Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
Design Interruption Process for HVDC System using Vacuum circuit Breaker
Based on Artificial Current Zero
1
2
3
Ali Raza , Waheed Aftab Khan , Manzoor Ellahi , Bilal Masood
4
1. Electrical Engineering Department, Superior Group of Colleges, aliraza9696@gmail.com
2. Faculty of Engineering & Technology, Superior Group of Colleges, waheed.khan@superior.edu.pk
3. Faculty of Engineering & Technology, Superior Group of Colleges, manzoor.ellahi@superior.edu.pk
4. Faculty of Engineering & Technology, Superior Group of Colleges, bilal.masood@superior.edu.pk
Abstract: This paper deals with the interruption
scheme designed using Vacuum Circuit Breaker
(VCB) based on zero crossing characteristics,
implemented electromechanically for High Voltage
Direct Current(HVDC) system. The operation is
carried out using Mechanical scheme as it has
minimum energy loss in comparison to Solid-State
and Hybrid schemes.TheMechanical scheme consists
of series connection of multiple modules, variable
resistors, VCB, and RC snubber circuit. The
commutation is performed through a commutation
branch that consists of LC circuitry and a switch. An
inverse current is injected in the system through
commutation branch at the instant of fault occurrence
to interrupt the arcing process. MATLAB is used for
designing the simulation of the proposed scheme.
The results of the simulation are analyzed and
improvements from previous schemes are discussed.
Keywords:HVDC,VCB,
branch, Interruption
1.
Hybrid,
Commutation
INTRODUCTION
The classical HVDC system relied on the point to
point distribution technology but, with the increased
focus on enhancing DC transmission, there is a need
of interconnection of multiple transmission lines.
This will cause to increase the transmission and
operating cost. In Current Source Converters
(CSC)based systems, complex filters and capacitors
were required to remove the harmonics of alternating
current (AC) [1]. Moreover the flow of power was
unidirectional. The reversal of polarity in this
technology was difficult while bidirectional flow of
power is the essential part of modern distribution
system [1, 2]. The solution of this problem is to use
multiple terminal high voltage DC (MTHVDC)
technology. Voltage source converter (VSC)
technology can be utilized in MTHVDC transmission
system. The design of VSC is based on the Insulated
gate bipolar transistors (IGBT). Voltage and power in
the VSC technology are controllable. The added
advantage in using VSC is the use of small size filters
than the CSC because only high frequency harmonics
rise in the VSC based systems.
In spite of many advantages, the rate of change of
short circuit current is very high because of low
inductance in the DC system [3, 4]. Therefore, there
is a need of reliable circuit breaker to interrupt the
fault current in a certain time otherwise whole system
will have to shut down [5]. The main issue of the DC
system is to design a reliable interruption scheme.
Nowadays, three interruption schemes are mostly
used such as, solid state (SS) DC interruption
scheme; electromechanical schemeand lastly hybrid
technique for interruption process. The Mechanical
scheme had a main constraint of very poor
interruption time.
The proposed scheme for DC interruption which is
based Artificial Zero-Crossing (AZC) is shown in fig
1. There are four branches in this scheme for
interruption of fault, vacuum circuit breaker (VCB),
metal oxide varistors and commutation branch
consists of pre charge capacitor C in series with
inductance L. Multiple VCBs are connected to
increase the voltage level. In the occurrence of fault,
VCB is opened first. When impulse of current
reaches a certain safe value then contacts of VCB
gets open and switch S of commutation branch is
closed. High frequency arc is generated due to
discharge of commutation capacitor through the
inductance and branch switch S. These high
frequency oscillations are overlapped on the VCB
current. Thus, artificial zero crossing scheme forces
the arc which is produced due to VCB to quench.
There is a high arc extinguish ability of vacuum due
to perfect insulation.
