transient voltage recovery - trv understanding -alexander

TRV What is it? Why should I care?

Current - TRV - Recovery Voltage

CURRENT

TRANSIENT RECOVERYVOLTAGE

RECOVERY VOLTAGE

TRV (Transient Recovery Voltage)

• TRV is the voltage across a current interrupting device (circuit breaker, switch, fuse, etc.)

immediately after current interruption.• TRV is the power system response to a

sudden change in network topology. • TRV can be analyzed by the method of

current injection. A current equal but opposite to the interrupted current is injected where the circuit breaker is.

What difference does TRV make to a Switching Device?

• The TRV capability of a switching device, compared to the TRV imposed by the power system, will determine whether or not the switching device will break the current it is asked to break.

What TRV means to a CB or switching device

• When the current is broken, the interrupter must recover dielectric strength faster than the circuit can build voltage across it.

• This applies to Switches, Fuses, Circuit Switchers, any device that interrupts current flow.

• Including solid state, vacuum, or plasma devices.

Prospective TRV

• This is the TRV inherent in the circuit withoutthe influence of the interrupting device.

• The interrupting device itself can modify the TRV.

• The more it modifies the TRV, the more will be the switching losses.

• When TRV studies are done they obtain prospective TRV

• Current interrupting devices are tested against prospective TRV

Current Zeros

• current zero - the normally occurring instantaneous zero values of current which occur twice each cycle in an alternating current circuit. (120 per second for 60Hz)

• The ideal AC circuit breaker would stop the current at the naturally occurring current zeros.

Alternating current through CB

I

RL

CB

Current zeros

CB Current Ratingsand related capabilities

• Continuous Current– Overload Current

• Short Circuit Current (rms symmetrical)– Total Current: AC + dc componentsdc time constant of 45 ms is standard– 1 or 2 s of short circuit current while closed– Close and latch peak current 2.6 x SCC

Short Circuit CurrentWhat does it look like?

• The short circuit current is heavily influenced by the power sources on the system.

• At the present, these are for the most part, rotating machines.

• An initial current burst may come from energy stored in system capacitance. – Normally the capacitive discharge is a small

contributor to the short circuit current, but in any case, it is pretty much over before the short circuit is detected, much more so by the time a circuit breaker actually opens.

Short circuit current waveformsS hor t Ci r c ui t Cur r e nt

-1.5

-1

-0.5

0

0.5

1

1.5

2

1 42 83 124 165 206 247 288 329 370 411 452 493 534 575 616 657 698 739 780 821 862 903 944 985 1026 1067

Symmetrical (fault initiates at voltage peak)

Offset (fault initiates at voltage zero)

CB Voltage Ratingand related capabilities

• Rated maximum voltage is the maximum line to line voltage for application– Dielectric test values : line to ground and open gap

• Lightning Impulse (LIWL) [BIL]• Switching Impulse (SIWL)• 1 minute 60/50 Hz withstand

– Transient Recovery Voltage (TRV) Capability; keyed to short circuit rating. Higher SC ratings have faster TRV Capability at a given current.

– Capacitance current switching capabilities• Line dropping• Capacitor bank • UG Cable dropping

Other CB Ratings

• Interrupting Time = time from initiation of a trip signal to the interruption of current in all poles.

• Reclosing (1/3 second)• Capacitance Current Switching• Shunt Reactor Current Switching• Out of Phase Switching Current [typically

0.25 x interrupting rating with a TRV peak 2.5 p.u. and a time to crest 2 x normal]

Background to Understanding TRV

• Energy storage in electric networks

• Transient Analysis• Elementary RLC Circuits• Elementary Traveling wave Principles

Where is energy stored in an electric network?

• If there were no energy storage, there would be no TRV.

• Magnetic field (where is that?) arises from?

• Electric field (where is that?) arises from?

