ECE 449 - Power Systems Laboratory

# Experiment 8: Scott Connections of Transformers

## Objectives

1. To obtain balanced two-phase supply from three-phase supply by Scott arrangement of two single phase transformers.
2. To perform resistor load test (unity power factor) for both balanced and unbalanced loads and compare the test results with calculations.
3. To obtain single-phase supply from three-phase supply by Scott arrangement and perform load test at unity power factor (resistor).
4. To observe and calculate quadrature of secondary side voltage of transformers.
5. To acquire the phase angle relationship between the secondary load currents of transformers.

## Prelab

Read the information below.  Draw the Phasor diagrams for three phase (3φ) to two phase (2φ) conversion system using Scott tap of transformers with balanced and unbalanced resistive load (unity power factor).

## Theory:

cott connection of two-single phase transformers is employed for conversion of a three-phase system to two phase system or vice-versa. Rating of one transformer should be 15% greater than that of the other, but in practical two identical transformers are used for interchangeability and spares. The connection scheme, known as Scott connections is as shown in the figure 1, 50% tap of one transformer (Main transformer) is connected to 86.6% tap of the other transformer (Teaser transformer). The secondary windings of transformers for balanced supply system have equal number of turns. Consider the Scott connection of two single-phase transformers with turn’s ratio $N_1:N_2$ as shown in figure 1. The phase diagram of line voltages on the primary side, $V_{AB}, V_{BC}, V_{CA}$ form an equilateral triangle.

Let,

$V_{AB} = V$ (1)
$V_{BC} = V(-120^o)$ (2)
$V_{CA} = V(120^o)$ (3)

The secondary voltage of the main transformer is given by,

$$V_b = \frac{N_2 V_{CB}}{N_1} = \frac{N_2}{N_1}V(60^o)$$ (4)

The voltage $V_{AM}$ is given by,

$$V_{AM} = V_{AB} + \frac{V_{BC}}{2}$$ (5)
$$V_{AM} = V + \frac{V}{2}(-120^o)$$
$$V_{AM} = V \Big(\frac{3}{4} - j\frac{\sqrt{3}}{4}\Big)$$
$$V_{AM} = \frac{\sqrt{3}}{2}V(-30^o)$$ (6)

This voltage $V_{AM}$ is across $\Big(\frac{\sqrt{3}}{2}\Big)*N_1$ turns. Therefore, the primary voltage (across $N_1$ turns) of teaser transformer is given by,

$$V_{AA'} = \frac{2}{\sqrt{3}} V_{AM}$$
$$V_{AA'} = V(-30^o)$$ (7)

Hence, the secondary voltage of the teaser transformer is given by,

$$V_a = \frac{N_2}{N_1} V_{AA'}$$
$$V_a = \frac{N_2}{N_1} V(-30^o)$$ (8)

Hence, from equations (4) and (8) we can see that, for a balanced three-phase supply on the primary side, the voltages on the secondary side of the transformers are equal in magnitude but 90 degrees out of phase. Therefore we got a balanced two-phase supply from balanced three-phase supply using Scott connection.

If the secondary load currents are Ia and  Ib, then the primary currents can be obtained as follows,

$$I_A = \frac{2N_2}{\sqrt{3}N_1} I_a$$ (9)
$$I_{CB} = \frac{N_2}{N_1} I_b$$ (10)
$$I_B = -I_{CB} - \frac{I_A}{2}$$ (11))
$$I_C = I_{CB} - \frac{I_A}{2}$$ (12)

To get single-phase voltage supply, short negative polarity side of teaser transformer and positive polarity of main transformer on secondary side and take the voltage across positive polarity of teaser transformer and negative polarity of main transformer on secondary side. This single phase voltage is given by,

$$V = \sqrt{V^2_a + V_b^2}$$ (13)

Since, $V_a$ and $V_b$ are 90 degrees apart.

In case of single-phase configuration, the secondary currents of teaser and main transformer are same i.e.

$$I_a = I_b$$ (14))

Substituting equation (14) in equations (9), (10), (11) and (12) we can get the currents on primary side.