Back-up Circuit Breaker
VCB
MOV+Inductor
VCB
MOV+Inductor
Communication Branch
Fig. 1: Proposed Interruption Scheme
Back-up Circuit Breaker
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
Solid state circuit breaker based on semiconductors
and this circuit breaker performs very fast switching.
This technology can also be said ultra-fast switching
technology.
IGBTs or different semiconductor based switches are
mostly used in this type of circuit breaker. These
switches are used in series and parallel combination
to control the circuit. Further research is carried on to
make better the circuit breaker [6, 7].
Fig.2: SS CB with surge arrestor [6]
2.
Literature Review
A significant research has been performed on HVDC
circuit breakers. An important development is
attained by ABB. Modularized scheme is used by
ABB for designing of HVDC circuit breaker. Each
module has 80KV capability. In this scheme
mechanical switch and ultra-fast switch technique of
power electronics is used. An experimental work is
carried out for verification of this scheme on 320KV
with 2.6KA rated current in [8]. Y. Niwa and fellows
have designed a dc circuit breaker using vacuum for
arc quenching purpose. Commutation is created by
thyristor using as a switch. The rating of this breaker
is 1500KV with 100KA. It is tested on traffic power
system railway track [7]. 3KV high speed DC
vacuum circuit breaker is designed by M. Batosik and
fellows [9]. Shi et is performed an experiment using
triggering gap on voltage. In this scheme current was
stressed to attain zero limits at 30KA while the
voltage level was about 3KV [10].
2.1 SS circuit breaker parallel with a surge
arrestor
In this type of circuit breaker solid state
semiconductor switch T plays a main circuit breaker
parallel with an arrestor as shown in fig. 2 [6]. In
normal operating condition switch T is in on state and
current flows through the switch. When fault occurs
in a system switch T will detect the fault and gets
open and load current will flow from surge arrestor.
It increases the voltage across surge arrestor. If we
know the fault current, arrestor voltage and dc
voltage then we can find the opening time of circuit
breaker and we can also find the absorbed energy by
the arrestor.
2.2 SS circuit breaker with freewheeling
diode
In this type topology a freewheeling diode and surge
arrestor are connected in a branch while this branch is
parallel across voltage dc source as shown in fig. 3.
In normal operating condition switch is closed and
current flows through switch T. In the occurrence of
fault switch gets open and current flows through the
diode. The inductance starts to demagnetize through
the surge arrestor and fault will go to decrease. The
surge voltage can be expressed as a sum of arrestor
voltage and dc voltage source. If we know the
inductance and fault current then we can find
absorbed energy during interruption process.
W=(1/2)*Li^2.
T
Ldc
DC
Surge Arrestor
This paper has been divided into six sections,starting
from Literature Review and Electromagnetic
Repulsion (EMR) in VCB is given in section 2 and 3
respectively. Sections 4 and 5 give details of
Research Methodology and Interruption Process.
Simulation results are presented and discussed in
section 6, followed by conclusion.
+
-
IL
D
Short Circuit
Facult
Fig.3: SS CB with freewheeling diode [6]
2.3 Passive Resonance Circuit Breaker
Passive resonance circuit breaker shown in fig. 4was
an important part for arc quenching at lower current
values in CSCHVDC systems as discussed in [12].
Surge Arrestor
Ia
Cc
Lc
L
Io
Is
CB
Fig.4: Pure mechanical HVDC circuit breaker [13]
3.
Electro Magnetic Repulsion (EMR) in
VCB
The fig. 5shows the structural diagram of Vacuum
Circuit Breaker (VCB) [6]. A pulse of current in the
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
coil is generated when a fault occurs in the system.
The alternating current creates magnetic field around
the coil. Thisinduced magnetic field in the metal plate
produces eddy current causing a repulsive force is
between the coil and metal plate.Since the current
flows due to changing magnetic field, it always flows
in the opposite direction toits cause. Therefore
moving plate moves rapidly.
(2)
Where, S shows the displacement of metal plates.
