EE 394J-10 Distributed Technologies

1
EE 394J-10 Distributed Technologies
Micro-grids architectures, stability and
protections
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Microgrids architectures and operation

• dc vs. ac
• The discussion refers to the systems main bus.
  Remember the discussion in the first class about
  Edisons electric system.
• No frequency/phase control is necessary in dc
  microgrids.
• From a general point of view dc systems are
  simpler to control.
• Lack of a monitoring variable may complicate
  fault detection and autonomous controls
  implementation.
• Since most distributed sources and energy
  storage devices have an inherently dc output, dc
  architectures are a more natural option for
  integration of such components.
• Most modern loads inherently require a dc input.
  Even the most classical ac loads, induction
  motors, rely more on inherently dc input variable
  speed drives (VSDs) to achieve a more efficient
  and flexible operation.

3
Microgrids architectures and operation

• dc vs. ac
• Availability dc is several times more reliable
  than ac (NTT data from 30,000 systems H. Ikebe,
  Power Systems for Telecommunications in the IT
  Age, in Proc. INTELEC 2003, pp. 1-8.)
• Efficiency
• Efficiency gains in energy conversion interfaces
  makes dc systems 5 to 7 more efficient than
  ac systems.
• dc powered VSDs are 5 more efficient than
  equivalent ac powered VSDs because the
  rectification stage is avoided.
• Dc systems tend to be more modular and scalable
  than ac systems because dc converters are easier
  to control and to parallel.
• dc systems components tend to be more compact
  that equivalent ac ones because of higher
  efficiency and for not being frequency dependent.

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Microgrids architectures and operation

• dc vs. ac
• Modular design makes dc systems more flexible
  and easier to expand, allowing for a more
  effective capital investment management and a
  better planning of the entire facility power
  installation.
• Well designed dc grids can achieve both hardware
  and operational cost savings over equivalent ac
  systems.
• Power conditioning to improve quality tends to
  be simpler in dc systems.
• Stability control tends to be simpler in dc
  systems.

5
Microgrids architectures and operation

• Stability issues
• Stability issues are more prevalent in
  microgrids than in a large electric grid because
  power and energy ratings are much lower.
• Analysis of stability issues in ac microgrids
  follow the same concepts than in the main grid
• Voltage and frequency values need both to be
  regulated through active and reactive power
  control.
• If sources are traditional generators with an ac
  output and are connected directly without power
  electronic interfaces, stability is controlled
  through the machine shafts torque and speed
  control.
• In dc systems there is no reactive power
  interactions, which seems to suggest that there
  are no stability issues. System control seems to
  be oriented to voltage regulation only

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Conventional (ac) datacenters

• Typical configuration
• Total power consumption gt 5 MW (distribution at
  208V ac)

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Conventional (ac) datacenters

• Data centers represent a noticeable fast
  increasing load.
• Increasing power-related costs, likely to equal
  and exceed
• information and communications technology
  equipment cost in the
• near to mid-term future.
• Servers are a dc load
• 860 W of equivalent coal power is needed to
  power a 100 W load

8
New (dc) datacenters

• Use of 380 Vdc power distribution for
• Fewer conversion stages (higher efficiency)
• Integration of local sources (and energy
  storage).
• Reduced cable size

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Data centers efficiency comparison dc vs. ac

• A 380Vdc power distribution standard is
  currently under study by the IEC

Brian Fortenbery and Dennis P. Symanski, GBPF,
2010
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New distributed (dc) datacenters

• Many small distributed data centers powered
  locally and with a coordinated operation
• Energy is used more effectively.
• Generation inefficiencies is energy that is not
  harvested (i.e. converted), contrary to
  inefficiencies in conventional power plants which
  represent power losses.

