facts upto 12th nov

7/30/2019 Facts Upto 12th Nov

1/103

Need for Transmission Interconnections-- To connect the load centres to generation locations

- Taking advantage of diversity of loads-- Minimize Total generation capacity

-- To minimize the cost per unit of electricity

-- Improve the reliability of power supply

-- Enables sharing of reserve capacities

-- Forms an effective electric gridWhat are the problems with AC/DC interconnections?-- As power transfers grow, power systems grow in size and complexity

-As the system becomes more complex, difficult to operate

-- System becomes less secure for riding major outages-- Large power flows with inadequate control

-- Excessive reactive power requirements

-- Full potential of transmission networks cannot not utilized due to

dynamic swings between different parts of the system


2/103

- Due to lack of electrical storage, generation load must balance

all the times.

-power flow is based on the inverse of the various transmission

line impedances

-To some extent, the electrical system is self-regulating. Ifgeneration is less than load, the voltage and frequency drop, and

thereby the load, goes down to equal the generation minus the

transmission losses

-When adequate generation is available, active power flows from

the surplus generation areas to the deficit areas, and it flows

through all parallel paths available which frequently involves extra

high-voltage and medium-voltage lines

Flow of power in AC systems


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With Thyristor controlled series capacitor


5/103

With Thyristor controlled Phaseangle regulator

With Thyristor controlled seriesreactor


6/103

Basic Applications of FACTS controllers power flow control,

increase of transmission capability,

voltage control,

reactive power compensation,

stability improvement,

power quality improvement,

power conditioning,

flicker mitigation,

interconnection of renewable anddistributed generation and storages


7/103

Limits of loading capability: Thermal/ Dielectric/ Stability

Thermal capabil i tyof an overhead line is a function of the ambient

temperature, wind conditions, condition of the conductor, and ground

clearance. It varies perhaps by a factor of 2 to 1 due to the variable

environment and the loading history.

Dielectr ic Lim it

For a given nominal voltage rating, it is often possible to increase

normal operation by +10% voltage (i.e., 500 kV-550 kV) or even

higher. Care is then needed to ensure that dynamic and transient

overvoltages are within limits

Stabil i tyThere are a number of stability issues that limit the transmission

capability. These include:

Transient stability

Dynamic stability

Steady-state stability

Frequency collapse

Voltage collapse

Subsynchronous resonance


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9/103

The FACTS technology is not a single high-power Controller, but rather a

collection of Controllers, which can be applied individually or in coordination

with others to control one or more of the interrelated system parameters

FACTS technology opens up new opportunities for controlling power and

enhancing the usable capacity of present, as well as new and upgraded,

lines.

FACTS Controllers to control the interrelated parameters that govern the

operation of transmission systems including series impedance, shunt

impedance, current, voltage, phase angle, and the damping of

oscillations at various frequencies below the rated frequency

FACTS technology also lends itself to extending usable transmission

limits in a step-by-step manner with incremental investment as and when

required.


10/103

Flexibi l i ty of Electr ic Power Transm issio n.

- The ability to accommodate changes in the electric transmission system or

operating conditions while maintaining sufficient steadystate and transient

margins.

-The ability to expand the capacity of the network in flexible manner

Flexible AC Transm issio n System (FACTS)

. Alternating current transmission systems incorporating power electronic-

based and other static controllers to enhance controllability and increase powertransfer capability


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12/103

simple two-machine system

current flow perpendicular to the driving voltage


13/103

Active and reactive power flow phasor diagram


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15/103

power angle curves for different values ofX


16/103

injecting voltage perpendicular to the line

current mostly changes active power;

regulating voltage magnitude mostlychanges reactive power


17/103

injecting voltage phasor in series with the line


18/103

Rating of series FACfS Controllers would be a fraction of the

throughput rating of a line.

For example,

a 500 kV (approximately 300 kV phase-ground), 2000 A line has a

three-phase throughput power of 1800 MVA,

and, for a 200 km length, it would have a voltage

drop of about 60 kV.

For variable series compensation of say, 25%, the series equipment

required would have a nominal rating of 0.25 X 60 kV X 2000 A = 30

MVA per phase, or 90 MVA for three phases, which is only 5% of the

throughput line rating of 1800 MVA


19/103

Control of the line impedanceX (e.g., with a thyristor-controlled series capacitor)

can provide a powerful means of current control.When the angle is not large, control ofX or the angle substantially provides the

control of active power

Control of angle , which in turn controls the driving voltage, provides a powerful

means of controlling the current flow and hence active power flow when the

angle is not large.

Injecting a voltage in series with the line, and perpendicular to the current

flow, can increase or decrease the magnitude of current flow. Since the current

flow lags the driving voltage by 90 degrees, this means injection of reactive

power in series, (e.g., with static synchronous series compensation) can provide

a powerful means of controlling the line current, and hence the active power

when the angle is not large.

