1. Understanding of Harmonics in Power Distribution System
Dr. Adel. M. Sharaf
Department of Electrical & Computer Engineering
University of New Brunswick

2. Outline Power System Harmonics? Why Harmonics are Troublesome?
Nonlinear Loads Producing Harmonic Currents
Harmonic Distortion?
Negative Effects of Sustained Harmonics
Mitigation of the Effects of Harmonics
Evaluation of AC Power System Harmonics?
Conclusions
References
2/23/2006
EE 6633 Seminar 1

3. What are Power System Harmonics?
Harmonic: a mathematical definition, generally used when talking about Integral orders of Fundamental frequencies
Power system harmonics: currents or voltages with frequencies that are integer multiples (h=0,1,2,…N) of the fundamental power frequency [1]
1st harmonic: 60Hz
2nd harmonic: 120Hz
3rd harmonic: 180Hz
Figure: 1 [2]
2/23/2006
EE 6633 Seminar 1

4. How are Harmonics Produced ?
Power system harmonics: presenting deviations from a perfect sinusoidal-waveform (voltage or current waveform).
The distortion comes from a Nonlinearity caused by saturation, electronic-switching and nonlinear electric loads, Inrush/Temporal/Arc/Converter/Limiter/Threshold Type Loads.
Figure: 2 [1]
2/23/2006
EE 6633 Seminar 1

5. Why Bother about Harmonics?
50-60% of all electrical Ac Systems in North America operate with non-linear type loads
Power-Quality-PQ Issues & Problems
Damage to Power Factor Correction capacitors
Waveform Distortion can create SAG/SWELL/NOTCHING/RINGING/…
All can cause damage effects to consumer loads and power systems due to Over-Current/Over-Voltage or Waveform Distortion
Additional Power/Energy Losses
2/23/2006
EE 6633 Seminar 1

6. Loads Producing Harmonic Currents
Electronic lighting ballasts/Controls
Adjustable speed Motor-Drives
Electric Arc Welding Equipment
Solid state Industrial Rectifiers
Industrial Process Control Systems
Uninterruptible Power Supplies ( UPS )systems
Saturated Inductors/Transformers
LAN/Computer Networks
2/23/2006
EE 6633 Seminar 1

7. Current vs. Voltage Harmonics
Harmonic current flowing through the AC Power System impedance result in harmonic voltage-drop at the load bus and along the Feeder!!
Figure: 3 [3]
2/23/2006
EE 6633 Seminar 1

8. How to Quantify Harmonic Distortion?
Total Harmonic Distortion-THD: the contribution of all harmonic frequency Currents/Voltages to the fundamental current. [3]
The level of THD-for Current or Voltage is directly related to the frequencies and amplitudes of the Offending Quasi-Steady State persistent Harmonics.
Individual Distortion Factor-(DF)-h quantify Distortion at h –harmonic-order
2/23/2006
EE 6633 Seminar 1

9. Calculation of THD (Eq-1)
THD: Ratio of the RMS of the harmonic content to the RMS of the Fundamental [3]
(Eq-1)
Current THD-I
(Eq-2)
Voltage THD-V
(Eq-3)
2/23/2006
EE 6633 Seminar 1

10. Negative Effects of Harmonics
Overheating and premature failure of distribution transformers [1]
Increasing iron and copper losses or eddy currents due to stray flux losses
Overheating and mechanical oscillations in the motor-load system [1]
Producing rotating magnitude field, which is opposite to the fundamental magnitude field.
Overheating and damage of neutral ground conductors [2]
Trouble sustained type Harmonics: 3rd, 9th, 15th …
A 3-phase 4-wire system: single phase harmonic will add rather than cancel on the neutral conductor
Malfunction/Mal-Operation of Sensitive Tele-control and Protection Relaying
2/23/2006
EE 6633 Seminar 1