The movement EMR can also be calculated using
equation 3,
(3)
Fdamp represents the resistance between the
movements of EMR, Fhold is the permanent magnet
force and m is the mass of moving part.
4.
Fig.5: Structure diagram of VCB [14]
VCB circuit topology works on the principle of RLC
oscillatory discharging mode. Current is flowing in
positive direction to avoid reverse charging of
capacitor. This works in RL discharging mode. The
fig. 6 shows the equivalent circuit diagram for the
metal plate that is composed of series inductance and
resistance.
M
C
L1
D
+
R1
I1
L2
I2
The block diagram of proposed scheme is show in
fig. 1. This scheme is divided into four pieces, such
as main VCB used as central circuit breaker (CCB),
secondly the varistors branch, thirdly commutation
branch to create zero crossing in the scheme for
interruption purpose and lastly the backup circuit
breaker to eliminate the residual current. The
monitoring and control section is not given in fig.2.
The proposed scheme is in shown in Fig. 7. The
scheme is based on the modular principle. Therefore
CCB contains multiple modules in series up to “n”
number of modules, which are based on the design of
the system voltage. Each module comprises of two
parallel branches a low voltage designed VCB and
varistor. The working principle of VCB is based on
repulsion by electromagnetism using permanent
magnet. If fault occurs in a system then
electromagnetic repulsion system respond quickly.
R2
SCR
Fig.6: The equivalent circuit of EMR [14]
The pulse of current during the process can be written
such as in equation 1,
(1)
While α=R/2L and
Research Methodology
=
Where,Uo represents the pre-charged voltage and Leq
shows the equivalent inductance of metal plate.
According to law of conservation of energy [15], the
power supply is the sum of heat loss, magnetic
energy and working process.
The electromagnetic force is calculated by using
following equation 2,
The usage of modularization of VCB has two reasons
over a single high voltage VCB. This technique has
two advantages over previous schemes. Firstly, if
contact stroke is increased from 2mm than dielectric
strength between vacuum gaps increases nonlinearly
[16]. The design of high voltage VCB is difficult to
achieve due to this behavior of vacuum gap. To
eliminate this problem, multiple VCBs are connected
in series [16, 17]. The vacuum gap of multiple small
VCBs can be equal or greater than main single circuit
breaker with some factor. The enhancement factor of
breakdown voltage can be derived as follows. The
breakdown voltage of vacuum gap can be calculated
as [18],
(4)
Where Ub is break down voltage while the range of
“α” is 0.4-0.7 and k is constant. Then n numbers of
vacuum gaps are connected in series to calculate
breakdown voltage described in following equation
[18].
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
(5)
The breakdown voltage Ubn for single vacuum gap
and enhancement factor are given in below equations.
(6)
Secondly, in this scheme a commutation capacitor is
pre-charged by this back up circuit breaker.
(7)
In another respect, modularize scheme is more power
efficient then single VCB gap due to small axial
dimensions of vacuum circuit breaker at low voltage
level. Therefore the opening speed of circuit breaker
will be increased during fault interruption. Moreover
the bearing transient recovery voltage is also on small
scale as compared to single long vacuum gap.
It indicates that commutation current is injected at
small value after separation of contacts. It reflects
that fault current can interrupt rapidly due to high
speed of contacts opening and small commutation
stroke.
VCB1
Ro
L
BCB
MCB
L MCB
5.
Interruption Process of the Proposed
Scheme
To explore the behavior of residual current due to
weak arc quenching capability of triggered sphere
gap through switch S, simulation is performed.
Therefore arc will be interrupted at forced zero
current by MCB (using VCB). During the
interruption process back up switches are used to
interrupt the residual current. The interruption
process is as follows.
BCB
Uo
RL
S
Lastly, the backup circuit breaker acts as an isolator.
It isolates the electrical supply from the DC circuit
breaker.
T0:
The system is operating in normal state
while 650A current is flowing and commutation
capacitor is pre-charged up to 80KV.
