1
Tutorial on IEEE Std. C57.158 Guide for Application of Tertiary and Stabilizing Windings IEEE PES Transformers Committee Performance Characteristics Subcommittee Jacksonville, FL, USA October 18, 2018 Enrique Betancourt R. Xose M. Lopez-Fernandez Krishnamurthy Vijayan Hamid Abdelkamel
2
Agenda 1. Scope of the Guide and Systems Engineering Framework– E. Betancourt 2. Function and Application of tertiary and Stabilizing Windings – Dr. X. Lopez-Fernandez 3. Specification and Testing of Tertiary and Stabilizing Windings – K. Vijayan 4. Utility Examples – H. Abdelkamel 5. Conclusion and further work- Enrique
3
1. Scope of the Guide and Systems Engineering Framework Enrique Betancourt
4
Scope and Purpose for the Guide •
Scope – To address application of tertiary and stabilizing windings in liquid-immersed power transformers, as covered by IEEE Std C57.12.00 – Also, provide recommendations to evaluate the need or convenience of having such windings – Primary application is for transformers and autotransformers with wye-wye-connected windings, with or without a delta-connected tertiary or stabilizing winding – The guide does not address tertiary windings in conventional delta-wye, or delta-delta connected transformers.
•
Purpose – This guide provides users with a conceptual framework and recommendations for the specification, application, and performance evaluation of tertiary and stabilizing windings
5
Need for a Guide • Tertiary and stabilizing windings are often confused in function and application • Need for a stabilizing winding not clearly defined in standards for transformers • Thermal rating of stabilizing windings can be ambiguously interpreted from specifications • Continuous rating of tertiary windings is frequently over-specified • Practical concepts and recommendations to simplify SW’s and TW’s testing and application were not included in a single source
6
Types of Y-connected transformers • Transmission, interconnection transformers and autotransformers – Can have a tertiary to supply local loads – Some users do not apply TW or even SW (for any type of core)
• Primary substation transformers – Normally have SW, not TW – Some specifications don’t call for SW (three legged cores)
• Windfarm collector transformers – Some with TW, most with SW, some others with none of them Note- Two-primary step-up transformers, or two-secondary step-downs are not in the scope of the guide.
7
Power systems design sequence • • • • • • • •
Start with identification of loads and available sources Choice of voltages and basic layout (circuits, substations) Load studies to define conductor sizes based on allowable losses and voltage drop Choice of system grounding Z0? Short circuit studies to confirm selection of breaker interrupting ratings Load flow and motor starting studies to define conductor and transformer ratings and to confirm that bus voltages are within acceptable values System studies to define BIL based on transient and continuous overvoltage Specification of components (transformers, conductors, breakers, etc.)
8
System grounding (1) • Grounding: the intentional connection of a phase or neutral conductor to earth • It provides control of voltage on live lines during ground faults, as well as detectable current to operate protection devices • Transformers’ functions in power systems – – – –
Convert voltage VH:VL Decouple grounding systems Z0? Convert phase angle Combine input power sources, or split output power among different loads
9
System grounding (2)
Grounding systems (areas) [See IEEE Std. 142 1991])
Grounding of neutral in power systems
10
Line to ground faults
VNEUTRAL- GROUND = f ( Z0/Z1 ) Voltages to ground under steady-state conditions [See IEEE Std. 242 2001]
VL-G = VL-N + VNEUTRAL-GROUND
11
Effectively grounded systems • The system is grounded at one of the lines or neutral, to avoid overvoltages beyond established limits during line to ground faults • By convention, “effectively grounded” requires that for every point at the system: Z0/Z1 < 3, and R0/Z1 < 1 • For a neutral-grounded system that means: VLG < 0.8 VLL = 1.4 PU (See IEEE Std. C57.12.80)
12
Analysis of 3-phase balanced systems • Balanced power systems can be represented by one single-phase equivalent circuit • In IEEE C57 series, most references to Transformers’ operating conditions are actually based on the three-phase balanced case [See IEEE Std. 399-1997]
13
Unbalanced systems • Circuit analysis: Mesh and Node equations using Kirchhoff’s Laws • Network analysis with impedance/admittance matrix transformations • Symmetrical components Three-phase system
Neutral conductor Self impedance
Mutual impedance
General three-phase network with self and mutual impedances
[Figure from Happoldt, H., Oeding, D., “Elektrische Kraftwerke und Netze”, Springer, 1978.]
14
Symmetrical components Reduced system equations:
• The symmetrical three-phase network can be analyzed as three independent “sequence” networks • Every component has three “sequence” equivalent circuits • In transformers, sequence 1 and 2 are equal, if they have similar magnetic and electrical parameters
•
Faults or unbalanced loads can be represented by interconnection of the sequence networks
[Figure from Happoldt, H., Oeding, D., “Elektrische Kraftwerke und Netze”, Springer, 1978.]