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Utility dc distribution
Jonbok Bae, GBPF 2011
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dc Homes

• dc in homes allows for a better integration of
  distributed generation, energy storage and dc
  loads.
• With a variable speed drive air conditioners can
  be operated continuously and, hence, more
  efficiently (about 50)

WIND GENERATOR
PV MODULES
MAIN DC BUS
LED LIGHTS (DC)
REFRIGERATOR (LOAD)
ENERGY STORAGE
ELECTRIC VEHICLE
AIR CONDITIONER
EPA 430-F-97-028
FUEL CELL
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ac microgrids stability

• Consider an ac microgrid with one ac generator
  and one load.
• The simplified equivalent circuit for the
  generator and its output equation is
• From mechanics

LOAD
Electric power provided to the load
Assumption Infinite bus (simplifies the analysis
but not true for micro-grids). Also during shorts
or load changes E is constant
Moment of inertia
electrical torque
angular acceleration
mechanical torque
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ac microgrids stability

• If a synchronous reference frame is considered
  then
• Swing equation
• where p.u. indicates per unit and
• So if decreases and if
  increases

Mechanical equivalent of its electrical
homologous variable
Synchronous speed
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ac microgrids stability

• Equal area criterion and analysis during faults
  or sudden load changes (particularly load
  increase). Lets see the most critical case a
  fault.
• After reaching will oscillate until
  losses and load damp oscillations and
• If the generator looses stability.

4) Because of rotor inertia increases up to here
Both areas are equal
1) Initial condition
3) Fault is cleared here
2) During the fault pe 0
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ac microgrids stability

• In ac systems, active and reactive power needs to
  be controlled to maintain system stability.
• Since frequency needs to be regulated at a
  precise value, imbalances between electric and
  mechanical power may make the frequency to
  change. In order to avoid this, mechanical power
  applied to the generator rotor must follow load
  changes. If mechanical power cannot follow load,
  energy storage must be used to compensate for the
  difference.
• Reactive power is used to regulate voltage.
• Some autonomous control strategies will be
• discussed in the future.

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Microgrids architectures and operation

• distributed and centralized architectures
• Power systems with distributed architecture have
  their power distribution and conversion functions
  spread among converters and the distribution is
  divided among two or more circuits.
• There are two basic structures in distributed
  architectures
• Parallel structures are used when the design
  focuses on improved availability.
• Cascade structures are used to improve
  point-of-load regulation, reduce cost, and
  improve system efficiency. Hence, they have at
  least two conversion stages among three or more
  voltage levels.

Cascade structure
Parallel structure
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Microgrids architectures and operation

• distributed and centralized architectures
• The possibility of having different connection
  structures and different conversion stages makes
  distributed architectures more flexible than
  centralized architectures.
• Hence, distributed architectures are the natural
  choice in systems requiring integration of a
  variety of energy sources with several different
  loads.
• When power converters are modular, the
  distributed architecture allows the system
  capacity to expand gradually as the load
  increases over time.
• Thus, distributed architectures have lower
  financial costs than equivalent centralized
  architectures.

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Microgrids architectures and operation

• distributed and centralized architectures
• Examples of distributed and centralized
  architectures can be found in telecommunications
  power plants (remember that telephony grids can
  be considered a low power dc grid).

Only (centralized) bus bars
Centralized architecture
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Microgrids architectures and operation

• distributed and centralized architectures
• Examples of distributed and centralized
  architectures can be found in telecommunications
  power plants (remember that telephony grids can
  be considered a low power dc grid).

Each cabinet with its own bus bars connected to
its own battery string and loads. Then all
cabinets bus bars are connected
Distributed architecture
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Power architectures topologies

• Some examples of different power distribution
  architectures

Radial
Ring
Ladder
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Dc microgrids stability

• Stability issues
• Consider a cascade distributed architecture. The
  point-of-load (POL) converter tightly regulates
  the output voltage on the actual resistive load.
  If Vout is kept fixed regardless of the input
  voltage and R does not change, then the power
  dissipated in the load resistance PL is constant.
  If the POL converter is lossless its input power
  is constant so it acts as a constant-power load
  with input voltage and current related by

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Dc Microgrids Stability

• In reality CPLs have the following form
• For the analysis we will assume that Vlim is
  close to zero.
• Then the dynamic impedance is
• Hence, CPLs introduce a destabilizing effect.

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Dc Microgrids Stability

• Consider the following simplified system of a
  POL converter behaving like a CPL and a buck
  converter regulating the main bus voltage that
  equals the POL converter input.
• Consider also the following circuit parameters
  E 400 V, D 0.5, C 1 mF, L 0.5 mH, PL 5
  kW (the POL converter and load resistance are
  represented by this parameter).
• The system will behave in two possible ways
  depending the initial conditions for the inductor
  current and capacitor voltage

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Dc Microgrids Stability

• If the initial capacitor voltage is high enough
  the systems state variables may oscillate. If
  the initial capacitor voltage is low enough
  and/or the power and inductance are also high
  enough and/or the capacitance is low enough, the
  inductor current will take very high values and
  the capacitor voltage will tend to zero.

iL(t)
iL(t)
vC(t)
vC(t)
vC(0) 120 V
vC(0) 50 V
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Dc Microgrids Stability

• The phase portrait for a buck converter with a
  constant power load with a fixed duty cycle looks
  like this
• For all dc-dc converters it looks similar.

