M1: Three Phase Transformer Back to Back · PDF file · 2012-09-19M1 Three Phase...

Electrical Machines Lab Third Year M1: Back to Back Test

Faculty of Engineering Cairo University Electrical Power and Machines Dept.

1

M1 Three Phase Transformer Back to Back Test Objective To determine the parameters of large transformers and obtain their performance characteristics. Background Two very simple tests are used to determine the constants of equivalent circuit and the power losses in the transformer. These consist in measuring the input voltage, current, and power to the primary, first with the secondary short circuited and then with secondary open circuited. The core losses are determined from the open circuit test. The copper losses are determined from the short circuit test. Stray load loss consists of the losses arising from the non-unifrom current distribution in the copper and the additional core losses produced in the iron by distortion of the magnetic flux by the load current. It is difficult to determine such losses accurately by conventional no-load and short circuit load tests. To obtain exact equivalent circuit and losses, the input and output parameters are directly measured under different loading conditions. This is easy for small rating transformers. However for large transformers, it is difficult and expensive to take direct measurements. A Back to Back Test is used in this case. This test requires two identical transformers having some tapping in the windings. Why Back-to-back test is used in case of large transformers ? First, the short circuit test is difficult to be applied, since applying a reduced voltage is very difficult and unpractical. Second, this test can simulate the loading conditions on the transformer without using real loads. Third, a large transformer supplying large essential loads has usually a second identical transformer installed in the same location for back-up, so using back-to-back transformer in this case is very practical. Procedure This test requires two identical three phase transformers. Take complete particulars of either of the two transformers. Connect the two transformers as shown in figure 1 and check the correct polarity using a voltmeter connected across the switch. The reading of the voltmeter should be zero for correct polarity connection. Then carry out the following procedures: 1. With the switch open, apply the rated voltage and read the various instruments. The input

power in this case covers the iron losses of the two transformers, thus the parameters Rc

and Xμ of the transformer equivalent circuit can be determined. The turns ratio of the transformer can be calculated from the reading of the primary and secondary voltages.

2. It is to be noted that if no taps are used and the corresponding terminals of the two

transformers are connected together, by closing the switch no circulating current will pass and the transformers behave as if they were open circuited.



2

3. With the terminals of either of the two transformers remain unaltered and changing the taps of the other, a small voltage will appear across the switch when it is open. This small voltage results in a circulating current in the transformer windings if the switch is closed. It is to be pointed out here that the voltage which appears across the switch (when it is open) is the open circuit voltage calculated when Thevenin's theory is applied. Correspondingly the circulating current which passes after closing the switch is due to the impedance seen from the switch terminals. This impedance is nothing but the short circuit impedance of the two transformers as if they were connected in series. This input power in this case covers the iron losses of the two transformers as well as their corresponding copper losses.

4. Repeat step 3 using other tapping points.

5. From the readings of steps 2 to 4, the resistance and leakage reactance of the transformer

referred to the secondary side can be determined. Hence along with the results of step 1 the complete equivalent circuit referred either to the primary or the secondary side can be obtained.

Report

1- From the results obtained determine: a- The parameters of the equivalent circuit of the transformer under test referred to the

primary and secondary sides. b- Draw the loss curve of either of the two transformers (power losses vs. load factor). c- The efficiency curves at unity power factor, 0.8 lagging, and 0.8 leading. d- The regulation curves at different power factors at x=0.8 and x=1.

2- Comment on the shape of the curves obtained.

3- Comment on the method used to determine the equivalent resistance and reactance and compare with that used in the normal short circuit test.

Comment on your results whenever necessary.



3

Connection Diagram



4

Induction Regulator A stationary 3-phase induction machine can also be used as a source of variable 3-phase voltage if it is connected as an induction regulator as shown in Figure (2). The phasor diagram shown illustrates the principle. As the rotor rotated through 360o, the output voltage Vo follows a circular locus of variable magnitude. If the induced voltages E1 and E2 are of the same magnitude (i.e. identical stator and rotor windings) the output voltage may be adjusted from zero to twice the supply voltage. The induction regulator has the following advantages over a variable auto transformer:

• A continuous step-less variation of the output voltage is possible. • No sliding electrical connections are necessary.

Figure 2



5

M1 Back-to-back transformer Lab results

V1o = V2o = turns ratio (a) = = V (V) I1 (A) I2 (A) W1 (W) W2 (W)

Tap 1 (no-load)

W10 = W20 =

Tap 2

Tap 3

Tap 4

At No-load (Tap 1) =

At loading conditions (other taps) [(W1 + W2) - (W10 + W20 ) ] = 2 * 3 I2

2 Req Req = 2Zeq = = Zeq = Xeq =

PcuFL= 3 I22 Req * (

𝑰𝟐𝑭𝑳𝑰𝟐

)2 I2FL = 𝑺𝑭𝑳√𝟑𝑽𝟐

=

PcuFL= ReqLV= XeqLV =

ReqHV= XeqHV =

Transformer Ratings S =

V1/V2 =

f =

Sb = Sratedφ = S/3 V1b = V1ratedφ V2b = V2ratedφ

φ

φ

2

1

VV

ooI

V φcos2

3 110

oo

cI

I φcos32

=oo

mI

I φsin32

=

==c

oc I

VR 1 ==

m

om I

VX 1 =ironP

b

bb S

VZ

21

1 =b

bb S

VZ

22

2 =

22

2

2 eqeq XRIV

+=

32

1

o

o

VV

221 oo WW +



6

Loss Curve Plosses = Piron + Pcu

Pcu = x2PcuFL where x = load factor = 𝑰𝑰𝑭𝑳

= 𝑺𝑺𝑭𝑳

Piron = PcuFL = Plosses = Piron + x2PcuFL

x 0.1 0.3 0.4 0.6 0.7 0.8 1

Plosses (w)

