3.25 recovery voltage power-frequency voltage which appears across each set of main switching or transition contacts resistor type tap-changer or transfer contacts reactor type tap-chan
Temperature of tap-changer environment
Unless more onerous conditions are specified by the purchaser, liquid immersed tap-changers shall be regarded as suitable for operation over the ranges of temperature given in Table 1
Table 1 – Temperature of tap-changer environment
NOTE 1 For the definitions of the tap-changer, see 3.55 and 3.56
NOTE 2 The value of 105 °C quoted above is based on the maximum top oil temperature in case of normal cyclic loading as specified in IEC 60076-7
NOTE 3 Minimum and maximum liquid temperatures of step-voltage regulators established in
Temperature of motor-drive mechanism environment
Unless more onerous conditions are specified by the purchaser, motor-drive mechanisms shall be regarded as suitable for operation in any ambient temperature between –25 °C and
For more onerous conditions for tap-changer or motor-drive mechanism environments, reference should be made to IEC 60214-2.
Overload conditions
Tap-changers which comply with this standard and are selected and installed in accordance with IEC 60214-2 shall not restrict emergency loading of the transformer according to
IEC 60076-7, which could result in top oil temperatures as high as 115 °C
See Note 3 in Table 1 in which maximum liquid temperature limit of step-voltage regulators relates to 200 % overload for half an hour
5 Requirements for on-load tap-changers
General requirements
Change-over selector recovery voltages
When installing a pressure relief device, it is essential to consider the addition of ducting or trunking to safeguard personnel from any displaced liquid The implementation of these protective measures should be mutually agreed upon by both the manufacturer and the purchaser.
Limiting devices for the protection against transient overvoltages
Manufacturers of on-load tap-changers with transient overvoltage limiting devices must provide comprehensive details about the protective features and any testing limitations applicable to the finished transformer.
When spark gaps are used, care shall be taken to ensure that, after spark-over, the discharge is quenched automatically
Change-over selector recovery voltages
Coarse or reversing change-over selectors temporarily disconnect the tap winding, which can lead to high recovery voltages across the selector contacts during separation, caused by capacitive coupling with adjacent windings The manufacturer of the on-load tap-changer must specify any limiting switching parameters for the change-over selectors used in the device.
NOTE IEC 60214-2 gives further details of selection, control circuits and devices and transformer testing recommendations
Leakage inductance in coarse fine regulation arrangements
When transitioning between the end of the tap winding and the end of the coarse winding using resistor type tap-changers, high leakage inductance may occur due to the two windings being in series opposition This situation can lead to a phase shift between the switched current and the recovery voltage of the diverter or selector switch, potentially resulting in prolonged arcing of the switch.
The on-load tap-changer manufacturer shall declare any switching limitations
NOTE IEC 60214-2 gives further details of selection and winding configurations regarding leakage inductance.
Type tests
Type-test certificate
– full details of the test arrangements adopted (for example, assembly, arrangement and drying out) with explanatory sketches as necessary;
– full details of all tests applied in accordance with 5.2.2 to 5.2.8;
– full details of limiting devices for transient overvoltages, where appropriate, see 5.1.5;
– photographs of all contacts breaking and commutating currents
The wear of contacts in vacuum interrupters and other current commutating contacts must be documented and must remain within the limits specified by the manufacturer Additionally, there should be no evidence of arcing on contacts that are not designed for such occurrences.
Routine tests
The following routine tests shall be performed on each completed on-load tap-changer:
NOTE Attention is drawn to tests to be carried out on on-load tap-changers after assembly on transformers, which are detailed in 11.7 of IEC 60076-1:2011
With the on-load tap-changer fully assembled but without the contacts energized, ten complete cycles of operation shall be performed without failure
In section 5.3.2 of the routine mechanical test, the sequence of operations for the on-load tap-changer must be documented, with the diverter or selector switch's operation recorded oscillographically The outcomes of this recording should closely align with the results obtained from the sequence type test outlined in section 5.2.6.3.
The tap-changer auxiliary circuits shall withstand without failure a separate source a.c withstand voltage test of 2 kV applied for 1 min between all live terminals and the frame
All liquid or gas containing compartments shall be tested at a pressure and vacuum declared by the manufacturer
6 Requirements for motor-drive mechanisms for on-load tap-changers
General requirements
Tap-change in progress indication
If required, a device may be provided which, after a possible interruption of the supply voltage, will complete a tap-change operation once it has been initiated
Operation counters must be appropriate for their intended use, considering environmental conditions and the specified number of operations for the on-load tap-changer The operational count for the on-load tap-changer can be supplied as electrical information.
