comparison of amorphous & crgo core transformer · pdf filecomparison of amorphous &...

Comparison of Amorphous & CRGO Core Transformer Losses Under Nonlinear Load Condition

Vinod Gupta, O. Naresh Kumar, K. Ashok Kumar*, U C Trivedi, T P Govindan Electrical Research and Development Association Vadodara, * Vijai Electricals Ltd. Hyderabad

Abstract: This project was undertaken in order to evaluate and compare the performance of transformer especially in terms of losses of transformers made up of two different core materials namely, CRGO and Amorphous Metal under Non-linear (harmonic) conditions. Where the criteria for evaluation of transformer load losses, while feeding non-linear load current are, well defined in IEEE Std C57.110-1998, similar criteria do not exist for measurement core losses. In this project, an effort has been made to evolve a methodology for computation of core losses with distorted voltage waveform, which is independent of design data. The benefits are to utilities in selection of site-specific optimized performance of transformer. In this paper comparative study was carried out for CRGO & Amorphous core transformer in terms of losses under harmonic conditions. Various experiments were conducted on two transformers of (1 phase, 10 kVA, 11/0.240 kV & 3 Phase 25 kVA, 11/0.433 kV) having different cores (Amorphous and CRGO). The coil construction was same in both the cases. The rresults are presented in the paper.

INTRODUCTION: The performance of transformer, the most ancient power system static element, is affected due to presence of distortion in the input supply voltage and non-linearity in the load current. Due to extensive use of modern power electronics controlled devices, the degree of non-linearity in the load current has increased in recent years. This has resulted in substantial increase in two of the major loss components namely core and coil losses. It is well known that transformer efficiency is as high as 99.5%, where as efficiency decreases manifold in above situation. While the criteria for evaluation of the transformer load losses while feeding non-linear load current are well defined in IEEE Standard C57 110, 1998 [6] , the same is not true for the core losses. Journals7 and Handbooks[4]-[5] gives formulae which requires the design data like thickness of the lamination, t, mass of the core, m, resistivity,ρ, density of the core material, δ, coefficient of hysteresis loss, σh and value of

Steinmetz constant, n; the resultant delivers specific core loss only. The actual value of core loss can only be computed if the value of Building Factor (which ranges between 1.07 to 1.29) [5] is known, which depends on geometry of construction, stacking factor, mass of the core not in the rolling direction and its angle with respect to rolling direction.

The journals, handbooks and standards use proportion of Hysteresis Loss to Eddy Current Loss as 50:50 for cold rolled material. During the sets of experimentation conducted at Author’s laboratory, it is found that this ratio differs depending upon the material and geometrical configuration of the core. So there is a need to develop a novel method for evaluation of core losses of Distorted Input Voltage Fed Transformer. The methodology evolved finds the coefficients of Hysteresis and Eddy Current Loss with distorted waveform. This methodology is independent of design data. The methodology finds certain constants which remain valid for all the transformers with same design facing any combination of distortions in voltage and current. The benefits are to utilities in selection of site specific optimized performance of transformer.

ERDA has developed a simple method to find the empirical constants required to evaluate core losses of a Distorted Voltage Fed Transformer through experimental results taking design value of flux density as input. The expression model is as below:

(1)

For a given design, the empirical co-efficient so derived in above mathematical model through sets of experiments are valid for any given situation having different combinations of voltage and current distortions.

The above mathematical model is independent of design variables such m, ρ, δ, t, coefficient of

W = k1 . Bsp . f + k2 . Bs

q . f m

16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 1

Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.

hysteresis loss, σh used in theoretical formula [9] given below:

(2)

METHODOLOGY The authors` laboratory has got manufactured model transformers of 1 phase, 10 kVA, 11/0.240 kV rating, for experiments. The core material used is CRGO in one transformer and Amophorous Magnetic Alloy in other.

The core loss was measured using 0.03% accurate Power Analyzer at 50, 60, 70, 80, 90 and100 Hz by maintaining constant rated flux density.

For computation of various constant we write eq. (1) as ::

(3)

W= mff ⋅+⋅ βα (4)

So, 1−⋅+= mff

W βα (5)

So, by plotting equation (5) the constants `α’ & `β’ are available from curve fitting, the value of m is calculated by substituting the values of `α’ & `β’ in equation (5).

A different set of experiment with half the rated flux density was done at same set of frequencies as mentioned above to obtain two different values of constant α; α 1 and α2 and constant β; β1 and β2 which is obtained from equations (3) and (4) as

psBk 111 ⋅=α (6)

psBk 212 ⋅=α (7)

Where Bs1 and Bs2 are the rated and half the rated flux density.

