Transmission Lines
Transmission
Lines
Once there was a question from the audience as under.
Inductance and capacitance of transmission line depends on
its length, conductor size, separation and configuration etc. But particular
line has fixed R, L & C. While in operation this line is acting as source
or sink of the reactive power. How this happens?
This requires review of transmission line fundamentals.
As resistance of line has no role in this aspect, we
concentrate only on L & C of line.
Inductance of the line is along the conductor and capacitance is between the conductors spread over the line as in the figure. Only one pair i.e. two conductors are shown for clarity. In fact each of the three conductors has inductance and each of three pairs of conductors has capacitance.
Line capacitance is between the conductors as seen in above equivalent diagrams. Hence capacitance gets charged whenever line receives supply from any one end. Therefore act of switching line from any one end is called charging of the line. Till the other end is open, there is no through path and power is not transmitted. In this condition line has only capacitive reactive power as under.
Line capacitance = C, Capacitive reactance Xc = 1/2πfC, Reactive power = V2/Xc
This remains fixed irrespective
of the load on the line. This appears as fixed capacitive power in the chart
below load line. As per convention, inductive power is considered as positive.
So capacitive power is negative. Long line under such charged condition has
higher voltage at other end compared to charging end voltage. This phenomenon
is known as Ferranti Effect. Line reactors are
provided at the ends of long EHV lines to control high voltage under such only
charged condition. Power flow starts only after the line is switched ON at
other end. Direction and amount of power flow depending on system parameters.
Practically line reactor has no role once the line is in service and power flow
starts. Some time line is fully loaded and system voltage is also poor. In such
condition disconnection of line rectors can improve voltage profile. But yet cannot
be disconnect because tripping of line only at one end have Ferranti effect.
Inductive element is along the
conductor and carries load current as it comes in series with the load.
Line inductance = L, Inductive reactance XL= 2πfL, Reactive power = I2XL
This is load dependant and proportional
to square of the load current. This appears as inductive power increasing with
load in the chart.
Chart shows variations in capacitive, inductive and resultant net reactive power along with the load. It is seen that at a particular load capacitive and inductive power are equal and cancels and net reactive power is zero. This load is known as Surge Impedance Loading (SIL). In this condition line behaves as if only resistive. Therefore it is ideal operating conditions and always preferred to have line loading close to SIL. Whenever load on line is lower than SIL it behaves like capacitive element delivering reactive power to the system. Similarly whenever load on line is higher than SIL it behaves like inductive element absorbing reactive power from the system. Such effect is predominant when load on line is far from SIL on either side. But load on the line depends on network conditions and many parameters. So load on line may not be matching SIL for all the time and hence advantage of ideal operation not available.
However it may be possible to
modify SIL to match line loading as under.
Line capacitance = C, Capacitive reactance Xc =
1/2πfC, Reactive power = V2/Xc
Increase of C means decrease of
Xc and increase of reactive power. It is seen in the chart that increase in
capacitive power will shift SIL to higher load.
Line inductance = L, Inductive reactance XL= 2πfL, Reactive power = I2XL
Increase of L means increase of
XL and increase of reactive power. It is seen in the chart that
increase in inductive power will shift SIL to lower load
So addition of capacitance or
inductance to line can change its SIL. It may be possible to adjust SIL to line
load by variable L and C.
This additional inductive
element and capacitive element can be connected in series or shunt with the
line. Elements in series with line are subjected to load current of the line
including overload and fault current. Therefore they should have current rating
accordingly. The reactive power of the series element varies with the load. Capacitors
in series can have self regulating feature. However it can have provision for
adjustment as per situation. Reactive power of shunt corrective element remains
fixed and needs to be adjusted as required. But its current capacity requirement
may be to its own capacity only as do not to carry load current. But it is subject
to full line voltage and surges and needs insulation system accordingly.
Arrangement as above enables transmission
line to operate in ideal condition at any load. Transmission line behaves like flexible
to adjust itself for ideal operating condition and hence is called Flexible Alternating Current Transmission System (FACTS).
Corrective elements used for
the purpose are called FACTS Devices. Provision
of such devices on line is also beneficial to optimize system operation as condition
requires. Various devices are available and employed according to suitability.
These are Thyristor Controlled Series Capacitors (TCSC), Static Synchronous
Series Compensators (SSSC), Static VAR Compensator (SVC), Static Synchronous Compensator
(STATCOM), Unified Power Flow Controller (UPFC), Distributed Power Flow
Controller (DPFC), Thyristor Controlled Phase
Shifting Transformer (TCPST) etc.
Use of above devices enables
line flexibility in many aspects such as Enhance Power Transfer Capability, Increase
Stability, Controllability, Congestion Management, Loss Optimization, Damping
to inter area oscillations, Fast Reactive Power Control etc.