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A Technical Paper Presentation
On
Power Quality Improvement With
Solid State Transfer Switches
.
KUPPAM ENGINEERING COLLEGE,K.E.S NAGAR,
KUPPAM-517425
Submi tted by :
. Ph: +91 8688780435
G.AJAY KUMAR
III E.E.E
Mail : [email protected]
PH: 8688780435
P.SASI KANTH
III E.E.E
. K.E.S.Kuppam
Mail:[email protected]
PH: +91 9652361267
ABSTRACT:
The solid state transfer switch (SSTS)
is designed to replace the mechanical auto
transfer equipment currently in use to switch
major industrial and commercial facilities
from one feeder to another - a process that
typically takes 0.3 to several seconds. A SSTS
can also provide large customers with a cost-
effective alternative to an in-house
uninterruptible power supply system. This
paper presents a new SSTS concept based on
bi-directional control thyristor (Bn)
technology for the mitigation of voltage
deviations (sags or swells), interruptions and
other power supply system faults. A utility
application of the solid state transfer switch is
described and its performance discussed.
INTRODUCTION:
Power quality issues are currently
receiving a great deal of attention in the light
of deregulation, liberalization and
privatization of the electrical energy market.
The overriding concern is that poor power
supply quality may cause disruption to a
consumer's process leading to a loss in
revenues. An essential prerequisite to a
profitable process-oriented industrial
operation is therefore a safe, reliable and clean
power supply.
The solid state transfer switch (SSTS)
is designed to replace the mechanical auto
transfer equipment currently in use to switch
major industrial and commercial facilities
from one feeder to another - a process that
typically takes 0.3 to several seconds. For
sensitive electronic based electrical equipment
this transfer time and momentary interruption
result in voltage deviations which may be
beyond the designed ride-through capabilities
of the equipment. A SSTS can also provide
large customers with a cost-effective
alternative to an in-house uninterruptible
power supply system.
The first section of the paper provides
a functional description and typical circuit
configurations of the SSTS. A comparison of
the different power electronics technologies
that are applicable for a SSTS implementation
is then presented. Finally, an application of a
solid state transfer switch in a utility provided
power quality solution is described and its
performance is discussed.
FUNCTIONAL DESCRIPTION OF
SSTS AND TYPICAL, CIRCUIT
CONFIGURATIONS:
The SSTS as shown in Figure I is a
high-speed, open transition switch which
enables the transfer of electrical loads from
one AC power source to another within a few
millisecond.
Fig :Solid State
Power Quality Improvement with Solid State Transfer Switches 2
Transfer Switch System
The open-transition property of the
SSTS means that the switch breaks contact
with one source before it makes contact with
the other source. The advantage of this
transfer scheme over the closed-transition
mechanical switch is that the electrical
sources are never cross-connected
unintentionally. The cross connection of
independent AC sources, with the alternate
source switching on to a faulted system is
discouraged by electric utilities.
The solid state transfer switch consists
of two three phase ac thyristor switches. The
thyristor, operating in its two modes, forms
the key component of the SSTS. In the ON-
state mode, low impedance forward
conduction of current takes place. In the OFF
stage mode, an open circuit with almost an
infinite impedance occur in the thyristor.
The basic ON-state and OFF-state
properties of the thyristor are used to form an
intelligent switch which can choose between
two upstream power sources providing the
better quality of supply available to the
electrical load downstream. The basic
configuration is based on anti-parallel
thyristors group on the preferred and alternate
sides of the switch. A thyristor allows
conduction only in the forward direction.
Figure 2 illustrate how the thyristors of
transfer switch 1 can conduct either in the
positive or the negative half cycle of the AC
sinusoid and the supply path is indicated by
the bold line.
Fig. 2: Thyristors of SSTS Conducting in
Positive
& Negative Half cycles of the Preferred Source
During normal operation, thyristors
associated with the preferred source are in the
ON-state normally closed (NC) position,
while those associated with the alternate
source are in the OFF-state normally open
(NO) position.
Current sensing circuits constantly
monitor the states of the preferred and
Power Quality Improvement with Solid State Transfer Switches 3
alternate sources and feed information to the
monitoring high speed controller. Upon
detecting the loss of the preferred source or
voltage that is not within the preset range, the
controller blocks the firing impulse signals to
the gate-driven thyristors of transfer switch 1
and instructs the thyristors of transfer switch 2
to turn ON with a fail-safe interlocking
mechanism. Power then flows via the path as
indicated by the bold line in Figure 3.
The transfer from the preferred to the
alternate source occurs rapid enough, that
even the most sensitive electrical OT
electronic loads are not disrupted.
Fig. 3: Thyristors on the Alternate Supply are
Turned ON on
Sensing a Disturbance on the preferred source
The mechanical bypass equipment
provides conventional transfer switch
functionality when the SSTS is in a thermal
overload condition or is out of service for
testing or maintenance.
