Optical current and voltage sensors are proving an alternative to conventional iron-core instrument transformers. Major advantages include improved accuracy over a broader dynamic range, wider band-width, zero risk of explosive failure and lighter weight. Additionally, ferroresonance, current-transformer (CT) saturation and the hazards of an open secondary are eliminated. BC Hydro (Vancouver, British Colombia, Canada) has been testing the latest second-generation prototype fiber-optic current and voltage sensors from NxtPhase Corp. (Vancouver) to further develop this technology.

BC Hydro began work with the optical sensors seven years ago, exploring the technology to monitor high-voltage bushings and CTs to prevent catastrophic failures. Early work saw a monitoring system tested at BC Hydro on 500-kV CTs using optical sensors for a voltage-phase measurement. The project evolved to a full-development program for complete voltage and current sensors. Commissioned in May 2000, the 230-kV combined current and voltage sensors have been in service at the Ingledow Substation as an ongoing technology field trial. The program's goals are to:

• Verify performance of the optical sensors in a substation environment.

• Verify stability and robustness of the sensors over time and temperature.

• Compare performance of the optical sensors with conventional instrument transformers.

• Incorporate improvements into the sensors based on the initial installation.

• Evaluate the application of optical technology in areas such as power-quality monitoring, high-precision metering, protection and disturbance monitoring.

It was important to select a location that would minimize impact on operations should there be any reason to have to take the optical sensors out of service. BC Hydro selected a section of parallel bus in the 230-kV portion of the Ingledow Substation, which would allow installation at any time without disrupting operations. Existing unused structures provided a ready base for the sensors. The sensors use hollow-core composite insulator columns filled with dry nitrogen with no SF₆ gas, oil or paper insulation.

The installation itself was fairly straightforward and was completed during a brief outage (Fig. 1). The sensors weigh only a few hundred pounds and proved easier to transport and erect than heavier conventional units. For installations in new substations, opportunities exist to reduce the amount of civil work and structure requirements.

A fiber-optic-cable assembly connects the sensors to the electronics in the control house. NxtPhase pre-terminated the fibers with optical connectors at the factory, eliminating the need for splicing or for delicate optical work in the field. To prevent damage, NxtPhase installed the fiber cables spooling the excess fiber in a cabinet at the base of the columns.

The sensors are passive optical devices designed and manufactured for minimal maintenance. All the intelligence of the system resides in an electronics package in the control building. Here, the optical signals originate and pass through the fibers to the columns in the yard where electric and magnetic signals influence the polarity of the light (see sidebar). The electronics package interrogates returning signals and creates digital outputs that represent the primary current or voltage.

Interfaces are a key issue for the new digital technology. In the existing analog environment, signals are typically 5 A for CTs, and 69 or 115 V for voltage transformers (VTs) with VA ratings that exceed the needs of modern relays, meters and other secondary equipment. All of this secondary equipment has become intelligent and digital, but the digital world exists only midway through secondary devices — all are equipped with analog front ends.

Waiting for the world to go all digital, with established protocols for sensors and digital secondary devices, is a bit like issuing a specification for both chickens and eggs and hoping for divine simultaneous delivery.

You have to start somewhere. In this case, the starting point was taking the digital signals from the device and interfacing them with a PC-based data-acquisition system. The devices provide low-energy analog signals (4 Vrms) and high-energy signals (1 A for current and 100 V for voltage) tied into meters from Power Measurement (PML) (Saanichton, British Columbia). An additional PML meter was used for data capture from conventional bushing type CTs and CVTs located elsewhere on the bus for comparison.

The electronic interface from the high-energy amplifiers allowed the use of the sensors with today's commercially available metrology. The digital source means the instruments are “digital ready” for when new digital metrology becomes available. A fully integrated digital system will reduce the number of components, improving reliability and operating flexibility.

Each of the sensors went through a full suite of tests at Powertech Labs (Surrey, British Columbia), a full-service facility with high-voltage, high-current and high-power labs. Tests included high-voltage withstand, partial discharge, accuracy, mechanical, thermal rise and polarity. The sensors also passed additional tests not generally performed on conventional instrument transformers. These tests included verification of accuracy over temperature and accuracy with disturbed electric fields.

The testing itself proved more challenging than anticipated. All the dielectric testing was straightforward and successful, but the optical sensors pushed the boundaries of conventional accuracy testing. For the trial units, it was not clear whether the accuracy of the current sensors or the test equipment was being measured. NxtPhase has since done extensive work with Powertech to create a series of overlapping test setups with precision CTs for low currents and shunts for high currents to demonstrate better than a 0.2% amplitude accuracy from 2 A to 3600 A. Protection accuracy also was verified.

Accuracy is one of the primary advantages of optical sensors, but there were other performance characteristics that have proved interesting. Figure 2 shows a switching transient that was captured from both the optical sensor and CVT. It is interesting to note the CVT's delayed response and tendency to resonate or ring. All indications suggest the optical sensor provides a more accurate representation of actual line conditions. Improved performance could lead to better security for protection applications.