Generator Exciter Replacement and Troubleshooting
Efforts -- A Capstone Experience
M.M. A Rahman1 and T. Petersen
School of Engineering, Grand Valley State University, 301 W Futon Street
Grand Rapids, MI 49504, rahmana@gvsu.edu 1
Abstract
Recently, an exciter system for a pumped storage facility was replaced as a fault occurred to one
of the generators. It was piloted as a part of a capstone project, a graduate course, offered by the
School of Engineering at Grand Valley State University in collaboration with a regional
company. The objective of this paper is to highlight this troubleshooting effort and experiences
gained during the replacement of a vintage 350 MW generator exciter assembly for the facility.
This work identifies that because of a long time frame between the removal and replacement of
the exciter assembly, conditions were created that challenged reinstallation efforts. Once
difficulties were experienced, due to a combination of factors, troubleshooting efforts took
longer than anticipated. This paper provides a step-by-step account of the actions taken and the
rationales for decisions made during troubleshooting efforts for this capstone project. The
specific reasons for missteps are identified. The paper concludes by providing recommendations
to ensure successful and timely installation of vintage equipment in the future. This was an
industry-university collaboration project that is very important to enhance engineering education
and research.
Keywords: Generator, Exciter, Troubleshooting
I. Introduction
The pumped storage facility is a 1,872 MW power plant site. The unit was built and made
operational in 1973. Original equipment [1] is installed and has 6 independently operated
turbine/generator units located at the base of a 27 billion-gallon reservoir. At night, when
customer demand for electricity is low, 6 reversible pump-turbines move water 363 feet uphill
through 6 large pipes to the reservoir. During the day when customer demand for electricity is
high water is released from the reservoir to flow back downhill through large pipes. This is used
to turn turbines in the powerhouse to produce electricity. Each of the 6 turbines is 460,000 hp
turning at 112.5 rpm. Each alternating current generator/motor set is rated at 350 MW, 2000
Volts, 3 phases, 60 cycles/s, 9382 Amps, with 500 V excitation systems. A schematic diagram of
this facility is presented in Figure 1-1.
An alternating current generator requires direct current to energize its magnetic field. An
excitation system must be able to provide the direct current output to provide the magnetic field
(flux) for a generator. In large alternating current generators this field is obtained from a separate
source called an exciter. Usually, alternating current generators above 250 MW take their
excitation from an alternating current source. This alternating current is converted to direct
current before being applied to the rotor of the main alternating current generator. The control of
this excitation is accomplished with an automatic voltage regulator. This paper provides a stepProceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
by-step account of the actions taken and the rationales for decisions made during troubleshooting
efforts for this project.
Fig. 1-1 A schematic diagram for the pumped storage facility
II. Problem Statement
On Friday July 17, 2009 the pumped storage Unit-3 DC Main Exciter experienced a fault in the
armature winding. Inspection and cleaning was successful (in clearing the ground condition) but
could not be sustained during operation. After a careful investigation, a consensus between
corporate engineering and site management was reached that this unit was no longer fit for duty
and required a rebuild. As the exciter assembly had exceeded its design life it was determined
that it was necessary to rewind the 3 major components to include the DC Exciter, Pilot Exciter,
and Permanent Magnet Generator (PMG). The 4160 V motor was not included in the rebuild
plan.
The rewind shop usually contracted by the company for exciter rebuilds was at capacity
and declined the work. An alternate shop commonly used by the company for motor rewinds was
contacted. While they had the required capacity available, the alternate shop informed the
company that a long lead time was required to rebuild an exciter of this size. In the meantime, a
500 Amp capacity trailer mounted static exciter was identified and installed so as to keep Unit-3
available for operation. An engineering team was put together to identify wiring changes needed
to remove power and controls from the permanent exciter and to allow for the necessary power
and control wiring installation for the trailer mounted exciter. All associated plant drawings were
identified and construction revisions were created using a CAD program.
III. Replacement Process, Observations, and Discussions
After several months the rewinding of the DC Exciter, Pilot Exciter, and PMG was complete.
