TRV What is it? Why should I care? CURRENT
TRANSIENT RECOVERY VOLTAGE
Current - TRV - Recovery Voltage
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 without the 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
L R
Current zeros
C B
CB Current Ratings and related capabilities • Continuous Current – Overload Current
• Short Circuit Current (rms symmetrical) – Total Current: AC + dc components dc 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 Current What 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 waveforms S hor t Ci r c ui t C ur r e nt
2
1.5
1
Symmetrical (fault initiates at voltage peak)
0.5
0 1
-0.5
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
Offset (fault initiates at voltage zero)
-1
-1.5
CB Voltage Rating and 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)
L R
C
CB
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
Simple oscillatory LRC Circuit • Natural frequency f = 1/(2π√LC ) • Natural impedance = √(L/C)
L R
C
CB
TRV HV CIRCUIT BREAKERS CURRENT
TRANSIENT RECOVERY VOLTAGE
RECOVERY VOLTAGE
Current - TRV - Recovery Voltage
TRV HV CIRCUIT BREAKERS •
During the interruption process the arc rapidly loses conductivity as the instantaneous current approaches zero. Within a few microseconds after current zero, current stops flowing in the circuit.
•
The power system response to the current interruption is what generates 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 lumped circuit elements will produce different TRV waveshapes.
•
When interrupting a fault in at the circuit breaker terminal in an inductive circuit, the supply voltage at current zero is maximum. The circuit breaker interrupts at current zero, at a time when the power input is minimum, and the voltage on the supply side terminal reaches the supply voltage in a transient process called the transient recovery voltage. TRV frequency is
1 2π
LC
with L = short-circuit inductance, C = supply capacitance.
TRV HV CIRCUIT BREAKERS
CURRENT
Supply voltage
TRANSIENT RECOVERY VOLTAGE
Current and TRV waveforms during interruption of inductive current
TRV HV CIRCUIT BREAKERS If a pure resistive circuit is interrupted, the supply voltage is zero at the time of interruption, therefore there is no TRV.
CURRENT
RECOVERY VOLTAGE
Current and TRV waveforms during interruption of resistive current
TRV HV CIRCUIT BREAKERS 2,5 2 1,5
CAPACITIVE CIRCUIT
1 0,5 0 -0,5 -1 -1,5 -2
RESISTIVE CIRCUIT
INDUCTIVE CIRCUIT INDUCTIVE CIRCUIT with stray capacitance
TRV and recovery voltage in resistive, inductive or capacitive circuits
TRV HV CIRCUIT BREAKERS •
There is a TRV for any interruption not just for fault interruptions.
•
Fault interruptions are often considered to produce the most onerous TRVs. Shunt reactor switching is one of the exceptions.
•
TRVs can be oscillatory, triangular, or exponential and can occur as a combination of these forms. A dc offset may also be present as is the case for lines with series capacitors.
•
A network can be reduced to the simple parallel RLC circuit for TRV calculations. This representation is valid for short-time frames until voltage reflections return from remote buses.
TRV HV CIRCUIT BREAKERS (Vcb)
L
C
R
•
The TRV in the parallel RLC circuit is oscillatory (underdamped) if R 〉
•
1 L/C 2
The TRV in the parallel RLC circuit is exponential (overdamped) if 1 R ≤ L/C 2
TRV HV CIRCUIT BREAKERS TRV (p.u.) 2
0.5
R / (L / C)
1,8
= 10 4
1,6 2
1,4 1
1,2
0.75
1 0,8 0,6
0,5
0,4 0,3
0,2 0 0
1
2
3
4
5
6
7
8
t / RC
9
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 the interrupting chamber(s), it is possible to effectively reduce the rate-of-rise of TRV. This possibility has been widely used for many years to ease the current interruption by air-blast circuit breakers.
•
When longer time frames are considered, typically several hundreds of micro-seconds, reflections on lines have to be taken into account. Lines or cables must then be treated as components with distributed elements on which voltage waves travel after current interruption. These traveling waves are reflected and refracted when reaching an open circuit or a discontinuity.
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 BREAKERS 7.2
The system TRV may exceed the standard capability curve, which is described by a two-parameter envelope where uc and t3 are defined in ANSI C37.06 for 10% short-circuit breaking capability, maximum voltage. For currents between 10% and 30% of rated short-circuit current, values of uc and t3 can be obtained by linear interpolation. 400 350
TRV CAPABILITY FOR A STANDARD BREAKER
300
VOLTAGE (kV)
•
Series reactor limited fault
250 200 150 100 SYSTEM TRV CURVE
50 0 0
10
20
30
TIME (µs)
40
50
60
Simple oscillatory LRC Circuit • Natural frequency f = 1/(2π√LC ) • Natural impedance = √(L/C)
L R
C
CB
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.
TRV HV CIRCUIT BREAKERS
Traveling waves on a faulted line after current interruption
TRV HV CIRCUIT BREAKERS
Voltage distribution on the line at different times after current interruption
TRV HV CIRCUIT BREAKERS VOLTAGE (p.u.) 2
VOLTAGE AT CIRCUIT BREAKER TERMINAL POINT C x = 0
tL/4
0.5 tL
3 tL/4
0
tL
1.5 tL
TIME VOLTAGE 3/4 OF WAY TO FAULT x= 0.75 L
-2
VOLTAGE HALF WAY TO FAULT x = 0.5 L
Time variations of voltages at three locations on the faulted line
TRV HV CIRCUIT BREAKERS Annex 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 that obtained for a bus fault due to the additional reactance of the line.
•
IT = fault current through the circuit breaker for a single-phase fault at the breaker terminal
•
IL = the reduced current for a line fault.
XS VLG
XL × λ
V LG I L= XL λ + XS
V LG XS = IT IL =
V LG X L λ + V LG / I T
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 Z0
is the positive sequence surge impedance is the zero sequence surge impedance
d = 2ω
d=
v ω
Z eff XL v
VCDo + VCDp VCDo is the velocity of light is 2 π × system power frequency (377 rad/s for a 60 Hz system)
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)
