Reliable under exceptional network conditions
In every power supply network, transformers are strategic nodes.
They are in operation in long
distance lines and complex intermeshed power grids which have to contend with exposure to quasistationary low DC currents or even high amplitude geomagnetically induced currents (GIC) caused by recurrent solar magnetic disturbances. Siemens accepted the challenge and provides specially adapted design measures to improve transformer performance and resistance capability under exceptional network conditions.
Areas of GIC impacts
GIC is a risk to electric infrastructures worldwide. The moment, intensity and location of a GIC event cannot be precisely calculated. As the shortest solar cycle predicts every 11th year, the next GIC event is forecast for
2012–2014.
Solar wind effects:
• GIC turbulences: can last for days, with alternating intensity and polarity
• Basic GIC Level 10–50 Amp DC for minutes to hours
• Extreme GIC peaks up to 200 A DC in transformers
GIC –
Geomagnetically induced currents
Space weather events like solar winds and sunspots cause geomagnetically induced currents. Electrically charged particles in both magnetosphere and ionosphere experience enormous variations, influencing earth’s magnetic field.
• Coronal mass ejections create solar winds and charged particles
• Interaction with earths magnetosphere
• Variation of earth magnetic field of low (quasi stationary) frequency
(0.1 to 0.001 Hz)
• Solar events follow 11-year cycles
• Predicted solar cycles of higher magnitude in 2012–2014
• High latitude countries
(Canada, Scandinavia, Russia) are particularly vulnerable to magnetic field turbulence (aurora borealis) loss. Since this GIC event Siemens Transformers has researched the phenomenon and developed GIC-safe power transformer designs to reduce the risks of electric
outage caused by such events.
GIC and the grid
High voltage overhead transmission lines, earth return paths and grounded high voltage HV windings are linked to geomagnetic field changes
(dB/dt ... 200 to 400 nT/minute).
Historic GIC events – the start of Siemens’ research
In the year 1859, a solar superstorm, known as the Carrington event, was the most powerful solar storm in recorded history. Auroras were seen all around the world, telegraph systems around the world failed, pylons threw sparks and telegraph paper caught fire. On March
13th 1989 a geomagnetic storm caused the collapse of the Canadian Hydro-
Québec power grid and, in a few seconds, six million people were left without power for nine hours, with significant economic
The magnitude of GIC depends on:
• Induced voltage (up to 2 to 10 V/mile)
• Network topology
• Resistivity of soil
• High voltage transmission line resistances
• Angle between HV transmission line and magnetic field
GIC flow from grounded transformer
neutrals through the HV windings into the transmission lines with some impacts on the transformer.

EARTH SURFACE POTENTIAL
75
70
65
60
55
50
0 5 hotspot temperature in winding
10 15 20 25 30 35 time in min
40 hotspot @ 120A DC hotspot @ 200A DC
45 50 55
5,0
4,0
3,0
2,0
1,0
60
0,0
10,0
9,0
8,0
7,0
6,0
Back effects from transformer on the grid
• Higher magnetizing current – reactive power consumption up to 50 MVA
• Harmonics occur
GIC effects on transformers
• Additional noise (15 to 20% enhancement) with characteristic spectrum: odd multiple frequencies of 60 Hz in spectra ... 300 Hz; 420 Hz).
Example: Noise increase with
2 A DC/Phase is 15 dB(A)
• Tank vibrations
• Critical eddy loss heating in specific metallic parts
• Tie bars near core are most vulnerable
(even at low DC levels)
• Clamping plate, tank cover, windings are vulnerable at extremely high DC levels only
From GIC calculability to transformer design reality
Siemens Transformers started its GIC research in 1989, intensified the development of GIC safe power transformers and have been carrying out case studies and specific GIC tests since 1996. While the actual GIC event still cannot be calculated, the calculation of it’s possible impact on the transformer today is highly advanced in terms of:
• Heating of different structural parts
• Winding hotspot rise for any given
GIC on/off load cycle
• Reactive power consumption in combination with analysis
For detailed examination of different
GlC effects, two different calculation models are used. With transient FEM-3D
(finite element method) hot spots in tie plates and clamping plates could exactly be identified, where eddy current heated these metallic parts. For GIC effects on the transformer in combination with the grid, transient electromagnetic network models are required. In that case the transformer is modeled as a magnetic subsystem of the total grid to allow determination of the reactive power consumption and the harmonics of the current.
Furthermore, network models deliver the possibility to calculate eddy losses in single winding parts for a detailed winding hotspot calculation.
Verification through testing
The GIC-safe designed transformer is then put through its paces in the test lab.
GIC withstand capability verification through:
• DC and GIC injection tests with hotspot measurements
DGA analysis before and after tests with specified accep tance levels used to verify the success of the GIC-safe power transformer design
“For knowledge, too, itself is power”
In GIC cases, Siemens’ expertise is unique and ensures GIC safety to the smallest detail:
• Specific steps against critical heating caused by GIC is calculated using FEM
• GIC-safe transformers are manufactured for up to 200 A extra DC capability
• The use of non-magnetic steel inserts in tank base and clampings reduces high saturation levels at extreme DC levels, and lowers eddy current losses and prevents overheating due to lower specific losses
Trick the GIC
Since Siemens succeeded in addressing
GIC effects on power transformers, the grave danger from outer space which could seriously damage electrical supply, is no longer a great threat. To keep customers’ power grids going, the choice must be the safest transformers and shunt reactors, which endure GIC impacts even at extreme DC levels.
Published by and copyright © 2014:
Siemens AG
Energy Sector
Freyeslebenstrasse 1
91058 Erlangen, Germany
Siemens AG
Energy Sector
Power Transmission Division
Transformers
Katzwanger Strasse 150
90461 Nuremberg, Germany www.siemens.com/energy
Power Transmission Division
Order No. E50001-G640-A217-X-4A00
Printed in Germany
Dispo 19200, c4bs No. 7480
TH 101-140116 XX 472581 DB 0314X.X