Conditions were now satisfactory for their reinstallation. In the meantime there had been a
number of changes in the make-up of the project team. Both the original project manager and
one of the lead electrical engineers assigned to the exciter rewinding had been re-assigned. The
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
project manager was working for another division of the company at a remote location but was
available by phone. The lead electrical engineer was available by phone but was leaving on a
business trip out of the country. Since the trailer mounted exciter was providing excitation needs
the rebuilt components were mechanically installed with the unit available for operation. After
completion of the mechanical installation a seven day outage was provided. The outage allowed
for demobilization of the trailer mounted exciter with concurrent electrical installation, startup,
and testing of the rebuilt components.
There are two leads each for the DC Main Exciter windings: the buck windings and the
boost windings. Connection to each winding is matched both by color and by number. Further,
the leads are polarity dependent. Towards the end of the first day while electrically connecting
these 6 field pole leads to their power feeds the electricians noted:
• The 6 field pole leads numbered 1-6 (representing the 3 separate winding circuits for the
main, buck, and boost source of excitation) replaced during rewind did not have the site’s
maintenance taped color coded.
• The 6 field leads numbered 1-6 replaced during rewind were not installed into the lead
mounting block in the same sequence as when sent to the rewind shop.
• The 6 leads on this unit were labeled (metal tags with numbers) in an opposite direction to
that of the 5 other DC generators on site.
The electricians continued to connect the wiring by matching number to number.
However, exhibiting the questioning attitude trait desired by the company the electricians
contacted management and informed them of the discrepancy.
On the morning of the following day the electrical engineer reviewed the situation. Due
to the leads not being re-taped for color code pairing nor sequenced correctly within the
mounting block this machine could not be re-installed with enough confidence to prevent
damaging equipment or of possibly harming personnel. The engineer was not in possession of
notes from when the exciter was shipped off site. As a result he did not have a drawing of the
markings when the exciter left, nor did he have resistance measurements for the windings. When
called, the rewind shop stated that all the leads were marked exactly as they arrived in the shop.
The rewind shop had a startup technician on site who suggested that it is quite common for the
rewind technician to land the lead as it comes out of the winding into the lead block without
concern for numbering. He further suggested that the technician probably never noticed that the
block had numbers. So, if the six position wires with yellow tape happened to exit the core at the
one wire location then he would land the yellow wire in the block at the easiest location, in this
case ‘1’.
A team was established consisting of an electrical engineer following the rewind of the
exciter components, another electrical engineer overseeing the wiring changes from the trailer
mounted temporary exciter, three site maintenance supervisors, an electrical laboratory
technician, and a lead instrument technician. Support, as needed, was provided by plant
electricians, mechanical repairpersons, and operations personnel. The team project manager and
the site operations and maintenance manager provided oversight in a hands-off/big picture
capacity to ensure safety and logic of thought. The team asked for resistance measurements. It
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
was found that two of the windings matched at a low resistance while one of the windings was of
significantly higher resistance. This made sense as the buck and boost windings were equal but
wound opposite, while the DC Main Exciter winding had more wire. The DC Main Exciter
winding was now identified and determined to be landed appropriately at the positions ‘3’ and
‘4’. However, polarity was still a question, as well as determining which winding was the boost
and which was the buck. It was now late in the second day. All power and control wiring for the
trailer mounted exciter was disconnected. With the exception of the DC Main Exciter wiring
discrepancy, all original plant wiring was restored and independently verified to be in accordance
with the project prints. During the morning hours of the third day it was found that the rewind
vendor could not locate any documentation identifying winding resistances or winding markings
that should have been recorded upon receipt of the exciter at their shop. The vendor stated that it
is their common practice to take these readings and that they were at a loss for where the data
might be. They stood by their assertion that the exciter was delivered back to the site in the same
configuration as which it was received.
