1. Introduction to Power Quality
Module: Power Quality and Harmonics
A/Prof. Mohammad A.S. Masoum, CURTIN

2. Purpose of this Presentation
To understand the definition, propagation, causes and classifications of power quality
To introduce and study voltage sag including it’s characteristics, causes, effects.
To perform voltage sag calculations and introduce mitigation approaches
Solve tutorial problems related to voltage sag calculation

3. Overview Definition of Power Quality (PQ) Propagation of Harmonics
Causes of Disturbances in PS
Classification of PQ Issues
Voltage Sag (Definition & Causes)
Definition, Causes & Effects
Characteristics
Calculations
Mitigation Techniques
Summary
Tutorial Problems

4. Definition of Power Quality
Electric PQ is an important aspect of PS and electric machinery with direct impacts on efficiency, security and reliability
Despite many related papers, articles, and books, its definition has not been universally agreed upon
Judging by the different definitions, "power quality" is generally meant to express quality of voltage and/or the quality of current
PQ can be defined as:
The measure, analysis and improvement of the bus voltage to maintain a sinusoidal waveform at rated voltage and frequency.

5. Propagation of Harmonics in PS
Propagation of harmonics (generated by a nonlinear load) in PS

6. Propagation of Harmonics in PS

7. Causes of Disturbances in PS
Unpredictable Events (~60%): faults, lightning, resonance, ferroresonance, and GICs. All utility related problems!
The Electric Utility: Generation (maintenance, planning, capacity and expansion constraints, scheduling), transmission (lightning, flashover, voltage dips), distribution (voltage dips, spikes and interruptions)
The Customer (considerable portion): Includes harmonics (nonlinear loads), poor power factor (highly inductive loads), flicker (arc furnaces), transients ( switching, electrostatic discharge and arcing), improper grounding, misapplication of technology…
Manufacturing Regulations: Includes “lack of standards” and “equipment sensitivity”

8. Classification of Power Quality Issues
Different classifications, each using a specific property to categorize the problem.
Some classify events as "steady-state" & "non-steady-state" phenomena.
Some (e.g., ANSI C84.1) use "duration of the event".
Some (e.g., IEEE-519) use the wave shape (duration and magnitude).
Other standards (e.g., IEC) use the frequency range of the event for the classification.

9. Magnitude-duration plot for classification of power quality events
The magnitude and duration of events can also be used to classify power quality events:
The voltage magnitude is split into three regions:
interruption: voltage magnitude is zero,
under-voltage: voltage magnitude is below its nominal value, and
over-voltage: voltage magnitude is above its nominal value.
The duration of these events is split into four regions:
very short,
short,
long, and
very long.
The borders in this plot are somewhat arbitrary and the user can set them according to the standard that is used.
Magnitude-duration plot for classification of power quality events

10. PQ Classification by IEC

11. PQ Classification by IEEE

12. Impulsive Transient Current Caused by Lighting Strike, Result of PSpice simulation

13. Low-frequency Oscillatory Transient Caused by Capacitor Bank Energization

14. Low-frequency Oscillatory Transient Caused By Ferroresonance of a Transformer at No Load, Result of Mathematica Simulation

15. Momentary Interruptions Due to a Fault
Instantaneous voltage swell caused by a single line-to-ground fault

16. Voltage Sag Caused by a Single Line to Ground Fault

17. Waveform Distortion Steady-state deviation from a sine wave:
DC Offset - Enforced by GIC and employment of rectifiers, forcing even harmonics.
Harmonics - Sinusoidal voltages or currents with frequencies that are integer multiples of f0.
Interharmonics - Their frequencies are not integer multiples of f0.
Notching - A periodic voltage disturbance caused by line-commutated thyristor circuits.
Electric Noise - Unwanted electrical signals (<200 kHz), caused by faulty connections, arc furnaces, electrical furnaces, power electronic devices, control circuits, welding equipment, improper grounding, turning off capacitor banks, adjustable-speed drives…

18. Voltage Fluctuation and Flicker
Voltage Fluctuation - systemic variations of voltage envelope or random voltage changes (0.9<Vmag<1.1), classified into “step-voltage changes” & “cyclic or random voltage changes”, caused by “pulsed-power output”, “resistance welders”, “start-up of drives”, “arc furnaces”, “drives with rapidly changing loads” and “rolling mills”.
Flicker - Continuous and rapid variations in the load current magnitude which causes voltage variations. It is most common causes of voltage fluctuations. May be caused by an arc furnace.

