE

P1601 covers many aspects of optical instrument

transformers, including HV and dielectric requirements,

keeping consistent with system requirements given in IEEE

C57.13 for magnetic instrument transformers [21]. It also

includes terminology and convention for specifying accuracy

over wider dynamic ranges, beyond what has been covered

under IEEE C57.13. P1601 is intended to use significant

reference to IEEE C57.13, IEC 60044-7, IEC 60044-8, and

IEEE C37.92 in order to keep consistency and avoid

redundant requirements. It, however, captures and

emphasizes requirements specific to optical instrument

transformers, introduces new requirements relevant to optical

sensors, and eliminates traditional requirements that are not

relevant to optical instrument transformers. A draft of P1601

is expected to be available for survey in late 2006, targeted for

balloting in 2007.

III. OPTICAL SENSORTECHNOLOGY AND APPLICATION

EXPERIENCE

The optical CT (OCT) uses an in-line fiber optic

interferometric design described in detail in [1] and [2]. The

sensing head is an optical fiber encircling the current carrying

conductor in one or several complete turns. It accurately

integrates the magnetic field around the conductor(s) that it

encircles, determining the current through its opening. The

sensing fiber can be packaged in fixed size windows or in a

flexible cable. The optical VT (OVT) uses a shielded

distributed electric-field sensor design described in detail in

[3] and [4] for accurate determination of voltage between itstwo terminals.

Details of the OCT columns structure, its physical

characteristics, and its applications flexibility are provided in

[5]. In summary, the OCT column consists of a dry type

composite insulator (no gas or oil insulation used), with a

fiber-optic window CT at the top. The OVT and the combined

optical VT/CT (OVCT) are described in details in [11]. The

OVT and OVCT columns consist of hollow composite

insulators, slightly pressurized (200 kPa) with dry nitrogen,

containing a number of electric field sensors. Optical fibers

connect the columns with the associated electronics located

remotely in the control room. The OVCT is identical in

structure to OVT, except a fiber optic CT head is added on thetop and its associated fiber(s) is routed through the OVT

column. The optical sensors use various outputs, including

digital, LEA, and HEA interfaces to transfer voltage and

current data from the electronics chassis of the sensors in the

control room to various secondary devices such as revenue

meters, protection relays, and disturbance recorders.

References [5] and [11] provide a comprehensive review of

the application of these optical sensors in the past 5 years. A

brief summary with more emphasis on interface is included

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below.

Figure 1 shows the result of a 108 kA fault measurement in

a laboratory environment. The OCT had two fiber turns and

had a 2000A:200mV ratio (scale factor) for its LEA output.

The reference was provided using a laboratory grade shunt

resistor current measurement system. The results show a

near-perfect match between the output of the OCT and the

reference when measuring the fault including its near DC

(decaying DC) component.

0 1000 2000 3000 4000Time (arb.)

-80000

-40000

0

40000

80000

120000

Current(A)

CT

Ref

S017 8/25/01 17:17

Figure 1. Verification of OCT fault current measurement using LEA

interface and a reference shunt at 108 kA peak. The reference shunt and the

OCT waveforms match very well.

In an field trial application, an OVCT system was used for

monitoring switching waveforms on a 230 kV shunt capacitor

bank using LEA interface [8]. Relays and data recorders were

used to record and analyze the performance. Several lessonswere learned from this early trial. In summary, the

performance was very satisfactory and consistent with

expectation. The bandwidth of the sensor, specifically the

ability to reproduce low-frequency and near DC signals, was

shown using the recorder. One of the side lessons learned was

that if the lower frequency signals are reproduced by the

sensors (to show the true nature of the primary waveform), the

inputs of the relays had to be modified to avoid saturation of

their input transformers. This is a direct result of the fact that

traditional magnetic instrument transformers used in the

industry do not reproduce the near-DC signals and,

accordingly, most relays have not been designed to tolerate

and capture those frequency elements. The alternative is to

use a high-pass filter in the output of the optical instrument

transformers to block the near-DC components, but that may

limit other information that a user may be interested in,

including dynamic performance of the grid. A third

alternative is to provide several independent LEA outputs

from the same sensor, each filtered specific to the application

and the secondary device it is being connected to. Additional

information on this project can be found in [8].

Another early 230 kV class OVCT system was used in

Arizona for both metering application and protection

application [8]. Two sets of output were provided from the

same sensors. One set of LEA outputs for protection

application and another set of LEA outputs to drive the

amplifiers (HEA outputs) used for revenue metering

application. The performance for both applications was quite

satisfactory. The LEA output was connected to a line

protection relay, shadowing a conventional protection system

(using a similar relay with magnetic instrument transformers).