Injecting voltage in series with the line and with any phase angle with respect

to the driving voltage can control the magnitude and the phase of the line

current. This means that injecting a voltage phasor with variable phase angle

can provide a powerful means of precisely controlling the active and reactive

power flow. This requires injection of both active and reactive power in series.


20/103

Because the per unit line impedance is usually a small fraction of the

line

voltage, the MVA rating of a series Controller will often be a small

fraction

of the throughput line MVAWhen the angle is not large, controlling the magnitude of one or theother line voltages (e.g., with a thyristor-controlled voltage regulator) can

be a very cost-effective means for the control of reactive power flow

through the interconnection.

Combination of the line impedance control with a series Controller and

voltage regulation with a shunt Controller can a provide a cost-effectivemeans to control both the active and reactive power flow between the two

systems

The required MVA size of the series Controller is small compared to the

shunt Controller, and, in any case, the shunt Controller does not provide

control over the power flow in the lines

Series-connected Controllers have to be designed to ride through

contingency and dynamic overloads, and ride through or bypass short

circuit currents. They can be protected by metal-oxide arresters or

temporarily bypassed bysolid-state devices when the fault current is too

high, but they have to be rated tohandle dynamic and contingencyoverload


21/103

TYPES OF FACTS CONTROLLERS

Based on type of connection

Series Controllers

Shunt Controllers

Combined series-series Controllers

Combined series-shunt Controllers

Based on type of source

V

oltageSource

Converter

CurrentSource

Converter


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24/103

Flexibility of Electric Power Transmission. The ability to

accommodate changes in the electric transmission system or

operating conditions while maintaining sufficient steady stateand transient margins.

Flexible AC Transmission System (FACTS).Alternating current

transmission systems incorporating power electronic-based andother static controllers to enhance controllability and increase

power transfer capability.

FACTS Controller. A power electronic-based system and other

static equipment thatprovide control of one or more A C transmission system

parameters


25/103

Sta

ticSynchronousseriescomp

ensator

TCSC/ TSSC

TCSR / TSSR


26/103

Combined Shunt and series Connected Controllers

Thyristor controlled phaseshifting transformer (TCPST)

Unified Power Flow controller(UPSC)


27/103

Diode Transistor IGBT

MOSFETGTOThyristor

MOSturn-offThyristor

Emitter Turnoff Thyristor

MOS controlledThyristor


28/103

Characteristics of Devices

Voltage and Current Rating

A 125mm device may have a current-carrying capability of3000-4000 amperes and a voltage-withstand capability in

the range of 6000 -10,000 volts.

The highest blocking capability along' with other desirable

characteristics is somewhere in the range of 8-10 kV.for

thyristors, 5-8 kV for GTOs, and 3-5 kV for IGBTs. In a

circuit, after making various allowances for overvoltages

and redundancy, the useable device voltage will be about

half the blocking voltage capability


29/103

Most of the devices made with tum-off capability, are made with

no reverse blocking capability.

Without the reverse voltage capability requirement the devicecan be thinner, have lower forward conduction and lower

switching losses

Current-Sourced Converters, needdevices with reverse voltage

withstand capability. It is not uncommon for many industrialapplications with a focus on the first cost, to consider use of a

diode in series with the asymmetric main device to obtain

reverse blocking capability.


30/103

Losses and Speed of Switching

Forward-voltage drop and consequent losses during full

conducting state (onstate losses). Losses have to be

rapidly removed from the wafer through the package andultimately to the cooling medium and removing that heat

represents a high cost

High dv /dt jus t after turn andhigh di /dt dur ing the turn-off

are very impo rtant parameters.

They d ictate thesize, cost, and losses of snubber circuits

needed to soften high dv /dt and d i/dt.

They decide the ease of series connection o f devices, andthe useable device cu rrent andvoltage rating


31/103

During the turn-on, the forward current rises, before the

forward voltage falls

During turn-off of the turn-off devices, the forward

voltage rises before the current falls. Simultaneousexistence of high voltage and current in the device

represents power losses

The gate-driver power and the energy requirement are avery important part of the losses and total equipment cost.

With large and long current pulse requirements, for turn-on

and turn-off, not only can these losses be important

in relation to the total losses, the cost of the driver circuit

and power supply can be higher than the device itself


32/103

During the turn-on, the forward current rises, before the

forward voltage falls

During turn-off of the turn-off devices, the forward

voltage rises before the current falls. Simultaneousexistence of high voltage and current in the device

represents power losses

The gate-driver power and the energy requirement are avery important part of the losses and total equipment cost.

With large and long current pulse requirements, for turn-on

and turn-off, not only can these losses be important

in relation to the total losses, the cost of the driver circuit

and power supply can be higher than the device itself


33/103

Compared to the self-commutating converter, the line-

commutating converter must have an ac source

connected to the converter, it consumes reactive power,

and suffers from occasional commutation failures in theinverter mode of operation

Converters applicable to FACTS Controllers would be of the

self-commutating type.

