A new controller provides front-end digital control to GE busfed power bridges.
By Richard C. Schaefer, Basler Electric Co., Highland, Ill.
Many power generation plants are faced with parts obsolescence, high maintenance, and down-time due to problems with the excitation system. These problems can include dc field breakers, motorized rheostats, rotating exciter failures, commutator deterioration, vibration, and obsolescence of the existing voltage regulator. Problems like these can affect machine availability and have the potential to result in extended down-time of the generator system. In addition, performance can be an issue, as more and more systems are required to have power system stabilizers.
The excitation system provides dc into the rotor of the generator to create a magnetic field necessary to build voltage at the generator output. Many past excitation systems were brush-type rotating exciters whose output was rectified by commutators and via brushes applying power to the generator's rotor. Over the years, it has been common to replace the existing rotating exciter for a static exciter that works into the generator main field or for those new installations, specifying a static exciter system. The static exciter is designed to provide all of the power required by the generator main field to support changing load. The static exciter system is designed for a wide range of machine ranges—from a couple hundred amps field output to as many as several thousand amps, depending upon the size and the speed of the generator.
The static exciter includes either a 3 silicon-controlled rectifier (SCR)/3 diode bridge or, as is more commonly provided today, a new 6 SCR bridge, a power potential transformer to step down generator voltage acceptable levels for the power rectifier bridge, and a controller and firing circuit. The digital controller contains a 0.25% automatic voltage regulator to maintain constant generator voltage regulation, manual control for standby, excitation limiters, protection, and data recording software. As system obsolescence prevails today, multiple solutions are available to retrofit older static exciter technology. Approaches may include:
Replacing the complete static exciter system including the power potential transformer
Keeping the power potential transformer and replacing the voltage regulator control and power rectifier bridge(s)
Retrofitting only the analog controls and keeping the power rectifier bridge(s) and power potential transformer.
Reusing rectifier bridges has become increasingly popular, as old rectifier bridge technology has exhibited high reliability and has remained in excellent operating condition. Hence, replacing only the analog controls due to obsolescence and keeping the power SCR bridge(s) may be a favorable economic solution for an excitation upgrade.
This article addresses a retrofit solution where a GE busfed static exciter manufactured in the 1980s was upgraded with a new controller for the digital excitation system, including a new digital firing circuit and gate amplifier board, to interface with the existing half-wave 3 SCR and 3 power diode rectifier bridge, ac breaker/field flash contactor, and power potential transformer.
In Corona, Calif., a GE Frame 5, 64 MVA, 13.8 kV, 3600 rpm co-generation plant required an upgrade to new controls to improve the performance and efficiency of the turbine generator. The excitation system was one of the elements that required replacement. The static exciter was located in a compact, restricted area containing two parallel convection-cooled rectifier bridges, an ac field breaker, a field flash contactor, and a rack-mounted analog voltage regulator assembly mounted on the front door. It also had a power supply, while a var/power factor controller was mounted in the cabinet interior. The busfed automatic voltage regulator (AVR) equipment was obsolete, and new performance requirements dictated the need to replace the voltage regulator.
The SCR bridge, however, was in fine operating condition with available replacement parts, hence the voltage regulator controls were the focus of replacement.
Retrofitting the controller
The original equipment represented a 1970s technology design and incorporated many transformers and huge power supplies to provide dc to the busfed card rack (Figure 2). The demolition included the elimination of all of the analog controls up to the SCR bridge.
The new equipment included a controller mounted on the front door to replace the old AVR rack, a new IFM 150 digital firing module programmed for a 3 SCR application with SCRs on the positive bus, and a gate amplifier board to provide distribution pulses to two existing convection-cooled rectifier bridges.
Digital technology offers substantially improved performance and features over its analog predecessor. Voltage regulation, excitation limiters, manual control, and communications are included in the digital controller to aid control and to streamline plant operations. Modbus communications provide metering, control, and annunciation, while generator voltage matching enables faster synchronization without the need for operator interface to manually adjust and match generator voltage.
Operating software is the key to commissioning the excitation system quickly and efficiently. The controller's BESTCOMS communication program uses a laptop computer for setup and commissioning, and testing tools are built in to BESTCOMS to aid startup and eliminate otherwise external-connected test equipment such as chart recorders.
The demolition included the elimination of all of the analog controls that made up the original excitation system, except the rectifier bridges (Figure 3).
The controller was installed on the front door of the cabinet. Along with a sequence panel that included interposing relays, the system included a firing circuit chassis, control transformers, and a field isolation module for monitoring field current and field voltage. The package was specially designed to fit on the back of the excitation cabinet door.
A gate amplifier board was mounted on the cabinet interior back wall to amplify firing pulses to gate the power SCRs for control of the field power. The controller interfaced to the existing ac field breaker and field flash contactor/field flash resistor. Figure 1 shows the cabinet lineup with the mounted controller.
A special interface transformer was used to match the power potential transformer secondary voltage to the required voltage of the new digital IFM150 firing module. The digital firing module has the means to be programmed for half- or full-wave SCR bridge operation.
After installation—and when wire checkout was completed—the excitation system and generator were ready to build voltage. The two-channel chart recorder in BESTCOMS was used to monitor voltage buildup and to perform the voltage step tests needed to determine excitation/generator performance. Unlike the older analog excitation systems that required voltage buildup in manual mode, the new excitation system safely built voltage in AVR mode without concerns of voltage overshoot.
Using the two-channel real-time chart recorder, voltage was built up in voltage regulation mode (Figure 4). Voltage step tests were performed to verify performance based upon the selected gains for the controller. Here, the generator voltage response was 0.3 sec with a 5% voltage step change in the positive direction. The analysis screen in BESTCOMS allows up to 10% voltage step tests to be performed, as well as time duration for the length of voltage step change. Comparing the performance, the analog system exhibited approximately 1 sec voltage response time, whereas the new system demonstrated three times faster voltage recovery that would aid generator relay coordination after a system disturbance.
Data are saved in either a screenshot pasted into a word-processing document or a data file for future record. Also available is oscillography that saves the information in a COMTRADE or log file.
The under excitation limiter (UEL) was tested at a lower calibrated value and dynamic performance and stable operation were verified.
The performance for the UEL responded in less than 1 sec with only a single, small under-damp swing. Upon conclusion of the commissioning, the excitation system was placed in var mode for normal operation.
The successful replacement of the analog controls with state-of-the-art digital technology met the expectations of the project and enabled the generator and excitation system to be commissioned quickly and efficiently with new features to enhance the system's reliability for many more years of successful operation.
Schaefer is senior application specialist in excitation systems for Basler Electric Co. He has been a senior IEEE member for 15 years, and holds an associate's degree in engineering technology. Since 1975, Schaefer has been responsible for excitation product development, product application, and the commissioning of numerous plants. Basler Electric Co. designs and manufactures excitation control systems, protective relays, and engine generator controls.
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