- Approximate for the separatrix
- Necessary but not sufficient condition for
oscillations
Buck LRC with a PL  5 kW, E 400 V, L 0.5 mH,
C 1 mF, D 0.5.
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Dc Microgrids Stability

• Regulating the output with a PI controller
  yields bad results

Simulation results for an ideal buck converter
with a PI controller both for a 100 W CPL
(continuous trace) and a 2.25 O resistor (dashed
trace) E 24 V, L 0.2 mH, PL  100 W, C 470
µF.
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Dc Microgrids Stability

• Model for a buck converter with a CPL

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Dc Microgrids Stability

• Linearization yields that in order to achieve a
  stable regulation point two conservative
  conditions are
• where Ri equals the sum of RSD, RD(1-D), and RL.
  
• Hence, stability improves if
• L is lower
• C is higher
• PL is lower
• Ri is higher
• R0 is lower (higher ohmic load)

(Predominant condition)
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Dc Microgrids Stability

• Consider the following dc microgrid

E1  400 V, E2  450 V, PL1 5 kW, PL3  10 kW,
LLINE  25 µH, RLINE  9 mO, CDCPL  1 mF, and
buck LRCs with L  0.5 mH, C  1 mF, D1 0.5,
and D2 0.45.
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Dc Microgrids Stability

• With an open loop control (fixed duty cycles)
  the system shows again important oscillations,
  well distant of the desired dc behavior.

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Dc Microgrids Stability

• OPTION 1 add resistors in series with the
  circuit inductors or in parallel with the circuit
  capacitors.
• Resistors damp the resonating
• excess energy in the circuit
• This solution is very inefficient.
• A minimum resistive load is needed

1 Ohm resistor added in parallel with the CPL
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Dc Microgrids Stability

• OPTION 2 Add filters, particularly capacitors.
• Oscillations decrease with
• increased capacitances
• Since the oscillation
• frequency s in the order of
• hundreds of hertz, increasing
• capacitance is expensive.
• Large capacitors tend to be
• unreliable.

60 mF capacitance placed at the CPL input and at
the buck converters output.
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Dc Microgrids Stability

• OPTION 3 Bulk energy storage (primarily
  batteries) directly connected to the main bus.
• Telecommunications power systems
• is a typical example of this solution.
• This solution tends to be expensive.
• This solution is more suitable for
• energy systems. For power systems,
• such as microgrids, bulk energy
• storage is not well suited.
• Additional disadvantages in microgrids
• are issues related with reliability, safety,
• and protections when stacking several
• battery cells in series to reach dc bus
• voltages over 150 V. Inadequate cell
• equalization is another disadvantage
• (indirect connection does not work).

1 F ultracapacitor located at the CPL input
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Dc Microgrids Stability

• OPTION 4 Load shedding.
• As PL decreases the oscillation
• amplitude also decreases.
• .
• This solution is not suitable for
• critical mission loads.
• This solution is equivalent to
• load shedding in ac systems

PL3 dropping from 10 kW to 2.5 kW at t 0.25 s
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Dc Microgrids Stability

• OPTION 5 Linear controllers.
• Linear controllers refer to PID-type of
  controllers in which output voltage regulation is
  achieved by creating a duty cycle signal by
  comparing the measured output voltage with a
  reference voltage and then passing that error
  signal through a PID-type controller.
• PD controllers can stabilize constant-power
  loads. The controller adds damping without losses
  through virtual resistances embedded in the
  controller gains.
• An additional integral action is used to provide
  line regulation and to compensate for internal
  losses.
• In some situations (particularly with buck
  converters) a PI controller is enough. But, in
  general, stability is not ensured.
• Advantages
• Simple
• Cost effective
• Disadvantages
• Stability is still not global
• Derivative term create noise susceptibility.

