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
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
Editor's Notes
The deadband is normally chosen somewhat above the nominal voltage to allow voltage drop along the medium-voltage and low-voltage feeders.
The horizontal axis gives the distance of HV Line from the main grid
The vertical axis gives the voltage
These principles are shown schematically in Figure,
The deadband is normally chosen somewhat above the nominal voltage to allow
voltage drop along the medium-voltage and low-voltage feeders. As the voltage for the most remote customer during maximum load should be above the
undervoltage limit. The voltage for remote customers can be boosted up to5%by using A distribution transformer with a different turns ratio. At the same time, the voltage
during low load should not exceed the overvoltage limit for any of the customers.
Hosting Capacity is the maximum amount of generation that can be connected without resulting in an unacceptable quality or reliability
for other customers.
From this it follows that when considering overvoltage's, the hosting capacity is the amount of generation that gives a voltage rise equal to the
overvoltage margin.
The introduction of distributed generation will result in less voltage drop whenever the generator is producing active power. The result is that the overvoltage margin
becomes smaller and the maximum voltage may even exceed the overvoltage limit. In that case, the power system no longer fulfills its primary aim to maintain the voltage
magnitude within limits. In other words, the hosting capacity is exceeded.