TUTORIAL SERIES FOR PRIMARY EQUIPMENT TESTING

TUTORIAL 1: TRANSFORMER

Testing the power transformer in the initial, periodic or post-repair mode to ensure the accuracy and reliability of test results, the safety of people and equipment. This guide provides specific requirements on conditions and test contents for power transformers installed indoor and outdoor, operated on high-voltage and low-voltage grids.

1. Definition
a. Power transformer: is an electrical equipment used to transfer the alternating power energy among circuits through electromagnetic induction. The transformer consists of a primary winding and one or more secondary windings. When a current with specified voltage is applied to the primary winding, induced current will be created in the secondary windings;
b. New test: is the test to evaluate the quality of electrical equipment for the first time before test running and put into use;
c. Periodic test: is the test for electrical equipment after a period of operation as prescribed in order to assess current quality of electrical equipment to prevent incidents that may occur due to the quality loss of electrical equipment;
d. Post-repair test: is the test for electrical equipment after being repaired in order to assess the quality of electrical equipment to ensure that problem has been fixed and the device is ready to re-operate;
e. Primary equipment: is the specified voltage applied into or created in the state of no-load between the connectors of the winding with no tap changer or the winding with tap changer that is connected at the tap changer; for the three-phase windings, it is the voltage between phase connectors;
f. Nominal voltage (Ur): is the conventional value of apparent power assigned to winding of transformer that with the nominal voltage of the winding, this capacity determines the nominal current of the winding;
g. Norminal capacity (Sr):is the conventional value of apparent power assigned to winding of transformer that with the nominal voltage of the winding, this capacity determines the nominal current of the winding;
h. Norminal current (Ir): is the current running through the phase connector of the winding; this current is calculated from the nominal capacity Sr and nominal voltage Ur for that winding;
i. Nominal frequency (fr): is the frequency at which the transformer is designed to work;
j. PV: power transformer;
k. M: Manufacturer;
l. TC: Test candidate;
m. NGM: Not guaranteed measurement.

2. Reference
a. IEC 60060-1: 2010: General definitions and test requirements (Subclause 4/Page 11: General requirements; Subclause 6/Page 22: Tests with alternating voltage);
b. IEC 60076-1:2011 (TCVN 6306-1:2015): Power transformers – Part 1: General (Article 10 table 1; Article 11);
c. IEC 60076-3: 2013 Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air (Article 11);
d. TCVN 6306-11: 2009 (IEC 60076-11: 2004): Power transformers – Part 11: Dry-type transformers;
e. QCVN QTĐ-5: 2009/BCT – National technical regulations on electrical engineering – Ep. 5: Testing equipment and electrical systems;
f. Minutes of experiments for finished transformers;
g. Report of results for the first time and/ or tests /testing earlier.

3. General provisions
a. Tested transformer:
i. Must be electrically completely isolated before testing;
ii. Test object must be well-defined, fully labeled and with specifications such as rated voltage, current, manufacturer, manufacturing date,…
iii. For dry-type transformer:
– Before measuring, the transformer should be in a non-electrical state at a constant temperature for at least 3 hours.
– The resistance and winding’s temperature must be measured at the same time. Winding’s temperature must be measured by sensors placed in typical position, preferably inside the windings, for example, in a gap between high voltage winding and low voltage winding.
iv. For oil-filled transformer:
– After the transformer stays at a non-electrical state for 3 hours, the average temperature of oil should be determined and the temperature of the winding can be determined by the average temperature of oil, as the average temperature is taken by the average of the top and bottom temperature point of the oil. However, the deviation between above and below temperature does not exceed 5 degrees.
v. Environmental and test conditions:
According to the technical requirements of each test subject is governed by the manufacturer or if no separate provisions:
– Ambient temperature: (0-40) ⁰C;
– Humidity: (50-98)%;
– The test object must be in the temperature-stable state or able to control the temperature of the measured object reliably.
b. Safety measures:
i. TC receives the scene with the provisions of the electricity industry;
ii. TC checks the power supply of the testing vehicle before starting;
iii. TC checks whether the subjects requiring testing have been disconnected with electricity, completely isolated from the sources of the system and the electrical enclosure must be grounded;
iv. Get safety fence, sign boards and assign safety guard if necessary;
v. All staffs working in the team must perform strictly the occupational safety and health equipment.