15
Not practical for symmetrical components • Asymmetrical built components •
Non-transposed transmission lines
•
Asymmetrical transformer banks
•
Unbalanced ground capacitances and resonance phenomena
• Exciting current phenomena • Transients analysis Example of more general tools: EMTP, ATP computer codes
16
Z0 on Y-connected transformers with no delta (C57.12.80) zero-sequence impedance: An impedance voltage measured between a set of primary terminals and one or more sets of secondary terminals when a single-phase voltage source is applied between the three primary terminals connected together and the primary neutral, with the secondary line terminals shorted together and connected to their neutral (if one exists).
Actually, a “zero-sequence shortcircuit impedance”, as opposed to a “zero-sequence excitation impedance”.
[See Alexander, G.W, McNutt, W.J., “EHV Application of Autotransformers”, IEEE T PAS, 1967.]
17
Transformers zero sequence impedance circuits
18
Unbalanced loading of transformers • Not handled in C57.12.00, C57.12.10, C57.12.90 • Line to line loading does not involve neutral current • Only when there is neutral current there is circulating current in the stabilizing winding • Important role of primary grounding VECTOR GROUP
Yyn Zyn
CORE CONSTRUCTION
PERMISSIBLE LOADING OF NEUTRAL (% OF RATED CURRENT)
W/o stabilizing Wdg. Shell type, Core type single phase or five-legged core
< 10% ( can actually be low or medium, depending on primary grounding characteristics)
Core type, three-legged Yyn+d Dyn
With stabilizing winding
< 25% for up to 1.5 hr < 20% for up to 3 hr < 10% continuous loading < 33%, in case stabilizing winding is rated to 33% of main windings 100%
Example of a German utility recommendation (VDE) [See Schlabbach, J., K.-H. Rofalski, Power System Engineering, 2nd. Ed., Germany, Wiley-VCH, 2013.
19
Unbalanced loading by primary “floating” neutral
• By floating primary neutral, the delta winding carries 33% of the secondary single phase load.
20
2. Function and Application of tertiary and Stabilizing Windings
21
Dr. Xose M. LOPEZ-FERNANDEZ
[email protected] http://xmlopez.webs.uvigo.es IEEE PES Transformer Committee Jacksonville, Florida October 18, 2018
22
Classically For many years it has been a common practice to include a delta-connected Tertiary and or Stabilizing Winding (TW&SW) when used in three phase systems, transformer windings usually Y-connected: • to protect the transformer and system from an excessive third harmonic • to stabilize the neutral point of fundamental frequency voltages • to avoid overheating hazard on transformer due to zero sequence flux
23
Content - Exciting current on single-phase ferromagnetic cores - Exciting current on a transformer bank. Neutral grounded v.s. UNgrounded - Potential negative effects without stabilizing winding (SW)
- Stabilizing and tertiary windings in Y connections - Need for SW in modern transformers - Modeling and circuit analysis
- Heating hazard: faults and temporary unbalanced loads
24
Exciting current on single-phase ferromagnetic cores Source of harmonics and zero sequence components
Since the B/H curve of the core magnetic material is not linear, the magnetizing current will not be sinusoidal (fundamental component plus the third harmonic as predominate).
25
Exciting current on a transformer bank. Neutral grounded Source of harmonics and zero sequence components
The third-order harmonic component of the magnetizing current flow through the grounded neutral of a Y-Y-connected winding.
26
Exciting current on a transformer bank. Neutral UNgrounded Source of harmonics and zero sequence components
If the harmonics currents cannot flow, then the output voltage will contain the harmonic distortion.
27
Potential negative effects without SW Exciting current harmonics - Neutral shift and overvoltages (when the neutrals are not grounded). - Resonance (transformer effectively grounded and system floating). - Telephone interference (neutrals grounded). - False operation of ground relays (neutrals grounded). Unbalanced loading & Fault conditions If there is no closed path to ground, the zero sequence currents would induce overvoltages and overheating hazard.
28
Stabilizing and tertiary windings in Y connections The main guilty for adding SW or TW is the Zero-sequence flux Classically, to remove the presence of zero-sequence flux is to assemble a delta-connected winding on the transformer.
* *
It has become axiomatic to add in each three phase unit a delta-connected of 35% of the equivalent size of one of the other two windings
This practice has been followed so closely for so many years that it is generally taken for granted that the stabilizing winding is a necessary part of such transformers .
29
Need for SW in modern transformers (1) Two questions arise to be considered:
Is the SW actually needed? If SW is needed, what should be the size?
30
Need for SW in modern transformers (2) The benefits of eliminating the SW can include economics (cost 525%; losses by 2- 5 %), reduce the number of components exposed to faults (mechanical, thermal and dielectric).