## Equipment

1. Two single-phase transf. Hampden T-1000 or one Three phase transf. Hampden           T-100-3A-3φ
2. 3-phase Variable AC Voltage -Variac (in lab)
3. Multimeters(DVM)
5. Krohn-Hite Digital Phasemeter Model 6400A
6. Two Coax cable (BNC to Banana connector – Check out of stockroom (SR))
7. Power lab box with cables and Fluke meter (SR)
8. High Sensitivity, Fluke AC-DC Current Clamps Probe (SR)

## Procedure

1. Perform windings polarity test of the two single-phase applying a maximum voltage of 20 Vpk-pk (≈ 7.07 Vrms) to one of the transformer primary side windings. Display and record on a two channel oscilloscope the voltage and phase shift polarity waveforms of primary and secondary sides’ windings.
2. Experiment transformers has four main windings. Create a 2:1 ratio step down transformer by connecting the two primary windings in series and the two secondary windings in series. Hampden T-1000 transformer is rated at 1.0 KVA. The rated current for high voltage side (H) is 4.16A and 8.32A on the low voltage side (X). Likewise, Hampden T-100-3A-3φ is rated at 120 VA with a high voltage side (H) rated current of 0.5A and corresponding low voltage side (X) rated current of 1A.
3. Connect the circuit as shown in the figure 1 for three-phase (3φ) to two-phase (2φ) conversion. Each transformer has in high voltage side (H) a winding tapped at 32 volts to provide a Scott tap (86.6%) of the two windings in series.
4. The single phase load on secondary sides of teaser and main transformers should be made connecting the three resistor banks of resistive load cart in series to have more load combinations under rating currents of transformers.
5. Without load on the secondary sides, close circuit breaker (CB) of AC Variac to supply voltage on primary side of transformers. Gradually adjust supply voltage to 120 Vrms L-L (line-to-line) while observing primary sides’ currents to do not exceed ratings. Record the voltages on primary and secondary sides of teaser and main transformers including voltage $V_{AM}$ across teaser terminals.
6. Use Krohn-Hite Digital Phasemeter Model 6400A with proper settings of voltage level and waveform to measure the phase angle relationship between the voltages on the secondary side of the transformers.
7. Start with balanced load on the secondary side (ZL1 = ZL2). Record all the ammeter and voltmeter readings on both sides of transformers. Repeat this for 5 more sets of balanced loads. (Note: Keep observing primary and secondary currents to do not go above the corresponding rating of transformer.)
8. Using your last set of loads. Display and record the phase shift waveforms between the secondary currents of teaser and main transformer on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring one secondary current of teaser transformer (Ia) while the other measure secondary current of main transformer (Ib). Measure and record with the oscilloscope the time delay between the two signals.
9. Now apply unbalanced load on secondary side (ZL1 ≠ ZL2) and record all the voltmeter and ammeter readings on both sides of transformers. Repeat this for 5 more sets of unbalanced load. (Note: Keep observing primary and secondary currents to do not go above the corresponding rating of transformer.)
10. Using your last set of loads. Display and record the phase shift waveforms between the secondary currents of teaser and main transformer on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring one secondary current of teaser transformer (Ia) while the other measure secondary current of main transformer (Ib). Measure and record with the oscilloscope the time delay between the two signals.
11. Turn off the supply voltage to circuit (AC Variac).
12. For three-phase to single-phase conversion, short the negative polarity side of teaser transformer and positive polarity side of main transformer in the previous circuit (fig. 1).
13. Use one of the resistive load cart of step 4 and connected across positive polarity side of teaser and negative polarity side of main transformer. (Note: The single-phase voltage is higher than the secondary voltage in two-phase conversion (equation 13)). If necessary connect two loads in series to maintain the rated voltage of load greater than or equal to single-phase voltage.
14. Without load on the secondary side, close circuit breaker (CB) of AC Variac to supply voltage on primary side of transformers. Gradually adjust supply voltage to 120 Vrms L-L (line-to-line) while observing primary sides’ currents to do not exceed ratings.
15. Apply 5 or 6 sets of load on secondary side of transformer and record all the voltmeter and ammeter readings on both sides of transformer. (Note: Keep observing primary and secondary currents to do not go above the corresponding rating of transformer.)

## Discussion

1. Discuss and number the advantages and disadvantages of Scott-T connections transformers configurations.
2. Verify experiment results with theoretical predictions for 3φ-2φ conversion (balanced and unbalanced load) and 3φ-1φ conversion.
3. Calculate phase shift angle between the secondary currents of teaser (Ia) and main transformer (Ib) for balanced and unbalanced load of 3φ-2φ conversion. Data of steps 8 and 10 of Procedure.
4. Draw Phasor diagrams for one load set of experiment results of 3φ-2φ conversion (balanced and unbalanced load) and 3φ-1φ conversion.
5. Why Scott-T connections of transformers are not recommended as a connection for 3φ-3φ applications?

## References

1. Martin J. Heathcote, The J & P Transformer Book, Ch. 7, 13th Edition, Elsevier, 2007.