VCB2
L
Ic
L
might be some residual current flowing due to long
burning of arc in triggered sphere gap after
quenching of arc from main circuit breaker forcedly
to zero. This residual current
C
T1:
A short circuit fault takes place. The
monitoring and control system detects the fault and
gives the opening signal to EMR system of every
module of CCB. It starts operating to open the
contacts of breaker quickly due to the arc is
formation.
Fig.7: Schematic diagram of proposed scheme
The high increasing rate of fault current can be
controlled by series RL snubber circuit before the
modules of VCB. The metal oxide varistor is
designed to control the peak value of transient
recovery voltage. It is parallel connected with VCB
because it also absorbs the energy during the
interruption process.
Commutation branch contains inductor L and
capacitor C and their values can be designed on the
basis of desired frequency and magnitude of countercurrent, this single branch is paralleled to multiple
modules. To attain bi-directional interruption, a
trigger sphere gap is adopted as a switch S.A back up
circuit breaker is used on the left side of the main
circuit breaker.
As described above, the arc quenching capability of
triggered sphere gap (DCCB) is very weak in bidirectional arc extinguish technique. DCCB cannot
interrupt bidirectional current completely. Therefore
T2:
After reaching the short circuit current up to
4.6KA. Commutation switch is triggered as it injects
high frequency oscillating commutated current when
CCB approaches a certain safe limit to withstand the
transient recovery voltage
T2-T3: During the first commutation process currents
starts to shift from CCB to commutation branch. In
this process commutation capacitor starts to
discharge and its voltage continuously decreases.
T3:
At the end of first commutation process
current is completely transferred to commutation
branch. The arc in CCB quenches due to
forciblytaking the current to zero, as a result high
energy is generated. To absorb this energy a series
inductor with metal oxide varistor is used.
T3-T4: The capacitor starts charging after
discharging the capacitor during first commutation
process with positive voltage in reverse direction.
Due to increase of voltage of commutation capacitor,
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
the CCB voltage also increases. Therefore current
starts to move from commutation branch to varistor
branch.
T4:
This is the second commutation process in
which total current is shifted to arrestor branch.
T5:
The current through switch S crosses zero
crossing after flowing through varistor. Arc can be
quenched in commutation switch at zero crossing if
an auto switch is adopted as experimental and
simulation results shown in [8, 9]. In spite of zero
crossing there is no guarantee that arc can be fully
quenched. It is totally dependent on the history of arc
and geometry of sphere gap. It concludes that only
commutation switch S is incapable to cut off current
by itself in the simulation. As a result, residual
current is still flowing in the commutation branch.
T5-T6: The current frequency of commutation and
metal oxide varistor is up to 8 kHz. This current is
decreasing gradually. In this process, the total current
which is flowing is the sum of commutation current,
varistor branch current and charging capacitor.
T6:
The total flowing current will be zero after
cut off through the varistor branch.
In spite of the fact that the commutation switch can’t
quench the arc itself. Due to long time arc burning an
oscillating current up-to 430Acan flow in the DC
system. The oscillating frequency can be calculated
such as follows,
The arc in the triggered sphere gap can be quenched
at a certain current level. It depends on many factors
as discussed above. Therefore, triggered sphere gap is
not a dependable arc quenching switch. To eliminate
the residual current, backup circuit breaker is used as
shown in simulation results fig.11. If contacts of
backup circuit breaker BCB1 are opened at T6 then
arc will be quenched and current flow in triggered
sphere gap will also be zero.
At the end of process, the commutation capacitor
starts to discharge through the resistor, after that the
second backup circuit breaker BCB2 can also be
opened to isolate the whole circuit. By this way
reclosing is done.
