is energised. OLTC may be located in either the high voltage winding or the low voltage
winding, depending on the requirement of the user, and the cost of effectiveness of the
application and tap changer availability. OLTC, being a current interrupting device, requires
periodic inspection and maintenance. The frequency of inspections is based on the time in service, range of use and number of operations.
DEFINITIONS:
i. ON LOAD TAP CHANGER (OLTC)
A device for changing the tapping connections of a winding, suitable for operation,
while the transformer is energised or on load. Generally, it consists of a diverter
switch (with transition impedance) and a tap selector (which can be with or without a
change-over selector).The whole being operated by the transition impedance. In some
forms of tap-changers, the functions of the diverter switch and the tap changer are
combined in a selector switch.
ii DIVERTER SWITCH:
A switching device used in conjunction with a tap selector to carry, make and break currents in circuits which have already been selected. A
diverter switch of spring operated type includes an independent means of storing
energy for their operation.
iii. TAP SELECTOR:
A device designed to carry, but not to make or break, current used in conjunction with a diverter switch to select tapping connections.
iv. SELECTOR SWITCH:
A switching device capable of making, carrying and breaking current, combining the duties of a tap selector and a diverter switch.
v. CHANGE-OVER SELECTOR:
A device designed to carry, but not to make or
break, current used in conjunction with a tap selector or selector switch to enable its
contacts and the connected tappings to be used more than once when moving from
one extreme position to the other.
vi. TRANSITION RESISTANCE:
A resistor consisting of one or more units bridging the tapping next to be used for the purpose of transferring load from one tapping to the other without interruptions or appreciable change in the load current and at the same time limiting the circulating current for the period that both tappings are used.
vii. DRIVING MECHANISM:
The means by which the drive to the tap changer is actuated.
viii. MOTOR-DRIVE MECHANISM:
A driving mechanism as defined above which
incorporates an electric motor and control circuit.
ix. STEP-BY-STEP CONTROL:
Electrical and mechanical devices stopping the motor drive mechanism after completion of a tap change, independently of the operating sequence of the control switch.
x. TAP POSITION INDICATOR:
An electrical and / or mechanical device for
indicating the tap position of the tap changer.
xi. TAP CHANGE IN PROGRESS INDICATION:
A device indicating that the
motor drive mechanism is running.
xii. LIMIT SWITCHES:
Electro-mechanical devices preventing operation of the tap changer beyond either end position, but allowing operation towards the opposite direction.
xiii. OPERATION COUNTER:
A device indicating the number of tap changes accomplished.
xiv. MANUAL OPERATION OF MOTOR DRIVE MECHANISM:
Operation of the tap changer manually by a mechanical device, blocking at the same time operation by the electric motor.
TEST SCHEDULE AND FREQUENCY:
Operation checks:
Every year.
Inspection & Maintenance: Once in 3 years or 50,000 operations, whichever is earlier, as per
Preventive Maintenance Schedule.
ISOLATION REQUIRED:
The maintenance is carried out on full shut down of the transformer where both the (HV and LV) side breakers are OPEN, isolators are OPEN and necessary earthing has been done. All protections for the transformer are kept OUT by switching off the DC supply. The transformer bushings are connected to earth as an additional precaution. The testing shall be carried out during shut down period and all testing shall be done under proper/prior sanction from LD.
PRECAUTIONS:
Testing shall be carr5ied out during shut down period. Ensure the isolation of transformer for High Voltage and Low Voltage side with physical inspection of OPEN condition of the concerned isolators / dis-connections. In case tertiary is also connected, ensure the isolation of the same, prior to commencement of testing.
PROCEDURES:
OPERATIONAL CHECKS ON OLTC
i. TAP CHANGER HAND OPERATION:
Check hand operation of the tap changer
UP and DOWN in the full range before electrical operation is attempted and that the
handle interlock switch will not allow electrical operation while the handle is inserted.
Continuity check should be done for any discontinuity, during tap changing operation,
by connecting an analogue multi-meter across HV & IV bushing and change the tap
positions from maximum to minimum.
ii. MAINTENANCE CIRCUIT:
Check the maintaining circuit for correct sequence by hand winding unit half way through a tap and then remove the handle. Energise the drive motor and ensure that the motor continues to drive the tap changer in the same direction.
iii. LIMIT SWITCHES:
Check the operation of the limit switches at both ends of the
range.
iv. DRIVE MOTOR:
With the tap changer in mid position, check the direction of rotation and measure the start and running currents in both the raise and lower mode of operation and record their values. Set the motor overload to 10% above running
current.
v. RAISE & LOWER CONTROL
Step by Step relay operation: Check that the tap changer moves one tap at a time by
pressing R or L push button.
Out of Step Relay: Move one tap changer in the three phase bank to be one position
out of step with other two. Check that the tap changer faulty alarm is activated.
Repeat for other two phases. Firstly, check the satisfactory operation of the
local/remote switch. By local control, raise and lower the tap changer over its full
operational range. Repeat the above, using the remote raise and lower facility over the
full range. Check that either the local and remote switches can not operate the tap
changer when it is in the other mode and vice versa. Hold the raise and lower push
buttons in following a tap change to ensure it only moves one tap at a time, hence
checking the step by step relay.
