1. 5/21/2015
Using Simulation Tools to Predict and Prevent Vacuum Circuit Breaker Switching Induced Transformer Failures
Steven B. Swindler,
Thomas J. Dionise,
Steven A. Johnston,
Thomas E. McDermott,
Intelligent Ships Symposium; May 21, 2015
DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

2. Concerns with Switching Transformers at Medium-Voltage
5/21/2015
Due to a high prevalence of medium-voltage (MV) transformer internal winding failures in industry, IEEE developed IEEE Std C57.142™-2010: IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients.
C identifies the following conditions as indicating a vulnerability to switching-induced transformer failures
Switching device directly connected via cable to 1 or 2 transformers
Transformer unloaded, lightly loaded, or feeding non-linear loads
Load, when switched, is primarily inductive
Switching produces oscillations near transformer natural frequencies
Long or short cables between switch and transformer (many papers indicate that short cables are worse from an oscillation perspective)
Load is switched frequently
Additionally, vacuum circuit breakers (VCBs) typically used on MV systems have characteristics that may exacerbate switching oscillations
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

3. Overview Unique concerns of medium-voltage electric power systems
5/21/2015
Overview
Unique concerns of medium-voltage electric power systems
Vulnerability of dry-type transformers
Transient-producing behavior of vacuum circuit breakers
Mitigation with surge arresters and RC snubbers
New tests and simulations in the design process
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

4. 5/21/2015
Above 1000 Volts, new concerns arise with switchgear applications and transients.
Relatively recent higher transformer failure rate in industry can be traced to the increased proliferation of data centers as major loads.
Traditionally, Navy ships generated, distributed, and utilized power at the 450V level, switching with air circuit-breakers.
Higher power demands required that the Navy generate and distribute power at higher voltages (4160V and up)
These new shipboard electric systems can resemble data centers in the use of VCBs closely coupled by cables to dry-type transformers. Incidences of dry-type transformers failures can be caused by lightly damped high-frequency transient overvoltages or excitation of transformer resonant frequencies, which previously had not been a concern on low-voltage shipboard power systems.
2011 ISS IX Example:
Mitigating Transient
Recovery Voltage (TRV)
With Bus Capacitance.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

5. 5/21/2015
VCBs can “chop” several Amperes of current; producing overvoltages that are mitigated with capacitance.
Vacuum switchgear is typically used to switch medium voltage industrial transformers, due to its small size, economics and reliability. Despite recent improvements in the technology, vacuum and SF6 switchgear still tends to “chop current” on opening, and also tends to produce multiple re-ignitions. A current chop can produce a high transient voltage, limited by capacitance and surge arresters in the system. With reference to Figure 3, the VCB current is prematurely forced to zero when it falls below Ichop, instead of smoothly interrupting at a natural, 60-Hz current zero. The value of Ichop depends mainly on the VCB contact material and ranges from 3 to 5 amperes for interrupters using Cr-Cu, and from 15 to 21 amperes for interrupters using Cu.
When switching off a transformer, this current chopping traps energy in the transformer magnetizing inductance, Lm, and this energy must discharge in oscillatory fashion through the stray capacitance, Cxf.
Maybe discuss Tom’s equations 2-5 regarding natural frequencies and damping?
Because Cxf tends to be small, Vpeak can be high enough to cause either insulation failure or an arrester operation. The frequency of oscillation may also be high enough to excite internal transformer resonance.
For convenience, the formulas above define the lumped-circuit surge impedance, frequency of oscillation and damping ratio
A re-ignition, i.e. reestablishment of current flow across the opening contacts, produces a high-frequency voltage oscillation that is usually of low magnitude, but sustained for about ¼ cycle. Vacuum breaker prestrikes may also occur on contact closing. These produce transients similar to re-ignition, but less often and usually less severe
Adding Capacitance Suppresses the Peak Transient Voltage
Predictions of necessary capacitance from hand calculations of simplified circuit.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

