Test fixture having switching means for facilitating transformer turns ratio testing

This disclosure relates to a transformer terminal switching unit or fixture with specially designed output terminals and transformer attachment cables. The disclosed transformer terminal switching unit is especially useful as an accessory to the BIDDLE Transformer Turn Ratio (TTR), tester, catalogue number 55003. The design of the transformer terminal switching test fixture is especially useful to test delta-star connected, three phase utility transformers with subtractively connected winding polarity. A transformer with additive windings can also be tested with the disclosed testing fixture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to Transformer Turns Ratio Testing and 
more particularly to test fixtures for increasing the speed of Transformer 
Turns Ratio Testing. 
2. Description of the Prior Art 
During a useful life of 25 to 40 years a utility network distribution 
transformer, for example, is subjected to a variety of tests on many 
occasions starting with acceptance tests upon delivery, to verify name 
plate and other data. One of the tests of interest to utility companies is 
determination of the Transformer Turns Ratio (TTR). One type of 
transformer turns ratiometer uses a standard reference transformer which 
has accurately calibrated tappings used with a hand cranked exciting 
magneto, current and voltage meters for normalizing the flux level and a 
null indicator which indicates the exact transformer turns ratio to 0.1%. 
Another method of TTR measurement involves connection of the transformer 
primary and secondary windings in series with additive polarity. A means 
is provided for excitation of the windings with a standard current. A 
potentiometer across the series connected windings with the movable arm 
connected to the primary and secondary junction through an AC null 
indicator permits determination of the ratio of primary to secondary 
turns. 
A delta-star connected, three phase utility transformer has three primary 
terminals and four secondary terminals. Measurement of the ratio of three 
pairs of primary and secondary windings involves connection of four leads 
of the ratiometer to each of the six windings in turn and with correct 
polarity. 
The Biddle TT (Transformer Turn) Ratiometer is a four terminal testing 
device. The four inputs are extended to six feet with flexible instrument 
leads. Connection to the transformer primary is through a pair of 
insulated leads terminating in spring loaded clips. Each secondary 
connection is via an instrument lead which is a dual conductor consisting 
of a current wire and a potential or voltage wire. The Biddle 
potential/current leads are connected to the transformer lugs being 
measured with a specially designed "C" clamp. The stationary end of the 
"C" clamp is insulated from the clamp body and terminates in an insulated 
pressure pad which is connected to the potential lead. The frame of the 
"C" clamp is connected to the secondary current lead. The potential and 
current leads are connected to the Biddle Ratiometer, thus the connections 
to each transformer secondary lug is made at a point on each side of the 
terminal lug which may be anywhere from 1/2".times.1/2" to 3/4".times.4", 
depending on the KVA (Kilo-Volt-Ampere) rating of the transformer. 
The need to connect and disconnect, for example, the Biddle TT ratiometer 
primary leads and to clamp and unclamp the secondary connectors three 
times to perform turns ratio measurements on a common core three phase 
transformer or on three single phase transformers connected in a 
delta-star configuration, indicated that a need existed for a device which 
would permit the three required turns ratio measurements to be made 
quickly and without the need for making separate connections to the seven 
transformer terminals. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an improved test 
fixture for testing transformer turns ratios. 
It is another object of this invention to provide a test fixture which 
facilitates transformer turns ratio (TTR) measurements through the usage 
of seven test leads which extend the reach of the Biddle TTR unit from 6 
feet to 12 feet (or more if required in connection with the transformer 
measuring schedule). 
It is still another object of this invention to provide a test fixture 
which connects the four input terminals of the Biddle TTR unit to a 
correct selection of the seven extended test leads to the transformer 
being ratio tested. 
It is a further object of this invention to provide a test fixture which 
incorporates a switching means which permits the three primary and three 
secondary windings of the transformer under test to be sequentially 
connected to the Biddle TTR unit. 
It is a still further object of this invention to provide a test fixture 
having two switchbox output terminals which permit the dual secondary 
current and potential leads from the Biddle TTR unit with specially 
designed "C" clamps to be properly connected. 
It is still another object of this invention to provide a test fixture 
which incorporates the switching arrangement for transformer winding 
selection in a metal cabinet. 
It is another object of this invention to provide a test fixture having 
sufficient space in one end of the metal cabinet for storage of the three 
primary and four secondary transformer test leads when the fixture is 
transported from job to job. 
It is still another object of this invention to provide a test fixture 
which has the switches and terminals on the transformer testing fixture 
labeled to conform with NEMA standards for three phase transformer 
terminal designations. 
The foregoing, and other objects, features and advantages of the invention 
will be apparent from the following, more particular description of the 
preferred embodiment of the invention, as illustrated in the accompanying 
drawing.