(stored in a memory) or with a six-figure or greater non-resettable counter
If required, a device indicating the number of operations shall be provided (in case of electrical information)
Manual operation of the motor-drive mechanism
Motor-drive cubicle
In the event of a power supply failure, a device must be implemented to enable tap-change operations for the on-load tap-changer This device will effectively block the motor-drive mechanism, preventing any unintended operations such as remote control or voltage recovery.
The direction of rotation and further instructions shall be indicated adjacent to the point of engagement
The design of the device should permit the operation by one person without undue effort
NOTE This subclause does not apply to step-voltage regulators as defined in IEC 60076-21
The motor-drive cubicle shall meet the protection requirements of IP44 according to
IEC 60529 and shall be protected against condensation by suitable means
If required, higher degrees of protection according to IEC 60529 may be agreed between manufacturer and purchaser
Protective device against running-through
A device to prevent the motor-drive mechanism from running through in case of failure of the step-by-step control circuit shall be provided
Protection against access to hazardous parts
Driving mechanism cubicles fitted with doors shall continue to provide protection to at least category IP1X (according to IEC 60529) with any door open
NOTE This will provide protection against accidental “back of the hand” contact as a minimum
External drive shafts shall be protected with guards.
Type tests
The output shaft of the motor-drive mechanism must be subjected to the maximum torque of the on-load tap-changer it is designed for, or an equivalent simulated load torque cycle that reflects actual service conditions This load requirement necessitates that 500,000 operations be completed throughout the entire tap range.
Additional cooling of the motor-drive is permissible during this test
During this test, performed at rated frequency:
– 10 000 operations shall be performed at the minimum voltage as specified in 6.1.2;
– 10 000 operations at the maximum voltage as specified in 6.1.2;
The test will involve 100 operations conducted at a temperature of −25 °C, ensuring that the motor-drive cabinet does not exceed this temperature at the start of the test The motor-drive mechanism will be evaluated under rated voltage and frequency conditions Additionally, the internal temperature of the cabinet will be monitored throughout the test and documented in the final test report.
The devices specified in sections 6.1.6, 6.1.10, 6.1.11, 6.1.12, and 6.1.14 must be tested to ensure their proper functioning This test should be conducted without any failures or excessive wear on the mechanical components.
Normal maintenance according to the manufacturer’s handbook is permitted during the test
During the test, the heating system of the motor-drive mechanism shall be switched off
In the event of a failure of the motor-drive mechanism's limiting device, the additional mechanical limiting device or the on-load tap-changer will prevent operation beyond the end positions during a motorized tap-change operation, ensuring that no electrical or mechanical damage occurs.
Degree of protection of motor-drive cubicle
When applicable, the motor-drive cubicle shall be tested in accordance with IEC 60529.
Routine tests
The motor-drive mechanism must operate electrically for ten cycles under service conditions or with a simulated load without any failures During this testing, it is essential to verify that the mechanism functions correctly in accordance with the requirements outlined in sections 6.1.6, 6.1.10, 6.1.11, 6.1.12, and 6.1.14.
Following the initial test, two additional operational cycles will be conducted: one at the minimum rated voltage and another at the maximum rated voltage of the auxiliary supply Both cycles must be completed without any failures.
NOTE The mechanical tests can be performed on the motor-drive mechanism separately or as in 5.3.2
Auxiliary circuits, along with motors and other components tested at lower voltages per IEC standards, must undergo a separate a.c withstand test This test involves applying 2 kV r.m.s for 1 minute between all live terminals and the frame.
7 Requirements for de-energized tap-changers
General requirements
The rated characteristics are as follows:
De-energized tap-changers may comprise of hand or motor-drive operated mechanical rotary or linear switches
Drive mechanisms for transformers commonly utilize hand-wheels or hand cranks These handles can be directly attached to the de-energized tap-changer mounted on the transformer lid, affixed to the head cover of the tap-changer, or connected to a remote gland housing located externally In the latter scenario, the handles are linked to the de-energized tap-changer through drive shafts or cables.
The operating handle for hand operated de-energized tap-changers shall be mounted externally
The tap position must be clearly marked when the de-energized tap-changer is in the fully on position It is essential to indicate the direction of rotation for both raising and lowering the tap position Additionally, where applicable, the number of rotations required for a single tap-change operation should be specified.
A system shall be provided to positively latch the DETC in service position to carry full operating current
All sealing glands of the de-energized tap-changer between the liquid or gas filled transformer or tap-changer tank and the environment shall be liquid or gas tight
To ensure safety, a device must be installed to prevent unintentional or unauthorized activation of the equipment This device can include a locking mechanism on the manual drive that necessitates a deliberate action from the operator to disengage it.
