Download Reactive Power and Voltage Control

Reactive Power and Voltage Control
Prepared by Viren Pandya
•
•
•
•
Contents
Q-requirement for V-Control in Long Lines
Operational Aspects in Q & V-control
Basic Principle of System V-control
Qflow Constraints & their Implications in Loss of
Voltage
• Effect of Transformer Tap Changing in PostDisturbance Period
• Effect of Generator Excitation Adjustment in
Post-Disturbance Period
• Practical Aspects of Qflow Problems Leading to
Voltage Collapse in EHV lines
Q-requirement for V-Control in Long Lines
E  AV  BI R : This is Phasor form
A  A , B  B
 E  AV   BI R  R
 E  AV cos   jAV sin   BI R cos(  R )  j sin(  R )
Equating magnitudes only
E 2  ( AV cos  )2  2 AVBI R cos  cos(   R )  ( BI R cos(   R )) 2
 ( AV sin  )2  2 AVBI R sin  sin(   R )  ( BI R sin(   R )) 2
 E 2  A2V 2  B 2 I R2  2 AVBI R [cos  cos(   R )  sin  sin(   R )]
Now, cos  cos(   R )  sin  sin(   R )  cos(    R )
and cos(    R )  cos(   ) cos R  sin(   )sin R
P
Q
But cos R 
& sin R 
VI R
VI R
 E 2  A2V 2  B 2 I R2  2 AVBI R [cos(   ) cos R  sin(   )sin R ]
2
2


P

Q
2
2 2
2
E  A V  B 
 2 AVP cos(   )  2 AVQ sin(   )

2
 V

Operational Aspects in Q & V-control
Interconnected system with EHV lines has numerous generators,
transformers, reactors, capacitors etc that are directly or indirectly
rendering electricity to consumers at desired voltage level.
Each series element in this system has reactance which has reactive
power demand as loss proportional to square of current passing
through it.
Reactive power losses are system-wide phenomenon and can
escalate under heavy loading and may diminish during light loading.
Any series winding with L serves as SINK for Q and shunt capacitances
of EHV lines & cables act as SOURCE to supply Q
Operational Aspects in Q & V-control
Transmission voltage levels indicate the balance between supply and
demand of Q.
Although under specific operating condition, frequency is uniform
throughout the power system, voltage levels can vary at different
points of transmission network due to reactive power problem.
Reactive power mismatch gives rise to voltage control problems
under variations in operating conditions namely steady state, transient
or dynamic states i.e. change in load, transformer tap position,
generator outputs, switching of capacitors/reactors, outages etc.
Operational Aspects in Q & V-control
1. System MVAR mismatch
 Surplus Q
 Deficit Q
 Surplus Q will cause OVERVOLTAGE, if excessive →Insulation breakdown
 Countermeasure?
 Yes… Shunt reactors
Operational Aspects in Q & V-control




Q deficit
Reduction in voltage magnitude
Countermeasure?
Yes… Many more
 Generator terminal voltage increase
 Reactive power boost locally or globally
 Generator transformer tap changing
 Quick acting load transformer tap changing
 Strategic load shedding
(India is MASTER in
this !!
)
Operational Aspects in Q & V-control
2. Vulnerable System Disturbance
 Northern Grid Disturbance: 1984, 1987 and 1994
 Eastern Grid Collapse: 1989, 1991, 1997, 2000, 2004 and even later
 Normally it is observed that disturbance starts from distribution system
and/or transmission system in most of the cases.
 Generating system has been very rarely source of such disturbances.
Basic Principle of System V-control
 Continuous control of voltage levels
 For sub-transmission and distribution systems, the voltages are
controlled by tap-changing transformers.
 However the core of the network i.e. transmission system, the
voltage levels are maintained by drawing of Q from reserves of
the systems’ controllable plant which is made up of mainly
rotating units.
 Large Q-disturbances are met majorly with reactive reserves.
Qflow Constraints & their Implications in Loss of Voltage

State prior to disturbance
 System is presumed to remain under medium to heavy loading.
 Some of EHV lines are fully loaded or even might be overloaded.
 But system frequency is within tolerable limits.
 Case – I: Initiating event of disturbance in transmission system
 Event : loss of a highly loaded EHV transmission line
 What will be effect of this?
 Immediate extra loading of the adjacent EHV line(s) causing
substantial increased reactive burden on the system.
 Post disturbance situation:





Reduction in voltage levels at adjacent load centres
Significant load reduction
Less demand on generating station
System frequency may rise marginally till regularized by governor control
Mean time AVR of alternators would operate to restore generator terminal
voltage
 If system Q-reserve is not enough, voltage reduction will be observed at all
segments of the network down to loads at distribution levels.
 If tap changing operation has started, distribution voltage is slowly restored
to normal level.





But increase in distribution voltage level results into increase in
load MW and MVAR supplied by distribution transformers.
This extra load percolating through sub-transmission network,
will cause its voltage to fall as each tap change raises the
distribution voltage.
Hence it increases series reactive loss & it in turn raise burden on EHV
transmission lines of Q-demand & hence EHV levels falls down too,
voltage level : Distribution - fully restored, Sub-transmission – partly
restored
Transmission voltage level :Continue to fall with tap changing process

Remedy?
 Generator AVR and Gen-Trans OLTC
 But Gen. has overexciter limiter in AVR
 Hence severe voltage crisis in the system
 Case – II: Initiating event of disturbance in distribution system
 Load OLTC operation
 Distribution voltage and even sub-transmission voltage levels are
restored
 But main bus voltage is depressed.
 Leading to voltage collapse
Effect of Transformer Tap Changing in Post-Disturbance Period
 Timing of OLTC should be gradded such that higher the voltage,
the faster is the tap changing
 Load MW and MVAR overshoots may be avoided if EHV substation
OLTC is capable of restoring sub-transmission voltage before any
downside transformer tap changer functions.
Effect of Generator Excitation Adjustment in Post-Disturbance Period
 Overexcitation operation of generator
 May start heating of generator windings if Qmax is reached or crossed.
 At last strategic load shedding
 Leading to voltage collapse if everything is failed…
Practical Aspects of Qflow Problems
Leading to Voltage Collapse in EHV lines
 Long transmission lines: Light loading is major problem
 Radial transmission lines: Any of EHV line loss increases Q burden on
adjacent lines by enhancing system reactance X
 Shortage of local reactive power sources: FC, FACTs, Syn. Condensers etc
 Reactive power capability : Generator excitation limits
 High voltage problems: Already mentioned as Ferranti Effect
Congrats
for
tolerance