Taking ratio, p

s

sp

s

ps

BB

BkBk

⎥⎦

⎤⎢⎣

⎡=

⋅

⋅=

2

1

21

11

2

1

αα

(8)

⎟⎠⎞⎜

⎝⎛

⎟⎠⎞⎜

⎝⎛

=∴

2

1

2

1

log

log

s

sB

Bp

αα

(9)

Similarly, q

sBk 121 ⋅=β (10)

qsBk 222 ⋅=β

(11)

Taking ratio, q

s

sq

s

qs

BB

BkBk

⎥⎦

⎤⎢⎣

⎡=

⋅

⋅=

2

1

22

12

2

1

ββ

(12)

⎟⎠⎞⎜

⎝⎛

⎟⎠⎞⎜

⎝⎛

=∴

2

1

2

1

log

log

s

sB

Bq

ββ

(13)

k1 and k2 are calculated by substituting the value of p & q in equations (6) and (10) respectively.

Once the various constants like k1, k2, p, q & m have been found, it is possible to find separately Hysteresis & Eddy current loss for any given distortion/situation of input voltage.

EXPERIMENTS & RESULTS: Based on previous experience through experiments, specific transformers were designed and manufactured with identical coil (load) losses with CRGO and Amorphous Metal cores with 3 phase, 25 kVA, 11000/433 Volts transformer & 1 phase, 10 kVA, 11000/240 Volts ratings. This helped to assess the increase in losses due to core and eddy current losses. Authors have measured no-load losses of 10 kVA, 11000/240 Volts transformer CRGO and Amorphous Core Transformer (AMT) having identical coil at different frequencies and flux densities. Dedicated and sophisticated instruments are used, having measurement accuracy of 0.03% over entire frequency range of interest. A new mathematical model is developed at author’s laboratory for evaluation of transformer core losses in situation where supply voltage is distorted.

W = σh . Bsn . f . m + 1.65 Bs

2 . f2 . t2 . m ρ . δ

W = k1 . Bs p . f + k2 . Bs

q . f m



In this work, characterization & core loss analysis was conducted for transformer with CRGO and Amorphous cores having specification of 3phase, 25 kVA, 11000/433 Volts, & 1 phase, 10 kVA, 11000/240 Volts rating. Using method described above, the results obtained on CRGO and AMT core transformer are tabulated below in Table No 1:

Table No: 1 Results of Final Model

Point wise discussion on the result is as below:

(1) Constant p The Hysterisis loss

mfBorfBkfW nSh

pShy ⋅⋅⋅⋅=⋅= σα 1

(14)

Where p (or n) = Steinmetz constant

Standards or handbook do not give value of p (or n) for Amorphous Magnetic Alloy, and the same for cold rolled was originally taken as 1.6 but with modern materials and higher flux densities, this value of p can vary from 1.6 to 2.5 or higher [5]. The value p (Steinmetz constant) obtained through experimental work as varying between 1.9 and 2.4.

(2) Constant q

Value of `q’, power to flux density while computing Eddy Current Losses are given as `2’ in all standards & reference handbook [4]-[5]. Whereas value obtained through experimental analysis is varying from 1.8 to 2.2.

(3) Constant m

Value of m, power to frequency while computing Eddy Current Loss is given as 2 in all national / international standards, journals & handbooks4,5,7. The value obtained is also nearly 2.

It appears that the numerical value assigned in various standards, journals and handbooks to constants p, q, and m are valid for computation of specific core loss only. The values obtained through experiments have in-built the effect due to Building Factor which itself is ranging from 7% to 29%.

Site survey was conducted to select the site where maximum THD in voltage & current are present so that comparative evaluation of transformer can be possible & later on same was simulated in the laboratory. A total 380 nos. of Sites were surveyed as per Table No 2.

Maximum of 25% voltage THD (at 415 volts) & 76% of THD at 67 amp of load was observed

At Industrial sites, CRGO & Amorphous core transformer were evaluated for its losses. The result for industrial site is shown in Table No 3.

Table No 2- No of Sites Surveyed

The industrial site results are simulated in the laboratory & results obtained are given in Table No 4.

Final Model: W=k1Bspf + k2Bsqfm Constants

CRGO AMT

10 kVA 25 kVA 10 kVA 25 kVA k1 0.0118 0.0056 0.0073 0.0040

k2 0.0003 0.0002 0.00007 0.00003

p 2.2739 2.3985 1.9444 2.1053q 1.9235 2.0074 1.8193 2.1220 m 1.9998 2.0000 2.0000 2.0003 Bs 1.5900 1.5400 1.4000 1.4000

Sr. No

Types of Load

Gujarat Maharashtra

Andhra Pradesh

01 Agricultural 30 23 1

02 Commercial 23 26 21

03 Domestic 11 25 5

04 Industrial 105 25 44

05 Mix 16 20 5

Total 185 119 76

Grand Total 380



Table No 3- Industrial Site Testing Results Industrial site testing results for 3-phase 25 kVA transformers with %Vthd %Ithd

14.70 27.45

AMT CRGO

Parameters (1) Linear loss at rated condition (2)

