LOSS ESTIMATION FOR THREE 33/11kV TRANSFORMERS AT SCOTTISH & SOUTHERN ENERGY POWER DISTRIBUTION by SIMON RYDER Addressee: MACIEJ FILA (SCOTTISH& SOUTHERN ENERGY POWER DISTRIBUTION) Registered in England & Wales 4405148 Registered Office, 5 New Street Square, London EC4A 3TW
JOB INFORMATION REPORT REFERENCE CUSTOMER SITE JOB DESCRIPTION TEST ENGINEERS TEST DATE REPORT PREPARED BY DATE REPORT AUTHORISED BY DATE 9635/SSEPD ISSUE 3 Scottish & Southern Energy Power Distribution Causeway, Frome, and Ferndown 33kV substations Loss estimation for three 33/11kV transformers Stefan DRAGOSTINOV, Simon RYDER (01/02/2016 and 03/02/2016), and Steve WARD (12/02/2016) 01/02/2016 Causeway 03/02/2016 Frome 12/02/2016 Ferndown Simon RYDER 18/02/2016 (ISSUE 1); 07/03/2016 (ISSUE 2); 14/03/2016 (ISSUE 3) Stefan DRAGOSTINOV (ISSUE 1) John LAPWORTH (ISSUE 2) Richard HEYWOOD (ISSUE 3) 23/02/2016 (ISSUE 1); 10/03/2016 (ISSUE 2); 15/03/2016 (ISSUE 3) 2 9635/SSEPD
TABLE OF CONTENTS JOB INFORMATION... 2 INTRODUCTION... 4 CONCLUSIONS... 4 LOSS ESTIMATION METHODS... 4 RESULTS... 7 TABLES... 9 FIGURES... 13 CIRCULATION... 17 3 9635/SSEPD
INTRODUCTION Following initial discussions at Scottish & Southern Energy Power Distribution (SSEPD) offices in Reading on 09/11/2015, Doble PowerTest were asked to make a trial of a proposed method for loss estimation on three 33/11kV transformers at SSEPD primary sub-stations. The measurements were made between 01/02/2016 and 12/01/2016. This report presents an explanation of the proposed method and results of the trials. It was revised and corrected following closing discussions at SSEPD offices in Reading on 04/03/2016. CONCLUSIONS Based on the proposed method, Doble PowerTest estimate the following losses for the three transformers included in the trial: Causeway 33/11kV transformer 3 (Brush serial no. 81976/1, built 2007) Load loss on tap 9 (12MVA base, 75 C reference temperature) 79.8kW No-load loss at rated excitation 6.0kW Frome C1MT (Brush serial no. 82274/1, built 2008) Load loss on tap 9 (15MVA base, 75 C reference temperature) 88.1kW No-load loss at rated excitation 7.2kW Ferndown C1MT (Tironi serial no. E6601, built 1998) Load loss on tap 9 (12MVA base, 75 C reference temperature) 47.6kW No-load loss at rated excitation 8.2kW LOSS ESTIMATION METHODS Origins of Transformer Losses Transformer losses can be divided into no-load losses, which are present whenever the transformer is energised, and load losses, which vary as the square of the load current. No-load losses are essentially caused by magnetisation of the transformer core. Load losses have a variety of causes, of which losses owing to the winding resistance make up the largest part (approximately 80% on modern designs). Other causes of load losses include currents induced by the leakage flux in the windings and also in the core, frame, and tank. Transformer Losses over the Years In general, losses in medium and large power transformers used in the transmission and distribution networks have reduced over the years. There has been increased pressure on users to specify transformers with lower losses. Alternatively, there has been increased pressure on users to specify high loss capitalisation values for total cost of ownership calculations. To produce transformers with lower losses, manufacturers have worked to improve design and construction. Additionally, manufacturers have worked with suppliers to develop new materials. 4 9635/SSEPD