Solely when the de-energized tap-changer is in a proper position state, it shall be possible to operate, remove or reinstall the safety device.
If a motor-drive mechanism is used to operate the de-energized tap-changer, preference shall be given to automatic interlocks by means of electrical interlocking circuits
The de-energized tap-changer cannot be operated beyond its designated range to an unselected position To accommodate varying selectable positions, mechanical end stops or a mechanical mechanism must be integrated into the selector or manual drive system to prevent movement beyond the first and last positions.
Type tests
Routine tests
The fully assembled de-energized tap-changer must undergo two complete operational cycles without any failures During this testing phase, it is essential to verify the correct operation and settings of the end stops as outlined in section 7.1.6.
Tests shall be performed on all liquid-tight glands and levels shall be declared by the manufacturer A declared value of zero indicates this test has not been carried out
NOTE Pressure and vacuum tests on small de-energized tap-changers are often not carried out
8 Requirements for motor-drive mechanisms for de-energized tap-changers
General requirements
Type tests
The output shaft of the motor-drive mechanism must withstand the maximum torque for the de-energized tap-changer it is designed for, or an equivalent simulated load torque cycle that reflects actual service conditions This load requirement necessitates performing 20,000 operations across the entire tap range.
Additional cooling of the motor-drive is permissible during this test
During this test, performed at rated frequency:
– 1 000 operations shall be performed at the minimum voltage as specified in 8.1.3;
– 1 000 operations shall be performed at the maximum voltage as specified in 8.1.3;
The motor-drive mechanism will undergo 50 operations at a temperature of -25 °C, with the initial temperature inside the motor-drive cabinet also set to -25 °C Testing will be conducted at the rated voltage and frequency, and the internal cabinet temperature will be monitored throughout the test, with results documented in the test report.
The device's proper operation, as outlined in sections 8.1.5, 8.1.6, and 8.1.7, must be confirmed during testing, ensuring that the test is conducted without any failures or excessive wear on the mechanical components.
Normal maintenance according to the manufacturer’s handbook is permitted during the test
During the test, the heating system of the motor-drive mechanism shall be switched off
In the event of electrical limit switch failure, mechanical end stops ensure that operation does not exceed the designated end positions during motorized tap changes, thereby protecting the motor-drive mechanism from both electrical and mechanical damage.
Degree of protection of motor-drive cubicle
When applicable, the motor-drive cubicle shall be tested in accordance with IEC 60529.
Routine tests
The motor-drive mechanism must operate electrically under service conditions or with a simulated load for two cycles without failure This test will verify proper functioning in compliance with the requirements outlined in sections 8.1.5, 8.1.6, and 8.1.7.
Following the initial test, two additional operational cycles will be conducted: one at the minimum rated voltage and another at the maximum rated voltage of the auxiliary supply Both cycles must be completed without any failures.
NOTE The mechanical tests can be performed on the motor-drive mechanism separately or as in 7.3.1
Auxiliary circuits, excluding motors and components tested with lower voltages per IEC standards, must undergo a separate a.c withstand test of 2 kV r.m.s for 1 minute, applied between all live terminals and the frame.
Tap-changers (on-load and de-energized)
Each tap-changer shall be provided with a nameplate of weatherproof material fitted in a visible position showing at least the following items:
– number and year of the relevant national standard and/or this IEC standard;
– the rated step voltage (if applicable);
– the transition resistor value (if applicable);
– static vacuum and pressure capabilities of the tap-changer
The entries shall be indelibly marked, for example by etching, engraving, stamping or by a photo-chemical process
For small de-energized tap-changers, if space constraints prevent all necessary information from being included on the nameplate, a separate loose nameplate may be provided, or the details can be found in the manufacturer's instructions.
Motor-drive mechanisms
Each motor-drive mechanism must feature a weatherproof nameplate positioned visibly, displaying the required items as outlined in section 9.1 Additionally, the nameplate should include any relevant information as necessary.
– the rated voltage and rated frequency of the electric motor;
– the rated voltage and rated frequency of the control equipment;
NOTE In the case of a d.c supply, the symbol - can be used instead of the indication of the rated frequency
– the number of service tap positions
The entries shall be indelibly marked, for example by etching, engraving, stamping or photo- chemical process
10 De-energized tap-changer warning label
For de-energized tap-changers, it is essential to attach a warning label or provide a separate instruction label next to the operating handle This label must clearly state that the operation of the DETC is permitted only when the transformer is de-energized, ensuring safety during use.