System
Parameters
Voltage 80
Source
kV
(U)
Ro
ShortCircuit
Inducta
nce (Lo)
Load
Resista
nce
(RL)
MCB Module
Protectio
n
Voltage
.79
Oh
m
23m
H
150
Oh
m
80
kV
Commutation
Branch
Charging
-40
voltage of kV
Commutat
ion
capacitor
Commutat 7.77k
ion
Hz
frequency
Peak
4.1kA
Current
(Ip)
Capacitan
ce (C)
12µF
Inductanc
e (L)
35µH
Table 1: Scheme Parameters of the proposed Technique
Parameters have been adopted according to commutation
frequency. Commutation frequency is the point where
artificial zero crossing starts.
In the view of simulation results, current
commutation technique has a favorable interruption.
Commutation process depends on commutation
branch and mainly on frequency of commutation
current. If the frequency of commutation increases
then the value of capacitor and inductor will decrease
for the same value of voltage and current. This
indicates smaller volume and lower cost. It is also to
be noted that voltage across commutation capacitor
and peak of desired countercurrent should be greater
than the short circuit current.
As it is discussed above, triggered sphere gap has
weak arc quenching capability to achieve bi-direction
interruption. In this method, commutation current
will overlap to increase short circuit current of the
same direction in the first half cycle. The current is
interrupted in the next half cycle. More energy is
decomposed when superimposition of currents
occurs. This is a big obstruction in the way of
successful interruption. This drawback can be
overcome by injecting high frequency current. For
example 50 KHz frequency is used [16].
6.
Simulation Results
The results of proposed scheme are shown in
following figures. The following results describe the
effect of residual current and the significance of
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
backup circuit breaker. Firstly, the results are shown
without using the backup circuit breaker.
6.1 Interruption results
circuit breaker
without
Backup
Fig. 10: Commutation Voltage
Fig. 8: Source Current
A normal current of 650A is flowing while a fault
occur at 2ms,and then current starts to rise as shown
in Fig. 8. As it approaches the safe limit 4.2KA then
VCB contacts on and switch S is closed. Arc is
quenching at zero crossing at 6.8ms but residual
current continues to flow until backup circuit breaker
works.
Fig. 10 depicts that as commutation capacitor is
inversely pre-charged when switch S is on then
commutation capacitor starts to discharge to create
commutation. When all the current is transferred
from CCB to Metal Oxide Varistor then it gets
charged again in reverse direction. As residual
current continues to flow, therefore its voltage drops
again.
6.2 To interrupt residual current
Secondly, to interrupt the residual current backup
circuit breaker is used,
Fig. 11: Source Current with backup circuit breaker
Fig.9: Commutation Current
When the contacts of VCB open then the transient
recovery voltage approaches to certain safe limit as
depicted in Fig. 9. As a result high current flows and
at the same time commutation switch is closed. Due
to which oscillating current is injected in the system.
In Fig.11 shows that the residual current continues to
flows after breaking the circuit breaker, therefore it is
a need of backup circuit breaker to interrupt the
current zero. As shown in Fig. 11, at 7.8ms backup
circuit breaker interrupts the source current.
Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE
6.3 Commutation current with backup circuit
breaker
Fig 12: Commutation Current with backup circuit
breaker
The Fig. 12 gives the Commutation current with
backup circuit breaker. High transient current flows
in the interruption process due to high transient
recovery voltage. While after switching backup
circuit breaker operatesresulting in no current flow
through the system due to commutation current.
the commutation process. The residual current can be
controlled by using backup circuit breaker.
According to simulation results the residual current
can be damped more rapidly by using high frequency.
Moreover, the thermal joule losses i.e. integral I2*t
created due to residual current can be eliminated
using high frequency. The value of commutation
inductor and commutation capacitor can be different
from each other. According to simulation small
inductance and large capacitance with lower charging
voltage can be more effectivein increasing the rate of
change of transient recovery voltage and interruption
capability will become better.
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7.
CONCLUSION
This scheme works perfectly and interruption time is
4.8ms. As a comparison from ultra-fast switching the
interruption time is slightly greater but scheme works
much faster than previous mechanical circuit
breakers. Also,as compared to previous scheme it is
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