Voltage magnitude can vary due to changes in load, distributed generation output, or switching operations. Variations are categorized as long or short duration. Long duration variations last over a minute and cause over or under voltages. Short duration variations cause temporary voltage drops, rises, or interruptions lasting less than a minute. Distributed generation can cause over or under voltages depending on system characteristics. Utilities control voltage magnitudes through limits, reactive power support, and transformer tap changing to maintain stability.

1. Voltage Magnitude Variation and Stability

2. Voltage Magnitude
It is the highest value attained by the supply voltages or load current
in a half cycle either positive or negative side

3. Voltage Magnitude Variation
Voltage magnitude variations are typically displayed as RMS voltage
verses time.

4. Voltage Magnitude Variation
It is occurred by
 Load variation on the system,
 Switching operation of heavy load and
 DG output -increase or decrease.
It is categorized in Long duration variation and Short duration
variations

5. Voltage Magnitude Variation
Long duration variations
Long-duration variations cover deviations at power frequencies for
longer than 1 min.
 Over voltages or
 Under voltages
These over and under voltages are generally not the result of system
fault but are also caused by load variation on the system and
switching operation.

6. Voltage Magnitude Variation
Short Duration variation
It resulted in temporary voltage drops (sags), voltage rises (swells), or
a complete loss of voltage (interruptions)
Voltage Sag
It is sudden reduction of RMS voltage to value between 0.1 to 0.9 pu
for duration greater than half a main cycle or during a period shorter
than 1 minute.

7. Voltage Magnitude Variation
Voltage Swell
It is increase in RMS voltage to value 1.1 to 1.9 pu for duration
greater than half a main cycle or during a period shorter than 1
minute.
Interruption
An interruption occurs when the voltage or load decreases to less
than 0.1 pu for a period of time not exceeding 1 min.

8. Voltage Magnitude Variation
Short duration voltage variations are caused by:
 Fault conditions
The energization of large loads which require high starting
currents
Intermittent loose connections in power wiring

9. Impacts of DG on Voltage Magnitude
Over voltages
o Serious Concern specially for location away from Substation
o Injected Active power – proportion to resistive part of source impedance
o Fault level and low phase angle of network impedance
o Voltage regulator (Back up Operation), Capacitor Bank detect wrong direction
o PV Generation on full sunny day….

10. Impacts of DG on Voltage Magnitude
Under voltages
o Presence of a generator on one feeder will result in a under
voltage on the other feeders.
o Peak Load
o High phase angle of network impedance – Wind Turbines

11. Voltage Magnitude Variation
Principle of voltage control in a distribution network

12. Voltage Magnitude Variation
Principle of voltage control in a distribution network

13. Voltage Magnitude Variation
Principle of voltage control in a distribution network

14. Voltage Limitation
Over and Under Voltage Limit
USA - For MV feeders 95–105%
UK - For MV feeders 94–106%
Pakistan – For HV 95-105 %
France - For LV 90% and 106%.
Japan – For LV 94–106%
Pakistan – For LV 95-105 %

15. Voltage Stability
Voltage stability is the ability of power system to maintain
steady voltages at all buses in the system
Scenario
voltage stable state - Every bus in the system, the bus
voltage magnitude increases as the active power injection at
the same bus is increased.

16. Voltage Stability
Scenario
A system voltage are unstable if for at least one bus in the
system, the bus voltage magnitude decreases as the
reactive power injection at the same bus is increased.
It indicates that if, P-Q sensitivity is positive for every bus
the system is voltage stable and if P-Q sensitivity is negative
for at least one bus, the system is voltage unstable.

17. Voltage Stability
Short term voltage stability
Short-term voltage stability involves loads that require
increased active and reactive power at timescales of
seconds or less.
→The situation that leads to voltage collapse is when the
loading of the system increases with decreasing voltage
magnitude

18. Voltage Stability
Long term voltage stability
Involves high loads, high power imports from remote
generation and a sudden large disturbance which could be
in the form of loss of large generators and loss of
transmission lines.
→The disturbance causes high reactive power losses and
voltage sags in load areas

19. Methods of Voltage Stability
Planning of generation system
→Sitting generating plants in the load areas
Maintenance of generation system
→Over excitation and under excitation limiters
Operation of generation system
→During peak load period, power import over the
transmission network should be reduced

20. Methods of Voltage Stability
Reactive power compensation
→Shunt reactors
Capacitor bank
→Act as constant reactive power sources
Tap changing
→By changing the transformation ratio