19. Power-Frequency Variations
Deviation of the power system fundamental frequency from its specified nominal value (e.g., 50 or 60 Hz):
If balance between generation and demand is not maintained, frequency will deviate because of changes in the rotational speed of electromechanical generators.
The amount of deviation and its duration depends on the load characteristics and response of the generation control system to load changes.
Transmission system faults can also cause frequency variations outside of the accepted range for normal steady-state operation of the power system.

20. Voltage Sag Definition of Voltage Sag
Classified as short duration PQ events (e.g., short duration reductions in Vrms).
Defined by IEEE as “decrease in rms voltage at power frequency for durations from 0.5 cycles to 1 minute” although they typically only last 1 min to a few cycles.
Classified as short interruptions, as the period system is affected is shorter than a 3 minute duration.
Other standards (e.g., IEC) use the frequency range of the event for the classification.

21. Voltage Sag Due to a Three-phase Short Circuit Fault (Instantaneous Voltage in One Phase)

22. Voltage Sag Due to Induction Motor Starting

23. Note
The rms voltage is typically calculated every cycle or half-cycle of the power system.
We can conclude that magnitude and duration are the main characteristics of voltage sag.
However, the during-sag voltage also contains a rather large amount of higher frequency components.
It is important to note the difference between voltage sag and voltage dip:
North America states voltage dips as the amount nominal voltage declines
For example a voltage dip of 30% is the same as a voltage sag to 70%.

24. Voltage Sag Causes of Voltage Sag
Short interruptions and most long interruptions usually originate in the local distribution network.
However, voltage sag is much more of a “global problem” than interruptions. It can be causes by short-circuit faults hundreds of kilometers away in the transmission system.
Primary causes of voltage sags are (see figure next page):
Starting of large motors
Energization of heavy loads (e.g., arc furnace).
Transmission and distribution faults.
Local starting faults.
Protection system faults.
Load transferring from one power source to another.

25. Voltage sag causes and durations: 1) transmission faults, ) remote distribution faults, 3) local distribution starting faults, 4) starting of large motors, 5) short interruptions, ) fuses.

26. Voltage Sag Effects of Voltage Sag on Power Systems and Equipment
Generally speaking, electrical equipment work best under rated V and will stop operating if V=0 for a certain period.
So1.5.3 Effects of Voltage Sag on Power Systems and Equipment me equipment will stop within one second (desktop computers) and others longer (e.g., lap-tops for hours).
For each equipment it is possible to determine how long it operates after interruption by performing a simple test.
The same test can be repeated for different voltage magnitudes (e.g., 90%, 80%,…., 10% of Vrated).
Connecting points >>>>> “Voltage-Tolerance Curve”.
Equipment have different voltage-tolerance curves.

27. Voltage-tolerance Curve (Requirement) for Power Stations

28. Voltage Tolerance Range of Various Equipment
A voltage tolerance of “ ms, %” implies that the equipment can tolerate a zero voltage of “ ms” and a voltage of “%” of the nominal indefinitely.

29. Voltage-tolerance curve was introduced by Thomas Key (1978) for reliability of power supply to military installations >>> became well-known when Computer Business Equipment Manufacturers Association (CBEMA) used them >>>> CBEMA Curve
CBEMA Curve

30. In 1996 CBEMA curve was replaced by the ITIC Curve, as recommended by the Information Technology Industry Council.
ITIC Curve

31. Detrimental Effects of Voltage Sag on Equipment and Power Systems
Televisions - a black screen for up to a few seconds.
Compact Disk Players - reset or just wait for a new command.
Microwaves - loss of memory (settings).
Desktop Computers - tripping >>> loss of unsaved work.
Process Control Computers (e.g., of a chemical plant)- tripping >>> leading to restarting procedures of 48 hours plus sometimes very dangerous situations.
Equipment - tripping when the rms voltage drops below 90% for longer than one or two cycles.

