Overview of custom power devices

OVERVIEW OF CUSTOM POWER DEVICES
Introduction
When a power-conditioning device is applied in a medium-voltage distribution
system of an electric utility, its purpose is to protect an entire plant, an entire
feeder, or a block of customers or loads. These devices generally have voltage
input and/or output ratings between 1 kV to 38 kV, with load ratings in excess of
500 kVA. The concept of applying a power-conditioning device at this level is
known as custom power.
The following is a brief discussion of the types of custom power devices, their
application, and economic considerations.
Static Var Compensator:
There are three basic configurations of static var compensators (SVCs)
1. FC/TCR (Fixed Capacitor/Thyristor Controlled Reactor)
The FC/TCR shown in Figure -1 behaves like an infinitely variable reactor. The
unit consists of one reactor in each phase, controlled by a thyristor switch. The
reactive power is changed by controlling the current through the reactor by means
of varying the firing angle on the thyristor valve--that is, by controlling the
duration of the conducting interval in each half cycle by issuing gating pulses to
the thyristors. A fixed harmonic filter provides the capacitive VARs necessary for
voltage regulation under the worst design conditions. With the filter supplying
VARs, the TCR controls the amount of reactive power supplied.
Figure -1 FC/TCR (Fixed Capacitor/Thyristor Controlled Reactor)

2. TSC (Thyristor Switched Capacitor)
The TSC shown in Figure -2 consists of several sets of thyristor switched capacitor
(TSC) steps. The major components include capacitors, thyristor switches, fuses,
and possibly a soft-start resistor system. The control valve, or switch, is often an
anti-parallel connected thyristor/diode or thyristor/thyristor pair. A parallel diode
would keep the capacitors charged while in standby. When the control turns on a
capacitor step, the charged capacitor results in no voltage across the closing
thyristor. This is a result of the natural operation of the thyristor in which the
device, when gated, waits until the correct forward biasing voltage is applied
across its terminals (few volts). This results in no inrush current, no generated
harmonics, and no over duty on the capacitors. A capacitor is switched off at
current zero, leaving it charged and ready to be switched again. The controlled
switching allows for thousands of operation per day.
When the TSC is started, a resistor in series with the capacitors can ensure that
they are charged slowly, avoiding high inrush currents and system disturbances.
After the capacitors are initially charged, a contactor can automatically bypass the
resistor.
Figure -2 TSC (Thyristor Switched Capacitor)

3. TSC/TCR (Thyristor Switched Capacitor/ Thyristor Controlled Reactor):
As shown in Figure -3, a combination of TSC and TCR is, in the majority of cases,
the optimum solution. With a combined TSC/TCR compensator, continuously
variable reactive power is obtained throughout the complete control range, as well
as full control of both the inductive and the capacitive parts of the compensator.
This is a very advantageous feature, permitting optimum performance during
large disturbances in the power system.
Fig-3 TSC/TCR (Thyristor Switched Capacitor/ Thyristor Controlled
Reactor)

Static Shunt Compensation:
The term STATCOM (STATic COMpensator) is typically used to describe an
SVC used in both transmission and distribution applications. However, the term
DSTATCOM (Distribution STATCOM) specifically applies to equipment used
for power quality improvement in distribution applications.
The DSTATCOM is a shunt-connected, solid-state switching power converter that
exchanges reactive current with the distribution system. It uses three-phase
inverters to transfer leading and lagging reactive current with the distribution
system via a coupling transformer. The DSTATCOM supplies reactive power by
synthesizing its output for insertion into the AC power system via high-frequency
power-electronic switching. More specifically, the DSTATCOM employs a pulse-
width modulation (PWM) scheme to generate higher-than-fundamental-frequency
currents for injection into the distribution system. This injection of high-frequency
current allows the DSTATCOM to provide harmonic-load-current compensation.
Compensation Devices for Voltage Sags and Momentary Interruptions
Source Transfer Switch:
Source transfer switches have been used throughout the industry for many decades
for protecting critical loads from power system disturbances. However, within the
last decade, the technology available for such devices has broadened their
application. Solid-state switches can now be used for the switching operation, thus
decreasing the switching time and allowing for a more seamless transfer of load
from one source to another.
This section focuses on the newer technology available to utilities, specifically the
static source transfer switch. In addition, traditional automatic transfer switches,
high-speed vacuum-switched transfer systems, and hybrid (both solid-state and
electromechanical) systems will be addressed.