While maintaining communication as needed, the lead engineer decided at this point that
continued discussion with the rewind vendor would be unfruitful. Recognizing that all wiring
was connected to allow operation of the exciter, the team set on a course of action plans to test
the windings one at a time. It was decided that all three sets of winding leads (main, buck, boost)
would be isolated and tested for polarity with a known DC source. This testing required
connecting the driving DC source from the Automatic Voltage Regulator (AVR), with known
polarity, to each winding lead set individually while the other windings were open circuited. This
would involve disconnecting four of the six wires, releasing safety tagging from the equipment
and, dressing for potential breaker arc-flash, racking in the 480 volt breaker, performing
controlled testing on the equipment to verifying operation and polarity, racking the 480 volt
breaker out, and re-establishing safety tagging. Each of these evolutions required between 90
minutes and 2 hours. At the end of a very long third day it was found that two of the three sets of
field pole windings were tested satisfactory with output polarities matching input polarities.
These were the main and boost windings. Since an opposite polarity was measured at the output,
testing of the buck winding showed the lead polarities were incorrectly marked on the DC
generator by the vendor. Had the 6 leads been assembled as marked, the DC generator would
have boosted excitation every time it tried to buck. During startup this would have most likely
kept tripping the unit on over-excitation. To correct this reverse polarity, the team reversed the
input connections to suit. All of the leads were then re-numbered to match the numbering scheme
of the other units. Early on the fourth morning the exciter was completely reassembled and made
ready for startup and testing. A properly operating exciter will exhibit 270 volts from the AC
Pilot Exciter, and somewhere between 10 A and 35 A from the buck and boost windings,
depending on various settings. With the unit operating the AVR seemed to have difficulty
controlling the AC Pilot Exciter output voltage. The voltage was high at 360 V compared to the
expected voltage level of 270 V.
The team needed to identify whether the underlying AC Pilot Exciter voltage issue was
with the Unit-3 AVR or the Permanent Magnet Generator (PMG). In attempting to identify the
weak link it was decided to cross connect the Unit-2 AVR and PMG with the Unit-3 AVR and
PMG. The assumption was that if the Unit-2 PMG was connected to the Unit-3 AVR and the
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
Unit-3 AVR worked correctly then the Unit-3 PMG would be the failure point. Conversely,
connecting the Unit-3 PMG to the Unit-2 AVR with the Unit-2 AVR incorrectly operating, then
the fault would lie with the Unit-3 PMG. However, should the Unit-2 AVR worked correctly
when connected to the Unit-3 PMG then the Unit 3 AVR would proved to be the source of the
problem.
Testing was performed with the output of Unit-2 PMG connected to Unit-3 AVR PMG
input links. This ensured the Unit-3 AVR had all the connections needed to function normally.
The AC Pilot Exciter output voltage was monitored at the Unit-3 AVR. Upon startup the AC
Pilot Exciter output voltage was at 360 V. The AC Pilot Exciter voltage then dropped to the field
flashing voltage of 90 V for a brief period of time. The AC Pilot Exciter voltage then steadily
climbed reaching 380 V before the units were manually shutdown. The outcome of this test
suggested that the Unit-3 AVR was the cause of the high AC Pilot Exciter voltage.
To verify the previous test the Unit-3 PMG was connected to the Unit-2 AVR in an exact
manner while monitoring the AC Pilot Exciter output voltage on the Unit-2 AVR. The AC Pilot
Exciter output voltage was 360 V upon start-up. The AC Pilot Exciter output voltage then
dropped to the field flash level for a brief period of time before rising back to 360 V. The voltage
then just cycled between 90 V and 360 V until the unit was manually shut down. It was decided
that the tests were inconclusive as a result of the mixed results. No single component could be
isolated as the faulty equipment. After further investigation it was noted that cross-connecting
the units in this manner would never allow for the AVR to operate normally due to non matching
phases between two separate units. In explanation, the PMG and AC Pilot Exciter are mounted
on the same shaft. Upon start-up of the exciter set the PMG and AC Pilot Exciter field poles are
aligned and spin at the same rate. The AC Pilot Exciter and PMG then separately produce three
phase AC voltages/currents which are in phase with each other. The AVR SCR firing circuits are
built to expect the AC Pilot Exciter and PMG to be in phase. Cross-connecting the Unit-2 to
Unit-3 components as previously mentioned places the Unit-2 AC Pilot Exciter output out of
phase with the output of the Unit-3 PMG. Since its circuitry is built for specific phasing, the
AVR could not function in this configuration. This explains why neither the PMG nor the AVR
could be isolated as the faulty component, but not why the Unit-2 and Unit-3 AVR’s responded
differently when cross-connected.
