EFFECTIVE TESTING OF DISTRIBUTION TRANSFORMER USING DPATT-3Bi
- All electrical parameter for general testing.
Voltage ( Vpn) Phase to Neutral
– Vr,Vy,Vb & Average
Voltage ( Vpp) Phase to Phase Voltage
– Vry, Vyb, Vbr & Average
– Vr,Vy,Vb & Average
– Ir, Iy,Ib & Average
System Frequency ( Hz)
- DSP Based True RMS Technology
- Direct Input Phase to Phase Voltage: 10 – 2100V and 80 A AC
- Pdf Report Generation
- EQDC Certified
- Data Logger
- Real Time View
- Online Graphing
- Special direct keys for Transformer Testing
F2 – Transformer OC Test Page
In this page we achieve the target LT Voltage on rated frequency. Because of frequency is not constant over time the target value is calculated by DPATT-3Bi. Both values will also on Sven Segment Display for better view.
F1 – Transformer SC Test Page
In this page %Impedance is calculated. Actual HT is achieved by increasing voltage till the full load current is achieved. Both values will also show on Seven Segment LED Display for easy view.
F3 – Scaled Wattage for temperature Compensation Page
This page shows scaled value of wattage on 75°C. For this you have to calculate scale factor by formula given on page11 and update the Scale factor.
You can do below Three Tests using DPATT-3Bi:
Short Circuit Test
Full Load Test
Copper Loss Test
Winding Loss Test
Open Circuit Test
No Load Test
Iron Loss Test
Core Loss Test
Scaled Wattage for Temperature Compensation
(IS 2026 Part-1)
Let’s Start with Example of Transformer Name Plate –
- Rating – 200KVA
- % Impedance – 4.21
- Sr . No. – 1LNZ53 (Serial No of Transformer)
- Rated HV Voltage – 11KV
- Rated LT Voltage – 240V (P-N) or 415V (P-P)
- HV AMPS– 10.5A
- LV AMPS – 278.2A
- Rated Frequency – 50Hz
Name Plate Settings for Transformer Testing
To do OC and SC Test in DPATT-3Bi you have to fill Transformer Specification such as HV Rated Voltage, LT Voltage and Full Load Current etc.
Ratings Of Transformer Name Plate
Serial No: you can set the serial no. of transformer for reporting purpose.
KVA Rating: Here you can set the KVA Rating of Transformer. This is used for Calculation in SC and OC Test.
HT Volts: You can set the Rated HV Volts.
This is used for calculation in SC Test.
LT Volts (Phase to Phase): Here Phase to Phase Rated LT Voltage can be entered.
This is used for calculation in OC Test.
LT Volts (Phase to Neutral): Here Phase to Neutral Rated LT Voltage can be entered.
Full Load Current: Here full load current is to be entered for testing.
Full load current to be calculated for SC Test. Procedure to calculate Full load current is given further in Short Circuit Test.
Rated Frequency: Here Rated Frequency to be entered.
This is used for calculation in OC Test.
SHORT CIRCUIT TEST
Theory: The test is conducted on the high voltage side of the transformer. Where the secondary winding (usually of low-Voltage Side) of the transformer is short circuited. The wattmeter reading measures only the full load copper loss.
Steps to start the short circuit test
Step -1 : Calculations
First we have to calculate full load current.
Full Load current = KVA Rating X1000
√3 X HV LINE VOLTAGE
From the transformer Specification
KVA Rating = 200
HV LINE VOLTAGE = 11KV
So Full Load Current = 200 X 1000
√3 X 11000
Full Load Current = 10.49 A
Enter this Full load current to the DPATT-3Bi Transformer Specification->Full Load Current (See Table 1-E).
Target value for current (Full Load Current) will show on LED Display with current value.
Step – 2: make the connections as per the connection diagram in figure 2. Now Short the LT Side of the Transformer and apply the voltage at the HT Side.
Slowly increase the voltage until full load current is achieved.
Target Value with selected parameter value is also shown on the Seven Segment LED Display for better Viewing.
|Present Value (Image- 3A)||Target Value (Image -3B)|
The Meter clearly shows the present value of Amps achieved (Image -3A) and the target value (Image -3B) required as per test.