The conformity of the warning label (e.g drawing, symbols) with local or national law is in the responsibility of the transformer manufacturer
The transformer manufacturer is responsible to fit an appropriate warning label that is clearly visible, near the operating mechanism of the DETC on the transformer
A similar label shall be attached to motor-drive mechanisms
Do not operate while the transformer is energized
Such operation may result in failure of the transformer and injury or death to the operator!
The manufacturer shall provide a handbook to facilitate the safe and proper operation of the tap-changer including maintenance criteria
The handbook will address installation, operation, and maintenance guidelines while also highlighting potential hazards such as electric shock, stored energy devices, and the risk of unexpected mechanism activation after a power interruption.
Supplementary information on switching duty on main and transition contacts relating to resistor type tap-changers
Tables A.1 and A.3 illustrate standard contact configurations for diverter and selector switches, displaying a single pair of contacts for each function, which typically represents a complete set of contacts in practical applications.
Tables A.1 and A.3 present the total number of circuit-transfer operations conducted, along with the duty executed by each contact pair for every combination of switched current and recovery voltage, across multiple cycles corresponding to N tap-change operations.
In the expressions for current and voltage, the ‘+’ and ‘–’ signs represent vectorial addition and subtraction rather than algebraic operations The duty on the contacts is influenced by the load's power factor on the transformer, which determines the phase angle between the through-current I and the step voltage E The impact of the load power factor on the duty of various contacts is detailed in Table A.2 for non-vacuum type on-load tap-changers and in Note 2 of Table A.3 for vacuum type on-load tap-changers.
Table A.3 presents the breaking and making stresses for vacuum-type on-load tap-changers, highlighting their significance Additionally, Note 2 of Table A.3 discusses the impact of load power-factor on the operation of the various contacts in these tap-changers.
If the transition impedance is divided into two units, these are assumed to be of equal value, each equal to R
The configurations illustrated in Figure A.1 are not comprehensive, as there are additional arrangements available, including the multiple resistor cycle, which can serve as an extension of the fundamental principles discussed.
Figure A.1a – Diverter or selector switch with operating cycle number 1
Figure A.1b – Diverter switch with operating cycle number 2 Figure A.1c – Selector switch with operating cycle number 2
NOTE The numbering of the operating cycles only refers to Table A.1
Figure A.1 – Examples of current and voltage vectors for resistor type tap-changers
Table A.1 outlines the responsibilities of primary and transition contacts for non-vacuum resistor type tap-changers, detailing the type of switch, operating cycle number, connection diagram, and the order of contact operation.
The main duty of contact involves managing the transition contact, which is crucial for switching current and recovery voltage It is essential to consider the number of operations for effective contact switching, particularly in non-vacuum type diverter switches.
W br eak s W I RI N /2 X ẵ( E/ R + I ) E + RI N /4 Y m ak es ẵ( E/ R – I ) E – RI N /4 X b reak s Z I RI N /2 Y ẵ( E/ R + I ) E + RI N /4 Z m ak es ẵ( E/ R – I ) E – RI N /4 2
L m ak es J E/ R + I ẵ( E + RI ) N /4 K E/ R E N /2 J br eak s E/ R – I ẵ( E – RI ) N /4 M m ak es M E/ R + I ẵ( E + RI ) N /4 L E/ R E N /2 K br eak s E/ R – I ẵ( E – RI ) N /4 No n- vac uum ty pe sel ec tor sw itch
A ẵ( E/ R + I ) E + RI N /2 B br eak s C m ak es A br eak s C ẵ( E/ R – I ) E – RI N /2 B m ak es A m ak es 2
E/ R E N /2 T m ak es S br eak s E/ R + I or E /R – I (NO TE 3 )
E + RI or E – RI (NO TE 3 )
The basic circuits involving multiple resistors are not included, as they are extensions of the fundamental circuits For clarity, the diagram illustrates the connections and contact operating order for one direction of switch movement, while the expressions for contact duty and the number of operations account for movement in both directions Duties depend on the power flow direction and are provided for both directions The number of operations is specified under the condition that the power flow remains unchanged.
Table A.2 – Effect of load power-factor on circuit-breaking duty for resistor type tap-changers (non-vacuum type)
Type of switch Operating cycle number
Contact Effect of load power-factor Contact Effect of load power-factor
Non-vacuum type diverter switch
1 W and Z None X and Y Maximum duty at power-factor = 1,0
2 J and M Maximum duty at power-factor = 1,0 K and L None
Non-vacuum type selector switch
1 B None A and C Maximum duty at power-factor = 1,0
Maximum duty at power-factor = 0 for N/2 operations NOTE Non-vacuum type selector switches employing the operating cycle number 2 are normally used with load current flow in one direction only
Table A.3 – Duty of main and transition contacts for resistor type tap-changers (vacuum type) (1 of 2) T ype of swi tc h
O per at in g C ycl e num ber D iagr am of c onnec ti on s
The main contact's operating order duty involves transitioning from one state to another, specifically from state S0 to state Se or from state n to state n + 1.
