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 Post-Disturbance 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

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 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.

Overexcitation operation of generator 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