Step -3: You can directly do SC Test by DPATT-3Bi by pressing F2 Button.
Image – 4
%Impedance is also calculated directly by DPATT-3Bi and is shown in above image.
You can also calculate %Impedance by Below Formula.
Step-3: For example full load current is achieved at 463.1 V. This is also called impedance voltage .
So %impedance = Impedance Voltage X100
Rated HV Voltage
%impedance = 463.1 X 100
%impedance = 4.21
STEP4 – Copper losses are shown on W/PF page of DAPTT-3Bi. The Page will looks like image-5.
Result: Hence Full load current is 10.49 Amps, %Impedance is 4.21% and total copper loss 45.97 Watt.
OPEN CIRCUIT TEST
Theory: This test is important because whenever the transformer is ON it consumes electricity in the form of iron loss. This loss occurs round the clock and it should be minimized through better design of iron core.
The core loss and magnetizing current depends on applied voltage and frequency.
It is recommended, that since we have no control on variation of mains frequency the applied voltage is varied to keep the V/f ratio constant.
For example, in the figure above,
Standard voltage 240V P-N or 415V P-P is applied on LT side through variac keeping
HT side open.
Steps to start the open circuit test
Step 1: Make the connection diagram as given in image -6. HT Side of Transformer is kept open and apply voltage through variac on LT Side.
Note – In case Supply frequency is different from rated frequency, then voltage to be applied according to Supply frequency.
Step 2: DPATT-3Bi directly calculates the Voltage to be achieved according to Current Frequency and Rated Frequency.
You have to press F1 on DPATT-3Bi to do Open Circuit Test.
The page will look like below –
In above image LT Volt is current LT Side voltage, Frequency is current Frequency and Target is LT Voltage which we have to achieve.
Achieved Value with Target value is also showing on Sven Segment LED Display as shown below.
|Present Value (Image- 8A)||Target Value (Image -8B)|
We have to apply voltage through variac till the LT Volt and Target is same.
You can also calculate the target Voltage by below formula.
Va = VRT X Fs
Va = Applied Voltage
VRT = Rated LT Voltage
FRT = Rated Frequency
Fs = Supply Frequency
Now for example if
VRT = 415V
FRT = 50
Fs = 49.5
Then Va = 415 X 49.5
Va = 410.8V
Step-3 No Load Current Calculation
HT Voltage= 11KV
Rating of Transformer = 200KVA
Solve: Rated Voltage=415V
Full Load Current =10.49A
No Load Current= 2.5% of Full Load Current
(According to BEE)
STEP-4 Iron losses are shown on W/PF page of DAPTT-3Bi. The Page will looks like image-9.
Result: Hence No Load Current is 0.26A and total iron loss is 45.97 Watt.
Scaled Wattage for Temperature Compensation (IS 2026 Part-1)
Since all readings are taken at room temperature (20(°C) but according to IS2026 Standard for Transformer Testing Reading should be on 75°C.
So we have to multiply some factor (according to standard) to the watt which is called scaled factor.
In DPATT-3Bi there is a facility to scale watt parameter.
To view scaled wattage Press F3 Button on DPATT-3Bi.
The display will look likes Image -10.
Wattage is given by below formula
W = I2R
We know that resistance of wire is different for different temperatures.
Resistance of wire on temperature change is given by –
R = R0 (1 + K ΔT)
R = Resistance on rise of temperature
R0 = Resistance at given Temperature
K= Temperature Coefficient
ΔT = Change in temperature (°C)
W = I2R0 ((1 + K ΔT)
Generally transformer windings are of aluminum or Copper. They have positive temperature coefficient (K).
K Copper = .00386
K Aluminum = .00429
So on 75°C Scale factor for copper (1 + K ΔT) = (1 + .00386 x 55)
Scale Factor = 1.21
Note: Scale factor is to be calculated and updated in meter.
SCALE FACTOR WHICH YOU HAVE ENTERED WILL ONLY HOLD ON SCALED WATTAGE PAGE. IT DOES NOT AFFECT ANY OTHER PARAMETER READING OR WATTAGE ON ANY PAGE. THIS IS ONLY GIVEN FOR EASY CALCULATION OF SCALED WATTAGE AT 70°C TEMPERATURE.