Module 8 - Voltage and Reactive Power Control

Question 1

What is voltage?

Answer 1

• Pressure from electrical circuit’s power source that pushes current through in a loop enabling them to do work.
• voltage between two ends is total energy required to move an electric charge along that path divided by magnitude of the charge

Question 2

How voltage is controlled?

Answer 2

• controlling production, absorption and flow of reactive power at all levels in the system
• generating units provide the basic means of voltage control; the automatic voltage regulator controls the field excitation to maintain a scheduled voltage level at the terminals of the generators

Question 3

What is reactive power?

Answer 3

• Power required to establish and maintain an ac fluctuating magnetic flux with which no energy transfer takes place
• useless? results in increased line currents and losses
• power caused by energy storage components.
• doesnt contribute to transfer of energy, it loads the equipment as though it did consume active power

Question 4

Flow of reactive power?

Answer 4

• oscillates back and forth between generators and loads
• common practice to consider reactive power as flowing in a certain direction

Question 5

What is reactive power management?

Answer 5

control of generator voltages, transformer tap settings, shunt capacitors or shunt reactors to ensure reduction in losses and acceptable voltage levels

Question 6

True or false? For a synchronous generator to supply reactive power, it must be UNDER-EXCITED

Answer 6

false

Question 7

True or false?
For a generator at no load, when capacitor added, the generator should be over-excited to absorb reactive power from capacitor?

Answer 7

False

Question 8

True or false?
Voltage collapse is as a result of the unbalance between the reactive power demand and
the real power supplied by the system

Answer 8

False

Question 9

True or false?
The main functional similarity between synchronous condensers and static VAR
compensators (SVCs) is that they both have rotating parts.

Answer 9

FALSE

Question 10

True or false?
A synchronous condenser has an overload capability of 10% - 20% for more than 30
minutes.

Answer 10

False

Question 11

True or false?
Series capacitors could cause excitation of low-frequency oscillations (sub-
synchronous resonance), which may damage turbine-generator shafts

Answer 11

true

Question 12

True or false?
Series capacitors can increase the loadability of a line at only a fraction of the cost of
new transmission.

Answer 12

True

Question 13

True or False?
Off-load tap changing transformers are used when changes in voltage ration need to be frequent?

Answer 13

False

Question 14

what is the relationship between voltage magnitude and reactive power flow?

Answer 14

• Reactive power flows from a bus of higher voltage to a bus of lower voltage.

Question 15

How does SIL relate to reactive power?

Answer 15

If a lines loading is higher than SIL, the line will absorb reactive power which will lead to a DECREASE in voltage profile of line

-If a lines loading is lower than SIL, the line will produce reactive power and lead to an INCREASE in voltage profile of the line

Question 16

How to correct for undervoltages?

Answer 16

• Ensure all equipment are in service
• remove shunt reactors
• insert shunt or series capacitors

Question 17

How to correct for over voltages?

Answer 17

-Remove switchable shunt and series capacitors
- insert switchable shunt reactors
- adjust taps on ULTCs

Question 18

What is primary voltage control?

Answer 18

controlling reactive power production from a device so that voltage magnitude is kept at or close to the set value of the controller

Question 19

what is secondary voltage control?

Answer 19

selection of set values for the primary controller

Question 20

what is tertiary voltage control?

Answer 20

• adjusts the settings of the secondary controller
• does this thru overall system economic optimization using real time calculations.

Question 21

What is the response of static reactive power sources and give examples?

Answer 21

• Slow response

Examples:
-shunt reactors
-shunt/series capacitors

Question 22

What is the response of dynamic reactive power sources and give examples?

Answer 22

• fast response

Examples:
- synchronous condensors
- tap changing transformers
- synchronous generators or motors

Question 23

According to South African Grid Code, what is voltage ride through capability of RPP?

Answer 23

it is the capability of the RPP to stay connected to the network and keep operating, following voltage dips or surges caused by short circuits or disturbances on any or all phases.