11. Negative Effects of Harmonics (cont’ d)
False or spurious Relay operations and trips of circuit breakers [2]
Failure of the Firing/Commutation circuits, found in DC motor-drives and AC drives with silicon controlled rectifiers (SCR-Thyristor) [1]
Mal-Operation instability of voltage regulator [1]
Power factor correction capacitor failure [1]
Reactance (impedance)-Zc of a capacitor bank decreases as the frequency increases.
Capacitor bank acts as a sink for higher harmonic currents.
The System-Series and parallel Resonance can cause dielectric failure or rupture the power factor correction capacitor failure due to Over-Voltages & Over-Currents.
2/23/2006
EE 6633 Seminar 1

12. Harmonics and Parallel Resonance Circuit
Harmonic currents produced by variable speed motor-drives: can be amplified up to times in parallel resonance circuit formed by the capacitance bank and network inductance [5]
Amplified/intensified harmonic currents: leading to internal overheating of the capacitor unit.
Higher frequency currents: causing more losses than 60hz currents having same amplitude
Figure 4: Parallel resonance circuit and its equivalent circuit [5]
2/23/2006
EE 6633 Seminar 1

13. Harmonics and Series Resonance Circuit
The voltage of upstream AC Network can be also distorted due to series/parallel resonance formed by capacitance of the capacitor bank and System/load inductance : Ca cause high harmonic current circulation through the capacitors [5]
Parallel Resonance can also lead to high voltage distortion.
Figure 5: Series resonance circuit and its equivalent circuit [5]
2/23/2006
EE 6633 Seminar 1

14. Measure Equipments of Harmonics
Digital Oscilloscope:
Wave shape, THD and Amplitude of each harmonic
“True RMS” Multi-Meter:
Giving correct readings for distortion-free sine waves and typically reading low when the current waveform is distorted
Use of Harmonic Meters-Single Phase or three Phase
Figure 6: “True RMS” Multi-Meter [3]
2/23/2006
EE 6633 Seminar 1

15. Standards for Harmonics Limitation IEEE/IEC
IEEE Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems (Current Distortion Limits for 120v-69kv DS)
Table 1: Current Harmonic Limits [4]
Ratio
Iscc / Iload
Harmonic odd
numbers (<11)
numbers (>35)
THD-i
< 20
4.0 %
0.3 %
5.0 %
7.0 %
0.5 %
8.0 %
10.0 %
0.7 %
12.0 %
>1000
15.0 %
1.4 %
20.0 %
2/23/2006
EE 6633 Seminar 1

16. Standard of Harmonics Limitation (cont’d)
IEEE Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems (Voltage Distortion Limits)
Table 2: Voltage Harmonic Limits [4]
Bus Voltage
Voltage Harmonic limit as (%) of Fundamental
THD-v (%)
<= 69Kv
3.0
5.0 Kv
1.5
2.5
>= 161 Kv
1.0
2/23/2006
EE 6633 Seminar 1

17. Mitigation Of Harmonics [1]
Ranging from variable frequency motor- drive to other nonlinear loads and equipments
Power System Design:
Limiting the non-linear load penetration to 30% of the maximum transformer’s capacity
Limiting non-linear loads to 15% of the transformer’s capacity, when power factor correction capacitors are installed.
Avoiding/Detuning resonant conditions on the AC System:
(Eq-4)
hr = resonant frequency as a multiple of the fundamental frequency
kVAsc = short circuit current as the point of study
kVARc = capacitor rating at the system voltage
2/23/2006
EE 6633 Seminar 1

18. Mitigation the Effects of Harmonics [1] (cont’d)
Delta-Delta and Delta-Wye Transformers
Using two separate utility feed transformers with equal non-linear loads
Shifting the phase relationship to various six-pulse converters through cancellation techniques
Figure 7: Delta-Delta and Delta-Wye Transformers [1]
2/23/2006
EE 6633 Seminar 1

19. Mitigation the Effects of Harmonics [1] (cont’d)
Isolation-Interface Transformers
The potential to “voltage match” by stepping up or stepping down the system voltage, and by providing a neutral ground reference for nuisance ground faults
The best solution when utilizing AC or DC drives that use SCR/GTO/SSR.. as bridge rectifiers
Line Isolation-Reactors
More commonly used for their low cost
Adding a small reactor in series with capacitor bank forms a Blocking series Filter.
Use diode bridge rectifier as a front end to avoid severe harmonic power quality problems
2/23/2006
EE 6633 Seminar 1

