Coal, Gas, O&M

Twelve-Pulse Digital Exciter Raises Reliability

Issue 2 and Volume 104.

UGI Development Co. Recently Installed a 12-pulse Digital DC Static Excitation System at itsHunlock Power Station, replacing a mechanical exciter that suffered from old age, which raised maintenance costs and reduced reliability. The new static excitation system was installed on Unit 3, a 1959-vintage 50 MW Westinghouse turbine generator powered by a Foster Wheeler anthracite coal-fired boiler. Hunlock Power Station is located on the North Branch of the Susquehanna River, in Hunlock Creek, Pa.


UGI Development Co.’s Hunlock Power Station
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The original Westinghouse directly connected mechanical exciter had two bearings and was driven by the main generator through an insulated flexible coupling. The excitation system controlled the output power of the main generator by varying the generator field coil excitation through a large field rheostat. The greater the excitation or field strength on the field coil, the more output power is produced by the main generator. Control of the mechanical exciter output was accomplished by either a manual motor-operated field rheostat or by means of the Westinghouse automatic Powertrex buck-boost voltage regulator system.

Replacement Justification

UGI justified replacing the Westinghouse mechanical exciter because of increased maintenance expenses, decreased reliability, and decreased availability, which resulted in lost generation. Some of the specific technical reasons included:

  • The mechanical coupling between the exciter and generator frequently failed, causing high vibration readings on the exciter inboard bearing and resulting in forced shutdowns of the turbine generator. Also, the mechanical coupling required significant preventative maintenance labor in the form of frequent lubrication and brush replacement.
  • The mechanical coupling failures would occasionally damage the exciter thrust bearings. This would result in forced shutdowns of the turbine generator due to high bearing vibration.
  • The mechanical exciter relied on carbon brushes for the transmission of DC power from the exciter to the generator fields. When the carbon dust from the brushes mixed with the lubricant from the coupling, it would ground the exciter field coils. After a grounding occurred, the exciter would have to be dismantled and shipped to a motor repair shop for cleaning. Power generation (and the subsequent revenue) would be lost until the cleaning was completed, the exciter reassembled and the unit brought back on-line. This could result in up to five days of lost generation.
  • The exciter field coils were starting to fail. Rewind of the original exciter would have been more than half the cost of a new static excitation system.
  • The age of the original equipment already exceeded the life expectancy of the insulation, indicating that a complete rewind of fields and armature was needed. This would have been a very costly, time-consuming process. Furthermore, the rewind would neither benefit routine maintenance activities nor improve the operating efficiency of the old system.
  • The old regulator had become unreliable and the response time was too slow. Spare parts for the voltage regulator were obsolete and were no longer supported by the manufacturer.
  • The original exciter was expensive to maintain. Brushes needed to be changed weekly. During extended outages, the exciter would have to be repaired or reconditioned. Installation of electrical connections, mechanical bolting, and alignment of the exciter was labor-intensive. On average, installation required 16 man hours.
  • The new static excitation systems are 20 percent more efficient than the older mechanical exciters.

DC Exciters

DC static excitation systems are not new to the industry. They are more commonly known as DC drives or rectifiers, and are widely used in DC motor control applications. The purpose of the DC drive or rectifier is to convert AC power to pulsating DC power through the use of silicon controlled rectifiers (SCRs).

DC drives come with a variety of electronic control packages to enhance versatility. The packages include high-speed digital voltage regulators, microprocessors for controlling the firing rate of the SCRs, self diagnostic packages for troubleshooting problems, and fault protection modules. Due to the accuracy, speed and dependability of the digital voltage regulator, it is ideal for controlling the output of a main generator. Because of this special application for the power generation industry, the DC drive evolved into what is known as the DC static excitation system.

DC Rectifier

Rectifiers come in a variety of configurations: three-pulse, six-pulse and 12-pulse designs. The designation refers to the number of SCRs being fired in the DC drive. The percentage amplitude of the pulsating AC component remaining on DC power level produced during the rectification process is called the ripple factor. The ripple factor is a measure of how smoothly the DC power flows into a load, such as a motor.

Three-pulse rectifiers have a very poor waveform. Filtering must be used to smooth out the ripple. Other drawbacks of the three-pulse rectifier are large amounts of feedback noise from the pulsating SCRs, low efficiency and induced line disturbances. Three-pulse rectifiers are typically used for small control applications.

Six-pulse rectifiers have a ripple factor of 4.5 percent. Depending on the type of load, the six-pulse rectifier may not need any filtering, and feedback noise is reduced as compared to the three-pulse rectifier. The six-pulse rectifier is used for medium control applications.

