Feature
Medium-Voltage Circuit Breaker
Timing-Test Evaluations
Part I
W
ith today’s computer technology capabilities, the use of statistical
tools to evaluate test data is just a mouse click away. This paper
presents a new way to evaluate circuit breaker timing test results.
It provides methods for comparing test measurements, validating test data, and
determining acceptability of breaker performance.
NETA has developed specifications for testing the operation of mediumvoltage air, oil, vacuum, and SF6 circuit breakers. These specifications state that
time-travel analysis should be performed and that measured values should be
compared to the manufacturer’s published data and to previous test data.
Introduction
Time-travel analysis measures the rate of displacement of the contact assembly. Such testing has historically been performed on medium- and high-voltage
circuit breakers and switches to assess whether the contact operator mechanism
is performing correctly. Incorrect operation can result in catastrophic breaker
failure due to incorrect arc interruption or incorrect contact closure.
Unfortunately, time-travel measurements do not require measurement of
contact wipe and gap dimensions as a routine maintenance procedure. If incorrect wipe and gap dimensions go undetected and cause contact erosion, such
deterioration will continue to be a problem, even if the contacts (or vacuum
bottles) are replaced.
The timing test methods described in this paper are based on measurements
that can be routinely performed without removing the circuit breakers from
the switchgear. The timing analysis is based on a simple statistical model that
provides a determination of measurement accuracy and an evaluation of timing
test results. In this paper:
1. Circuit breaker timing test data and manufacturer’s wipe and gap settings are
used as acceptance criteria for new breakers. (Trending the annual timing test
results will also aid in future evaluations of in‑service circuit breakers.)
2. Problems that can be created by inadequate measurement resolution are identified. Remeasurement of the timing data with improved resolution is used to
identify wipe and gap adjustment limitations in Part II of this article.
by William Hays
Emerson Process Management
Electrical Reliability Services
Test Procedure
Description
The procedure developed was
initially used to test new General
Electric PowerVac VB1 circuit
breakers with ML-18H contact
mechanism. Additional tests
performed on General Electric
and Powell circuit breakers that
have been in service for several
years will be evaluated in Part
II. To perform these tests, timetravel analyzer equipment was
connected according to timing
set-up instructions and the following breakers were evaluated:
(GE mains 80, 81 and four feeder
breakers FDR A1, A2, B1, and
B2). Timing test data from these
GE circuit breakers are used as the
standard against which additional
breaker timing measurement results will be compared.
Next, slow close procedures
were used to verify closing sequence of the new GE breakers
in accordance with GE Operating
& Maintenance instructions. The
phase-B timing test results shown
in Figure 1 is a plot of the difference in close (D CLOSE) and
open (D OPEN) timing between
the outside poles and the center
(phase-B) pole contacts for these
six GE circuit breakers manufactured in the same lot.
3. A newly developed quality factor is explained and demonstrated. This quality
factor incorporates statistical methods to evaluate measurement accuracy.
www.netaworld.org Spring 2008 NETA WORLD
Phase B Timing Test Results
2.000
D CLOSE
D OPEN
1.000
The circuit breaker measurements identified as a FAIL
condition are shown in the Figure 2 bar graph where the
phase-B contact opening sequence does not meet the
manufacturer’s requirements to open first for three of the
six circuit breakers.
0.000
Open Timing
-1.000
81 MAIN 80 MAIN FDR A1
FDR A2
FDR B1
FDR B2
D CLOSE
0.500
0.750
0.625
0.750
1.000
0.750
D OPEN
-0.625
-0.750
-0.750
-0.750
-0.625
-0.500
Table 1
m-sec
Figure 1
The center pole contact operating sequence is shown in
Figure 1. The timing data verifies the manufacturer’s contact
wipe and gap adjustments results in a phase-B operating
sequence to be the last to close and the first to open.
All circuit breakers in this test group show the same
contact operating sequence. Phase-B is the first to open,
and phase-B is the last pole to close. The positive contact
closing times identify the additional time between the outer
poles closing and the center pole closing. The negative
contact opening times verify the center pole is the first to
close. This is the basic test data indicating acceptable contact
operating sequence.