Static Source Transfer Switch (SSTS):
The almost seamless transfer of the load from the preferred feeder to the alternate
feeder is made possible with the use of solid-state devices. The most common
solid-state device used in a static source transfer switch (SSTS) is the thyristor, or
silicon-controlled rectifier (SCR). The thyristor was first developed in 1957 by
General Electric Research Laboratories and offers one of the highest power-
handling capabilities of solid-state devices. The gate-turnoff (GTO) thyristor has
also been tested for use in static SSTSs.
Thyristor (SCR):
Figure -4 shows a schematic diagram of an SCR, which is one of the simplest of
devices to control. The thyristor only requires two things to operate it:
1) Forward biasing and
2) A gate current.
Applying a positive voltage to Vac forward biases the thyristor. Once the thyristor
is forward biased, a gate signal can be applied and the thyristor will begin to
conduct. The current path during conduction is from the anode to the cathode.
Once the device begins to conduct, it is latched on until it is reverse biased, and the
gate signal can be removed. The voltage drop across the thyristor during
conduction is typically 2 to 3 volts.
Figure -4 Thyristor (SCR)
The thyristor cannot be turned off from the gate. Only when the anode-to-cathode
current tries to go negative, under the influence of the circuit in which the thyristor
is connected, does the thyristor turn off.
Due to the high power ratings that have been available, thyristors have been the
preferred solid-state device used in SSTSs. The thyristor is a proven device and is
cheaper than devices of similar type (GTO, MCT, and so on).

Gate-Turnoff (GTO) Thyristor:
The gate-turnoff thyristor, often called GTO, is very similar to the thyristor in that
it requires a gate signal and forward bias in order to conduct, and ceases to conduct
when the anode-to-cathode current tries to go negative. The difference lies in
turning the thyristor off. Unlike the thyristor, which is a line-commutated device,
the GTO can be turned off with the gate. During conduction, if a large enough
negative gate signal is applied, the device will cease to conduct. The gate signal
required to turn the device off is typically one-fifth to one-third the amount of the
anode current.
Unfortunately, due to the nature of the device, such functionality sacrifices the
availability of high power ratings. GTOs are currently not available in ratings
equivalent to that of the thyristor, and in order to control when the GTO is turned
off, a significant amount of power is required. For these reasons, the GTO is not
currently being used in stand-alone SSTS systems, but has worked its way into the
design of other protection devices. For example, Superconductivity, Inc. uses a
GTO in the static switch of its SSD (Superconducting Storage Device).
Recently, due to the development of higher-rated thyristors, medium-voltage SSTS
systems have found their way into the industry (see Figure -5). These types of
devices can range up to 35 MVA at 35 kV, thus allowing the SSTS to be a facility-
wide solution.
Figure 3-5 Medium-Voltage Static Source Transfer Switch