Twelve-pulse rectifiers have a very low ripple factor. Filtering is not needed because the harmonic distortion is reduced to 10 percent that of a six-pulse rectifier. Twelve-pulse rectifiers are used for large control applications.

When determining what rectifier configuration is best suited for a given process, it should be recognized that if more SCRs are used during the rectification process, the ripple factor is smaller and less filtering is needed to smooth out the DC power. Conversely, as the number of SCRs decreases, more inherent harmonic distortion or feedback noise will be generated by pulsation of the SCRs.

Users should determine how much ripple and noise can be tolerated by the process load. For example, a critical load application such as a motor requires smooth, noise-free power. An application such as a battery charger, on the other hand, is less susceptible to ripple and noise and may not require filtering. Users should also determine what level of protection is needed for the process. Basic packages have instantaneous over current, over voltage, loss of field and feedback loss protection. Optional packages include self diagnostics for troubleshooting, frequency protection, internal drive fault protection for miscellaneous faults and over temperature protection.

Decision Criteria

The standard static excitation system in the power generation industry is the six-pulse configuration. In this application, six-pulse systems usually do not use any filtering because the 4.5 percent ripple factor is smooth enough for the generator field. The only drawbacks with the six-pulse systems are that any ripple introduced into the generator field is an efficiency loss in the form of heat. Because of this loss, more energy is consumed to sustain the field strength of the generator for producing power. The six-pulse systems also have a higher harmonic distortion level and a lower power factor when compared to the 12-pulse alternative.

Hunlock Station’s original mechanical exciter was an ideal DC machine because it produced no ripple or harmonic distortion. Several key factors convinced Hunlock Power Station management to purchase a 12-pulse excitation system:


Post-retrofit, all that’t left are cable connections to the digital exciter enclosure.
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  • The 12-pulse system would reflect similar characteristics to an ideal DC machine, including 1 percent ripple and significantly reduced harmonic distortion.
  • The 12-pulse unit would provide a better power factor along with the near total reduction of harmonics. The higher efficiency, reduced line disturbance and better waveform reduce the heating in the iron of the generator fields.
  • Maintenance on couplings, bearings and brushes would be eliminated along with their associated friction and drag losses.
  • Energy-burning rheostats would be eliminated.
  • A rewind of the fields and armatures, when the wiring insulation began to fail, would have been almost the same cost as the purchase of the new solid state exciter. The armature had been rewound once, but another failure was inevitable.
  • Mechanical parts such as bearings, vibration equipment and brushes would be eliminated from inventory since the 12-pulse electronic system requires no moving parts. Also, fewer spare parts would be needed in the 12-pulse system because of the redundancy of the parts used in the drive package.

Field Results

Due to the compact design of the drive package, the installation of all major components was accomplished while Hunlock Power Station’s Unit 3 was operational. The control wiring was installed during a scheduled four-week maintenance outage in September 1998. The control wiring was easily installed into the existing turbine trip and computer control systems.

With a little planning, set-up time for the new excitation system was greatly reduced by conducting preliminary testing on the drive before shipment and prior to returning the turbine generator to service. Testing consisted of energizing the drive, varying or simulating the generator load, and monitoring the output response to ensure that everything was functioning properly.

When the turbine generator was placed on-line at the conclusion of the four-week planned outage, the generator output was held at 3 MW for approximately four hours. At this power level, the new static excitation system was adjusted and tuned.

Operation of the new excitation system since installation in September 1998 has been virtually flawless. One minor control disturbance, which was corrected, did cause the turbine generator to trip, causing a one-day forced outage. Overall, reliability and integrity have been dramatically improved over the mechanical exciter, as demonstrated by Hunlock Power Station’s 97 percent service factor for 1999. The duration of forced outages associated with exciter failure has been reduced from seven days to one (which was caused by a unique problem that will probably never recur).

Bios

Stephen P. Kopcho is a staff engineer with UGI Development Co. He has 15 years of experience in instrumentation and control engineering, and holds a bachelor’s degree in electrical engineering from the State University of New York at Binghampton.

Peter Albert, president of Dutch Electric Corp., has been replacing rotating generators with solid state equipment for 35 years. He has taken part in the industry transformation from the use of selenium stack rectifiers, through germanium SCRs, and into the current silicon-based rectifier devices now controlled by microprocessor technology.

Ed Moody is vice president of business development with Multi-Level Technologies Inc. He has more than 35 years of experience in the area of instrumentation and control, with Honeywell, Renwell Industries, Teleflex and the Weston Division of Schlumberger.


Installation and maintenance of the old mechanical exciter was very labor-intensive.
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The new 12-pulse excitation system enclosure.
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