Current-sourced converters in which direct current

always has one polarity, and the power reversal takes

place through reversal of dc voltage polarity

Voltage-sourced converters in which the dc voltage always

has one polarity, and the power reversal takes place through

reversal of dc current polarity.


34/103

Conventional thyristor-based converters, being without turn-

off capability, can only be current-sourced converters,

whereas turn-off device-based converters can be of either

type

Since the direct current in a voltage-sourced converter flows

in either direction, the converter valves have to be

bidirectional, and also, since the de voltage does not reverse,

the turn-off devices need not have reverse voltage capability.


35/103

For the voltage-sourced converter, unidirectional dc voltage of a dc capacitor is

presented to the ac side as ac voltage through sequential switching of devices.

Through appropriate converter topology, it is possible to vary the ac output

voltage in magnitude and also in any phase relationship to the ac system voltage.

The power reversal involves reversal of current, not the voltage. When the storage

capacity of the de capacitor is small, and there is no other power source connected

to it, the converter cannot supply or absorb real power for much more than a cycle.

The ac output voltage is maintained at 90 degrees with reference to the ac current,

leading or lagging, and the converter is used to absorb or supply reactive power

only.

Voltage Source Converter

Current Source Converter

For the current-sourced converter, the de current is presented to the ac side

through the sequential switching of devices, as ac current, variable in

amplitude and also in any phase relationship to the ac system voltage. The

power reversal involves reversal of voltage and not current. The current-

sourced converter is represented symbolically by a box with a power device,

and a de inductor as its current source.


36/103

Principle of Voltage source converter

A valve with a combination of turn-off

device and diode can handle power

flow in either direction, with the turn-off

device handling inverter action, and

the diode handling rectifier action


37/103


38/103

1. From instant t1 to t2, with turn-off devices 1 and 2 on and 3 and 4 off, Vab is

positive and iab is negative. The current flows through device 1 into ac phase

a, and then out of ac phase b through device 2, with power flow from dc to

ac (inverter action).

2. From instant t2 to t3, the current reverses, i.e., becomes positive, and flows

through diodes l' and 2' with power flow from ac to dc (rectifier action).

Note that during this interval, although devices 1 and 2 are still on and voltage

Vab is +Vd, devices 1 and 2 cannot conduct in a reverse direction. In reality,

devices 1 and 2 are ready to turn on by turn-on pulses when required by thedirection of actual current flow.

3. From instant t3 to t4, devices 1 and 2 are turned off and devices 3 and 4 are

turned on, thereby Vab becomes negative while iab is still positive. The current

now flows through devices 3 and 4 with power flow from dc to ac (inverteraction).

4. From instant t4 to t5, with devices 3 and 4 still on, and 1 and 2 off, and Vab

negative, current iab reverses and flows through diodes 3' and 4' withpower

flow from ac to dc (rectifier action).


39/103

S f i l h t


40/103

AC current and voltage can have any phase relationship, that is, the converter

phase angle between voltage and current can cover all four quadrants, i.e.,

act as a rectifier or an inverter with leading or lagging reactive power

Summary of single phase converter

The active and reactive power can be independently controlled with control

of magnitude and angle of the converter generated ac voltage with respect

to the ac current

Diodes carry out instantaneous rectifier function, and turn-off devices carry

out instantaneous inverter function. When the converter operates as a

rectifier with unity power factor, only diodes are involved with conduction, andwhen it operates as an inverter with unity power factor, only turn-off devices

are involved in conduction.

When any turn-off device turns off, the ac bus current is not actually

interrupted at all, but is transferred from a turn-off device to a diode when the

power factor is not unity, and to another turn-off device when power factor is

unity

Turn-off devices 1 and 4 (or turn-off devices 2 and 3) in the same phaseleg

are not turned on simultaneously. Otherwise this would cause a "shootthrough"


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44/103

Operation of a phase-leg through four quadrants: (a) Phase-leg; (b)Waveforms and phasor diagrams through all four quadrants.


45/103

Static Shunt Compensators: SVC and STATCOM

It has long been recognized that the steady-state transmittable power can be

increased and the voltage profile along the line controlled by appropriate reactive

shunt compensation.

The purpose of this reactive compensation is to change the natural electrical

characteristics of the transmission line to make it more compatible with the

prevailing load demand

Var compensation is thus used for voltage regulation at the midpoint to segmentthe transmission line and at the end of the (radial) line to prevent voltage instability,

as well as for dynamic voltage control to increase transient stability and "damp

power oscillations


46/103

Th id i t h t ti i ifi tl i th t itt bl


47/103

The midpoint shunt compensation can significantly increase the transmittable

power (doubling its maximum value) at the expense of a rapidly increasing

reactive power demand on the midpoint compensator


48/103

It can be observed that the midpoint shunt compensation

can significantly increase the transmittable power (doubling

its maximum value) at the expense of a rapidly increasingreactive power demand on the midpoint compensator.