DETAILED DESCRIPTION OF THE SPECIFICATION 
FIG. 1 shows the test fixture with its switching panel or means, housing 
and cables. Transformer connection leads with test clips 26 and 28 and 
winding selection switches 24 and 24A are assembled in preferably a metal 
box having, for example, the dimensions 8".times.8".times.19". The 
carrying box or case for the test fixture may be a commercially available 
housing of ferrous sheet metal or a custom built housing of aluminum or 
other material (wood or plastic) of the same approximate dimensions. The 
primary (two) terminals for connection to the primary test clips to the 
Biddle TTR unit are shown at 20 of FIGS. 1 and 1A. The primary terminals 
20 are preferably plain or threaded 3/16" brass pillars mounted on 
insulating pads for the purpose of accommodating the primary test clips of 
the Biddle TTR unit (not shown). 
Specially designed terminals for accommodation of the "C" clamp two wire 
connectors to the Biddle TTR unit are shown at 22, FIGS. 1, 1A and 1B. The 
Biddle "C" clamp secondary input connector is shown at 22A (which is shown 
solely for illustrative purposes). 
The three double pole, on-off switches for primary winding combinations are 
shown at 24 in FIGS. 1, 1A and 2. Three four pole on-off switches for 
secondary winding combinations are shown at 24A, FIGS. 1, 1A and 2A. The 
three test clips terminating the three primary test leads are shown at 26 
in FIG. 2 (only one of which is shown in FIG. 1). The primary test clip 26 
(3 required for H.sub.1, H.sub.2, H.sub.3 see FIG. 2) should be similar or 
equal to the Mueller type 48C, 20 amp general purpose spring clip. The 
primary test leads connected to the primary test clips 26 for primary 
transformer connections should be similar or equal to Alpha 5857 type E, 
19.times.30 AWG, 600 volt Teflon (color selection to meet user lead 
identification requirements). 
Test lead termination for secondary transformer connections is shown at 28 
in FIGS. 1, 2A and 2B (only one shown in FIG. 1, but four are used as 
shown in FIGS. 2A and 2B). The test clip shown by reference numeral 28 
should be similar or equal to the Mueller type 25C, 75 amp, general 
purpose clip. Dual test leads are used for each transformer secondary 
connection. A single current and a single potential wire is terminated by 
parallel connection at each of the test clips 28. The four current and the 
four potential wires which comprise the secondary connection leads are 
carried through the switching matrix or panel without contacting each 
other. One of the current wires terminate as shown in FIG. 1B at 28A of 
terminal 22 and one of the potential wires are connected to terminal 22 at 
28B. There are two terminal 22 elements (see X.sub.1 and X.sub.2 of FIG. 
1A). The other two of the four current wires and the other two of the four 
potential wires terminate in the switch arrangement as described below. 
The current wire attached to secondary clip 28 should be similar or equal 
to Alpha 5859 type E, 19.times.25 AWG. 600 volt Teflon. The potential wire 
attached to the secondary clip 28 should be similar or equal to Alpha 
5857. The current and potential wires which make up the secondary test 
leads can be paired with spaced bands of heat shrink tubing or plastic 
tape. Color combinations should be in accordance with user requirements 
for lead identification. Space reserved in the carrying case for storage 
of test leads when the test accessory is not in use or during transport, 
is shown at 30 in FIG. 1. The grommet where test leads enter the switch 
test fixture or box is shown at 30A of FIG. 1. Shown at 32 in FIG. 1 is a 
binding post terminal for grounding the test fixture. For field or shop 
operation, a wire from 32 should be connected to the ground terminal on 
the Biddle TTR unit and also to a convenient external grounding point. 
Measurement of transformer primary to secondary turns ratio involves 
comparison of a variable ratio standard transformer with the transformer 
under test. Thus, in this test the absolute values of current or voltage 
are, within limits, not critical, so that lead resistance or lead lengths 
is not a factor affecting accuracy. A primary and secondary lead length of 
12 feet has been mentioned above as preferred, but this footage can be 
adjusted to conform with the bulk of requirements which will be 
encountered in the field or shop where the TTR unit and the test fixture 
will be used. 
Switches and terminal layout of the test panel is shown in FIG. 1A. 