Simple oscillatory LRC Circuit

• Natural frequency f = 1/(2π√LC )

• Natural impedance = √(L/C)

R

L

CCB

Simple transient circuit analysis

• Consider what the circuit behaves like in the steady state or quasi steady state immediately before and immediately after a switching event.

• Then consider how to get from before to after – Guess what the transient behavior could be.

Steady state voltage across and current through a closed CB

I

V

Steady state voltage across and current through an open CB

V

I




R

L

CCB

TRV HV CIRCUIT BREAKERS

Current - TRV - Recovery Voltage

CURRENT


RECOVERY VOLTAGE


• During the interruption process the arc rapidly loses conductivityas the instantaneous current approaches zero. Within a fewmicroseconds after current zero, current stops flowing in thecircuit.

• The power system response to the current interruption is whatgenerates TRV.

• The difference in the power system response voltage from the source side to the load side of the circuit breaker is the TRV.

• The breaking operation is successful if the circuit breaker is able to withstand the TRV and the power frequency recovery voltage.

TRV HV CIRCUIT BREAKERS• The nature of the TRV is dependent on the circuit being

interrupted, whether primarily resistive, capacitive or inductive,(or some combination). Additionally, distributed and lumpedcircuit elements will produce different TRV waveshapes.

• When interrupting a fault in at the circuit breaker terminal in aninductive circuit, the supply voltage at current zero ismaximum.

The circuit breaker interrupts at current zero, at a time when thepower input is minimum, and the voltage on the supply sideterminal reaches the supply voltage in a transient process calledthe transient recovery voltage.

TRV frequency is

with L = short-circuit inductance, C = supply capacitance.CLπ2

1


Current and TRV waveforms during interruption of inductive current

CURRENT


Supply voltage


If a pure resistive circuit is interrupted, the supply voltage is zero at the time of interruption, therefore there is no TRV.

Current and TRV waveforms during interruption of resistive current

CURRENT

RECOVERY VOLTAGE


TRV and recovery voltage in resistive, inductive or capacitive circuits

-2

-1,5

-1

-0,5

0

0,5

1

1,5

2

2,5

RESISTIVE CIRCUIT

INDUCTIVE CIRCUITwith stray capacitance

CAPACITIVE CIRCUIT

INDUCTIVE CIRCUIT


• There is a TRV for any interruption not just for fault interruptions.

• Fault interruptions are often considered to produce the mostonerous TRVs. Shunt reactor switching is one of the exceptions.

• TRVs can be oscillatory, triangular, or exponential and canoccur as a combination of these forms. A dc offset may also bepresent as is the case for lines with series capacitors.

• A network can be reduced to the simple parallel RLC circuit forTRV calculations. This representation is valid for short-timeframes until voltage reflections return from remote buses.


• The TRV in the parallel RLC circuit is oscillatory (underdamped) if

• The TRV in the parallel RLC circuit is exponential (overdamped) if

CLR /21

⟩

CLR /21

≤

R C

(Vcb)

L


0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 1 2 3 4 5 6 7 8 9

TRV (p.u.)

R / (L / C)0.5 = 10

4

2

1

0.75

0,5

0,3

t / RC

TRV HV CIRCUIT BREAKERS• By lowering the resistance in the equivalent circuit, for example

when adding a resistance of low ohmic value in parallel to theinterrupting chamber(s), it is possible to effectively reduce therate-of-rise of TRV. This possibility has been widely used formany years to ease the current interruption by air-blast circuitbreakers.

• When longer time frames are considered, typically severalhundreds of micro-seconds, reflections on lines have to betaken into account. Lines or cables must then be treated ascomponents with distributed elements on which voltage wavestravel after current interruption. These traveling waves arereflected and refracted when reaching an open circuit or adiscontinuity.

Key Descriptors of TRV

• TRV Peak• TRV Rate of rise (RRRV)• Long term (100ms) recovery voltage.