4. Test guide
Only perform tests when regulations specified in Provision 3 are fulfilled, including steps as follows:
a. External inspection:
i. Check the profile of the transformer:
– The instructions on operation and use;
– Diagrams and details, accessories required for testing;
– Special guide for transformer (if any).
ii. Check by observation: no damage caused by mechanic electric discharge and corrosion; Transformer must remain intact; poles are connected securely with no cracks; oil level of transformer must stay within the allowed limits,…
b. Insulation resistance measurement:
i. Setup the diagram to measure insulation resistance based on following diagram:

ii. Execute test:
Step 1: Connect devices and equipment based on the diagram. Launch device and set up parameters following the user manual.
Step 2: Measure and read the resistance value at 15 seconds, 60 seconds.
Step 3: Record the parameters in Power transformers’ test result form.
Bước 4: Remove the connections.
Bước 5: Use a grounded wire to discharge extra electricity on the object.
iii. End measurements and wait to make sure the circuit has completely finished discharging for at least 3 minutes. Disassemble the diagram and end the measurement.
iv. Fully record the original observations. Perform preliminary assessment and calculation if measured results are within the permitted limits. Similarly, continue to perform tests with other different voltage levels.
c. Check the reliability of dielectric
i. Prototype should be dry and clean, need to be conducted at ambient temperature.
ii. Only perform testing when the results of insulation resistance tests have  satisfied the requirements prescribed.
Table of test value for alternating voltage with industrial frequency varying nominal voltage levels as the following table:

Nominal voltage (kV)	Test voltage in 1 minute (kV)
0,72	3
1,2	6
3,6	10
7,2	20
12	28
17,5	38
24	50
36	70

iii. Using alternating high-voltage testing equipment that ensures testing requirements in terms of capacity.
Setup diagram for testing electrical durability testing as follows:

iv. Conduct test:
Step 1: Connect test devices and equipment following the diagram.
Step 2: Start the equipment and set up parameters. Select the testing voltage and leakage current limit conforming to voltage level of transformer’s winding. Select automatic or manual test mode.
Step 3: Perform high-voltage test for each phase following user manual for alternating high-voltage test equipment.
Step 4: Steadily increase the voltage to test value with growth rate 2kV / s and note whether discharge happens or not, maintain this voltage value within 1 minute.
Bước 5: Stop measurement, rapidly reduce voltage value to 0V and turn off CB.
Bước 6: End measurement, wait to make sure the circuit has completely finished discharging for at least 3 minutes. Disassemble the diagram and end measurement.
Bước 7: Record full test voltage value, time and leakage current / breakdown … comforming to Power transformer’s test results form. Perform preliminary calculation and assessment with test results, if experimental data is within the permitted limits. Similarly, continue to perform other electrical durability with industrial frequency tests,……
d. DC resistance measurement for cool windings:
i. Principle for DC resistance measurement method: If object is winding of transformer, use the pair of pumping lines of station box that are always clipping the outside of measured object properly, the pair of wires measuring voltage drop clipping the object to be measured, ensuring good contact.
ii. Connect the wiring diagram for measuring resistance of transformer’s winding conforming to the principle of Volt-Ampemet following these diagrams:

 

