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
9635/SSEPD
ISSUE 3
CUSTOMER
Scottish & Southern Energy Power Distribution
SITE
Causeway, Frome, and Ferndown 33kV substations
JOB DESCRIPTION
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
TEST ENGINEERS
TEST DATE
REPORT PREPARED BY
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)
DATE
REPORT AUTHORISED BY
DATE
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
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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.
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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.
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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.
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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.
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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.
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TABLES
Table 1 – Calibration Data for Measurement Equipment
Test
Equipment
Winding Resistance
DV Power RMO60T
Impedance
Doble M4000
Serial No.
DOB 0733
DOB 2068
Calibration Due
31/10/2017
20/08/2018
Table 2 – Causeway 33/11kV transformer 3, HV winding resistance (mohm)
Tap
A-B
A-C
1
571.9
571.3
2
564.4
564.0
3
558.0
557.4
4
550.5
550.6
5
543.9
543.9
6
536.7
536.8
7
530.5
529.6
8
523.0
522.8
9
516.3
516.0
10
508.8
508.8
11
502.3
501.9
12
495.0
495.0
13
488.5
488.0
14
480.8
480.9
15
474.0
474.5
16
467.5
467.3
17
464.7
460.9
B-C
573.5
565.9
559.4
551.9
545.3
537.9
531.2
523.7
517.0
509.8
503.1
495.6
489.0
481.1
474.4
467.6
461.3
Table 3 – Causeway 33/11kV transformer 3, LV winding resistance (mohm)
Tap
a-n
b-n
20.27
20.12
c-n
20.23
Table 4 – Causeway 33/11kV transformer 3, impedance
Tap
1
9
17
Impedance (%, 12MVA base)
12.92
12.56
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
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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
6.3
7.8
10.0
(BMAX = 1.7T)
Revised Estimate
4.1
6.0
7.4
(BMAX = 1.55T)
Table 8 – Frome C1MT, HV winding resistance (mohm)
Tap
A-N
1
198.7
2
195.7
3
192.8
4
189.8
5
187.0
6
183.8
7
181.0
8
177.8
9
175.0
10
172.0
11
169.2
12
166.1
13
163.4
14
160.4
15
157.6
16
154.6
17
152.1
B-N
198.4
195.4
192.6
189.6
186.7
183.6
180.8
177.7
174.9
171.9
169.1
166.1
163.3
160.3
157.6
154.6
152.1
C-N
198.7
195.6
192.7
189.6
186.7
183.7
180.8
177.7
174.8
171.9
169.0
166.0
163.2
160.2
157.5
154.4
151.8
Table 9 – Frome C1MT, LV winding resistance (mohm)
Tap
a-n
14.65
b-n
14.77
c-n
14.76
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Table 10 – Frome C1MT, impedance
Tap
1
9
17
Impedance (%, 15MVA base)
12.06
11.89
10.97
Table 11 – Frome C1MT, Load losses (kW at 15MVA, 75°C reference temperature)
Tap
Minimum
Expected
1
78.7
85.1
9
81.6
88.1
17
85.0
91.5
Maximum
92.6
95.7
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
Original Estimate
7.7
9.5
(BMAX = 1.7T)
Revised Estimate
5.0
7.2
(BMAX = 1.55T)
Table 14 – Ferndown C1MT, HV winding resistance (mohm)
Tap
A-N
1
158.0
2
155.8
3
153.6
4
151.6
5
149.5
6
147.8
7
145.4
8
143.4
9
141.3
10
143.2
11
145.3
12
147.3
13
149.4
14
151.9
15
154.0
16
156.1
17
158.3
11
B-N
159.2
156.3
154.2
152.1
150.1
148.0
145.9
143.8
140.8
144.5
146.6
148.7
150.8
152.9
155.0
157.2
159.3
Maximum
12.2
9.0
C-N
159.2
157.0
154.9
152.8
150.7
148.7
146.6
144.6
140.9
144.2
146.3
148.4
150.6
153.1
154.9
157.1
159.2
9635/SSEPD
Table 15 – Ferndown C1MT, LV winding resistance (mohm)
Tap
a-n
12.03
Table 16 – Ferndown C1MT, impedance
Tap
1
9
17
b-n
11.98
c-n
12.04
Impedance (%, 12MVA base)
11.73
11.16
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
Original Estimate
8.7
10.7
(BMAX = 1.7T)
Revised Estimate
5.6
8.2
(BMAX = 1.55T)
12
Maximum
13.8
10.2
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FIGURES
Figure 1 – Causeway 33/11kV transformer 3, general view
Figure 2 – Causeway 33/11kV transformer 3, nameplate
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Figure 3
Winding Resistance Results for Causeway 33/11kV Transformer
600
580
560
540
520
500
480
460
440
420
400
1
3
5
7
9
A-B
A-C
11
13
15
17
B-C
Figure 4 – Frome C1MT, general view
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Figure 5 – Frome C1MT, nameplate
Figure 6
Winding Resistance Results for Frome C1MT
210
200
190
180
170
160
150
140
1
3
5
7
9
A-N
B-N
11
13
15
17
C-N
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Figure 7 – Ferndown C1MT, general view
Figure 8
Winding Resistance Results for Ferndown C1MT
165
160
155
150
145
140
135
1
3
5
7
9
A-N
B-N
11
13
15
17
C-N
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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.
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