4 parameter TRV

Typical TRV problem apps• Series inductor (reactor) limited fault i.e. the fault

impedance is largely from a lumped inductor (eg. TLIs, CLRs, flow control Inductors) (RRRV concern)

• Short Line Faults (SLF) at lower currents (critical currents) (RRRV concern)

• Transformer limited faults ( where a transformer is a major part of the fault impedance and there are no parallel lines or cables to slow the TRV) (RRRV concern)

• Switching small inductive currents (shunt reactors) (current chopping, reignitions, RRRV)

• Lines with series capacitors (high TRV peak)

• Very long lines with a large Ferranti effect (>250 miles) (high TRV peak)

TRV HV CIRCUIT BREAKERS7.2 Series reactor limited fault

• The system TRV may exceed the standard capability curve, which isdescribed by a two-parameter envelope where uc and t3 are defined inANSI C37.06 for 10% short-circuit breaking capability, maximumvoltage. For currents between 10% and 30% of rated short-circuitcurrent, values of uc and t3 can be obtained by linear interpolation.

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60

TIME (µs)

VOLT

AG

E (k

V)

SYSTEM TRV CURVE

TRV CAPABILITY FOR A STANDARD BREAKER




R

L

CCB

Elementary Traveling Wave Concepts

• Surge impedance Z = the instantaneous ratio of voltage to current on a distributed parameter line

• Reflection and transmission coefficients – open circuit positive voltage reflection– short-circuit negative voltage reflection.

• Voltage reflection coefficient = (Z2 - Z1)/(Z2 + Z1)

• Voltage transmission coefficient = 2Z2/(Z1 + Z2)

– Z1 is the surge impedance of the source line– Z2 is the surge impedance of the "receiving" line

ITRV and Short Line Faults

• All SF6 breakers have some difficulty handling the steep rates of rise of TRV caused by short line faults. Line-to-ground capacitors and/or capacitors across the open gap have been used to delay the initial TRV ramp.


Traveling waves on a faulted line after current interruption


Voltage distribution on the line at different times after current interruption


Time variations of voltages at three locations on the faulted line

VOLTAGE AT CIRCUIT BREAKERTERMINAL POINT C x = 0

VOLTAGE HALF WAYTO FAULT x = 0.5 L

VOLTAGE 3/4 OF WAY TO FAULT x= 0.75 L

TIME

VOLTAGE (p.u.)

0

2

- 2

tL0.5 tL tL/4 3 tL/4 1.5 tL

TRV HV CIRCUIT BREAKERSAnnex D Calculation of SLF quantities

Result of digital simulation

TRV HV CIRCUIT BREAKERS• TRVs for line faults are determined on a single-phase basis.

• The fault current for a line side fault is somewhat reduced from thatobtained for a bus fault due to the additional reactance of the line.

• IT = fault current through the circuit breaker for a single-phase fault atthe breaker terminal

• IL = the reduced current for a line fault.

SL

LGL XX

VI

+=

λ

VLG

XS XL × λT

LGS IV

X =

TLGL

LGL IVX

VI

/+=

λ

TRV HV CIRCUIT BREAKERS• The transmission line parameters are given in terms of the effective

surge impedance, Zeff, of the faulted line and the amplitude factor, d

Zeff = (2Z1 + Z0)/3

Z1 is the positive sequence surge impedanceZ0 is the zero sequence surge impedance

v is the velocity of lightω is 2 π × system power frequency (377 rad/s for a 60 Hz system)

vXZ

dL

effω2=

CDo

CDpCDo

VVV

d+

=

CB Fundamentals• A real circuit breaker has at least four states:

– open - very low conductivity but finite dielectric strength– closed - high conductivity but limited current capability– interrupting - increasing dielectric strength, after current zero– closing - decreasing dielectric strength

• It takes a finite time to go from open to closed, or closed to open.

• If the circuit breaker gets "stuck" while interrupting, or at the wrong place while closing, it can be destroyed.

How CB Interrupters work

• Circuit breakers use Electric Arc dynamics to accomplish interruption.

