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Uninterruptible Power Supply
The Utilization of Thyristor-SCR Drives to
Protect Data Center-Based Products.
Kevin Oresick; Matt Logue
EE 435 – Introduction to Power Electronics
West Virginia University
Contents
 Introduction
 Background on Devices
 Protection of the Devices
 PSPICE Simulation of SCR Devices
 Simulink Simulation of SCR Devices
 Conclusions
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Introduction
• Over the past few decades, there has been considerable research
and advancement in uninterruptible power supplies.
• More importantly, with the advancements in modern power
electronic devices, engineers have been able to utilize different
drive technologies to enhance overall system security.
• This specific project is based primarily off of these advancements
and is inspired by the ever increasing usage of server-based
products, storage of large amounts of data, and the havoc that can
be caused on a business and their clients by an outage/emergency.
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Introduction
• An uninterruptible power supply (UPS) is essentially an electrical device
that will provide power to a load when the input source fails, usually during
an emergency.
• This particular design encourages the use of thyristors to create a
converter that stores energy in order to supply a base of servers during the
unfortunate occasion of a blackout, power outage, inclement weather, or
other unpredictable disasters that can potentially cost businesses a
fortune.
• Make note that a UPS differs significantly from that of a standby generator
or a back-up emergency source due to the design parameters.
• While both supply power during an outage, only a UPS will provide nearinstantaneous protection from any interruptions of the source, in which
energy is supplied from battery or flywheel storage. Because a UPS is
supplying energy to a large load, the UPS is only capable of supplying to
the respective load for a brief period of time.
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Background on Devices
 AC Power Supplies
 Thyristor SCR Devices
 Teccor brand Thyristor
 Fast-Acting Semiconductor Fuses (LittleFuse, Inc.)
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AC Power Supplies
•
AC power supplies are commonly used as standby sources for critical loads and in
applications where normal AC power supplies are not available. These standby
sources are not only reliable, but the conversion (or switch) happens almost
instantaneously.
•
There is ideally no need for breaking the supply if there is a failure. Something to
consider is the type of battery material utilized, however that is particularly out of the
general scope of this project.
•
The main focus is to choose appropriate switching devices and their respective
parameters.
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Thyristor SCR Devices
•
Since the first thyristor silicon-controlled rectifier (SCR) was developed, many
advances have been accomplished in power semiconductor devices. A thyristor has
three terminals; an anode, a cathode, and a gate. When a small current is passed
through the gate terminal to cathode, the device conducts.
•
There are many types of thyristors and each type has their own specific
characteristics and usages.
•
Once a thyristor is in a conduction mode, the gate circuit has no control and the
device will continue to conduct. Also, the forward voltage drop is very small, often
between 0.5V and 2V.
•
A functioning thyristor can be turned off by making the potential of the anode equal to
or less than the cathode potential.
•
The UPS design incorporates standard phase-controlled SCRs that are triggered with
few milliamps of current at less than 1.5V of potential [Littlefuse, Inc 2010].
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Teccor brand Thyristor
•
The Teccor SCR is an excellent unidirectional switch optimized for phase-controlled
applications. The typical applications are AC solid-state switching, industrial power
tools, exercise equipment, white goods and commercial appliances.
•
The SCR of choice for the UPS was a thyristor designed by Teccor. The device is a
Sxx20x and Sxx25x Series thyristor and can be purchased from LittleFuse
Incorporated.
Figure 1. Teccor thyristor.
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Teccor brand Thyristor
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Teccor brand Thyristor
Note: Device Sxx25L
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Fast-Acting Semiconductor Fuses
 Fast-Acting Semiconductor Fuses (LittleFuse, Inc.)
Model #: LA100P25-1
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Fast-Acting Semiconductor Fuses
•
Device LA100P25-1 applications
include but are not limited to
protection of UPS systems, AC/DC
drives, reduced voltage motor
starters and other 1000V or less
semiconductor devices.
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Protection of Devices
 Types of Device Protection
Which Device for Which Application?
 Thermal Protection
Cooling/Heat Sink
 Overcurrent Protection
Fusing Devices
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Types of Device Protection
•
Due to the reverse recovery process of power devices and switching actions in the
presence of circuit inductances, voltage transients occur in converter circuits. Even
thoroughly designed circuits can have short-circuit fault conditions, resulting in excessive
current flow through the devices.
•
The heat produced by losses in a semiconductor device must have the appropriate
apparatus to dissipate sufficiently to ensure that the device can operate efficiently within
the specific device temperature limitations.
•
In practical terms, power devices are designed to be protected from thermal runaway by
heat sinks, high changes in currents and voltages by snubber circuitry, reverse recovery
transients, and supply/load-side transients, and lastly, fault conditions by fuses.
•
For sake of the UPS design, thermal cooling and fuse protection will be discussed.
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Heat Sinks and Cooling Devices
•
Due to on-state and switching losses, heat is generated within the power device.
Inevitably, this heat must be transferred from the device to a cooling unit to maintain a
proficient operating environment. There are many ways to accomplish the heat transfer,
however this design explicitly uses convection cooling or the installment of a heat sink.
•
Heat must flow from a device, to the case, and then to a heat sink in the cooling medium.
To appropriately add a heat sink to a system, one must calculate the junction temperature
of the device  .
•
This temperature can be found by the following equation:
 =  ( +  +  )
where  = thermal resistance from junction to case,
 = thermal resistance from case to sink,
 = thermal resistance from sink to ambient,
 = ambient temperature.
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Heat Sinks and Cooling Devices
Figure 7. Thermal resistance characteristics of heat sink apparatus [Rashid, 2003].