During a fault, the relay connected to the optical system

performed the same as the relay connected to the main

protection system, verifying the functionality of the

optical/LEA scheme.

Several other optical sensors have been used for protection

application using LEA interface between the OCT and the

relay. Figure 2 shows a picture of an OCT used on a 420 kV

class circuit breaker in Italy. A key advantage of such

installation is significant installation and real state savings by

hanging the CT from the circuit breaker as opposed requiring

separate civil work for the CT erection. Similar installations

are made in an air insulated substation (AIS) in England [11],

and in gas insulated substations (GIS) in Austria, see figure 3.

The interface between the OCT and the relay or recorder is

LEA in all these cases. The performance of the optical

sensors is being compared with that of the traditional magnetic

instrument transformers, and the results to date are very

satisfactory.

Figure 2. OCT mounted on a 420 kV class live tank circuit breaker.

Reference [5] also provides performance information for an

OCT system used for a shunt capacitor bank current

unbalance protection application in Alberta, Canada. In this

application, the OCT is used as a low ratio 230 kV classwindow CT to detect small current differences down to 0.1 A

between two conductors. Use of the OCT allowed elimination

of safety concerns associated with low-ratio HV conventional

wire-wound CTs. This OCT provided measurement over a

very wide dynamic range, from 0.1 A to 1000 A, using two

outputs from the same OCT (see [5] for details). The first

output is an HEA output, with a CT ratio of 1A:1A, for

connection to an unbalance detection relay. The second

output is an LEA output with a CT ratio of 25A:200 mV, with

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the capability to measure instantaneous currents as high as

1000 A.

Optical systems are inherently linear and can provide

excellent accuracy over wide dynamic range; accordingly,

they are ideal solutions for high-voltage energy metering

applications. Reference [5]-[7], and [11] provide information

on optical sensors used for accurate revenue metering (mostly

using HEA outputs).

Magnetic CT

OCT

Magnetic CT

OCT

Figure 3. OCT integrated into 420 kV class GIS switchgear.

The wide bandwidth and DC capability of the OCT makes

it ideal for DC current measurement applications. For

measuring DC signals, the LEA interface is most commonly

used. Same HV OCT sensors used in AC applications can be

used, perhaps with different signal filtering, for HV DC

applications. A novel configuration of the OCT with a

flexible wrap-around sensor head is used for measuring very

high-currents (up to several 100 kA) in large conductors.

Figure 4 shows use of a wrap-around flexible head OCT in a

chemical plant in Magog, Quebec, on a 40 kA system [5].

This sensor is used for metering, protection, and process

control at 40 kA DC, using LEA output at 4V. The LEA

output was calibrated in factory and verified for accuracy on

site for less than 0.1% error.

Similar flexible head wide-bandwidth DC current sensors

have been used for laboratory applications in characterizing

power electronics and FACTS systems. Ref. [5] also provide

frequency response of an OCT used for measuring harmonic-

and DC-rich signals up to 100 kA with better than 0.2%

accuracy. The LEA output was used for this application. The

additional flexibility to change the ratio via software made this

sensor even more attractive for use in laboratory environment.

The 3-dB bandwidth of this sensor was in excess of 20 kHz.

The OVT has also been used as a portable reference VT for

calibration of other VTs. Figure 5 shows a portable 550 kV

OVT calibration system. The OVT is built into a mobile

trailer and is designed for live connection to HV lines. This

OVT has been prepared for calibrating 550 kV capacitive VTs

(CVTs) throughout a utilitys EHV network, in order to

improve the performance and convergence of the state

estimator, see [9]. It has both HEA and LEA outputs, with

selectable ratios for use at 550 kV, 230 kV, and 138 kV

voltage classes. The same device has also been used for

measuring power quality and harmonics on a 550 kV line next

to a static VAR compensator. Ref. [9] contains results of

harmonics measurement using the LEA output of this OVT on

a similar HV system. Figure 6 shows result of monitoring

voltage and total harmonic distortion on a 550 kV system for a

period of 4 minutes.

Figure 4. A wrap-around optical current sensor used in a high-current DC

application. This sensor is used for metering, protection, and process control

at 40 kA DC, using LEA output at 4V. The sensing head cable is routed

through an insulating conduit.

Figure 5. A 550 kV class OVT as a portable calibration reference.