32. Detrimental Effects of Voltage Sag on Variable Speed Drives
Drive controller (or protection)- detect the sudden change in operating condition and trip.
The drop in DC bus voltage (resulting from the sag) will cause maloperation or tripping of the drive controller or the PWM inverter.
The increased AC current during the sag (or the post-sag over-current) will charge the DC capacitor and enforce over-current trip or blowing of fuses.
The process driven by the motor will not be able to tolerate the drop in speed or torque variations due to sag.
After a trip (when the voltage comes back) some drives restart immediately, some restart after a certain delay and others have to be manually restarted.

33. Voltage Sag Voltage Sag Characteristics
Most common terms to define voltage sags are: sag magnitude, sag duration, and phase-angle jump.
Voltage Sag Magnitude
Main approaches to compute sag magnitude are from:
rms voltage,
fundamental voltage component, or
peak voltage.
These values can be computed over each cycle or half-cycle.
As long as the voltage is sinusoidal, it does not matter which approach is used.
But, especially during voltage sag this is not the case.

34. Sag Magnitude from Vrms
using a one-cycle window
using a half -cycle window
Voltage Sag

35. Sag Magnitude from Vfun
Voltage Sag
Sag Magnitude using a half -cycle window

36. Sag Magnitude from Vpeak
Voltage Sag
Sag Magnitude using a half -cycle window

37. Voltage Sag Voltage Sag Duration
Protection circuits require different fault-clearing time.
Downstream faults on transmission are cleared faster than in distribution, as differential and distance protection are used.
Fault clearing times (longer duration sags occur at lower voltage levels)
These longer sags are due to faults in local distribution (due to improper start of large induction motors). They are deeper and last longer than remote distribution faults (due to current-limiting fuses clearing and Ztransformer btw the fault and load).

38. Voltage Sag Calculations of Voltage Sag
Voltage Sag in Radial Systems
Typical distribution network with load
(letters) and fault positions (numbers)
(wehre z=Zfeeder, L=distance btw fault & pcc)

39. Sag Magnitude as a Function of Distance to the Fault
for overhead lines with different cross sections
for an overhead line at different fault levels
for underground cables with different cross sections

40. Voltage Sag Calculations of Voltage Sag
Voltage Sag in Non-Radial Systems
Radial systems are common in low- and medium-voltage networks. At higher voltage levels, other arrangements are common. Some typical cases will be discussed:
Voltage Sag with Local Generators
Voltage Sag in Subtransmission Loops
Voltage Sag in Branches From Loops
This will mitigate voltage sags of the load in two ways:
LG increases fault level (especially for a week system) at distribution bus which mitigates voltage sags due to faults on distribution feeders,
LG will also mitigate sags due to faults in rest of system by keeping up voltage at its local bus & feeding into fault.

41. Voltage Sag with Local Generators (LG)
Connection of a local generator to a distribution bus
Without LG:
Note:
With LG:

42. Voltage Sag in Subtransmission Loops
Example of a
Sub-transmission loop
Assuming E=1:

43. Voltage Sag in Branches from Loops
System with a branch away from a loop

44. Voltage Sag Calculations of Voltage Sag
Voltage Sag in Meshed Systems
When system becomes more complicated, closed expressions for voltage sag get very complicated/unfeasible.
Therefore, matrix calculations based on Thevenin’s superposition theorem and nodal impedance matrix.
Current & voltages during a sag are sum of two contributions:
Current & voltages before fault- which are due to all generators across the system.
Current & voltages due to change in voltage at fault position- which are due to the fault originate at a voltage source at fault position with all other sources short-circuited.

45. Main Equations (Sag in Meshed Systems)
Vduring fault
due to fault
place a short-circuit at
node f to model the fault
short-circuit all voltage sources
& place Vf(0) at the position
Vbefore fault c
& c
at fault position (k = f),
we know ΔVf= - Vf(0)
Pre-fault voltages are
normally close to unity
Therefore, calculation of sag magnitudes is very easy.
Drawback is that Z needs to be calculated:
can use a recursive procedure (Z is updated for each added branch)
or first calculate Y and then finding its inverse.

46. Voltage Sag Mitigation of Voltage Sag
There are various ways to mitigate voltage sag.
To understand them, the mechanism leading to an equipment trip must be understood:
short circuit
fault
equipment
trip
extensive voltage
drops at fault position
Short circuit faults >>> always cause sag for some consumers.
Faults in radial parts >>> interruption (due to protection device)
If resulting event exceeds a certain severity >>> equipment trip.
Other events such as capacitor switching >>> equipment tripping; however, majority are due to short-circuit faults.