Terminals shown at 20 and 22 are to permit connection to the Biddle TTR 
unit. Primary switches 24 in FIG. 1A marked H.sub.1 H.sub.3, H.sub.1 
H.sub.2, and H.sub.2 H.sub.3 should be similar or equal to Cutler-Hammer 
Switch Type 7320K2 or 7560K6, On-Off, 15/16 amp 125 volt. Secondary 
switches 24A in FIG. 1A marked X.sub.0 X.sub.1, X.sub.0 X.sub.2 and 
X.sub.0 X.sub.3 should be similar or equal to Cutler-Hammer Switch Type 
7690K8 On-Off, 15 amp 125 volt. If the alternate embodiment for the 
secondary switch wiring shown in FIG. 2B is used, the same switches for 
primary and secondary switching can be used i.e. Cutler-Hammer Type 7320K2 
or 7560K6. FIG. 1B shows construction details of one of the two special 
dual terminals designed to accommodate the Biddle "C" clamp connector. For 
example, two vertical No. 18 copper blades approximately 11/8" by 3" are 
bolted to an insulating mounting and separated by an insulating spacer 23 
of formica or a similar insulator. The blades are insulated from the 
switch panel by a fiber insulating grommet 23. The double terminal 
assemblies are preferably rigidly affixed to the switch panel at a height 
through the panel sufficient to accommodate the Biddle "C" clamps. The 
secondary current wires 28A from the winding selection switches (see FIGS. 
1B 2A and 2B) are attached to the terminal blades next to the outside wall 
of the carrying case. The inner blades are connected to the potential 
wires. In use, the "C" clamps are always attached to the terminals with 
the wing screws facing outward. 
Operation 
The wiring diagram of FIG. 2 shows that closure of any one of the double 
pole switches 24 (H.sub.1 H.sub.3 or H.sub.2 H.sub.1 or H.sub.2 H.sub.3) 
will place one of the delta connected transformer primary windings (not 
shown, but connected to the terminal clips 26) across terminals 20 
(H.sub.1, H.sub.2) connected to the Biddle TTR unit. Operation or closure 
of any two switches 24 simultaneously will short circuit one winding of 
the transformer delta. The circuit output terminals 20 are short circuited 
in the case of parallel closure of switches H.sub.1 H.sub.3 and H.sub.2 
H.sub.3. The wiring diagrams in FIGS. 2A and 2B show that operation of any 
one of the switches designated 24A will connect one transformer secondary 
winding to Biddle TTR unit input terminals X.sub.1 and X.sub.2 through the 
segregated current and potential leads. Due to the differences in the star 
or "Y" configuration and the delta connection, simultaneous operation of 
two or even three of switches 24A will only parallel the three secondaries 
without shorting either X.sub.1 or X.sub. 2 or any winding. The adverse 
phase relationship between windings would make such a parallel connection 
tantamount to a short circuit, so operation of two selector switches 
simultaneously is not called for in the test fixture measurement schedule. 
When the Biddle TTR unit test leads are connected to H.sub.1 and H.sub.2 
and to X.sub.1 and X.sub.2 of the test fixture and if the transformer 
under test is at a service location, removal of the transformer primary 
links of the transformer under test should be verified. Even with a nearby 
secondary isolation switch, it is probably desirable to disconnect the 
four X leads (X.sub.0, X.sub.1, X.sub.2, X.sub.3) of FIGS. 2A or 2B to 
avoid accidental secondary reconnection. After the extension test leads 
are connected to the correct transformer terminals, the six switchbox 
switches should be turned off. The Biddle TTR ratio switches and vernier 
dial (not shown) should be set to 00.000 and the magneto cranked to check 
the null balance of the unit. With the tap changer used on utility 
transformers for small voltage increments in the normal position (neither 
"up" or "down") one calculates the transformer ratio (as shown by the name 
plate data and sets the TTR unit ratio switch accordingly. The primary 
switches H.sub.1 H.sub.3 and X.sub.0 X.sub.1 are turned on and one 
measures the ratio of the A phase windings. This should conform within a 
few percentage points to the calculated ratio. The measurements for phase 
B and C are repeated. The tap changer is run up and down with the operator 
measuring and recording the transformer turn ratios on each tap. If it 
should not be possible to get a null on any phase or if one reading is out 
of tolerance, the switch fixture or box is disconnected and the Biddle TTR 
unit is connected directly to the primary and secondary windings in 
question, to verify the possibility of a shorted turn. 
Brief Summary of the Specification 
Thus, as described above with reference to the figures, the use of 
switching means with test lead extensions makes it possible to measure the 
ratios of three single phase transformers or the internal elements of a 
delta-star connected utility transformer with a commercially available TTR 
unit, without the need for rearrangement or reconnection of test leads 
during measurement. This results in a considerable saving of time and 
expense and in a reduction of measurement effort. 
In field work, a transformer undergoing tests may be only one of several 
other transformers on a platform or on a substation pad which may or may 
not be energized and represent a safety hazard to the operator making 
tests on the isolated transformer. The availability of test lead 
extensions and a reduction in the number of test lead attachments to the 
isolated transformer are considered a worthwhile safety feature. 
While the invention has been particularly shown and described in reference 
to the preferred embodiment thereof, it will be understood by those 
skilled in the art that changes in the form and details may be made 
therein without departing from the spirit and scope of the invention.