During discussions on the early evening of the 5th day of troubleshooting, the team came
to the conclusion that they had done nothing different when connecting the rewound exciter
components. They verified that the wiring matched the prints. Recognizing that people are
consistently inconsistent they concluded that if the DC Main Exciter leads were misidentified at
the rewind shop then it is likely that either the PMG or the AC Pilot Exciter, or both, had leads
also incorrectly wound and misidentified. The team went looking for wrongly labeled leads. The
leads were again visually verified at the Unit-3 PMG incoming connection points in reference to
the AVR. Wires marked ‘X’, ‘Y’, and ‘Z’ were found to be properly matched at the terminal
strip within the AVR. Yet, suspecting a wire mismatch somewhere in the system, voltage
readings were taken at these terminals and compared against voltage readings taken at the
corresponding Unit-2 terminals. It was discovered that the voltages were different among the
phases and different between the Unit-2 and Unit-3 readings. A phase meter was used to identify
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
that the phasing was proper. Looking at the control circuit (specially comparators) diagram in
Fig 1-2, it was recognized that the wiring could be in phase but landed out of sequence. If the
fault was caused by swapped wires then the three leads needed to be moved either to the left by
one phase or to the right by one phase. As an example, moving left one phase would require
moving PMG-X to AVR-Z, PMG-Y to AVR-X, and PMG-Z to AVR-Y. The incoming Unit-3
PMG leads to the Unit-3 AVR were moved to the left by one phase. The Unit-3 exciter and the
AVR now indicated the correct AC pilot voltage of 270 V. The AVR circuits were now able to
recognize the PMG in synchronization with the AC Pilot Exciter. This proved that either the
leads had been mislabeled by the rewind shop as compared to the as-received condition or the
shop connected the leads to different phase circuits in the PMG winding while failing to label the
leads to suit.
IV. Conclusions
After failure of the exciter, the major components were sent to a shop not normally used for
repair. The rewinding process was a long lead-time effort taking many months. Either prior to
shipping or at the rewind shop the leads were misidentified. In the meantime several members of
the original team had been reassigned and were not available for reinstallation of the equipment.
Complicating the reinstallation further, members of the team were not fully trained on the
operation of the equipment. Once problems were discovered during the installation, due to the
combination of several different key people, improperly identified equipment, and a lack of
detailed knowledge of how the equipment operates, troubleshooting efforts ended up taking
several days. In the end it was found that there were multiple problems, including violation of a
fundamental concept of 3-phase system, traceable to a common process deficiency in preparing
the components for shipment to the rewind vendor and at the rewind vendor facility.
V. Recommendations
This capstone experience leads to following five recommendations to prevent future recurrence:
(1) in terms of pedagogy, the fundamentals of engineering is always important and need to be
emphasized persistently, (2) the site should properly identify and document equipment
conditions (and lead markings) prior to shipment to vendor facilities, (3) while it is not always
possible for long lead-time projects, every effort should be made to maintain consistency in
staffing, (4) it is often the case that “old-timers” who understand “antiquated” technology and
who are familiar with the operation and maintenance of vintage equipment are not available
when problems occur. Training should be undertaken to ensure the engineers and maintenance
people become adequately trained on the circuitry and operation of vintage equipment, especially
when they have the luxury of time during long lead-time projects, and (5) finally, the initial,
periodic, and final inspections at vendor facilities should be conducted when dealing with
vintage equipment. This is specially true for first-time vendors who are unfamiliar with the
specific equipment they are receiving. Overall, this was an industry-university collaboration that
highlighted the value of fundamental engineering concepts to engineering education and research
significantly.
Bibliography
[1]
Hitachi, Ltd. Instructions of Thyristor Type Automatic Voltage Regulator (Hitachi Manual Number KS-03571). Tokyo, Japan: Hitachi, Ltd.
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education
Proceedings of the 2011 ASEE NC & IL/IN Section Conference
Copyright © 2011, American Society for Engineering Education