C ha ngi ng fr om S e to S o or fr om n + 1 t o n
S w it chi ng cur rent R ec ov er y vo lt age C los in g cur rent C los in g vo lt age
N um ber of ope ra tions C ont ac t S w it chi ng cur rent R ec ov er y vo lt age C los in g cur rent C los in g vo lt age
N um ber of ope ra tions
(w ith o ne trans iti on res ist or)
A m t rans fer s A m t rans fer s V m m ak es V m m ak es V t br eak s V t br ea ks E / R - I E - R I N /2 A t tr an sf er s A t tr an sf er s V t m ak es V t m ak es 2
A t tr an sf er s A t tr an sf er s V t m ak es V t m ak es V m br eak s V m br eak s E / R - I E - R I N /2 A m t rans fer s A m t rans fer s V m m ak es V m m ak es
E/R E 0 0 N /2 S m t rans fer s S t tr an sf er s V m m ak es V t m ak es V t br eak s V m br eak s E / R + I or E / R - I (N O T E 4)
0 0 E/R E N /2 S t tr ans fe rs S m t rans fer s V t m ak es V m m ak es
(wit h t wo t ra nsit ion re sis to rs)
V to 1/ 2( E / R + I ) E + R I 1/ 2( E / R + I ) E + R I N /4 V te m ak es V to m ak es 1/ 2( E / R - I ) E - R I 1/ 2( E / R - I ) E -R I N /4 A m t rans fer s A m tr an sf er s V te
1/ 2( E / R + I ) E + R I 1/ 2( E / R + I ) E + R I N /4 V to br eak s V te br eak s 1/ 2( E / R - I ) E - R I 1/ 2( E / R - I ) E -R I N /4 V m m ak es V m m ak es 2
V te m ak es V to m ak es E / R + I 1/ 2( E + R I ) E / R - I 1/ 2( E - R I ) N /2 V to E/R E E/R E
N /2 V m br eak s V m br eak s A m t rans fer s A m t rans fer s E / R - I 1/ 2( E - R I ) E / R + I 1/ 2( E + R I ) N /2 V te N /2 V m m ak es V m m ak es V to br eak s V te br eak s
(wit h t wo t ra nsit ion re sis to rs)
V m br eak s V m br ea ks S t2 tr an sf er s S t1 tr ans fe rs V t2 m ak es V t1 m ak es V t1 br eak s V t2 br eak s V t2
S m t rans fer s S m t rans fer s V m m ak es V m m ak es V t2 br eak s V t1 br eak s S t1 tr an sf er s S t2 t rans fe rs
S o , S e are the tap selector contacts
S m , S t1 , S t2 are selector contacts of a selector switch
V m is the main contact (vacuum interrupter)
V t , V to , V te , V t1 , V t2 are the transition contacts (vacuum interrupters)
A m , A t are the auxiliary transfer switches
The circuits featuring one or two transition resistors and two or three vacuum interrupters represent the fundamental designs for vacuum type tap-changers More complex circuits that incorporate additional resistors and a greater number of vacuum interrupters are considered extensions of these basic configurations and are not included in this discussion.
NOTE 2 Duties including (E/R+I) will be maximum at power-factor = 1,0, duties including (E/R-I) will be maximum at power-factor = 0 and duties including neither (E/R+I) or (E/R-I) are not affected by the power-factor
NOTE 3 The given contact duties in the upper row are valid for one switching direction, the duties given in the lower row are valid for the opposite direction
NOTE 4 Duties depends on the power flow direction and are given here for both directions
NOTE 5 The number of operations is given under the condition that the power flow direction will not change
Supplementary information on switching duty relating to reactor type tap-changers
Additional test parameters
Service duty test
The provisions outlined in section 5.2.3.2 include specific requirements for preventive auto-transformers, which must maintain a circulating current in the bridging position at 50% of the rated through-current or as specified by the manufacturer in the design test report Additionally, a power factor of 80% is mandated.
Breaking capacity test
The requirements outlined in section 5.2.3.3 include specific provisions for preventive auto transformers, which must maintain a circulating current in the bridging position at 50% of the rated through-current or as specified by the manufacturer in the design test report Additionally, the power factor is set at 0%, and the number of operations is limited to 40.