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Wide bandwidth and wide dynamic range make the NXVT

and the NXCT very useful tools as measurement equipment

for recording fault conditions. For example, [10] shows

details and waveforms measured using these optical sensors

during a staged fault test of a 500 kV series capacitor bank

system. The bandwidth and dynamic range of the sensors

with an LEA interface allowed accurate measurement of

MOV (metal oxide varistor) voltage and current, fault

currents, and various other parameters. The OCTs wide

dynamic range allowed for accurate recording of both primary

fault current (11 kA) and secondary arc currents (harmonic-

rich currents < 50 A). Figure 7 shows some waveforms

measured (energy calculated from measurements) during one

of these staged faults. More detailed information and

waveforms are given in [10]. The data acquisition system

used for recording this data had 16 floating input channels

suitable for connecting to LEA signals less than 20 V, and

collected data at 100,000 samples per second.

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

Time

TotalHarmonicDistortion(%)

302000

302200

302400

302600

302800

303000

303200

303400

303600

303800

304000

Line-to-GroundVoltage(V)

THD, Phase ATHD, Phase BTHD, Phase CVoltage, Phase AVoltage, Phase BVoltage, Phase C

.

Figure 6. Field measurement of voltage and total harmonic distortion (THD)

using a 550 kV portable OVT over a period of 4 minutes. Values up to the

25th harmonic were measured. The fundamental frequency of the system was

60 Hz. The LEA output of the OVT was used for the measurement. The

OVT was a 0.2% class device.

-15

-12

-9

-6

-3

0

3

6

9

12

15

-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Time (s)

CurrentandEnergy

-250

-200

-150

-100

-50

0

50

100

150

200

250

MOVVoltage(kV)

OVCT MOVEnergy (MJ)

OCT MOVCurrent (kA)

OCT FaultCurrent (kA)

OVT MOVVoltage (kV)

Figure 7. Measurements using OCTs and OVTs with LEA output during a

staged fault test on a 500 kV series capacitor bank. The OVT was a 145 kV

class OVT. [10]

IV. SUMMARY AND CONCLUSION

Several applications of OVTs and OCTs using low energy

analog interface are reviewed. The LEA interface provides a

flexible and easy to use option for optical voltage and current

transformers. This interface also exerts minimal cost and

performance limitation on exploitation of these sensors.

Several examples of use of these sensors with LEA outputs for

metering, monitoring, and protection application are provided.

These applications include regular over-current protection,

shunt and series capacitor bank monitoring and protection,

revenue metering, various DC measurements, and portable

measurement and monitoring applications. In all cases, it was

found that the performance of the optical sensors using LEA

interface were satisfactory as compared with specifications

and/or with conventional magnetic sensors (when relevant).

Furthermore, it was observed that to take maximum advantage

of the performance of the optical sensors, the interface and

input of the secondary devices connected to these sensors

need to have high performance capability comparable to the

sensors.

V. REFERENCES

[1] G. A. Sanders, J. N. Blake, A. H. Rose, F. Rahmatian, and C. Herdman,

Commercialization of Fiber-Optic Current and Voltage Sensors at

NxtPhase, 15th Optical Fiber Sensors Conference, Portland, OR, May

2002, pp. 31-34.

[2] J. Blake, P. Tantaswadi, R. T. de Carvalho, In-line Sagnac

interferometer current sensor, IEEE Transactions on Power Delivery,

vol. 11, pp. 116-121, Jan. 1996.

[3] P. P. Chavez, F. Rahmatian, and N. A. F. Jaeger, Accurate voltage

measurement by the quadrature method, IEEE Transactions on Power

Delivery, vol. 18, no. 1, pp. 14-19, Jan. 2003.

[4] P. P. Chavez, F. Rahmatian, and N. A. F. Jaeger, Accurate voltage

measurement with electric field sampling using permittivity shielding,

IEEE Transactions on Power Delivery, vol. 17, no. 2, pp. 362-368, Apr.

2002.

[5] F. Rahmatian and J. N. Blake, Applications of High-Voltage Fiber

Optic Current Sensors,Proceedings of the IEEE-PES General Meeting,

Montreal, Quebec, Jul. 2006, paper 1129.

[6] J. N. Blake and A. H. Rose, Fiber-Optic Current Transducer Optimized

for Power Metering Applications, Proceedings of the IEEE T&D

meeting, Dallas, TX, Sept. 2003, pp. 1-4.