Major improvements in design, construction, and materials include: No-load loss: Improved core steel Improved slitting and cutting of core steel Elimination of through bolts Use of so-called step-lap joints between limbs and yokes Reduced flux density Reduced internal clearances (and hence reduced mass) Load loss Use of continuously transposed conductors Single point earthing Use of flux shields Use of flux shunts Reduced internal clearances (and hence reduced mass) Compared with designs produced 50 years ago, contemporary designs typically have 30-50% of the noload losses and 40-70% of the load losses. Most of these changes have taken place gradually. One important exception is the reduction in no-load losses caused by use of reduced flux densities, which was the result of a change in specifications from 1966. As was mentioned above, no-load losses are essentially caused by magnetisation of the transformer core, and are thus strongly dependent on the properties of the core material. Early transformers had hot-rolled steel cores, the properties of which deteriorate with use. No-load losses in such transformers are likely to increase in service. Hot-rolled steel has been replaced by cold-rolled steel in modern designs, the properties of which do not deteriorate with use. Cold-rolled steel was first patented by Norman GOSS in 1935, and is highly likely to be used in any designs manufactured after the patent expired in 1955. (It may also be used in some earlier designs). As was also mentioned above, load losses have a variety of causes, of which losses owing to the winding resistance make up the largest part. Substantially all transformers have windings manufactured from copper, the properties of which do not deteriorate with use. It follows that load losses in such transformers will not substantially increase in service. Measurement of Losses At manufacturer s works, no-load losses are measured in the test laboratory by exciting the transformer using a powerful variable voltage supply (usually a rotary convertor, but static convertors are also used). Voltage and current are measured using highly accurate instrument transformers class 0.1 or better. Losses are then measured using a highly accurate power analyser. 5 9635/SSEPD
At manufacturer s works, load losses are usually measured in the test laboratory by short-circuiting one winding and supplying the other, using a powerful variable voltage supply. Voltage and current are measured using highly accurate instrument transformers class 0.1 or better. Losses are then measured using a highly accurate power analyser. Accurate determination of the load losses is particularly challenging owing to the low power factor of the transformer in the measurement configuration. Estimating Losses in the Field In the field, it would be very difficult to duplicate the methods for measuring transformer losses at the test laboratories in manufacturer s works. For smaller transformers, it might be possible to transport the transformer to a suitable test laboratory for measurement. For larger transformers, this would also be very difficult. To overcome difficulties in measuring losses on transformers in the field, Doble PowerTest propose to make measurements of some parameters using portable test equipment and then estimate the losses from these results. These estimates can be refined based on available design data, e.g. masses recorded on the nameplate or in the operating/maintenance manual. Measurements required are as follows: Winding resistance Impedance (mainly to confirm values marked on the nameplate) Load loss estimated using the following methodology: Winding resistance losses are calculated directly from measured winding resistance and currents, based on nameplate values of rated power. Losses induced in the windings by the leakage flux (so-called winding eddy current losses) are estimated from the winding resistance losses using a dimensionless multiplication factor based on medium power transformer designs reviewed by Doble PowerTest over the last 5-10 years. Analysis of 33 different designs by 10 different manufacturers suggest that this multiplication factor is in the range 0.030pu to 0.133pu with an average value of 