Hybrid Source Transfer Switch:
One company uses a hybrid static switch in parallel with a vacuum switch (see
Figure -6). During normal operation, the preferred-side vacuum switch conducts,
thus supplying power to the load. When the need for a transfer arises, the vacuum
switch opens and the appropriate thyristor is gated. The opening of the vacuum
switch produces an arc voltage, which in turn forward biases one of the preferred-
side thyristors. Once this occurs, the load current begins to conduct through the
preferred-side static switch. The load is then transferred to the alternate source
similar to the standard static SSTS. Once the alternate-side static switch picks up
the load, it is then transferred to the alternate-side vacuum switch. This method
increases efficiency to almost 100% and also eliminates the need for cooling
devices.
Figure 3-6 Hybrid Source Transfer Switch
High-Speed Mechanical Source Transfer Switch (HSMSTS):
Due to the increased cost of medium-voltage SSTSs, some manufacturers have
decided to reduce the cost of the device by replacing the traditional thyristor with a
vacuum switch. Although less expensive and more efficient (at approximately 99%
or greater) than the thyristor-controlled switch, the transfer time is longer. Typical
transfer times associated with the HSMSTS are on the order of 1½ cycles, with no
crossover time (paralleling of the two sources). Therefore, this approach is only a

viable solution if the particular load in need of protection can withstand a 1½-cycle
system disturbance.
Static Series Compensators:
The purpose of a static series compensator (SSC) is to mitigate the effect that
voltage sags and interruptions have on a sensitive customer loads. An SSC is a
waveform-synthesis device based on power electronics that is series-connected
directly into the utility primary distribution circuit by means of a set of single-
phase insertion transformers. An SSC can be configured to use line energy supply
(LES) to provide the energy from the utility feeder that is to be injected into the
distribution circuit. LES systems may incorporate energy drawn from the
incoming affected line, as in Figure -7. In this system configuration, when the
voltage of one or more phases of the incoming supply drops below a preset
threshold, the SSC injects a controlled amount of voltage into the affected phase
or phases to boost load voltage back to a more suitable level. The load, therefore,
is buffered from the disturbance.
Figure -7 SSC Using Line Energy Supply (LES)
LES is an alternative to stored energy supply (SES), where the injected energy is
provided from some form of onboard, pre-charged energy source such as DC
energy-storage capacitors, flywheel energy storage, superconducting magnetic
energy storage, or batteries. An SSC may be configured to operate as a standby

compensator, where the inverter is not actively in the circuit until triggered by a
power quality event that requires action to restore the incoming source.voltage to
acceptable quality. Alternatively, the SSC may be continually online providing
voltage injection during idle conditions that will be able to offset voltage drop
caused by sudden increase in load current through the series-insertion transformer.
Static Voltage Regulators:
A traditional step-voltage regulator is a regulating transformer in which the
voltage of the regulated circuit is automatically controlled in steps by means of
taps, without interrupting the load. Such a transformer can boost or buck the
voltage supplied to a load with a delay on the order of seconds. In contrast, a
static voltage regulator (see Figure -8) provides voltage boost during voltage sags
by using thyristor switches that rapidly change taps on three single-phase
transformers. This type of device is generally limited in design to provide up to
50% boost. The rating of an SVR needs to be the same as the full rating of the
load that it will protect, because during sag or swell events, the SVR will carry
the entire load. An SVR is not effective during voltage interruptions since there is
no voltage to transform.
An SVR can be configured to have a 1:1 transformer winding ratio to act solely
as a load protection device. However, it can also be configured to operate as a
step-down transformer, in lieu of a traditional step-down transformer.
During voltage sags in which the SVR switches to full 50% boost, the current
drawn by the unit can be twice as high as normal. Therefore, upstream protection
devices need to be coordinated with the SVR so that they do not operate in
response to the higher current levels. The SVR does not compensate for the
change in the voltage waveshape that occurs during voltage sags. If the load is
sensitive to changes in waveshape, such as phase angle jump, then it may still
malfunction during the event, even though the magnitude of the voltage is within
design requirements.

Since an SVR is designed to operate with individual phase control, an SVR can
also correct for unbalanced voltages during steady-state operation.
Figure -8 Static Voltage Regulator
Backup Energy Supply Devices:
Battery UPS:
In a traditional online UPS system, the load is always fed through the protection
unit. The incoming AC power is rectified into DC power, which is used charge a
bank of batteries. This DC power is then inverted back into AC power to feed the
load. If the incoming AC power fails, the inverter is fed from the batteries and
continues to supply the load. Generally, UPS systems are designed to provide five
to fifteen minutes of backup. In addition to providing ride through for power
outages, an online UPS system provides very high isolation of the critical load
from all power line disturbances. Online UPSs can be purchased in a variety of
sizes.