Nonlinear load at site (3)

Linear loss at equivalent condition (4)

Linear loss at rated condition (5)



Voltage (volts) 433.00 384.49 384.49 433.00 390.66 390.66

% loading 100.00 56.30 56.30 100.00 61.95 61.95

at Temp °C 75.00 42.50 42.50 75.00 45.00 45.00

No Load Loss (Watt) 26.90 19.58 85.10 62.82

Full Load Loss (Watt) 559.21 282.03 565.00 317.68

Total (Watt) 586.11 304.70 301.61 650.10 401.40 380.50

Increase in Watt 3.09 20.90

% increase in loss 1.03 5.49

% savings in case CRGO if replaced With AMT at Identical conditions (6-3) 24.09

% savings in AMT compare to CRGO under harmonic condition ((20.90-3.09)/20.90) 85.20

1. Increase in AMT from linear load to nonlinear load at same voltage, loading and temperature (3-4).

2. Increase in CRGO from linear load to nonlinear load at same voltage, loading and temperature (6-7).

3. Increase in total losses if AMT replaces CRGO in nearly identical condition (6-3).

This increase will be much more had the load be near to rated condition and system giving rated voltage.

Table No 4 Laboratory simulation Results of Industrial sites Laboratory simulation Results of Industrial sites for 3-phase 25 kVA transformers with %Vthd %Ithd

20.86 # 26.23

AMT CRGO

Parameters (1) Linear loss at rated condition (2)



Linear loss at rated condition (5)



Voltage (volts) 433.00 390.97 390.97 433.00 389.31 389.31

% loading 100.00 58.58 58.58 100.00 58.39 58.39

at Temp °C 75.00 27.00 27.00 75.00 27.00 27.00

No Load Loss (Watt) 26.90 19.58 85.10 62.82

Full Load Loss (Watt) 559.21 277.18 565.00 281.24

Total (Watt) 586.11 319.80 296.76 650.10 389.40 344.06

Increase in Watt 23.04 45.34

% increase in loss 7.77 13.18

% savings in case CRGO if replaced With AMT at Identical conditions (6-3) 17.87

% savings in AMT compare to CRGO under harmonic condition ((45.34-23.04)/45.34) 49.18

1. Increase in AMT from linear load to nonlinear load at same voltage, loading and temperature (3-4).

2. Increase in CRGO from linear load to nonlinear load at same voltage, loading and temperature (6-7).

3. Increase in total losses if AMT replaces CRGO in nearly identical condition (6-3).

This increase will be much more had the load be near to rated condition and system giving rated voltage.

# Vthd increases due to back to back connection



Temperature rise test was done for 25 kVA transformer with actual loading under harmonic conditions. The results of temperature rise test are as given in Table No 5. Table No 5: Temperature Rise test results with linear &

non-linear load

CONCLUSIONS: The method developed at authors’ laboratory is able to in-built the effect of Building Factor, the value of which is having large band from 7% to 29%. Also the computation of core losses at higher distortions is possible. Losses under harmonic condition in amorphous core are lower in comparison to CRGO core under non linear loss condition & same was verified in laboratory simulation & temperature rise test.

ACKNOWLEDGEMENT: The Authors are thankful to Vijai Electrical Ltd, Hyderabad for sponsoring research project. Thanks are also due to all the support extended by Dr. M. Ramamoorty, Ex-Director, ERDA

REFERENCES: 1. IS : 2026 – 1977,`Specification for Power

Transformers

2. CBIP Test Manual for Transformers, Section J,` Specification for Power and Distribution Transformers`

3. Transformer No Load Losses with Distorted Voltage Waves, The Brown Boveri Review December 1960 Vol. 47, No. 12, pp 875- 892

4. Transformers, 2nd Edition, BHEL, Publisher, Tata McGraw Hill Publishing Co. Ltd.

5. Large Power Transformers, K. Karsai etal, Publisher ELSEVIER 1987

6. IEEE C57.110:1998 ` IEEE Recommended Practice for Establishing Transformer Capability when Supplying Non-Sinusoidal Load Currents

7. The J & P Transformer Book, 12th Edition, Publisher Newnes 1998

8. Power Transformer Handbook, Edited by Bernard Hochart, Alsthom Transformer Division, France, Publisher, Butterworths

9. Richard Boll , “Soft Magnetic Materials: fundamentals, alloys, properties, products, applications.”, Heyden & Sons, Edison 1979

CRGO AMORPHOUS

27% THD

<1.5 % THD

27% THD

<1.5 % THD

Rise in Oil Temperature, ˚C

28.48 25.6 24.17 23.01

HV winding, ˚C 51.77 45.3 41.99 39.67

LV Winding, ˚C 48.75 43.18 42.91 39.94



comparison of amorphous & crgo core transformer · pdf filecomparison of amorphous &...

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