49/103

Improvement of Voltage Instability

I t f t i t t bilit li it


50/103

Improvement of transient stability limit


51/103

Equal area criterion to illustrate thetransient stability margin for a

simple two machine system without

compensation

with an idealmidpoint compensator


52/103

Power Oscillation Damping

In the case of an under-damped power system, any minor disturbance can cause

the machine angle to oscillate around its steady-state value at the naturalfrequency of the total electromechanical system. The angle oscillation, of course,

results in a corresponding power oscillation around the steady-state power

transmitted. The lack of sufficient damping can be a major problem in some

power systems and, in some cases, it may be the limiting factor for the

transmittable power.


53/103

Waveforms illustrating power oscillation damping by reactive shunt compensation:

(a) generator angle, (b) transmitted power, and (c) var output of the shunt

compensator


54/103

Summary of Compensator Requirements

The compensator must stay in synchronous operation with the ac

system at the compensated bus under all operating conditions

including major disturbances. Should the bus voltage be lost

temporarily due to nearby faults, the compensator must be able to

recapture synchronism immediately at fault clearing.

The compensator must be able to regulate the bus voltage for voltage

support and improved transient stability, or control it for power

oscillation damping and transient stability enhancement, on a priority

basis as system conditions may require.

For a transmission line connecting two systems, the best location for

var compensation is in the middle, whereas for a radial feed to a load

the best location is at the load end.

The Thyristor Controlled and Thyristor Switched Reactor (TCR and TSR)


55/103

The Thyristor-Controlled and Thyristor-Switched Reactor (TCR and TSR).


56/103


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58/103

Harmonics in TCR


59/103

Harmonics in TCR


60/103


61/103

The Thyr is tor Switched Capacitor (TSC)


62/103

The Thyr is tor-Switched Capacitor (TSC).


63/103

Transient free switching of Capacitor bank


64/103

Transient free switching of Capacitor bank


65/103


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67/103

Fixed Capacito r, Thyr isto r-Contro l led Reacto r (FC-TCR)


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69/103

Thy risto r Sw itch ed Capacito r, Thyr isto r-Contro l led Reacto r (TSC-TCR)


70/103


71/103

The possibility of generating controllable reactive power directly, without the use

of ac capacitors or reactors, by various switching power converters was disclosed

by Gyugyi in 1976.

These (de to ac or ac to ac) converters are operated as voltage and current

sources and they produce reactive power essentially without reactive energy

storage components by circulating alternating current among the phases of the ac

system.

Like the mechanically powered machine, they can also exchange real power with

the ac system if supplied from an appropriate, usually de energy source.

Because of these similarities with a rotating synchronous generator, they are

termed Static Synchronous Generators (SSGs). When an SSG is operated

without an energy source, and with appropriate controls to function as a shunt-

connected reactive compensator, it is termed, analogously to the rotatingsynchronous compensator (condenser), a Static Synchronous Compensator

(Condenser) or STATCOM (STATCOM).


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75/103

The inputs to the internal control are: the ac system bus voltage, v, the


76/103

p y g , ,

output current of the converter, io' and the reactive current reference,

IQRef Voltage v operates a phase-locked loop that provides the basic

synchronizingsignal, angle theta. The output current, io' is decomposed

"into its reactive and real components, and the magnitude of thereactive current component, I rQ, is compared to the reactive current

reference, I QRef The error thus obtained provides, after suitable

amplification, angle alpha, which defines the necessary phase shift

between the output voltage of the converter and the ac system voltage

needed for charging (or discharging) the storage capacitor to the dcvoltage level required.


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85/103

Concept of Series Capacitive Compensation


86/103

Concept of Series Capacitive Compensation


87/103


88/103

Voltage Stability Limit


89/103

Improvement of Transient Stability Limit

Thyristor-Switched Series Capacitor (TSSC)


90/103

The operating principle of the TSSC is straightforward: the degree of series

compensation is controlled in a step-like manner by increasing or

decreasing the number of series capacitors inserted. A capacitor is inserted

by turning off, and it is bypassed by turning on the corresponding thyristor

valve.

Thyristor-Controlled Series Capacitor (TCSC)


91/103

The TCR at the fundamental system frequency is a continuously variable

reactive impedance, controllable by delay angle a, the steady-state impedance of

the TCSC is that of a parallel LC circuit, consisting of a fixed capacitive impedance,

Xc, and a variable inductive impedance,XL(a),


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93/103

CONVERTER TYPE SERIES COMPENSATORS


94/103

The Static Synchronous Series Compensator (SSSC)


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