0.086pu. Losses induced in other parts of the transformer by the leakage flux (so-called other eddy current losses or stray losses) are estimated from the reactive power absorption of the transformer using a multiplication factor. The reactive power absorption is calculated from the impedance. As for winding eddy current losses, the multiplication factor is based on medium power transformer designs reviewed by Doble PowerTest over the last 5-10 years. Analysis of 33 different designs by 10 different manufacturers suggest that this multiplication factor is in the range 0.67kW/Mvar to 4.96kW/Mvar with an average value of 2.72kW/Mvar. Load loss estimates are thus made using the following formula: = + 1+ + % 100 where k1 is the dimensionless multiplication factor for winding eddy current losses and k2 is the multiplication factor for other eddy current losses. 6 9635/SSEPD
No-load loss estimates are based on the core mass. If no value for the core mass is available, this is assumed to be 60% of the core and winding assembly mass (sometimes also referred to as the untanking mass). This is multiplied by specific losses from manufacturer s data. For the first issue of this report, rated excitation was assumed to be 1.7T. This was found to be incorrect, following a review of results and a subsequent review of specifications. For the second issue of this report, rated excitation is now assumed to be 1.55T. Rated losses of available core lamination grades are in the range 0.75W/kg to 1.45W/kg at 1.7T. Alternatively, rated losses of available core lamination grades are in the range 0.50W/kg to 1.05W/kg at 1.55T. Lower values are used for more modern designs, to reflect changes in design practices and improvements in core lamination grades. This is also multiplied by a dimensionless multiplication factor often known as the building factor, to reflect additional losses caused by slitting and cutting the laminations; stacking the laminations; and especially partial saturation of laminations at the joints between the limbs and yokes. For modern designs with step-lap joints and without through bolts, the building factor is typically 1.1pu. For older designs the building factor may be as high as 1.5pu. No-load loss estimates are thus made using the following formula: = where ps is the specific loss and k3 is the dimensionless multiplication or building factor for no-load losses. RESULTS Causeway 33/11kV Transformer 3 Causeway 33/11kV transformer 3 is a 12/24MVA continuous emergency rated transformer built by Brush in 2007, serial no. 81976/1. Figure 1 shows a general view of Causeway 33/11kV transformer 3. Figure 2 shows a close-up view of the nameplate. Calibration data for the test equipment used is listed in Table 1. Results of winding resistance results are listed in Table 2 (HV) and Table 3 (LV). Selected results are also shown in graphical form in Figure 3. Ambient temperature at the time of the measurements was 13 C. Results of impedance measurements are listed in Table 4. Based on the measurement results, Doble PowerTest estimate that load losses for Causeway 33/11kV transformer 3 on tap 9 at 12MVA are 79.8kW at 75 C reference temperature. Load losses on other tap positions and also minimum and maximum expected values of load losses are listed in Table 5. Measured load losses from works test on tap 9 at 12MVA are 84.3kW at 75 C reference temperature. Measured no-load losses on other tap positions are listed in Table 6. Based on the untanking mass recorded on the nameplate and their experience with transformers manufactured around the same time, Doble PowerTest estimate that no-load losses for Causeway 33/11kV transformer 3 are 6.0kW at rated excitation. Minimum and maximum expected values of noload losses are listed in Table 7. Measured no-load losses are 5.7kW at rated excitation. 7 9635/SSEPD