The GE instruction manual states: “A properly adjusted
breaker will have more gap and wipe on the center pole
than on the outside poles.” With the breaker in the open
position, the gap between B‑phase moving and stationary
contacts is larger than the gaps between the A‑phase or
C‑phase contacts. This larger gap dimension for the center
pole results in the longer closing delay timing while a wipe
adjustment that results in increased wipe spring compression provides mechanical force for the center pole to be the
first to open.
The General Electric GEK-86123G Instructions Manual
provides the specification of the time delay from the initiation of the electrical trip/close to the opening/closing of the
circuit breakers contacts in Section 12 – Timing. Maximum
pole spread is specified to be two milliseconds for either
open or close operation. The maximum pole spread shown
for the initial test group is 1.000 millisecond for close and
the time from initiation of opening to contact opening for
phase-B is 0.750 millisecond. The minimum times for initial
phase-B contact opening/closing to the phase-A or phase-C
open/close operation is 0.500 millisecond.
Test Data Evaluation Method
The purpose of evaluating the timing test data is two-fold:
(1) to validate the accuracy of the data measurements and
(2) to provide guideline values to determine if the spread in
the data measurements indicate that contact gap and wipe
mechanical adjustments are required to meet the operating
sequence and the maximum pole spread requirements.
NETA WORLD Spring 2008
AVE 112.5
STD DEV +/- 36.194
160.000
140.000
GE 52-I
120.000
GE 52-G2
100.000
POW CB-101
POW CB-103
80.000
POW CB-105
60.000
POW CB-107
40.000
20.000
0.000
A
B
C
Phase
Figure 2
PHASE B OPEN 1st
FAIL
3
3
3
QF
number of phase
A & C OPEN
times outside
of the phB std
deviation
ph B OPEN
times that are
less than ph
A or C OPEN
times
phB OPEN
times that are
more than phA
and C OPEN
times
Stastical measurement value &
boundary limits
0
Table 2
The number of phase-B contact opening timing tests that
fail to meet the requirement to open first is shown in Figure
2 and Table 2. Three out of six circuit breakers in this bar
graph (i.e. GE-52-G2, POW CB‑105, POW CB-107) do
not meet the first to open contact phase timing condition.
Table 2 shows that three of the six measurements fail
to meet the requirement for phase-B to open first. This is
identified as a fail condition. The measurement quality factor
(QF) is zero for this group of circuit breakers. The quality
factor and the manufacturer’s established limit conditions
are used to determine the acceptability of the circuit breaker
timing tests.
The QF value is based on three statistical parameters.
The measurement accuracy (spread of values) along with
the mean and average of the measurement values is used
for this QF rating.
www.netaworld.org
Conclusions
A statistical evaluation of timing data to determine the
acceptability of data measurement accuracy for evaluating
wipe and gap adjustments has been presented. This evaluation method is to be used to evaluate circuit breakers with
different wipe and gap adjustments in Part II.
A quality factor calculation is demonstrated to be effective in establishing a measurement dependent value that
can be used for evaluating measurement of circuit breaker
timing data. The verification of circuit breaker wipe and gap
adjustments provides timing measurement data that can be
used to document NETA testing requirements for medium
voltage circuit breakers,
www.netaworld.org Additional evaluation of timing data is presented in Part
II. Establishing a database for judging the acceptability of
wipe and gap adjustments is required to verify the usefulness of this measurement evaluation method. The inherent
problem of using a small number of sample measurements
to establish testing values is recognized as having limited
acceptability without additional test data. Further correlation of measurements and settings factors with mechanical
inspection and circuit breaker timing tests is required to
validate this measurement evaluation method.
Bill Hays received his BS in Engineering from California State University in Los Angeles in 1971. He has provided inspection and testing
of power distribution equipment since 1989 with Emerson’s Electrical
Reliability Services group. Mr. Hays is a senior member of IEEE and
is a NETA Level 3 test technician. He is a field evaluation engineer for
Emerson’s Electrical Reliability Services in San Diego, California. Prior to
joining Emerson, Mr. Hays provided testing and failure analysis services
for the semiconductor industry.
Spring 2008 NETA WORLD