Timing is Everything
The role of the high-voltage (HV) circuit breaker has always been one of the most determining factors of HV network reliability. Its main role is to protect the network and installed electric equipment from destructive short-circuit current surges. A HV circuit breaker can stay in the closed position for years, but it is still expected to interrupt a powerful short-circuit current of many thousands of amperes in a fraction of a second. The nature of its operation places it among the most unpredictable equipment on the electric network.
To prevent a misoperation, engineers are continuously creating ingenious new methods to efficiently test this critical apparatus to uncover any improper function or defective part. While some methods succeeded, most failed.
One of the earliest and most successful test methods was the “timing test.” Still the preferred method, the timing test consists of measuring the mechanical operating time of the breaker's main contacts. The time measurements start from the beginning of an operational order until either the contacts make on a closing operation or part on a tripping operation. This article briefly revisits the history of timing tests and describes the modern technology that Hydro-Québec (Québec, Canada) uses in the field today.
Timing Test: A Brief History
A breaker, whether oil-insulated, SF
Hydro-Québec conducted first-generation timing tests by tracing curves on graduated paper film driven by a servomotor, with light deflected by mirrors fixed on galvos (current driving device). Engineers studied these curves extensively with hand-drawn calculations. Any deviation from the standard specifications would suggest a presence of an anomaly.
Second-generation testing began around 1988 and continued until about 1995 when microprocessor technology acquired and displayed data digitally. Stand-alone instruments that incorporated printers using thermal paper were built mainly for operation-sequence programming and printing results as graphs and in predefined, unchangeable tabular forms. These instruments quickly showed their limitations. In addition to the two main tasks, users demand that the instrument manage other routine tasks within the operating system:
Data entry management, using an incorporated console
Data display management, using an incorporated LCD screen
Data printing management
Data acquisition and saving
Calculation performance (most of the time inaccurate, because of signal interference or contact bouncing).
However, it was too much to ask from the heavily tasked microprocessors of these stand-alone instruments, and the need for analysis programs was becoming obvious.
Consequently, a short-term solution was a new technique called “computerized assisted testing.” This technique involved transferring the data from a small number of tests to a larger computer and then analyzing the results. The test was performed directly from the microprocessor test unit itself and not from the computer. This test required several external manipulations, and because the expensive software used in this method was strictly for analysis purposes, it was not used most of the time. An operator would spend a lot of manipulation time trying to conduct a simple test.
Today's Test Equipment
Based on the previous experiences, the third-generation test instruments completely separated recording/control acquisition from computer/analysis programming. Computer technology modernized the timing test to a point that it moves the power of the lab to the field. This is done not with the data acquisition part that is only electronics, but with powerful analyzing programs that simplify the analysis process into an automatic operation with accurate and reliable conclusions.
This modular concept has several advantages:
It takes advantage of the world's best technological advances, instead of being limited to the technology of the timing test instrument manufacturer. This means a colorful computer screen display, large memory capacity, hard disks with gigabytes of capacity, precise color printers, fast modems for data transfer and sharing test information and lightweight computers.
The evolution of the analysis programs is made independent of the timing test instrument itself, making it possible for the software to evolve at the speed of the hardware. Just look at the evolution of platforms such as DOS, Windows 3, 95, 98 and 2000.
Data are collected in raw form so they can be preserved independently of the analysis manipulations and be reviewed later with better software tools.
Modern timing test equipment is able to collect a multitude of information, most of which was only available in high-tech labs years ago. The information that today's test equipment can collect includes:
- Main and auxiliary contacts timing operation
- Resistor insertion time
- Travel, velocity and acceleration of the main contacts
- Operation coils current signatures
- Main circuit voltage and current signatures
- Operating mechanism's characteristics
- Pressure measurements
- Temperature measurements
- Mechanical strain forces measurements
- DC supply to breaker measurements
- Speed damping characteristics.
All of this is accomplished with a proper transducer connected to an analog data-acquisition channel. Once this is complete, a good analysis program enables the operator to reach interesting conclusions.
Testing at Hydro-Québec
Hydro-Québec requires test equipment to meet very strict requirements. For example, it must be usable in very-high-voltage environments (Hydro-Québec is one of the few utilities in the world to use 735 kV) and in harsh environments (-50°C/-58°F is not uncommon).