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Dc Microgrids Stability

• OPTION 5 Linear controllers.
• PI controller
• ki 1
• kp 0.1
• With PI controllers stability
• is not insured and results
• are poor.

vB1(t)
vB2(t)
vB3(t)
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Dc Microgrids Stability

• OPTION 5 Linear controllers.
• PID controller
• ki 1 10-3
• kd 50 10-6
• kP 0.5
• Fast dynamics are
• achieved thanks to the high
• proportional gain. However,
• this high gain can be
• usually implemented only
• in buck converters.

vB1(t)
vB2(t)
vB3(t)
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Dc Microgrids Stability

• OPTION 5 Linear controllers.
• PD controller
• kp 0.5
• kd 50 10-6
• Fast dynamics are
• achieved thanks to the high
• proportional gain. However,
• this high gain can be
• usually implemented only
• in buck converters.

vB1(t)
vB2(t)
vB3(t)
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Dc Microgrids Stability

• OPTION 6 Geometric controls (boundary).
• Geometric controls (e.g. hysteresis controllers)
  are based on event-triggered switching instead of
  time-dependent switching.
• With an hysteresis control, the output voltage
  (or inductor current) is controlled to be between
  a band. Whenever the voltage (or inductor
  current) crosses the bands boundaries a switch
  action is triggered (switch is closed or opened).
• Advantages
• High performance (fast)
• Global stabilization
• Disadvantages
• Complicated output regulation analysis may
  require to determine the trajectories. Overshoots
  caused by capacitances and inductances are
  difficult to control
• Lack of a fixed switching frequency

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Dc Microgrids Stability

• OPTION 6 Geometric controls
• A line with a negative slope achieves an stable
  regulating point for all converters. Regulation
  is simple to implement.

Boost
Buck
Buck-boost
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Dc Microgrids Stability

• OPTION 6 Geometric controls.

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Protections

• Consider as an example the power distribution
  system of a full electric ship (which can be
  considered as an islanded microgrid)

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Protections

• Circuit protection conventional approach based
  on switch gear. Issues
• Coordination
• Fault current detection and interruption

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Protections

• Circuit protection based on power electronics
  or solid state circuit breaker.

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Dc systems faults management

• In power electronic distributed architectures,
  faults may not be properly detected because,
  without a significant amount of stored energy
  directly connected to the system buses,
  short-circuit currents are limited to the
  converter maximum rated current plus the
  transitory current delivered by the output
  capacitor.
• If the latter is not high enough, the protection
  device will not trip and the fault will not be
  cleared.
• In this case, the converter will continue
  operation delivering the maximum rated current
  but with an output voltage significantly lower
  than the nominal value.
• Consider the following situation

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Dc systems faults management

• With C 600 µF, the fault is not properly
  cleared and voltage collapse occurs for both
  loads.

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Dc systems faults management

• To avoid the situation described above, the
  converter output capacitance has to be
  dimensioned to deliver enough energy to trip the
  protection element.
• One approach is to calculate the capacitance
  based on the maximum allowed converter output
  voltage drop. However, this is a very
  conservative approach that often leads to high
  capacitance values.
• Another option is to calculate the capacitance
  so that it can store at least enough energy to
  trip the protection device, such as a fuse.
• Fuse-tripping process can be divided into two
  phases
• pre-arcing
• Lasts for 90 of the entire process.
• During this phase, current flows through the
  fuse, which heats up.
• arcing
• the fuse-conducting element melts and an arc is
  generated between the terminals. The arc
  resistance increases very rapidly, causing the
  current to drop and the voltage to increase.
  Eventually the arc is extinguished. At this
  point, the current is zero and the voltage equals
  the system voltage.

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Dc systems faults management

• The energy during pre-arcing is
• where TF is the total fault current clearing
  time, RF is the fuse resistance before melting,
  and IC,F is the limiting case capacitor current
  during the fault.
• IC,F equals the fault current less the sum of the
  converter current limit and other circuit
  currents. For larger capacitances than the limit
  case, the converter current may not reach the
  rated limit value, so IC,F might be slightly
  higher than in the limit case.
• .If a linear commutation is assumed, the portion
  of the arcing phase energy supplied by the
  capacitor is
• Thus,

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Dc systems faults management

• With VS VF 50 V, IC,F 135 A, RF 1 mO,
  and considering a typical value for TF of 0.1 s,
  the minimum value of C is 900 µF. If the previous
  system is simulated with C 1mF, then
• Ringing on R2 occurring when the fault is
• cleared can be eliminated by adding a
• decoupling capacitance next to R2

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Dc systems faults management

• Additional simulation plots

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Series faults in ac systems

• Series faults occur when a cable is severed or a
  circuit breaker is opened, or a fuse is blown.
  Then an arc is observed between the two contacts
  where the circuit is being opened.
• The arc is interrupted when the current is close
  to zero.
• Due to cable inductances, voltage spikes are
  observed when the arc reignites.