6. 5/21/2015
Dry-type transformers reduce weight and risk of fire, but are not as well insulated as oil-filled transformers.
Dry-type transformers are typically fed by cables and switched by vacuum switchgear. Surge arresters are traditionally used to protect transformers from transient overvoltages due to switching, lightning, and any other sources.
Figure 1 shows the typical insulation strength vs. voltage time-to-peak, for typical 95-kV Basic Insulation Level (BIL) liquid-filled transformers, and typical 60-kV BIL dry-type transformers (IEEE C62.22). The BIL for dry-type transformers is usually less than for liquid-filled.
The BIL is tested for a voltage time-to-peak of 1.2 ms, but the insulation level of liquid filled transformers is higher than the BIL for faster surges, i.e. voltage time-to-peak less than 1.2 ms. This beneficial increase doesn’t occur in dry- type transformers, see Figure 1
However, arrestors only protect against over-voltages, they do not protect against internal transformer oscillations which may be excited by switching transients
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

7. 5/21/2015
The circuit model has to include transient (or high-frequency) behavior of VCB, cable and transformer.
Arrester
Refer to SFRA figure: A transformer winding has internal resonances that may be represented with a network of inductors, capacitors, and resistors (Degeneff, 2007). If an oscillatory transient at the terminal excites one of these resonances, internal voltages can rise to the point that a turn-to-turn or interwinding failure may occur, even if the terminal voltage does not approach the tested insulation strength. This mechanism has been verified with many post-failure transformer inspections, and can also be reproduced with computer simulation (Shipp et. al., 2011).
Sweep Frequency Response Analysis, which can be done readily available commercial test sets, indicates the transformers natural resonant frequencies.
To determine whether switching transients will excite these resonant frequencies, circuit simulations can be performed. These simulations may differ from those typically performed in that they focus on parasitic parameters, like cable and transformer line-to-ground capacitance and transformer magnetizing parameters, rather than the line-to-line voltages and line currents used in load flow, fault analysis, and typical transient analysis (which looks at responses to large load changes, etc.)
Distributed-Parameter Cable
Transformer Frequency Response Analysis
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

8. 5/21/2015
The Alternative Transients Program (ATP) supports detailed studies, with no software licensing cost.
Dr. McDermott teaches week-long courses at NAVSSES Philadelphia on simulation of electrical transients using ATP because of its low ($0) cost and high capabilities.
To validate the estimates and perform a more comprehensive analysis, transient simulation should be considered. The Alternative Transients Program (ATP) was developed at Bonneville Power Administration, and it’s the ancestor of commercial software that does the same function. ATP is now maintained by a developer community in Japan and Europe, with capability and reliability at least comparable to its competitors. The Navy qualifies for an ATP license at no charge. The ATP has a graphical interface called ATPDraw, but unlike its competitors, it can also be readily scripted and interfaced to different data files. (Note: MATLAB with the SimPowerSystems toolbox could also suffice. If using R2014b or later, the authors suggest the “specialized technology” version of SimPowerSystems.)
The ATP is a multi-phase, time-domain power system simulator using trapezoidal integration at a fixed time step. Frequency-dependent component models generally use rational function fitting, with recursive convolution in the time domain. Non-linear component models generally use the compensation method, injecting currents into the linear part of the network to match boundary conditions and device equations. The ATP includes model parameterization routines for overhead lines, cables, transformers, and surge arresters.
MATLAB / SimPowerSystems also works, but use the “Specialized Technology Library”
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