iii. Conduct test:
Step 1: Connect test devices following the diagram.
Step 2: Plug the power cord and start the device.
Step 3: Enter settings to select the necessary mode.
Step 4: Enter settings to select the necessary mode.
Step 5: Select “Start” to start the measurement.
Step 6: When the displayed value gets stable as selected according to the mode, the safe light turns on or choose “Stop” to stop measuring, record the results
Step 7: Record results in Power transformer’s test results form
Step 8: End of measurement.
iv. Turn off the power supply, disassemble the measuring line, clean and return the ground with original state before the test.
e. Measure variable ratio and determine the polarity:
i. Set diagram measuring variable ratio and determine the polarity as follows:
– For 1-phase transformer: The wires for the primary is X with 2 of the lines are X1 and X2, wires for the secondary with 2 of the lines are H1 and H2
– For 3-phase transformer: consisting of wires for the high side with 4 lines H1, H2, H3, H0 and wires for the lower side with 4 lines X1, X2, X3, X0. For Y or Δ connection, only use 3 lines H1, H2, H3 or X1, X2, X3. For Yo connection, use all 4 lines.
ii. Consuct test:
Step 1: Connect test devices following the diagram.
Step 2: Plug the power cord and start the device.
Step 3: Enter settings to select the pumping voltage required to test.
Step 4: Select the type of wiring and change ratio: select the type of wiring such as: singal phase, Δ / Y, Y / Δ,… Once selected, enter the input transformer ratio varying voltage levels and voltage values for the high and low side; Can choose auto-detect mode to find vector group. Selecting vector group and enter the transformer ratio as written on label or due to manufacturer.
If at this step, we need to detect vector group, select Auto test and continue to enter the transformer ratio.
Step 5: Choose starting to measure. During the process, there is continuously beep.
Step 6: Now the device will automatically start pumping voltage to the lower side winding and perform calculations. The results will appear on the screen after the process is finished.
Step 7: Perform tap changing, measure variable ratio at the other voltages similarly.
Step 8: Record results in Power transformer’s test results form.
Bước 9: End of measurement.
iii. Turn off the power supply, disassemble the measuring lines, clean and return the ground with original state before the test.

7. Evaluation of the test results:
a. External inspection:
i. Labels must be clearly readable;
ii. No damage caused by mechanic, electrical discharge and corrosion;
iii. Transformer must remain intact;
iv. The connecting poles are securely with no cracks;
b. Insulation resistance:
i. Insulation resistance value measured after converting to the same temperature must not be less than 30% of the the insulation resistance value of manufacturer or the laboratory / inspection earlier.
ii. In cases that there is no data for comparison, it is allowed to consult the minimum insulation resistance value stated in QCVN QTD -5: 2009 / BCT as follows (unit MΩ):

High voltage winding’s voltage level	Temperature of winding (0C)
	10	20	30	40	50	60	70
– Less than 35 kV voltage and 10000 kVA capacity	450	300	200	130	90	60	40
– 35 kV voltage and more than 10000 kVA capacity – More than 110kV voltage indepent of capacity	900	600	400	260	180	120	80

iii. If the insulation resistance values measured satisfy above conditions, we continue the experiment with next items.
iv. Formula for temperature by factor K1:

Δt=t2-t1	K1	Δt=t2-t1	K1
1	1.04	10	1.5
2	1.08	15	1.84
3	1.13	20	2.25
4	1.17	25	2.75
5	1.22	30	3.4

In which: – t1: temperature measured before or in profile
– t2: measured temperature measured after

• Δt> 0 when converted to t1 have to multiple with K1
• Δt< 0 when converted to t1 have to divide by K1

Converting method:      RCD (at temperature t1) = K1 * RCD (at temperature t2).
K1 is converted coefficient values with Δt = t2 – t1.
iv. In cases that the temperature difference is not in the table above, we can have a coefficient given by multiplying the corresponding coefficient..
For example: 9⁰C = 4⁰C + 5⁰C so we calculate:
K1(∆t = 9⁰C) = K1(∆t = 4⁰C) * K1(∆t = 5⁰C) = 1,17*1,22 = 1,42
c. Check reliability of dielectric:
Voltage test with industrial frequency in dry conditions for transformer is considered satisfactory when:
i. No supplying current oscillation, unusual irruption when increasing voltage or maintaining test voltage.
ii. No surface electrical discharge or insulation breakdown during testing.
iii. Insulation values measured after test are not impaired.
d. DC resistance measurement for cool windings:
i. DC resistance of cool winding measured is converted at the temperature of 75°C compared to the design data with no more than ± 10% deviation for the first time; for later experiments, no more than ± 2% deviations compared to the experimental data previously put into operation is satisfacton;
ii. If at temperatures not 75^oC, the value of DC resistance measured must be converted to the temperature of 75^oC by the formula:

In which: – R₇₅: resistance need to be converted to 75^oC
– R_t: resistance measured at temperature t
– t:  temperature of winding
e. Measure variable ratio and determine the polarity:
i. If the variable ratio value measured is positive, the primary and secondary are the same polarity, and otherwise.
ii. K_I%= (calculated ratio – measure ratio) / calculated ratio.
Calculated ratio is indicated on the label or due to the manufacturer. Deviations of variable ratio of each winding do not exceed 0.5% compared to the ratio on label.
Test results are recorded in Test results form.