Wheather the SW can be omitted entirely depends: 1) Whether the resulting zero-sequence impedance and thirdharmonic characteristics are compatible with the system (steady-state operation, relaying practice, grounding)
2) Whether the transformer can perform reliably under expected transient and emergency conditions (faults)
31
Need for SW in modern transformers (2) (Nowadays) the SW could be omitted when:
- Core steel exhibits very low exciting currents (harmonics are a small fraction). - Telephone ground return circuits have eliminate all interference possibilities. - The loads are much closer to being balanced. - The relaying equipment can discriminated the various components of voltages and currents. (Particularly) Three-phase, three-legged core transformers become less susceptible to line-to-neutral voltage distortion (Nevertheless, heating hazard on structural parts has to be controled: by design and/or relaying equipment)
32
Modeling and circuit analysis With or without SW under faults or unbalanced loads the concern/goal is to calculate: Zero-Sequence current Zero-Sequence flux
Windings thermal rating No three-legged (Conductor size) Voltage unbalanced cooling (Zero-sequence voltage)
Three-legged Heating hazard Structural metal parts (hot spots)
Short-circuit forces (stresses in winding conductors) (Conductor size) clamping structural strength
The worst case of permanent load would be a full singlephase load, and of temporary fault a single phase-toground fault with its duration taken into account.
33
Heating hazard: faults and temporary unbalanced loads Zero sequence currens + Zero sequence magnetic flux + Heating
34
Heating hazard: faults and temporary unbalanced loads Without magnetic shunts
With magnetic shunts
Three-phase, three-legged core transformers become less susceptible to line-to-neutral voltage distortion (Nevertheles, heating hazard on structural parts has to be controled: by design and/or relaying equipment)
35
Heating hazard: faults and temporary unbalanced loads Critical points to be considered: * Tank walls * Yoke clamps * Leg plates
36
Heating hazard: temporary unbalanced
Control the heating hazard 2.8 minutes base on transient heating: maximum permitted zero-sequence current in a certain time.
37
PTI Transformers LP
3. Specification and Testing of Tertiary/ Stabilizing windings Krishnamurthy Vijayan - Terminal Connections - Some special aspects - Testing
38
Buried or link outside tank
The stabilizing winding needs to be designed only for single phase loading and single-line-to-ground fault condition
39
Advantages of two terminals brought out • Ease of resistance measurement especially since all 3 phases are in series • Option to open delta if SW is not required at a later stage, and thus eliminate any current flowing in it
• Option to open SW loop at the later stage to reduce fault currents and thus lower circuit-breaker capacity
40
All three terminals brought out
Even at no load or very low station service load, the SW must be designed for a three-phase fault, unless terminals are isolated or insulated
41
SW with delta connection open
• Operating voltage is line-to-line voltage and not line-to-ground voltage • The ungrounded points must be adequately protected and insulated • Transient voltage to be considered for terminal voltage and selection of surge arrester
42
De-energized Taps in the Tertiary winding
Complicated and expensive so recommend to avoid this requirement if possible
43
Examples of Tertiary tap arrangement
Separate Tertiary DETC winding
DETC in stack of main TV winding
44
Effect on Tertiary with neutral end taps in Auto transformers Advantages: • • •
It is an economical option since LTC cost would be lower The regulating winding would be at the neutral end, and so the design is simpler with a low insulation level Due to the location of LTC taps, there is an inherently higher impedance for the tertiary, and so short-circuit forces would be lower
Disadvantage: •
•
The autotransformer becomes a variable flux design, and thus operates with higher flux densities at extreme taps The tertiary winding voltage varies with tap position and, if loaded, this aspect has to be considered
45
Keeping Tertiary voltage constant in a variable flux design
46
Test winding which is not a delta tertiary When delta tertiary/stabilizing winding is not specified, manufacturer may provide a test winding based on test plant limitation • It can be wye-connected so that no fault current flows in the winding during operation. • Such winding terminals are not brought out during operation, but must be appropriately insulated • May need to be protected through internal surge arresters. • The neutral point should be internally grounded.
47
Stabilizing winding rated voltage •
• • • •
The voltage rating to be low to reduce inductive transfer from LV/HV. This also helps to have higher conductor area for the same MVA and thus higher short circuit strength Being delta connected, the turns of the winding would be higher than wye The SW voltage to be lower than the phase voltage of LV so that SW turns are lower than LV turns and thus reducing inductive transfer For dielectric purpose, higher impulse levels can be specified but the rated voltage to be lower as per above considerations It is recommended that customer specifies only BIL level and manufacturer decides on the rated voltage to avoid high transient voltages
48
Testing of Stabilizing winding Loaded tertiary winding, tests performed as per IEEEE C 57.12.00 and C57.12.90 Following tests recommended on stabilizing winding: 1. 2. 3. 4.