9. 5/21/2015
Text-based modeling language code in ATP reproduces the multiple re-ignition behavior of the VCB.
High-frequency current interruptions;
No RC snubber.
Oscillatory voltages;
No RC snubber.
For this analysis, we developed a VCB model capable of modeling current chopping (5A) and realistic transient recovery voltage (TRV) characteristics.
During a simulated opening, if the voltage across the VCB contacts exceeds the dielectric withstand capabilities of the parting contacts, a re-ignition occurs, re-establishing electrical contact with the load. Multiple interruptions and reignitions can occur before the load is truly disconnected.
The details of this ATP model are shown in the paper.
From the paper:
VCB opening or closing often generates repetitive transients, called pre-strike (closing) or re-ignition (opening) that might excite a transformer internal resonance. When the vacuum (or SF6) breaker opens, transient recovery voltage (TRV) builds up across the contacts and may cause a breakdown, or re-ignition, if the voltage exceeds the TRV rating of the VCB. Vacuum breakers have a unique ability to interrupt high-frequency current following the breakdown, which may lead to a rapid series of breakdowns and interruptions before the breaker finally opens. A similar behavior can occur when the vacuum breaker closes, and this is called pre-ignition. However, re-ignition during opening is the more severe case. SF6 switchgear is considered less susceptible to multiple re-ignition and pre-ignition, but is not immune. The vacuum breaker voltage recovery and high-frequency current interruption model was obtained from (Greenwood & Glinkowski), with 200 A/us high-frequency interruption capability. This behavior defines the vacuum breaker ability to sustain reignitions and voltage escalation.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

10. 5/21/2015
Transformer voltage reduced from 61 kV (red) to 17 kV (green) by adding an RC snubber (0.125 mF, 50 W)
Adding a snubber (resistor in series with capacitor) at the terminals of the transformer can reduce the magnitude and frequency of voltage at the transformer.
In this paper, the snubber capacitance of mF sufficed to limit current chopping and reignition transients, and it also reduced the system resonant frequency below 1000 Hz. With more information available from a SFRA, the value of C could be further tuned, and 0.25 uF is a typical value resulting from that process. It is not desirable to increase C more than necessary, because that may increase the risk of ferroresonance and of harmonic resonance.
The snubber resistance is set to approximately match the cable surge impedance, but it’s not critical to match exactly. Larger values of R help to mitigate ferroresonance, but as shown later, that was not completely effective in this case. In the rest of this paper, a resistance of 50 ohms is always included in the snubber.
This section presents the results of ATP simulations of current-chopping transients on a load center transformer, with 50-foot cable between transformer and VCB. Figure 10 shows the chopped current (right axis) and transformer voltage (left axis) on the first pole to open, which is phase C. The chopped current is 1.13 Amperes without the snubber, and 0.58 Amperes with the snubber because Csnub is large enough to noticeably compensate for Lm. The voltage is not zero at the instant of interruption in either case; it’s actually phase shifted due to the transformer core resistance. The voltages begin to change after current interruption, but this is barely noticeable on the time scale of Figure 10. The actual peak transformer voltages are shown in Figure 11. The simulated peak values (61 kV, 17 kV) and the times-to-peak are comparable with the earlier estimated values (62 kV, 18.8 kV).
Chopping transformer magnetizing current; 1.13 Amps.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

11. 5/21/2015
The VCB may chop up to 5 Amperes, which increases the transient voltage.
20 kV with Arrester only, but high-frequency oscillations.
51 kV with RC snubber only, and slower oscillations.
Figure 12 shows the peak transformer voltage if the chopped current is increased to 5 Amperes, which is the maximum level of chop current assumed for the VCB. To chop that much current, a small inductive load (0.1 power factor) was added to the transformer secondary. Note that with no protection, the peak voltage is 150 kV, which is enough to fail the transformer. With arrester protection only, the peak voltage is limited to about 20 kV. However, note that the arrester does nothing to reduce the voltage rate-of-rise of between 0 and 0.1 ms, which is fast enough to excite internal resonances above the transformer’s first series resonant point. Also note that when the arrester comes out of conduction at about 2.1 ms, there is an oscillatory transient at the same frequency as with no protection. With a snubber only, the peak voltage is 41 kV and the frequency of oscillation is also reduced. Actual transformer internal resonant frequencies may not always be known, but judicious selection of snubber parameters can reduce the system natural frequencies to values that “could not” excite a transformer resonance.
150 kV with no mitigation.
Chopping 5 Amps; opening under light load.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