Duty of switching contacts
Tables B.1 to B.4 respectively, show the duty on switching contacts for reactor type tap- changers with the following types of switching:
– selector switch and equalizing winding;
– diverter switch and tap selector;
– vacuum interrupter and tap selector
Similarly, Figures B.1 to B.8 show the sequence and vector diagrams for the four types of reactor type tap-changers
Table B.1 – Duty of switching contacts for reactor type tap-changers with selector switch – Switching direction from P1 to P5
(NOTE 1) Contact operation Contact Switching current Recovery voltage
(selector switch opens) H ẵ I (NOTE 2) ẵ IZ
NOTE 1 P1, P3 and P5 are service tap positions
NOTE 2 I is the load current
NOTE 3 E T /Z is equal to I C , the circulating current, Z is the impedance of the preventive autotransformer and E T is the tap voltage
NOTE 4 When the transition to on-tap is in the reverse direction, that is, from P5 to P1, the switching current at the
G contact is ẵI and the corresponding recovery voltage is ẵIZ (P4) The switched current at H contact is E T /Z – ẵI and the corresponding recovery voltage is E T – ẵIZ (P2)
NOTE 5 See Figure B.1 for the operating sequence diagrams and Figure B.2 for the vector diagrams
NOTE 6 All additions shown in the table are vector additions
C is the selector switch (2 in total)
Figure B.1 – Operating sequence of reactor type tap-changers with selector switch ϕ
The system voltage progression during the transition steps for two tap position change operations is illustrated in Figure B.2b, with key points labeled (a) to (e) Points (a), (c), and (e) indicate quiescent operation, while points (b) and (d) reflect momentary operations caused by reactance drop.
NOTE 2 Vectors (a-b') and (e-d') represent reactor voltage due to transformer action
Figure B.2 – Current and voltage vectors for reactor type tap-changers with selector switch
Table B.2 – Duty of switching contacts for reactor type tap-changers with selector switch and equalizer windings – Switching direction from P1 to P5
(NOTE 1) Contact operation Contact Switching current Recovery voltage
(selector switch opens) H ẵI + ẵE T /Z
NOTE 1 P1, P3 and P5 are service tap positions
NOTE 2 I is the load current
NOTE 3 ẵ E T /Z is equal to I C , the circulating current, Z is the impedance of the preventive autotransformer and E T is the tap voltage ẵ E T is the equalizer winding voltage
When transitioning to on-tap from P5 to P1, the switching current at the G contact is given by \$\Delta E_T / Z - \Delta I\$ and the recovery voltage is \$\Delta E_T - \Delta I Z\$ (P4) Similarly, the switched current at the H contact is \$\Delta E_T / Z - \Delta I\$ with a corresponding recovery voltage of \$\Delta E_T - \Delta I Z\$ (P2).
NOTE 5 See Figure B.3 for the operating sequence diagrams and Figure B.4 for the vector diagrams
NOTE 6 All additions shown in the Table are vector additions
A is the reactor (2 in total)
X is the selector switch (2 in total)
Figure B.3 – Operating sequence of reactor type tap-changers with selector switch and equalizer windings ẵ I ϕ
The system voltage progression during the transition steps for two tap position change operations is illustrated in Figure B.4b, with key points labeled (a) to (e) Points (a), (c), and (e) indicate quiescent operation, while points (b) and (d) reflect momentary operations caused by reactance drop.
NOTE 2 Vectors (a-b') and (e-d') represent reactor voltage due to transformer action
Figure B.4 – Current and voltage vectors for reactor type tap-changers with selector switch and equalizer windings
Table B.3 – Duty of switching contacts for reactor type tap-changers with diverter switch and tap selector – Switching direction from P1 to P7
(NOTE 1) Contact operation Contact Switching current Recovery voltage
Selector moves to bridge taps 1 and 2 G - -
NOTE 1 P1, P4 and P7 are operating positions
NOTE 2 I is the load current
NOTE 3 E T /Z is equal to I C , the circulating current, Z is the impedance of the preventive auto-transformer and E T is the tap voltage
NOTE 4 When the transition to on-tap is in the reverse direction, that is, from P7 to P1, the switching current at the
G contact is ẵI and the corresponding recovery voltage is ẵIZ (P6) The switched current at the H contact is ẵI – E T /Z and the corresponding recovery voltage is E T – ẵIZ (P3)
NOTE 5 See Figure B.5 for the operating sequence diagrams and Figure B.6 for the vector diagrams
NOTE 6 All additions shown in the table are vector additions
C is the tap selector (2 in total)
D is the diverter switch (2 in total)
G and H are the diverter switches
Position P1 shows operation on tap 1
Position P4 shows taps 1 and 2 being bridged
Position P7 shows operation on tap 2
Figure B.5 – Operating sequence of a reactor type tap-changer with diverter switch and tap selector ẵ I ϕ
The system voltage progression during the transition steps for two tap position change operations is illustrated in Figure B.6b, with key points labeled (a) to (e) Points (a), (c), and (e) indicate quiescent operation, while points (b) and (d) reflect momentary operations caused by reactance drop.