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Series faults in ac systems

• Visually, arcs in ac series faults are not very
  intense

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Series faults in dc systems

• In dc arcs last longer (because there are no
  zero crossings for the current) but no voltage
  spikes are generated.

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Series faults in dc systems

• Dc arcs last longer than ac ones, are much more
  intense and may damage the contacts.

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Solid state switches

• DC currents are more difficult to interrupt than
  equivalent ac currents when using conventional
  switchgear (physical separation of contacts).
• Proposed solution solid state circuit breakers.
• Solid state circuit breakers do not provide a
  physical disconnection, but such disconnection
  can be implemented by adding a conventional
  disconnect switch in series with the solid state
  circuit breaker. The conventional disconnect
  switch acts after the solid state switch
  interrupts the current.
• Other issues with solid state circuit breakers
• ON-conduction losses
• May fail as a short circuit (although
  conventional switches may also fail in this
  way). Solution redundancy
• For higher voltages, series connection of
  devices is necessary leading to coordination
  issues (perfect on-off action) and over-sizing
  to prevent device damage from excessive voltages
  during transients due to switching
  incoordination.
• Advantages of solid state switches
• Allow for many ON-OFF cycles.
• Contacts are not worn out (because there are no
  contacts).
• Act quickly.

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Solid state switches

• Examples

ac GTO
ac IGBT (no redundancy)
dc IGBT
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Solid state switches

• Switch technologies MOSFET, IGBT, Thyristors
  (SCR, GTO, GCT, MCT).
• MOSFET Intended for low power and voltage
  applications (in which semiconductor switches may
  not be competitive to conventional circuit
  breakers). Conduction losses are resistive and
  resistance increase with higher voltage ratings.
  For a given device the ON-resistance increases
  with current so in order to achieve low losses it
  may be needed to operate it below about 60 of
  its rated current.
• IGBT Voltage ratings higher than in MOSFETs.
  Losses are relatively higher than thyristor-based
  technologies but lower than MOSFETs as current
  increases. Control is simpler than in other
  technologies and limits the current in an
  inherently way (as oppose to thyristor-based
  devices in which the current is not limited by
  the gate voltage as it happens with
  transistor-based devices). A varistor is used to
  absorb energy and clamp the voltage during
  turn-off
• SCR Rated at higher voltages than other
  solutions and have lower conduction losses than
  other technologies in equivalent conditions. In
  principle it is not suitable for dc (how to turn
  it off?). Can still be used with force commutated
  circuits to reduce the current to zero fast but
  usually requires a capacitor (expensive at higher
  voltages and less reliable). Force commutation
  circuit increases the cost.

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Solid state switches

• GTO Voltage ratings tend to be higher than in
  IGBTs. Limited turn-off capabilities (additional
  circuitry is required to implement a fast
  turn-off because although thyristor-based devices
  can conduct a given large current, they may be
  damaged when attempting to interrupt such
  current). A snubber may not be necessary provided
  that the circuit inductance is high enough to
  yield a low di/dt (i.e. snubber may not be
  necessary if the load includes a power
  transformer). However, when having multiple
  thyristors in series individual snubbers for each
  device may be needed in order to ensure equal
  voltage distribution among devices.
• GCT (also known as Integrated Gate-Commutated
  Thyristor (IGCT)) Very similar to a GTO (the
  main difference is smaller cell size built with a
  p-n-p configuration on turn-off for the IGCT vs.
  a n-p-n-p configuration for GTO) and lower gate
  inductance). In equivalent conditions it has the
  lowest conduction losses of all of these
  technologies. High-cost and limited availability.
• MCT Similar to GTO, it has lower conduction
  losses than transistor-based technologies (IGBT
  and MOSFETs). Gate is controlled based on voltage
  instead of current as it is observed in the other
  thyristor technologies. The MCT requires a
  positive voltage applied to the gate in order to
  remain in the OFF condition. High-cost and
  limited availability.