12. 5/21/2015
A caveat with RC snubbers; more capacitance can worsen ferroresonance if one or two poles of the VCB delay opening.
As the final example result, Figure 15 shows the three-phase transformer voltages during ferroresonance with a mF surge capacitor. This is a type of series resonance between a capacitance and a non-linear inductance, where high voltages occur across each L and C component. In this scenario, cables and surge capacitors provide the capacitance, and transformers provide non-linear magnetizing inductance. Ferroresonance can occur if one or two poles of the VCB fail to open or close; this event is called a “hung pole”. Figure 16 illustrates the ferroresonance path from the source across the transformer windings, through the capacitors to ground and back through the source ground. The distorted waveforms are caused by energizing the non-linear inductance in series with a capacitance, leading to rapid flux reversals that generate the square-wave appearance of two phase voltages in Figure 15. Once it begins, ferroresonance usually continues until the “hung pole” finishes opening or closing, a backup breaker opens, or some equipment fails. When present, surge arresters are usually first to fail due to excessive energy discharge. Ferroresonance can also lead to transformer failure because its power frequency insulation strength is less than the BIL.
With no surge capacitor, ferroresonance did not occur and all three transformer voltages were in phase, and equal to the red trace. Ferroresonance occurs in many different ways and is hard to predict. However, it is possible for vulnerability of the transformer to the periodic form of ferroresonance to be determined by examining the transformer inductance and the transformer terminal capacitances. There is a range of transformer terminal capacitance that leads to vulnerability. In this example, ferroresonance occurs at 0.125, 0.33 or 0.5 mF, but not for 1 nF or 0.25 mF. These results are only illustrative and do not necessarily mean that 0.25 mF would be a “safe” value in all cases. Sometimes the snubber resistance is enough to suppress ferroresonance, but not in this case. Mitigation of ferroresonance, summarized in (Lister, 1973), could also include resistive loads on the order of 10% of the KVA rating of the transformer, winding connections, or grounds, changing capacitance values, and regular VCB maintenance. ATP is a good tool for performing this analysis.
Sustained non-linear overvoltages may cause transformer or other equipment to fail.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching

13. This is still new to the Navy Needs for transient simulation
5/21/2015
Conclusion – new engineering concerns arise with adoption of medium-voltage electric power systems.
This is still new to the Navy
Consult IEEE Standards C37.011, C57.142, C62.22
Needs for transient simulation
Non-linear effects of transformers, switchgear, loads and possibly surge arresters
Optimize the R and C parameters of a snubber
Investigate high-resistance grounding
Needs for new tests
Transformer / system interactions
Require frequency response tests (SFRA) on new transformers
To summarize, oscillatory voltage transients at levels less than transformer impulse voltage ratings may produce transformer failures, and surge arresters don’t necessarily protect against these transients. RC snubbers can protect a transformer against these oscillatory transients, at a reasonable cost. The snubber capacitors draw little current at steady state. However, they require adequate space to install. They also increase the risk of ferroresonance, especially in cases where one or two breaker poles “hang” on opening. The snubber resistance can reduce or sometimes avoid the ferroresonance associated with using surge capacitors without resistors.
In a comprehensive transient study, the switching frequency determined by simulation is compared to the internal resonant frequencies determined by SFRA testing. Then the RC snubber parameters are chosen through an iterative process to avoid one or more internal resonant frequencies. The expected number of switching operations, and the target reliability for the transformer, should also play a role in the decision to install snubbers or not.
These concerns are new to the Navy, and they come about from increasing the voltage level of shipboard electric power systems above 1 kV, the use of vacuum rather than air breakers, the implementation of new designs to reduce footprint, i.e. primary breakers close-coupled to transformers and transformers with higher efficiencies. The ATP software used for the examples in this paper has no licensing cost, and it includes both a graphical user interface and high-frequency / transient modeling features. ATP can provide a cost-effective simulation platform for shipboard electric power systems to help address these new medium-voltage issues.
5/21/2015
ISS 2015: VCB Switching; Distribution A: Approved for public release; distribution is unlimited
ISS 2015: VCB Switching