Frome C1MT Frome C1MT is a 15/30MVA continuous emergency rated transformer built by Brush in 2008, serial no. 82274/1. Figure 4 shows a general view of Frome C1MT. Figure 5 shows a close-up view of the nameplate. Calibration data for the test equipment used is listed in Table 1. Results of winding resistance results are listed in Table 8 (HV) and Table 9 (LV). Selected results are also shown in graphical form in Figure 6. Ambient temperature at the time of the measurements was 7 C. Results of impedance measurements are listed in Table 10. Based on the measurement results, Doble PowerTest estimate that load losses for Frome C1MT on tap 9 at 15MVA are 88.1kW at 75 C reference temperature. Load losses on other tap positions and also minimum and maximum expected values of load losses are listed in Table 11. Measured load losses from works test on tap 9 at 15MVA are 92.6kW at 75 C reference temperature. Measured no-load losses on other tap positions are listed in Table 12. Based on the untanking mass recorded on the nameplate and their experience with transformers manufactured around the same time, Doble PowerTest estimate that no-load losses for Frome C1MT are 7.2kW at rated excitation. Minimum and maximum expected values of no-load losses are listed in Table 13. Measured no-load losses are 7.1kW at rated excitation. Ferndown C1MT Ferndown C1MT is a 12/24MVA continuous emergency rated transformer built by Tironi in 1998, serial no. E6601. Figure 7 shows a general view of Frome C1MT. The nameplate was illegible. Information on the untanking mass etc. was obtained from a drawing in the operation and maintenance manual. Calibration data for the test equipment used is listed in Table 1. Results of winding resistance results are listed in Table 14 (HV) and Table 15 (LV). Selected results are also shown in graphical form in Figure 8. Ambient temperature at the time of the measurements was 6 C. Results of impedance measurements are listed in Table 16. Based on the measurement results, Doble PowerTest estimate that load losses for Ferndown C1MT on tap 9 at 12MVA are 47.6kW at 75 C reference temperature. Load losses on other tap positions and also minimum and maximum expected values of load losses are listed in Table 17. Measured load losses from works test on tap 9 at 15MVA are 43.9kW at 75 C reference temperature. Measured no-load losses on other tap positions are listed in Table 18. Based on the untanking mass recorded on the nameplate drawing and their experience with transformers manufactured around the same time, Doble PowerTest estimate that no-load losses for Ferndown C1MT are 8.2kW at rated excitation. Minimum and maximum expected values of load losses are listed in Table 19. Measured no-load losses are 8.6kW at rated excitation. 8 9635/SSEPD
TABLES Table 1 Calibration Data for Measurement Equipment Test Equipment Serial No. Calibration Due Winding Resistance DV Power RMO60T DOB 0733 31/10/2017 Impedance Doble M4000 DOB 2068 20/08/2018 Table 2 Causeway 33/11kV transformer 3, HV winding resistance (mohm) Tap A-B A-C B-C 1 571.9 571.3 573.5 2 564.4 564.0 565.9 3 558.0 557.4 559.4 4 550.5 550.6 551.9 5 543.9 543.9 545.3 6 536.7 536.8 537.9 7 530.5 529.6 531.2 8 523.0 522.8 523.7 9 516.3 516.0 517.0 10 508.8 508.8 509.8 11 502.3 501.9 503.1 12 495.0 495.0 495.6 13 488.5 488.0 489.0 14 480.8 480.9 481.1 15 474.0 474.5 474.4 16 467.5 467.3 467.6 17 464.7 460.9 461.3 Table 3 Causeway 33/11kV transformer 3, LV winding resistance (mohm) Tap a-n b-n c-n 20.27 20.12 20.23 Table 4 Causeway 33/11kV transformer 3, impedance Tap Impedance (%, 12MVA base) 1 12.92 9 12.56 17 11.82 Table 5 Causeway 33/11kV transformer 3, Load losses (kw at 12MVA, 75 C reference temperature) Tap Minimum Expected Maximum 1 69.8 76.0 83.2 9 73.4 79.8 87.1 17 78.0 84.4 91.8 9 9635/SSEPD