TYPICAL CIRCUIT
CONFIGURATIONS FOR SSTS
INSTALLATIONS:
Circuit configurations of a SSTS
installation are dependent on several factors
such as the availability of an existing alternate
source, the size of critical loads, the need to
protect single or several separate critical
loads, whether it will be a customer or utility
provided solution and economical
considerations. Three typical circuit
configurations of SSTS are shown in Figures
4,5, and 6.
The dual service circuit configuration
shown in Figure 4 is the most widely
implementation of the SSTS system. Under
normal supply conditions, the critical load
is supplied by the preferred source. When a
disturbance or voltage sag on the preferred
source occurs, due for example, to an
upstream fault, the critical load is
transferred to the alternate source in less
than half a cycle.
For the bus coupler circuit
configuration shown in Figure 5, separate
critical loads are served independently
from the two separate sources A and B.
The supply paths are indicated by the bold
lines. When a disturbance occur on source A,
the solid state switch for critical load feeder A
opens, the bus coupler solid state switch
closes and the entire load is then served by
source B with minimal disturbance to the
loads. Note that for this case, the switches on
both the feeders must be dimensioned to carry
the total current of both the loads.
Power Quality Improvement with Solid State Transfer Switches 4
Fig. 4: Dual service circuit configuration Fig. 5: Bus coupler circuit configuration The third circuit configuration for the
SSTS system as shown in Figure 6 is different
from the previous two by additionally
providing protection against disturbances and
sags from a downstream load side fault. In the
case of a disturbance on the source on BUS A,
the NC switches connected on BUS A will
open and the NO ones on BUS B will close.
The load feeders 1 and 2 will then be supplied
by SOURCE B. When a disturbance occurs on
one downstream load feeder, for example
feeder 3, the NC switches on the downstream
side BUS B of load feeder 4 connected to
source B opens.
The upstream solid state NO switch on
BUS A of load feeder 4 then closes within a
few milliseconds, allowing the load on this
feeder to be supplied by SOURCE A as
shown by the dotted line in Figure 6.In this
way, the healthy critical feeder connected to
the same source as the faulted feeder will not
be disconnected and hence there is an increase
in the overall availability of energy for the
plant even for faults on the downstream
feeders. The fault is cleared within the
duration associated with the operation times
of downstream relays and circuit breaker
located on feeder 3.
Fig. 6: Bus transfer circuit configurationBesides these basic configurations,
many other customized configurations are
possible depending on specific project
requirements.
Power Quality Improvement with Solid State Transfer Switches 5
THYRISTOR TECHNOLOGIES FOR
SSTS IMPLEMENTATION:
The thyristor technology most
suitable for a particular SSTS application
depends on the response time required, the
load current, the short-circuit requirements
and application voltage level.
Thyristor technologies that are
considered in this section are:
1. Conventional Phase Control
Thyristors,
2. Bi-Directional Control
Thyristor (BCT),
3. Integrated Gate Commutated
Thyristor (IGCT).
Phase Control Thyristors (ET) is the
most common thyristor technology being
applied to medium voltage SSTS systems.
Systems based on the PCT will transfer within
a quarter cycle for under voltage conditions
and may take up to half a cycle for over
voltage or large out of phase conditions.
These systems have large current carrying and
short-circuit withstand capability. Since PCI'
technology is a mature technology, the
reliability and cost of SSTS systems based on
this thyristor technology are well established.
Bi-directional Control Thyristor
(BCT) is a new development in high power
phase control thyristor
technology. Two anti-parallel high power
thyristors are integrated onto one single
silicon wafer and are assembled into one
housing. This new feature enables SSTS
system based on BCT to meet higher demands
concerning size, reliability and cost. Figure 7
shows on the left a PCT and right the BCT
stack assemblies. For the stack itself, the BCT
solution needs only 50 8 of the mechanical
and electrical parts that are used in the PCT
solution.
Fig. 7: Reduction of components in thyristor stacks due
to the use of BCT instead of PCT technologyIntegrated Gate Commutated
Thyristors (IGCTs) are state of the art in gate
turn-off (GTO) thyristor technology. They
combine the low conduction losses associated
with thyristors and the snubber less excellent
turn-off capability of transistors. Thus for
sensitive loads requiring demanding transfer
time of under a quarter cycle, IGCT based
SSTS systems can achieve this performance
target. IGCTs have current carrying capability
of up to 2200 A, turn-off currents of up to 6
kA and a short-circuit withstand of up to
25kA. Cost of IGCT based SSTS systems are
higher than systems based on the previous two
technologies but this price premium is being
rapidly eroded with increased applications and
development. Figure 8 shows an IGCT unit
capable of meeting the most demanding SSTS
application.