After long and thorough evaluations, Hydro-Québec approved a few timing test systems that met its requirements, without compromising the new computer technologies available. Presently, Hydro-Québec is using Model CBA-32P from Zensol Automation (Saint Laurent, Québec) in the Québec City area.
Today's testing equipment allows the operators to create the testing plan prior to traveling to the testing site, thus reducing the actual testing time they spend in the field. Most test plans include:
Information on the breaker being tested
Operator's name
Place and date of the test
How data are to be collected (identifying the acquisition channels, sampling rate, number of points and acquisition time)
How data are to be displayed
How calculations are to be made
How the operations' sequences are to be executed.
The operator controls the execution of each test through simple clicks of the computer mouse. The data slave test instrument records are transferred to the computer immediately. A powerful Windows-based analysis program processes the complex calculations almost instantaneously and presents it in a graphical format. This allows the operator to concentrate only on useful conclusions. The slave instrument (with no keyboard, no screen) is considered a computer peripheral, the same as a printer or a scanner.
In substations where the level of interference is very high (800 kV), the fiber-optic link cable isolates the computer. With an extra-long 61-m (200-ft) cable, the operator stays indoors without exposure to cold, snow, rain, sun or hot, humid weather. The operator can conduct the test safely and comfortably in the presence of dangerous breakers.
Furthermore, the operator can transmit the data files over the network or through the Internet so additional and more experienced personnel can investigate the results while the test is still in progress. This contributes to making the best decisions, thus reducing intervention time and achieving higher accuracy in the test results.
Analysis Programs
Graphics are a great way to have a global view of the breaker's parameters. Multiple contact traces on the same page show any discrepancies between contacts. In addition, they are the fingerprints of the breaker's parameters in operation. An abnormal trace curve suggests a matter to investigate.
The figure to the right shows a hard rebound at the end of a travel curve on a trip operation of a dead tank breaker. This suggested a lack of damping at the end of travel. Excess energy was not being absorbed well and damage may have already occurred.
After examining the internal parts, damage had indeed happened on the main rod of the moving contact. The root cause was a defective dashpot.
Comparison is a fundamental requirement of any timing test analysis. Contacts are compared to each other in the same test to verify synchronization between poles or with previous tests to identify evolving anomalies. Most of all, contacts are always compared to reference values. To simplify comparison, most analysis programs allow the operator to superimpose graphics of the same nature, which makes it easy to spot any divergence in behavior.
The manufacturer supplies a time-specifications chart with each breaker. On a timing test, these times have to be respected within specified tolerances. In the old days, time calculations were done the hard way, with rulers and pencils for each trace. Speedy calculations needed more experienced personnel, because they had to use more complex formulas. Current analysis programs do that automatically. If you want them to, they will plot the values needed beside the graph. They can even raise a flag where needed to attract attention. Various calculations programs can be prepared in advance to be available for each breaker type or specific situation.
Several tools are available to help complete analysis tasks:
Scaling to help get a better view of small details
Manual calculating to check a special value
Exporting data to more specialized programs, such as Microsoft Excel, to increase scope of analysis or for better presentation
Keeping records in its electronic form, the gathered data can be saved in a convenient and practical manner
Transferability to remote places by communication networks (phone or satellite networks)
Photos and illustrations of various types of breakers
Batch testing for repetitive, unattended testing (for manufacturers)
Integrated on-line help.
Timing tests for HV circuit breakers will always be important in predictive maintenance. Technological advances helped greatly in their evolution to a point such that analyses that previously were exclusive to high-tech labs can now be conducted in the field. The new generation of field testers is more focused on finding the problem instead of on long and tedious routine operations such as calculations and calibration of the testing equipment.
Hydro-Québec's experience has been that all the operators that used this new way for breaker testing only once would never choose an old way.
Every utility can benefit from a quality testing program, because it minimizes circuit-breaker misoperations and improves the overall reliability of the HV electric network.
Bruno Girard is an engineer with the Trans-Energie Group, the Transmission Division of Hydro-Québec. Girard is in charge of giving technical support to the electrical maintenance employees of the Québec City area. As a part of his responsibilities, he also provides his expertise when major failures occur on the apparatus. Girard has more than 22 years of experience at Hydro-Québec, including 19 years as an engineer-technical support in apparatus. Girard graduated in 1980 from the University of Laval, Québec, Canada.
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