Table 6 Causeway 33/11kV transformer 3, Load losses (kw at 12MVA, 75 C reference temperature) Tap Works Test 1 78.9 9 84.3 17 85.6 Table 7 Causeway 33/11kV transformer 3, No-load losses (kw at rated excitation) Flux Density Minimum Expected Maximum Original Estimate (B MAX = 1.7T) 6.3 7.8 10.0 Revised Estimate (B MAX = 1.55T) 4.1 6.0 7.4 Table 8 Frome C1MT, HV winding resistance (mohm) Tap A-N B-N C-N 1 198.7 198.4 198.7 2 195.7 195.4 195.6 3 192.8 192.6 192.7 4 189.8 189.6 189.6 5 187.0 186.7 186.7 6 183.8 183.6 183.7 7 181.0 180.8 180.8 8 177.8 177.7 177.7 9 175.0 174.9 174.8 10 172.0 171.9 171.9 11 169.2 169.1 169.0 12 166.1 166.1 166.0 13 163.4 163.3 163.2 14 160.4 160.3 160.2 15 157.6 157.6 157.5 16 154.6 154.6 154.4 17 152.1 152.1 151.8 Table 9 Frome C1MT, LV winding resistance (mohm) Tap a-n b-n c-n 14.65 14.77 14.76 10 9635/SSEPD
Table 10 Frome C1MT, impedance Tap Impedance (%, 15MVA base) 1 12.06 9 11.89 17 10.97 Table 11 Frome C1MT, Load losses (kw at 15MVA, 75 C reference temperature) Tap Minimum Expected Maximum 1 78.7 85.1 92.6 9 81.6 88.1 95.7 17 85.0 91.5 99.0 Table 12 Frome C1MT, Load losses (kw at 15MVA, 75 C reference temperature) Tap Works Test 1 86.5 9 92.6 17 91.6 Table 13 Frome C1MT, No-load losses (kw at rated excitation) Flux Density Minimum Expected Maximum Original Estimate (B MAX = 1.7T) 7.7 9.5 12.2 Revised Estimate (B MAX = 1.55T) 5.0 7.2 9.0 Table 14 Ferndown C1MT, HV winding resistance (mohm) Tap A-N B-N C-N 1 158.0 159.2 159.2 2 155.8 156.3 157.0 3 153.6 154.2 154.9 4 151.6 152.1 152.8 5 149.5 150.1 150.7 6 147.8 148.0 148.7 7 145.4 145.9 146.6 8 143.4 143.8 144.6 9 141.3 140.8 140.9 10 143.2 144.5 144.2 11 145.3 146.6 146.3 12 147.3 148.7 148.4 13 149.4 150.8 150.6 14 151.9 152.9 153.1 15 154.0 155.0 154.9 16 156.1 157.2 157.1 17 158.3 159.3 159.2 11 9635/SSEPD
Table 15 Ferndown C1MT, LV winding resistance (mohm) Tap a-n b-n c-n 12.03 11.98 12.04 Table 16 Ferndown C1MT, impedance Tap Impedance (%, 12MVA base) 1 11.73 9 11.16 17 10.70 Table 17 Ferndown C1MT, Load losses (kw at 12MVA, 75 C reference temperature) Tap Minimum Expected Maximum 1 41.3 46.0 51.3 9 42.9 47.6 52.9 17 52.6 57.5 63.2 Table 18 Ferndown C1MT, Load losses (kw at 12MVA, 75 C reference temperature) Tap Works Test 1 42.9 5 43.1 9 43.9 12 45.4 17 52.3 Table 19 Ferndown C1MT, No-load losses (kw at rated excitation) Flux Density Minimum Expected Maximum Original Estimate (B MAX = 1.7T) 8.7 10.7 13.8 Revised Estimate (B MAX = 1.55T) 5.6 8.2 10.2 12 9635/SSEPD
FIGURES Figure 1 Causeway 33/11kV transformer 3, general view Figure 2 Causeway 33/11kV transformer 3, nameplate 13 9635/SSEPD
Figure 3 600 Winding Resistance Results for Causeway 33/11kV Transformer 580 560 540 520 500 480 460 440 420 400 1 3 5 7 9 11 13 15 17 A-B A-C B-C Figure 4 Frome C1MT, general view 14 9635/SSEPD
Figure 5 Frome C1MT, nameplate Figure 6 210 Winding Resistance Results for Frome C1MT 200 190 180 170 160 150 140 1 3 5 7 9 11 13 15 17 A-N B-N C-N 15 9635/SSEPD
Figure 7 Ferndown C1MT, general view Figure 8 165 Winding Resistance Results for Ferndown C1MT 160 155 150 145 140 135 1 3 5 7 9 11 13 15 17 A-N B-N C-N 16 9635/SSEPD
CIRCULATION Doble PowerTest UK Stefan Dragostinov Richard Heywood (File) Scottish and Southern Energy Power Distribution Maciej Fila Sarah Rigby Alistair Steele Copyright Doble PowerTest All rights reserved. No part of this publication may be produced, stored in a retrieval system, or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of Doble. Indemnity This document may not be distributed or used outside the client for whom it is prepared, except with written authorisation from Doble. Doble disclaims all liability for any loss, damage, injury or other consequence whatsoever arising from any unauthorised use howsoever caused, including any such resulting from error, omission or negligence in its application. 17 9635/SSEPD