Power Quality Improvement with Solid State Transfer Switches 6
Fig 8: A 4.5 kV. 3kA IGCT unit
A SSTS UTILITY APPLICATION:
In this section a utility provided solid
state transfer switch solution based on BCT
technology is described. An industrial
customer connected on the 6 kV level of the
distribution system is supplied via a T
junction from a 50 kV meshed network. When
a fault in the 50 kV
systems occurs, an attempt to clear the
fault by auto-reclosure is initiated using the
breakers U1 and U 2 if the fault is within the
reach of the distance relays. The momentary
interruption due to the auto enclosure process
causes voltage sag ' with the effect that
process control equipment in the
manufacturing facility fail resulting in
production disruptions.
Fig. 9: A utility provided solid state transfer switch solution
to mitigate momentary interruptions
Three solutions were considered, one
with the construction of a second distribution
line, the another with the installation of a UPS
for sensitive loads and finally a proposal for a
solid state switch system. Of the three
proposals, the SSTS system was deemed the
most attractive from the economical point of
view since there exists an independent 16 kV
distribution line of sufficient capacity, in the
vicinity of this manufacturing plant.
This proposed SSTS solution shown in
Figure 9 requires the installation of a new
1616 kV transformer TR2, a solid state switch
on the secondary of the each of the
transformers TRl and TR2. The transfer
switch 1 is normally closed and the transfer
switch 2 normally open. The transformer TR2
is connected to the 16 kV network even for
normal operation but under no load condition.
If a disturbance in the 50 kV network requires
the transfer of the critical load to the 16 kV
supply, inrush currents and any circuit breaker
switching times related to the transformer
TR2 will be avoided. A consideration that
needs to be taken into account is the no-load
losses of the transformer TR2 during standby
mode. Bypass circuit breakers similar to the
SSTS configuration shown in Figure 4 are
needed for maintenance and testing purposes.
The activation of the SSTS system is
through the detection of voltage sag on the 50 Power Quality Improvement with Solid State Transfer Switches 7
kV side of the network. The control system is
made up of a signal sampling block
synchronized by a phase locked loop (PLL), a
fault detection algorithm and a transfer inhibit
block. The SSTS also performs continuous
diagnostics by monitoring thyristor status
(open or shorted conditions) and calculated
device temperature. If multiple thyristor
failures or over temperature condition is
detected within the SSTS any transfer
command will be inhibited and the system
will be safely placed into a bypass mode.
The following requirements must be
met:
Transfer to the alternate source
will take place when the 50 kV
voltage level for any of the three
phases fall below 80 5% the pre-
fault level.
Prevent transfer during load
side (downstream) fault condition.
The static switch is designed to
ride through a fault long enough to
let downstream protection devices
operate and to keep the SSTS as
transparent to the system as
possible with regards to system
protection.
Prevent transfer when the
voltage of alternate source is
outside programmed limits and
keeping the load on the preferred
source.
Prevent transfer when both
sources are out of phase more than
the programmed limits.
On detection of a fault, the firing
signal to transfer switch 1 is blocked to turn it
off and firing impulses sent to transfer switch
2 to activate it. A command is simultaneously
given to open the circuit breaker Ls3. The
power flows through solid state switch 2 from
the 16 kV systems after a successful transfer.
Simulation of the operation of this
SSTS system was carried out using the
Electromagnetic Transient Program. Figure 10
shows the preferred supply with three phase
voltage sag at the 6 kV side of transformer
TR1 and the load voltage during and after
being transferred to the alternate source.
Fig. 10: (a) Preferred supply voltage during three phase fault,
(b) Load voltage during and after transfer.
CONCLUSION:
The basic medium voltage solid state
transfer switch configurations and technology
have been presented and a specific SSTS
utility application described, The new BCT
and IGCT technologies described can be used
to meet the complete range of requirements Power Quality Improvement with Solid State Transfer Switches 8
demanded by even the most stringent SSTS
applications. These SSTS systems provide
high reliability due to well proven
components and low part counts. Using BCT
units, SSTS systems can be more
economically constructed and implemented
due to the encapsulated anti-parallel
thyristors.
REFERENCES:
1. Chan, K.; Kara, A.; Wirth E.,
"Innovative System Solutions for
Power Quality Enhancement", ABB
Review, 3/98 Schwartzenberg, J. W.;
De Doncker, R. W., "15 kV Medium
Voltage Static Transfer Switch", IEEE,
May/June 1995.
2. Backlund, B. et. al., "Bi-
Directional Control Thyristor",
Product Information, ABB
Semiconductors AG, Switzerland,
January 1997.
3. Linder, S. et. al., "A New
Range of Reverse Conducting Gate-
Commutated Thyristors for High-
Voltage Medium-Power
Applications".
4. Proceedings of the 7' European
Conference on Power Electronics and
Applications, Trondheim, Norway,
September 1997.
Power Quality Improvement with Solid State Transfer Switches 9