An offline UPS allows the utility to power the equipment until a disturbance is
detected and a mechanical switch transfers the load to the battery-backed inverter.
The transfer time from the normal source to the battery-backed inverter is
important. Since there is a very short-duration interruption during the time it takes
to detect a mains failure, start the inverter, and transfer the load to battery power, a
load with some inherent ride-through capability is required for the interruption to
go unnoticed. A standby UPS utilizing a static, rather than mechanical, switch can
provide nearly seamless transfer from utility power to battery power during utility
events. Currently available medium-voltage UPSs are standby using a static switch
(Figure -9). When applying a standby UPS, it is important to remember that it does
not provide any transient protection or voltage regulation.
Fig-9 Medium-Voltage Standby UPS
SMES:
In superconducting magnetic energy storage (SMES) system, energy is stored
within a magnet that is capable of releasing megawatts of power within a fraction
of a cycle to replace a sudden loss in line power.
When the SMES is in standby mode, the current continually circulates through the
normally closed switch of the voltage regulator and power supply, and back to the
magnet. The power supply provides a small trickle charge to replace the power lost
in the non-superconducting part of the circuit. When a voltage disturbance is
sensed, the controller directs real and reactive power from the inverter to the load,

while automatically opening the solid-state isolation switch within two
milliseconds. When the voltage across the capacitor bank reaches a pre-set level,
the switch closes. This sequence repeats until normal voltage from the utility
feeder is restored. This systematic transfer of energy from the magnet to the load
keeps the load interruption free for optimum performance of your critical
processes.
The SMES recharges quickly and can repeat the charge/discharge sequence
thousands of times without any degradation of the magnet. Recharge time can be
accelerated to meet specific requirements, depending on the system’s capacity.
Currently, there is no manufacturer of SMES-based UPS systems, as the only
manufacturer has redirected its focus on other applications of SMESs.
Flywheel:
Kinetic energy is stored in a rotating mass (the flywheel), and this energy is drawn
upon when needed to smooth the operation of the machine. A flywheel, in
essence, is a mechanical battery storing kinetic energy. Until recently, flywheels
were only useful for short ride through or output modulation on mechanical
devices. Modern power electronics, bearing and composite technologies are
making flywheels a viable way to provide ride-through and mitigate power quality
events.
An electric motor is used to accelerate the flywheel up to speed, converting
electrical to mechanical energy. When the flywheel reaches its "fully charged"
speed, it enters a mode comparable to a chemical battery’s "float" state, in which a
small amount of power is drawn from the supply to maintain a constant RPM
(overcoming the friction losses of the bearings). This same motor is then used as a
generator to convert mechanical energy to electrical energy when needed. In order
to provide useful ride-through duration and power, flywheel systems are run at
high speeds (>3600 rpm), and therefore use what is effectively an ASD to control
the speed and acceleration of the flywheel whenever line power is available. When
energy is removed from the flywheel, these same electronics are used to modulate

the power returned to the supply, ensuring consistent required voltage levels for as
long as possible.
Because retrieving stored power from a flywheel requires only mechanical
slowing and no chemical reactions, flywheels have several advantages when
compared with batteries:
· Frequent cycling of flywheel systems does not appreciably diminish useful life.
· Flywheels are relatively unaffected by ambient temperature extremes.
· The rate at which energy can be exchanged into or out of the flywheel is
limited only by the motor-generator and power electronics design. Therefore,
it is possible to withdraw large amounts of energy in a far shorter time than
with traditional chemical batteries.
· Environmental issues present with batteries are not present with flywheel
systems.
For all of their advantages, Current flywheels have relatively low specific energy,
making them more suited to dealing with momentary PQ events rather than to
long outages.

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