NOTE 2 Vectors (a-b') and (e-d') represent reactor voltage due to transformer action
Figure B.6 – Current and voltage vectors for reactor type tap-changers with diverter switch and tap selector
Table B.4 – Duty of switching contacts for reactor type tap-changers with vacuum interrupter and tap selector – Switching direction from P1 to P11
(NOTE 1) Contact Contact operation Switching current Recovery voltage
NOTE 1 P1, P6 and P11 are operating positions
NOTE 2 I is the load current
NOTE 3 E T /Z is equal to I C , the circulating current, Z is the impedance of the preventive autotransformer and E T is the tap voltage
When transitioning to on-tap from P11 to P1, the switching current at the V contact is denoted as \$\Delta I\$ and the associated recovery voltage is \$\Delta V_Z\$ (P9) The switched current at the V contact is calculated as \$E_T / Z - \Delta I\$ with the corresponding recovery voltage being \$E_T - \Delta V_Z\$ (P4).
NOTE 5 See Figure B.7 for the operating sequence diagrams and Figure B.8 for the vector diagrams
NOTE 6 All additions shown in the table are vector additions
A is the reactor (2 in total)
B is the by-pass switch (2 in total)
Figure B.7 – Operating sequence of a reactor type tap-changer with vacuum interrupter and tap selector ẵ I ϕ
The system voltage progression during the transition steps for two tap position change operations is illustrated in Figure B.8b, with key points labeled (a) to (e) Points (a), (c), and (e) indicate quiescent operation, while points (b) and (d) reflect momentary operations caused by reactance drop.
NOTE 2 Vectors (a-b') and (e-d') represent reactor voltage due to transformer action
Figure B.8 – Current and voltage vectors for reactor type tap-changers with vacuum interrupter and tap selector
Method for determining the equivalent temperature of the transition resistor using power pulse current
To ensure accurate temperature measurement of the resistance material in an on-load tap-changer or thermally equivalent setup, position the resistor appropriately Additionally, place thermocouples or thermometers for measuring the cooling medium's temperature at least 25 mm below the lowest point of the resistance material.
Measure and record the temperature of the resistance material and of the cooling medium at the start of the test
The test shall be performed with current I p , the r.m.s value of which is obtained from
The current value \( I_i \) represents the loading on the transition resistor during various steps of the switching sequence To calculate the specific currents, the through-current must be set to 1.5 times the maximum through-current, as outlined in section 5.2.5 The time \( t_i \) indicates the duration for which the currents \( I_i \) are active, and these values should be averaged from the service duty test specified in section 5.2.3.2 The coefficient \( k \) is selected based on the testing requirements for the resistor, with a recommended value below 5; values between 5 and 10 may only be used if the heating phenomenon remains adiabatic.
It has to be considered that the current I i and the time t i are depending on the operating cycle of the diverter/selector switch
The resistor will be tested with the specified current for a duration equivalent to half of one operational cycle The length of time for the current application will be established based on this criterion.
The rest period without current flowing through the resistor must match the minimum time interval between two successive operations of the tap-changer.
To determine the peak temperature, extrapolation of recorded values may be necessary
Simulated a.c circuits for service duty and breaking capacity tests
General
Figures D.1 and D.2 illustrate two validated simulated test circuits: the transformer method in Figure D.1 and the resistance method in Figure D.2, as referenced in section 5.2.3.5 These figures are provided for informational purposes, and the use of alternative circuits remains permissible.
Transformer method
To comply with sections 5.2.3.2 and 5.2.3.3, it is essential to monitor and adjust the current and voltage values at the four contacts, as illustrated in Figure D.1 This adjustment may involve modifying the U ED, X a, and R values, as well as the mutual phase of the voltage vectors, to account for the circuit and supply reactances.
1 and 4 are the main contacts R is the transition resistor
2 and 3 are the transition contacts X a is an adjustable reactor
5 is the supply from a generator or network U AB = U BC = U CA is the three-phase supply voltage
6 is the auto-transformer, or transformer, with step adjustable voltages U DF is the step voltage relevant to I t
7 is the diverter switch I t is the test current to be adjusted by means of U ED and X a
Figure D.1 – Simulated test circuit – Transformer method
Resistance method
To comply with sections 5.2.3.2 and 5.2.3.3, it is essential to monitor and, if necessary, adjust the current and voltage values at the four contacts, as illustrated in Figure D.2 This adjustment can be achieved through minor modifications to the R1 ohmic value, taking into account the circuit and supply impedance.