20. Mitigation the Effects of Harmonics [1] (cont’d)
Harmonic-Shunt or Trap Filters:
Used in applications with a high non-linear ratio to system to eliminate harmonic currents
Sized to withstand the RMS current as well as the value of current for the harmonics
Providing true distortion power factor correction
Figure 8: Typical Harmonic Trap Filter [1]
2/23/2006
EE 6633 Seminar 1

21. Harmonic Trap Filters (cont’d)
Tuned to a specific harmonic order such as the 5th, 7th, 11th,… etc to meet requirements of IEEE Standard
The number of tuned filter-branches depends on the offending steady-state harmonics to be absorbed and on required reactive power level to be compensated
Figure 9: Typical Filter Capacitor Bank [5]
2/23/2006
EE 6633 Seminar 1

22. Harmonics Filter Types [6]
Isolating harmonic current to protect electrical equipment from damage due to harmonic voltage distortion
Passive Filter-Low cost:
Built-up by combinations of capacitors, inductors (reactors) and resistors
most common and available for all voltage levels  
Active Power Filter APF:
Inserting negative phase compensating harmonics into the AC-Network, thus eliminating the undesirable harmonics on the AC Power Network.
APF-Used only for for low voltage networks
2/23/2006
EE 6633 Seminar 1

23. Harmonic Filter Types (cont’d) [7]
Unified Switched Capacitor Compensator USCS:
The single line diagram (SLD) of the utilization (single-phase) or (three-phase- 4-wire) feeder and the connection of the Unified Switched- Capacitor Compensator (USCS) to the Nonlinear-Temporal Inrush /Arc type Loads or SMPS-Computer/LAN-Network loads.
Figure 10 [7]
2/23/2006
EE 6633 Seminar 1

24. Harmonics Filter Types (cont’d) [7]
The USCS is a switched/modulated capacitor bank using a pulse-width modulated (PWM/SPWM) Switching Strategy. The switching device uses either solid state switch SSR-(IGBT or GTO).
Figure 11 [7]
2/23/2006
EE 6633 Seminar 1

25. Need To Evaluate System Harmonics? [1]
The application of capacitor banks in systems where 20% or more of the load includes other harmonic generating equipment.
The facility has a history of harmonic related problems, including excessive capacitor fuse operation or damage to sensitive metering/relaying/control equipment.
During the Planning/Design stage of any facility comprising capacitor banks and nonlinear harmonic generating equipment.
2/23/2006
EE 6633 Seminar 1

26. When to Evaluate System Harmonics? [1] (cont’d)
In facilities where restrictive Electric Power Utility Company Standards/Guidelines limit the harmonic injection back into their system to very small magnitudes.
Industrial/Commercial Plant expansions that add significant harmonic generating nonlinear type equipment operating in conjunction with capacitor banks.
When coordinating and planning to add any emergency standby generator as an alternate/renewable power source
2/23/2006
EE 6633 Seminar 1

27. Conclusions
The harmonic distortion principally comes from Nonlinear-Type Loads.
The application of power electronics is causing increased level of harmonics due to Switching!!
Harmonic distortion can cause serious Failure/Damage problems.
Harmonics are important aspect of power operation that requires Mitigation!!
Over-Sizing and Power Filtering methods are commonly used to limit Overheating Effects of Sustained Harmonics.
2/23/2006
EE 6633 Seminar 1

28. References
[1] www-ppd.fnal.gov/EEDOffice-w/Projects/CMS/LVPS/mg/8803PD9402.pdf
[2]
[3]
[4]
[5] Harmonics_and_Reactive_Power_Compensation_in_Practice.pdf
[6]
[7] A.M. Sharaf & Pierre Kreidi, POWERQ UALITYE NHANCEMEUNSTI NGA UNIFIEDSW ITCHED CAPACITOCRO MPENSATOR, CCECE CCGEI 2003, Montreal, Mayimai 2003
/03/$ IEEE
2/23/2006
EE 6633 Seminar 1

29. Question ?
2/23/2006
EE 6633 Seminar 1