[7] F. Rahmatian, G. Polovick, B. Hughes, and V. Aresteanu, FIELD

EXPERIENCE WITH HIGH-VOLTAGE COMBINED OPTICAL

VOLTAGE AND CURRENT TRANSDUCERS, in Proc. CIGRE

General Session 40, Aug. 29 - Sep. 3, 2004, paper A3-111.

[8] A. Klimek and C. Henville, Early Experiences with Protection

Applications of Optical Current & Voltage Transducers, in Proc. 2003

Western Protective Relay Conference.

[9] F. Rahmatian, J. H. Gurney, and J. A. Vandermaar, PORTABLE 500

kV OPTICAL VOLTAGE TRANSDUCER FOR ON-SITE

CALIBRATION OF HV VOLTAGE TRANSFORMERS WITHOUT

DE-ENERGIZATION, inProc. CIGRE General Session 41, Aug. 29 -

Sep. 3, 2006, paper A3-103, to be published.

[10] F. Rahmatian, D. Peelo, G. Polovick, B. Sunga, and J. Lehtimaki

OPTICAL CURRENT AND VOLTAGE SENSORS IN EHV SERIES

CAPACITOR BANKS APPLICATION, in Proc. CIGRE SC A3 & B3

Joint Colloquium, Tokyo, Japan, Sep. 26-27, 2005, pp. 164-169.

[11] F. Rahmatian and A. Ortega, Applications of Optical Current and

Voltage Sensors in High-Voltage Systems, Proceedings of the IEEE-

PES T&D Latin America, Caracas, Venezuela, Aug. 2006, paper 471.

[12] Instrument Transformers Part 8: Electronic Current Transformers,

International Standard IEC 60044-8:2002, first-edition, 2002-07.

[13] Communication networks and systems in substations - Part 9-1: Specific

Communication Service Mapping (SCSM) - Sampled values over serial

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unidirectional multidrop point to point link, International Standard IEC

61850-9-1:2003, first-edition, 2003-05.

[14] Communication networks and systems in substations - Part 9-2: Specific

Communication Service Mapping (SCSM) - Sampled values over

ISO/IEC 8802-3, International Standard IEC 61850-9-2:2004, first-

edition, 2004-04.

[15] Implementation Guideline for Digital Interface to Instrument

Transformers using IEC 61850-9-2, UCA International User Group, R3-

0, 2005-08-25.[16] Standard for Analog Inputs to Protective Relays from Electronic Voltage

and Current Transducers, IEEE Standard C37.92-2005, 2005.

[17] Instrument Transformers Part 7: Electronic Voltage Transformers,

International Standard IEC 60044-7:1999, first Edition, 1999-12.

[18] Draft: Standard Requirements for Optical Voltage and Current Sensor

Systems, IEEE P1601/D06, Working group draft D06, Sponsored by

IEEE PES Power Systems Instrumentation and Measurement, 2006-06.

[19] Instrument Transformers Part 7: Electronic Voltage Transformers,

CSA standard CAN/CSA-C60044-7, 2006, ballot draft.

[20] Instrument Transformers Part 8: Electronic Current Transformers,

CSA standard CAN/CSA-C60044-8, 2006, ballot draft.

[21] IEEE Standard Requirements for Instrument Transformers, IEEE

Standard C57.13-1993, 1993.

VI. BIOGRAPHIES

Farnoosh Rahmatian (S89, M91) was born in 1969. He received B.A.Sc.

(Hon.), M.A.Sc., and Ph.D. degrees from the University of British Columbia,

Vancouver, BC, Canada, in 1991, 1993, and 1997, respectively, all in

electrical engineering. From 1997 to 2004, he was a Director of Research &

Development at NxtPhase Corporation, also in Vancouver, working on

precision high-voltage optical instrument transformers for use in high-voltage

electric power transmission systems. Since 2004, he has been the Director of

Optical Systems at NxtPhase T&D Corporation, focusing on application and

commercial use of optical voltage and current sensors.

Dr. Rahmatian has also been an adjunct professor at the Department of

Electrical and Computer Engineering at the University of British Columbia,

and a member of: IEC TC38 working groups on instrument transformers,

Standards Council of Canada, Canadian Standards Association, CIGRE, IEEEPower Engineering Society, and IEEE Lasers and Electro-Optics Society. He

is the acting co-chair of IEEE/PES working group on optical instrument

transformer systems. Dr. Rahmatian has received an R&D 100 award for the

development of the optical fiber current and voltage sensor in 2002, and has

authored or co-authored over 50 scientific and technical publications.

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