Tap Change Position Indicator:
Check accurate indication of mechanical position indicator in all positions. Also check that the remote instrument operates in the same sequence with the same accuracy. Without remote instrument connected, operate the tap changer and measure the output from each binary coded matrix.
Tap Change Incomplete Alarm:
Check the operation of the tap changer incomplete alarm, including the flag relay, by winding the unit by hand half way through a tap change and monitoring their correct operation and time to operate.
Operation Counter:
Check that the tap changer counter operates and records readings correctly.
Remote Indication:
Check that the remote indication and control facility is provided
to the outgoing terminals of the marshalling kiosk.
Tap Changer (Surge) Protective Relay:
Check the tripping function of the relay.
Open the cover and press button “Trip”. Check that all circuit breakers of transformer
operate properly. Press push button “Reset”, close the cover and tighten it.
INSPECTION AND MAINTENANCE OF OLTC
Normally the temperature of the OLTC compartment may be few degrees Celsius less than the main tank. Any temperature, approaching or above that of the main tank, indicates an internal problem. Prior to opening the OLTC compartment, it should be inspected for external symptoms of potential problems. Things such as integrity of paint, weld leaks, oil seal integrity, pressure relief device and liquid level gauge are all items which should be inspected prior to entering the OLTC.
Following de-energisation, close all valves between oil conservator transformer tanks and tap changer head. Then lower the oil level in the diverter switch oil compartment by draining of the oil for internal inspection. Upon opening the OLTC compartment, the door gasket should be inspected for signs of deterioration. The compartment floor should be inspected for debris that might indicate abnormal wear and sliding surfaces should be inspected for signs of excessive wear.
The following check points / guidelines for inspection and maintenance should be addressed and the manufacturer’s service engineer should be consulted for details of maintenance / overhauling activity to ensure the absence of problems and proper operation in future.
a. Function of control switches.
b. OLTC stopping on position.
c. Fastener tightness.
d. Signs of moisture such as rusting, oxidation or free standing water.
e. Mechanical clearances as specified by manufacturer’s instruction booklet.
f. Operation and condition of tap selector, changeover selector and arcing transfer
switches.
g. Drive Mechanism operation.
h. Counter operation.
i. Position indicator operation and its co-ordination with mechanism and tap selector
positions.
j. Limit switch operation.
k. Mechanical block integrity.
l. Proper operation of hand crank and its interlock switch.
m. Physical condition of tap selector.
n. Freedom of movement of external shaft assembly.
o. Extent of arc erosion on stationary and movable arcing contacts.
p. Inspect barrier board for tracking and cracking.
q. After filling with oil, manually crank throughout entire range.
r. Oil BDV and Moisture Content (PPM) to be measured and recorded.
Finally, the tap selector compartment should be flushed with clean transformer oil and
all carbonization, which may have been deposited, should be removed. Min. BDV
should be 50 kV and Moisture Content (PPM) should be less than 20 PPM.
Quarterly- Oil in Tap changer selector compartment (if provided separately)- check
for dielectric strength. Minimum values prescribed as per IS: 1866-2000 are as below
for various equipment voltages:
a. 145 KV and above: 50 KV
b. 72.5 KV & less than 145 KV: 40 KV
Below 72.5 KV: 30 KV
Take suitable action to restore quality of oil. Procedure for testing of dielectric
strength of oil is given in Appendix-A.
Quarterly or after 5000 operations whichever is earlier- Oil in OLTC Diverter Switch
compartment- Check for dielectric strength. Minimum BDV value should be 30 KV-
Take suitable actions for restoring BDV of Oil to 50 KV.
Half Yearly- Oil in tap changer selector compartment (if provided separately) - Check
for the following:
a. Dielectric strength:
Minimum values given at Item No.10
b. Water Content: Maximum values for various voltage class of transformers are given
below:
i. 25 PPM for 145 KV & above.
ii. 35 PPM for below 145 KV
iii) 1.0 (100%) for below 145 Kv.
c. Resistivity: Minimum value=0.1x1012 Ohm cm. At 90C
d. Tan Delta : Maximum value=0.2 (20%)for 145 KV class & above
e. Acidity: Maximum value =0.5 mg.KOH/per gram.
f. Sludge content: No sediment or precipitable sludge should be detectable
g. Flash Point : Decrease of 15C (Max.) from the initial value, minimum value 125C)
h. Inter facial Tension: Minimum Value=0.018 N/m Dissolved Gas Analysis
Procedure for sampling of oil for these tests is given in Appendix-B.
a. Take suitable action to restore quality
b. Recondition, if the value of Ten Delta permits, or otherwise reclaim or replace
c. Recondition, if the value of specific resistivity permits, or otherwise reclaim or
replace oil
d. Reclaim or replace oil
e. Recondition oil, if sediment is detected
f. Replace oil, if more economical for other tests