• There are further assessments for cooling devices, and more options are
available (discussed in oversight).
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Fusing Devices
•
A thyristor requires a minimum time to spread the current conduction uniformly throughout
the junctions.
•
If the rate of rise of anode current is fast compared with the spreading velocity of a turnon process, excessive heating may occur due to high current density and the device may
possibly fail as a result of excessive temperatures [Rashid, 2003].
•
The chosen semiconductor device (thyristor SCR) may be protected by carefully choosing
the locations of the fuses.
•
As a safety measure, most manufacturers strongly suggest placing a fuse in series with
each device.
•
This provides individual protection that permits better coordination between a device and
its fuse, allows conforming utilization of the device, and obviously protects from shorting
faults.
•
Not all fuses are identical, and choosing the right fuse can be a daunting task. However,
by knowing a few parameters, the correct fuse can be obtained.
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Fusing Devices
•
In selecting a fuse, it is necessary to estimate the fault current and then to satisfy the
following requirements:
1) The fuse must carry continuously the device rated current.
2) The i2t let-through value of the fuse before the fault current is cleared must be
less than the rated i2t of the device to be protected.
3) The fuse must be able to withstand the voltage, after the arc extinction.
4) The peak arc voltage must be less than the peak voltage rating of the device.
•
Some things to note are the following: the i2t value is termed as the let-through energy
which is responsible for melting the fuse.
•
For the UPS operation, a fast-acting fuse is needed. A general rule of thumb is that a fastacting fuse with an RMS current rating equal or less than the average current rating of the
thyristor normally can provide adequate protection under fault conditions [Rashid, 2003].
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PSPICE Simulations
•
The following section includes all preliminary data simulated, schematic design of
the SCR drives, and output waveforms of the RMS voltage no load/full load, the
system current at full load, the SCR current at no load/full load, and the SCR gate
voltage pulsed 180 degrees apart.
NOTE: Tables are excerpt from design documentation.
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PSPICE Simulations
•
The following figure is the circuit schematic of the SCR designed specifically for
utilization within the UPS system. Although the following diagram is the design of a
thyristor rectifier, the device design requires eight thyristors total (four thyristor
rectifiers and four thyristor inverters).
Figure 8: Thyristor –SCR rectifier design schematic.
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PSPICE Simulations
Figure 9: RMS Voltage waveform at No Load.
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PSPICE Simulations
Figure 9: RMS Voltage waveform at Full Load
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PSPICE Simulations
Figure 10: Current waveform at Full Load
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PSPICE Simulations
Figure 11: SCR Current waveforms at Full Load
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PSPICE Simulations
Figure 12: SCR Current waveforms at No Load
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PSPICE Simulations
Figure 13: SCR Gate Voltages when fired 180 degrees apart.
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PSPICE Simulations
Heat Sink – Fuse – Power Calculations
Figure 14: Desired heat sink and appropriate parameters
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PSPICE Simulations
Heat Sink – Fuse – Power Calculations
Figure 15: Fuse selection parameters.
NOTE: Power loss based on all thyristors in system
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Simulink - An Alternative Approach
Figure 16: Simulink design of functional UPS system.
Figure 17: UPS Layout container.
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Simulink - An Alternative Approach
Figure 18: Simulink design of thyristor rectifier.
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Simulink - An Alternative Approach
Figure 19: Simulink design of thyristor inverter.
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Simulink - An Alternative Approach
Figure 20: Simulink scope of current output at UPS activation.
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Simulink - An Alternative Approach
Figure 21: Simulink scope of RMS voltage output at UPS activation.
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Closing Remarks
Future Design Consideration
Electromagnetic Interference (EMI)
High-Power Application Cooling
Oversight?
Overall Design Conclusion
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Future Design Considerations
 Electromagnetic Interference
•
Power electronic circuits switch on and off large amounts of
current at high voltages and thus can generate unwanted electrical
signals which affect other electronic systems.
•
These unwanted signals occur at higher frequencies and give rise
to EMI, also known as radio frequency interference (RFI).
•
Numerous sources of EMI (atmospheric noise, lightning, radar,
radio, television, etc).
•
Any power converter is a primary source of EMI.
•
In the future, we will research appropriate approaches to minimize
EMI generation within a desired system.
• Shielding, advanced control techniques for minimizing
input/output harmonics, operating at unity input power factor,
soft switching, and lower total harmonic distribution (THD).
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Future Design Considerations
 High-Power Application Cooling Techniques
•
Devices are more effectively cooled by liquids, normally oil or water.
•
Water cooling is very efficient and approximately three times more effective
than oil cooling.
•
However, it is necessary to use distilled water to minimize corrosion to some
applications and antifreeze to obviously avoid freezing.
•
Oil cooling, which may be restricted to some applications, provides good
insulation and eliminates the problems of corrosion and freezing (However,
oil is flammable).
•
Heat pipes and liquid-cooled heat sinks are commercially available.
•
These are just some alternatives to the conventional heat sink design.
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Conclusive Thoughts
Overall Design Conclusion
•
The UPS system will function appropriately for approximately 15 minutes.
• This is an adequate amount of time to activate alternative sources or
shut down data centers, servers, etc.
•
The main purpose of the UPS system is to ensure that a load is
continuously powered during the rise of a emergency or outage.
• Not designed to operate for long periods of time.
• Only long enough to activate a secondary back-up generator or
alternative source.
•
Ultimately, the UPS proves to be a reliable system that can save companies
multitudes of data and valuable hardware.
• More importantly, this fail-safe makes clients feel confident that data is
safe.
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Conclusive Thoughts
•
Questions?
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