47. Voltage Sag Available Mitigation Approaches
Reducing the number of short-circuit faults- by replacing overhead lines by underground cables, cover wires…. (already performed by most utilities).
Reducing fault-clearing time- this will not reduce number of events but only their severity.
Changing the power system- very costly.
Installing mitigation equipment- such UPS, DVR,& StatCom. Popular (only place customer has control over situation).
Improving equipment immunity- most effective solution; but not a short time solution (customer finds out about equipment immunity after equipment has been installed).

48. Summary
Definition of PQ: measure, analysis and improvement of Vbus to maintain a sinusoidal waveform at Vrated & Frated .
Causes of disturbances: utility, customer & manufacturer.
I (h) >>> propagation of harmonics >>> V (h).
Different classifications of PQ.
Voltage sag (definition, characteristics, causes, effects, calculations & mitigation).

49. Tutorial Problem 1.1: Sag calculation (radial power system)
An example of a radial power system supplying an industrial customer with several large ac and dc adjustable-speed drives is shown in Figure E The dc and ac drives are fed via dedicated transformers at 420 V and 660V, respectively. System information including source impedance, feeder and transformer data are provided in Tables E1.1.1 to E1.1.3.
Identify pcc points for faults on one of the 11 kV, 33 KV, 132 kV and 400 kV feeders.
Compute the critical distances (Lcrit) for sag magnitudes of 10%, 30%, 50%, 70% and 90% on the 11 kV, 33 KV, 132 kV and 400 kV feeders.
Magnitude of the most shallow sag due to a fault at 11 kV.
Plot the sag magnitude versus distance for faults at various voltage levels in the supply.
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50. Tutorial Problem 1.1: Sag calculation (radial power system)
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51. Tutorial Problem 1.2: Sag Calculation (with local generators)
An example of a system with on-site generation is given in Figure E The industrial loads are fed from a 66 kV, 1700 MVA substation via two parallel 66/11 kV transformers. The fault level at the 11 kV bus is 720 MVA, which includes the contribution of two local (on-site) 20 MVA generators with a transient reactance of 17%. The industrial loads are fed from the 11 kV bus. The feeder impedance at 66 kV is 0.3 /km.
With reference to Figure 1.9 (and Eqs. 1.9) of the notes, we get the following impedance values referred to 66 kV: Z1=2.56 , Z3=6.42 , Z4=18.5 , and Z2=(0.3 /km) Lfault where Lfault is the distance of the fault from pcc (the 66 kV bus) in kilometers.
Calculate the minimum sag magnitude without and with the on-site generator.
For fault distances of Lfault=10 km and Lfault=20 km, compute the sag magnitude without and with the on-site generator.
Plot the sag magnitude versus distance without and with the on-site generator.
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52. Tutorial Problem 1.2: Sag calculation (with local generators)
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53. Tutorial Problem 1.3: Sag Calculation (for a subtransmission loop
Consider the system of Figure E1.3.1 where a 125 km, 132 kV loop is connecting a number of substations. Only the substation feeding the load of interest is shown which is located at 25 km from the main substation. The fault level at the point of supply is 5000 MVA and the feeder impedance is 0.3 /km.
Calculate sag voltage for a fault (on 100 km line) located 40 km from the main substation.
Calculate sag voltage for a fault (on 25 km line) located 10 km from the main substation.
Plot sag magnitude versus the fault position for the 100 km and 25 km lines.
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54. Tutorial Problem 1.3: Sag Calculation (for a subtransmission loop
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55. Tutorial Problem 1.4: Sag Calculation for Meshed Systems
Consider the circuit diagram shown in Figure E1.4.1 representing a 275/400 kV mesh system. Nodes 1 and 2 represent 400 kV substations; nodes 3, 4 and 5 represent 275 kV substations; the branches between 1 and 3 and between 2 and 4 represent transformers (the later two transformers in parallel); and the impedance values are in percent at a 100 MVA base.
Build the node admittance matrix.
Compute the node impedance matrix.
Compute the voltage at node 5 due to a fault at node 2.
Construct a table showing voltages at any node of the system due to fault at any other node.
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56. Tutorial Problem 1.4: Sag Calculation for Meshed Systems
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