Efficiency Curves 𝜼 = 𝒙 𝑺𝑭𝑳 𝐜𝐨𝐬𝝓

𝒙 𝑺𝑭𝑳 𝐜𝐨𝐬𝝓+𝑷𝒊𝒓𝒐𝒏+𝒙𝟐𝑷𝒄𝒖𝑭𝑳 𝜼 = 𝒙 𝐜𝐨𝐬𝝓

𝒙 𝐜𝐨𝐬𝝓+ 𝟏𝑹𝒄 𝒑𝒖

+𝒙𝟐𝑹𝒆𝒒 𝒑𝒖

x 0.1 0.3 0.4 0.6 0.8 1

η (0.8 lag)

η (unity)

Regulation Curves 𝑹𝒆𝒈% = 𝒙 𝒁𝒆𝒒 𝒑𝒖 𝐜𝐨𝐬(𝜽𝒔𝒄 ±𝝓) × 𝟏𝟎𝟎 - lagging pf / + leading pf

𝜽𝒔𝒄 = 𝐭𝐚𝐧−𝟏 𝑿𝒆𝒒𝑹𝒆𝒒

=

lag unity lead

Φ (degrees) 80 30 10 0 10 20 30

Reg (x=0.8)

Reg (x=1)



7

Loss curve



8

Efficiency curves



9

Voltage regulation curves

8

1 2

3

4

5

6

7

8

9

11

Oil-immersed 630-kVA distribution transformer TUMETIC:View into the inside

1

2

3

4

5

6

7

8

9

10

11

Core and windingsBoth are held together by a pressed structure and bolted together with the tank lid. The complete unit can be lifted out of the tank.

CoreHigh-quality electric sheet steel, most modern core design and optimized lamination provide for low-loss and noise-optimized operation.

WindingsConstruction and materials guarantee a long service life.

Tap changerUsed to adjust the ratio to the local voltage conditions. It can be adjusted from outside in de-energized condition.

Low-voltage bushings

High-voltage bushings

Thermometer bagImportant accessory for temperature monitoring.

TankThe TUMETIC design shown here is hermetically sealed. Elastic corrugated walls take up the volume changes of the transformer fluid.

TruckPlain transport wheels can be aligned for longitudinal or transversal movement.

ConservatorIn the TUNORMA type, it is equipped with an oil level indicator and a filling socket.

Corrosion protectionThe surface gets a multicoating in the standard color cement gray (RAL 7033). Special colors or galvanization are possible.

From oil to ester: The transformer fluid

Insulating and cooling – these tasks represent high requirements for the transformer fluid: It must be insensitive to high temperatures, to the influence of air oxygen and catalysts; furthermore it has to be resistant to aging and non-corrosive. Siemens uses the adequate fluid for each customer requirement and application:

Mineral oil, which complies with the specifications of the international standards for insulating oils, IEC publication 60296 – for distribution transformers without special require-ments.

Silicone oil, which is self-extinguishing in case of fire. Due to its high fire point above 300 °C it has been classified as a K-liquid according to IEC 61100.

Ester, which is nonhazardous to water and has a very good biodegradability. Additionally, ester offers high fire safety due to its high fire point above 300 °C, and has also been classified as a K-liquid according to IEC 61100.

9

H1

11

7

10

9

3 8

B1

2N2U2V2W

1U 1V 1W H1

10

51

7

3 8

B1

2N 2U2V2W

1U 1V 1W

Perfectly prepared for any task – whether “standard” or “special”

TUMETIC and TUNORMA – two transformer types, many applications. The standard design already covers a wide range of requirements, but as we know from long years of experience, special applications ask for special solutions. That is why Siemens offers a wide variety of useful additional solutions, assembly appliances and devices. Here are some examples:

Safe-to-touch outside-cone and inside-cone plug-in bushings on the high-voltage side instead of porcelain bushings.

Transformer connection terminals on the low-voltage side with or without covers.

Bar bushings on the low-voltage side with or without covers.

Air-filled cable connection boxes for special protection requirements.

All relevant protection and monitoring devices for distribution transformers.

2 E8

E

6

A1

4

8

EE2

9

6

A1

TUMETIC distribution transformers(hermetically sealed tank)

TUNORMA distribution transformers(with conservator)

1 Oil level indicator 2 Oil drain valve 3 Thermometer pocket 4 Buchholz relay (on order)

5 Dehydrating breather (on order) 6 Adjustment facility for tap changer 7 Rating plate (movable) 8 Earthing terminal

9 Pulling lug, ø 30 mm10 Lashing lug11 Filling tube

M1: Three Phase Transformer Back to Back · PDF file · 2012-09-19M1 Three Phase...

Documents

Transcript of M1: Three Phase Transformer Back to Back · PDF file · 2012-09-19M1 Three Phase...