Turns ratio (Performed during manufacturing) Winding resistance (Performed during manufacturing if delta closed inside) Applied voltage test if any terminal brought out Design values provided for impedance in most cases
Note: Depending on test plant limitation, all 3 terminals of the SW winding can be brought out for test purpose. After test, the unit is connected for the operating condition
49
Dielectric testing Testing 1. Impulse test on SW not required if the corner is to be grounded during operation 2. If all terminals of the SW are not brought out, impulse and induced tests to be performed in the operating condition Design considerations 1. During HV and LV impulse test, the ungrounded corners of SW may have higher transient voltages 2. Any current limiting reactor in SW may result in higher transient voltages at the connecting points
50
Frequency response measurement
In units with Stabilizing winding some caution to be applied during this test on HV and LV terminals 1. Recommended practice is to keep the delta corner closed but not grounded 2. Grounding of a corner introduces additional asymmetry between phases 3. Opening or closing of delta corner can cause difference in the frequency response 4. Test in operating condition can be done as an additional test
51
Frequency response - Example1
52
Frequency response- Example2
53
4. Utility Examples
Stabilizing or Tertiary Winding Effect on Relaying and Arresters Hamid Abdelkamel, PE, PMP Transmission Substation Design Engineer
[email protected]
10/18/18
Stabilizing Winding Effect on Relaying and Arresters 1) Ground over current coordination Issues 2) Delta back feed 3) Stabilizing winding sizing
Transformer Zero Sequence Models: YG-YG
YG-D-YG
Y-D-YG
Transformer Zero Sequence Models (Cont’ed) • D-YG
• YG-YG AUTO (w/o Stabilizing)
• YG-D-YG AUTO (w/ Stabilizing)
Sequence Network for L-G Fault:
-
For Transformer low-side L-G fault, how ground current passes through transformer dependent on winding configuration:
,
Effect on Ground Overcurrent Relaying: • YG-YG HS Pickup > LS Pickup x Turns Ratio HS Pickup = 240A < LS Pickup 1200 x 69/138=600A Possible mis-coordination • YG-D-YG HS Pickup > LS Pickup x Turns Ratio x Current Divider Per case shown: Possible mis-coordination • Y-D-YG or D-YG Low-side Ground fault Current does not reflect to high-side
LS=low side & HS=high side
240A
240A
Effect on Delta Back-feed Relaying: • Ground Fault Detection on Delta back-feed systems requires zero sequence overvoltage detection in order to prevent failure of LA and Transformers from over-voltage:
Effect on Delta Back-feed Relaying: • Due to lower zero sequence impedance to ground ‘YG-D-YG’ can prevent correct 59N operation, which can result in over-voltage failure of LAs and Transformers: 1) Overvoltage may exceed rating of LA 2) Overvoltage will overexcite transformer
3) Very hard to detect
Effect on Delta Back-feed Relaying:
• YG-YG and Y-D-YG have minimal effect on 59N operation:
Stabilizing Winding Short Circuit Bracing: • Current through 𝑍𝑇 represents current through Stabilizing • Stabilizing winding needs to be braced for Z1(sys) = Z2(sys) =0 with a range of Z0(sys) for worst case LG faults • Misconception is that infinite bus represents worst case—not true for ground faults. Assuming infinite bus will lead to under sizing stabilizing winding
Recommendation (Continued)
For wye-wye XFMR designs: – It is strongly recommended to have a discussion between transformer design engineer and protection engineer on whether or not a stabilizing winding is required as well as requirements if one is required
– All impedance calculations (Z0, Z1, Z2, T-model) should be submitted for end user review with proposal and modeling
65
5. Conclusion and Further Work
66
Conclusion •
•
• •
Stabilizing windings reduce the zero-sequence impedance of Y-connected transformers and improve their performance under unbalanced operating conditions As those windings impact cost of the equipment, their application should be driven by zero-sequence needs for system grounding, and control of third harmonic phenomena The Guide C57.158 is a source of practical recommendations for specification, testing and application of stabilizing and tertiary windings Systems and protections engineers should be involved in decisions that impact the zero-sequence performance of power transformers
67
Further work Still to consider recommendations for: • Thermal effects of circulating currents on delta windings of transformers subjected to DC currents • Comparison of behavior of Y-connected transformers subjected to switching transients, with and without a delta winding • Circulating currents in transmission transformers by temporary bi-polar operation of the lines • Thermal behavior of SWs subjected to extended loading • Role of delta windings in Y-connected transformers during ferro-resonance conditions • Closer coordination with protection relaying practice
68
Bibliography - See Annex A of C57.158 -