• What is an electric arc?:– a high density plasma (fully ionized gas)– charge carriers: ions and electrons– charge moves due to the electric force

One of the features of ac circuit breakers is that arc interruption naturally takes place very close to the normally occurring current zeros on the ac wave.

Contact opening

• As contacts begin to open, the contact force decreases, and the area actually making contact decreases.

• Both these cause contact resistance to increase. • Eventually the energy dissipated in the contact

will cause a molten metal bridge to form. • Magnetic forces will enlarge the molten bridge,

which assumes a tubular shape, until the forces cause it to explode, and an arc forms.

CB Dielectric Recovery• The objective is to "quench" the arc. Have it go from a highly

conductive state to an insulating state at just the right moment, fast enough to accomplish successful interruption.

• A stable arc is in an energy balance. The energy removed from the power system is balanced by the cooling energy. A 10,000 amp arc with 100 volts drop consumes 1 MW.

• As the current approaches zero the input energy also approaches zero, and the plasma column rapidly loses conductivity. – If cooling is sufficient the voltage across the cooling plasma column will

not drive enough current to sustain the temperature required for conducting the next half cycle and the arc will extinguish.

– These few microseconds around current zero are referred to as the energy balance or thermal region.

Temperatures of circuit breaker arcs are in the range of 10,000 - 20,000 °K

From Conducting Plasma to Insulating Gas

• Thermal Region - the first few tens of microseconds after current cessation

• Characterized by removal of charge carriers from the plasma

• Transition region is characterized by recombination of ions and electrons.

• Dielectric region (after 100 microseconds) Characterized by cooling gas, increasing density and dielectric strength.

Puffer vs self blast

Pure Puffer• Interrupting effort not

dependent on arc current• Good performance

throughout current range• If it can do 100% of rated

it will do anything less• Requires strong

mechanisms to develop the “puff pressure”

Self Blast• Interrupting effort heavily

dependent on arc current• at currents around 20 –

30% of rated, it may struggle

• Possibility of “critical currents”

• Mechanism can be about 20% the energy of a full puffer mech

Vacuum vs SF6

Vacuum CB• Low operating energy• No interrupting “window”• Flash of open contacts

will self clear• Performance based on

contact material, purity, and cleanliness

• Very fast TRV capability 1µs

SF6 CB• Moderate operating

energy• Limited interrupting

window• Flash of open contacts

won’t clear • Performance based on

Gas flow dynamics• “weak” on Fast TRVs

Vacuum vs SF6

Vacuum CB• Low capacitance current

inrush capability 7kA for class C2

• Requires “semiconductor level” cleanliness and purity in manufacture of interrupter

SF6 CB• Extreme capacitance

current inrush capability 100kA?

• Interrupter can be assembled in “normal clean” environment

Oil vs SF6

OIL CB• Pretty good with fast TRV• Restrikes almost every

operation with capacitor switching

• Good for at most 5 full fault operations

• 500 operation mechanical life

• High maintenance if frequently operated

SF6 CB• Weak on fast TRV• Good for capacitor

switching• Good for at least 20 full

fault operations• 2000 – 10000 operation

mechanical life • Often won’t need

maintenance for 12 + years

Oil vs SF6

OIL CB• Heavy clunky device• Uplift on fault clearing• Made to “old” standards

and will not perform up to 1999 standards

• Can literally explode if operated beyond its short circuit rating

SF6 CB• Reasonably light weight

device• Almost no external

reaction forces when fault clearing

• Breaker may burn down but no hydrogen explosion or fire if overdutied

References• IEEE C37.04 CB Requirements• IEEE C37.06 CB Ratings• IEEE C37.06.1 Fast TRV• IEEE C37.011 TRV application guide• http://ewh.ieee.org/soc/pes/switchgear/TechPres near

bottom of page 3rd entry under “tutorial on TRV” 2003

• RWA Engineering [email protected], [email protected](rwayengineering.com)

transient voltage recovery - trv understanding -alexander

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