05-09-2012, 04:26 PM
Analysis of Selected Motor Event and Starting Reports
Analysis of Selected.pdf (Size: 2.02 MB / Downloads: 66)
Abstract
Motors are estimated to be one of the most
numerous components of the electric power system. As the device
that takes electrical energy and converts it to the mechanical
energy needed to power processes, a motor that is unnecessarily
out of service can bring an entire process to a halt, resulting in a
significant loss of revenue. Conversely, the expense and time to
replace a large motor damaged beyond repair mean that failing
to quickly and dependably protect a motor is also a concern.
Because there are many common failure modes (mechanical,
electrical, thermal, and so on), root-cause analysis of a motor
failure can be involved.
This paper investigates several real-world events with data
from both motor starting reports and event records. The data
demonstrate the value of having devices capable of recording
motor data during starts and fault events and of capturing and
reviewing such data for the purpose of determining root cause.
Lessons learned are shared to help in troubleshooting motor
problems and to avoid potential misoperations in motor
protection.
INTRODUCTION
Michael Faraday is well known for his discovery of
electromagnetism and for developing the first electric motor.
William Sturgeon, Joseph Henry, Andre Marie Ampere, and
Thomas Davenport built on Faraday’s discoveries to further
develop direct current (dc) motors. Nikola Tesla was the first
to invent the alternating current (ac) motor that is commonly
used today in industry [1].
Since the invention of the electric motor, engineers have
discovered numerous uses for this valuable tool. Today,
electric motors are used in every industry, accounting for more
than 50 percent of the electrical load in the United States.
Most industrial consumers can attribute more than 85 percent
of their electricity bill to electric motors [2].
Electric motors play an important role in the world as we
know it. Without them, many conveniences we take for
granted would not be available. Therefore, it makes sense that
every effort should be made to provide adequate protection
and monitoring of these valuable assets.
EVENT 2: CURRENT UNBALANCE ELEMENT TRIPS
DURING STARTING
In April 2010, a 375 hp motor suffered an unbalance trip
while starting. It was assumed that two current transformer
(CT) secondary wires were rolled going to the relay.
Therefore, the A- and C-phase wires were swapped in an
attempt to correct the assumed problem, and the motor was
started again. Despite the wiring change, once again, the relay
tripped on unbalance. Assuming the wrong wires had been
swapped, the B- and C-phase wires were rolled and the motor
was started for a third time, and the relay tripped on unbalance
for the third time.
EVENT 5: FULL PHASE DIFFERENTIAL TRIP
DURING STARTING
In May 2011, a 6,000 hp motor protected by several relays,
including a differential relay, started successfully. However,
the differential relay indicated a TRIP target. Upon further
review of the relay data, it appeared that the differential
element asserted. If the differential relay indicated a trip, why
did the breaker not open immediately? Also, why did the
differential element assert in the first place when the motor
continued to operate after the relay operation with no visible,
audible, or measurable damage?
The oscillograph from the filtered event report is shown in
Fig. 14. Notice how the ground currents are not sinusoidal and
decay throughout the event report. Whenever we see ground
current that decays, it is an indication that CTs may be
saturating.
CONCLUSION
Since the invention of the motor 180 years ago, we have
been using this great tool in numerous applications. Today, ac
motors are critical to the operation of power, industrial, and
manufacturing plants. A false trip or, worse, the failure of a
motor can halt a major process, costing thousands or even
millions of dollars in lost revenue and environmental fines.
Therefore, as discussed in this paper, it is essential that the
event report data provided by microprocessor-based motor
protection relays be analyzed in detail to determine root cause
for each operation. This should be done regardless of whether
company policy is to automatically replace a failed motor.
This paper presents several real-world events to illustrate
the lessons that can be learned from the stories told by event
data. Protection schemes and operating practices can be
improved based on these lessons to reduce outages and
damages. This makes operations safer and more reliable,
leading to less downtime and lost revenue.