Other waveforms collected in the course of the trial indicate the presence of surges on the secondary wiring of both the conventional CTs and CVTs. Figure 3 shows a relatively high surge voltage on the secondary wiring from the conventional CVT. These “phantom transients” were seen repeatedly on the inputs to the meters monitoring the conventional signals but were not evident from the optical sensors. Optical fibers provide galvanic separation of the high voltage and the control electronics, thus eliminating concerns with transients and false trips.

Waveforms also were captured as the bus was energized and de-energized. Figure 4 shows the signal from the optical-voltage sensor during de-energization. The trapped charge occurs when a conductor with some amount of capacitance is de-energized. This trapped charge can pose a significant problem until the line is reclosed. At such a point, the CVT can be driven into a ferroresonant condition because of the trapped charge redistributing as a dc voltage across the transformer portion of the CVT. The ferroresonance appears as persistent oscillations in the secondary output that can adversely affect system protection resulting in false trips. The use of the optical sensor avoids the risk of ferroresonance as there are no magnetic components. The optical sensor is able to measure the trapped charge. These measurements could lead to the development of “smart” reclosing of lines, thus reducing system transients and stress on high-voltage equipment.

Sometimes it takes a field installation to identify problems that are difficult to identify in the lab. BC Hydro encountered reduced accuracy in the voltage sensor in fog and snow conditions that were not identified in lab simulations. A study of the issue led to a design modification for the commercial version of the product, and subsequent testing indicates the problem has now been solved.

As a result of the field trial, several other areas have improved, including:

• More compact electronics to minimize space needed in the control building.

• Enhancements to allow ratio changes via software in the field.

• Interchangeability improvements to allow switching of various components while maintaining calibration.

• Improved metrology in the high-current lab for measuring accuracy.

• Full documentation for installation and operation.

During the 18 months that the original units have been in service, there has been no forced outage for maintenance or repair activities. The units have proven to be reliable and robust. This pre-commercial field trial allowed the vendor opportunities to upgrade and enhance the product before commercial release.

BC Hydro's long-term vision includes substations that consist of optical transducers, switchgear, and a robust processing platform that will perform all functions of protection, metering, operation and online condition monitoring. BC Hydro views this work as a first step towards that goal.

Greg Polovick received the BE degree from the University of Saskatchewan in 1970, then joined Westinghouse Canada as a power-transformer design engineer. In 1974, he joined BC Hydro working first in the Quality Control and Inspection Department and later in the Stations Engineering Division, where he is currently a specialist engineer responsible for the procurement and application of reactive equipment. He has worked on several CEA, CSA and CIGRÉ working groups related to reactive equipment standards.

Optical-voltage sensing (Fig. 5) is accomplished using three Pockels cells (miniature optical electric field sensors), strategically positioned within a hollow nitrogen-filled insulator. Circularly polarized light enters the electro-optic crystal. When an electric field is applied, an induced phase difference is created between the two principal polarizations traveling in the crystal, resulting in an elliptical polarization at the output. By measuring the degree of “ellipticity,” an accurate measurement of electric field can be derived. The three electric field measurements are then combined using an appropriate weighting factor to provide the voltage measurement from line to ground.

Optical-current sensing (Fig. 6) is accomplished using two linearly polarized light waves that are sent from the control building opto-electronics package through optical fiber to the sensing head, which encircles the conductor at the top of the insulator column. Each light wave passes through a quarter wave plate creating right and left circularly polarized light. The two light waves traverse the fiber-sensing loop, reflect off a mirror at the end of the fiber loop and return along the same path. The magnetic field induced by the current flowing in the conductor creates a phase shift in direct proportion to the current. The closed-loop electronics circuit demodulates the light waves, determines the phase shift and produces an output signal representing the current that flows through the line.