The calculated current and voltage values occurring in the whole tap-change operation on the four contacts should be used to calculate the power divider (see Figure D.2)
1 and 4 are the main contacts
2 and 3 are the transition contacts
5 is the supply from a generator or network
U s is the single phase supply voltage
R 1 R 8 are resistors forming the power divider where
I 1 and I 2 are the switched current r.m.s values of contacts 1 and 2;
U 1 and U 2 are the recovery voltage r.m.s values of contacts 1 and 2;
U 3 and U 4 are the applied voltage r.m.s values of contacts 3 and 4;
I 3 and I 4 are the making current r.m.s values of contacts 3 and 4
Figure D.2 – Simulated test circuit – Resistance method
In the case under consideration (four-contact diverter switch with operating cycle number 1 according to Table A.1), the equation for the most onerous conditions is given in the following equations:
Example of a synthetic test circuit for service duty test of vacuum type tap-changers
Definitions with relevance to the synthetic test circuit
Synthetic test circuit
test circuit with a power supply other than an a.c generator or a transformer
Simulated a.c test circuit
test circuit according to Annex D
Pre-arc
arc that appears between closing contacts when the distance has become so small that a flashover occurs between the contacts
Making voltage
voltage applied across closing contacts
Example for the test setup of a synthetic test circuit
Figure E.1 shows an example of an appropriate synthetic test circuit
Figure E.1 – Synthetic test circuit for service duty test of vacuum type tap-changers
The method involves utilizing direct current (d.c.) and direct voltage from charged capacitors, rather than alternating current (a.c.) and voltage Each operation requires charging the capacitors, with the discharge process regulated by inductors and resistors.
In the breaking section, DC1 functions as a direct current generator that charges capacitor C1 Thyristor T2 controls the discharge of C1, either blocking or initiating it The combination of inductance L2 and resistor R2 is adjusted to produce a discharge current that closely resembles a sine wave, with a period time nearly matching 50 Hz or 60 Hz.
Rp vacuum interrupter making part breaking part
In the construction phase, the DC2 serves as a direct current generator that charges capacitor C2 Thyristor T1 is responsible for either blocking or initiating the discharge of C2 Additionally, inductance L1 and resistor R1 are carefully adjusted to ensure that the current derivative closely resembles that of an actual transformer.
The thyristors POL1:1, 1:2, 2:1, and 2:2 are designed to change the polarity of the test object The resistor Rp ensures that the thyristors in the making part remain conductive until a re-strike or the vacuum interrupter is closed.
Figure E.2a – Breaking current Figure E.2b – Making current
Figure E.2 – Currents of the synthetic test circuit
The breaking current is tuned to look as shown in Figure E.2a In this case, it is approximately
The making current is tuned to look as shown in Figure E.2b In this example, it rises to
3 200 A in about 0,05 ms, giving a current derivative of about 70 kA/ms and a time constant of
This example illustrates the closing conditions for a vacuum type tap-changer with a current rating of 1,300 A, detailing the operating cycles The closing voltage is defined as \$E + RI\$, while the closing current is represented by \$\frac{E}{R} + I\$, as shown in Table A.3.
Example for the breaking/making condition during a switching operation
In the example below (Figure E.3), the recovery voltage and the making voltage are the same
These two voltages could be different and in such case two different circuits generating different recovery voltage and making voltage shall be applied main vacuum interrupter
0 10 20 30 40 50 60 time [ms] auxiliary contact breaking current making voltage making current
Figure E.3 – Example of the synthetic test for a switching operation with equal voltages for breaking and making duty
At approximately 9 ms, the breaking current is activated, followed by the opening of the main vacuum interrupter at 10 ms This process creates an arc lasting 9 ms, which can be adjusted between 0 ms and 10 ms by varying the interrupter's opening time Just before reaching 20 ms, the arc is extinguished, providing a clear indication of its termination through the measurement of the arc voltage.
Within 0.1 ms after the arc is extinguished, the recovery voltage is activated If there is no re-ignition, this voltage remains until the contacts are closed, serving as the making voltage As the vacuum interrupter begins to close and the contact distance narrows to a few millimeters, a pre-arc forms, allowing the making circuit to discharge and create optimal closing conditions.
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IEC 60076-12, Power transformers – Part 12: Loading guide for dry-type power transformers
IEC 60076-15, Power transformers – Part 15: Gas-filled power transformers
IEC 60376, Specification of technical grade sulfur hexafluoride (SF 6 ) for use in electrical equipment
IEC 60599, Mineral oil-impregnated electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis