Harmonisa

HARMONISA

Outline • • • • • • • • • • • •

Introduction THD and TDD Displacement and True Power Factor K-Factor and Transformer Derating When should you be concerned? Application of IEEE 519 Standard Harmonics Measurements Industrial Customers Commercial Customers Revisions to IEEE 519 IEC Standards Conclusions Harmonics

2

What Are Harmonics ?  Harmonics are due to distortion of the voltage or current waveform  The distortion comes from nonlinear devices, principally loads I(t)

Nonlinear Resistor

V(t)

V I

Harmonics

3

Linear Load

Harmonics

4

Nonlinear Load

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5

Harmonic Components 60 Hz (h = 1)

+ +

180 Hz (h = 3) 300 Hz (h = 5)

+ 420 Hz (h = 7)

+ + +

540 Hz (h = 9) 660 Hz (h = 11) 780 Hz (h = 13)

+

· · ·

Harmonics

6

Harmonics

Harmonics

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Fourier Series Representation • Fourier series 

x(t )  co  2

n

sin  nt   n 

n 1

• Average value • RMS Value

C

x  co X  co2 





C n2

n 1

Harmonics

8

Periodical nonsinusoidal waveforms • Most of power voltage and current waveforms have no dc component • Most of normal voltage and current waveforms have no even-order harmonics

Harmonics

9

RMS Values • Under sinusoidal condition:

Vrms  Vmax / 2 • Under nonsinusoidal conditions: Vrms

1 T 2  v (t )dt   T 0

Harmonics



2 V  n n 1

10

Examples 1) x(t )  100 cos100t  X rms  100 / 2 2) x(t )  100  100 2 cos100t  X rms  1002  1002  100 2 3) x(t )  220 2 cos100t   50 2 cos 300t  2

2

X rms  220  50  .... Harmonics

11

Total Harmonic Distortion  Defines the total harmonic content of current or voltage  Ratio of the RMS of the harmonic content to the RMS of the fundamental, as % of fundamental 

THD (%) 

2 V  h h2

V1

100%

2 Vrms  V12 THD (%)   100% V1 Harmonics

12

Total Demand Distortion Factor (TDD)

Applies for current distortion only. The total rms harmonic current distortion, in % of the maximum demand load current (15 or 30 min demand) 2 I rms  I12 TDD(%)   100% I max

Harmonics

13

Harmonic Sources Harmonic sources are nonlinear loads - Saturated transformers and inductors - Switching regulators - Switching power supplies - Variable Speed Drives - Electronic ballast

Harmonics

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Harmonic Sources

Harmonics

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Rectifiers AC source

Uncontrolled

DC Load

DC - DC Converter

rectifier

(a) Switched - mode dc power supplies

AC source

Uncontrolled rectifier

PWM Inverter

Induction Motor

(b) Variable - speed AC drives.

AC source

Controlled

PWM Inverter

rectifier

AC Load

(c) Uninterruptible Power Supplies (UPS) Controlled

AC source

DC Load

rectifier

(d) DC power supplies.

AC system

Phase - Controlled rectifier

Phase - controlled inverter

Harmonics (e) DC power transmission systems.

AC system

16

Single-Phase Rectifiers

Harmonics

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Input Current Harmonics of Single-Phase Rectifiers • The input current has no dc component nor even-order harmonics • The input current harmonic is dominated by the 3-rd order harmonic. • The displacement power-factor is unity but the true-power factor is not unity.

Harmonics

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Harmonic Profile of Personal Computer

Harmonics

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Electronic Ballast

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Electronic ballast Line Current for Electronic Ballast

Current (Amps)

1.00

Max: 0.784 Min: -0.792 Avg: 0.305828 Abs: 0.792 RMS: 0.334094 CF : 2.37059 FF : 1.09242

0.75 0.50 0.25 0.00

-0.25 -0.50 -0.75 -1.00 0

10

20

30

40

50

Time (mS)

Line to Neutral Voltage for Electronic Ballast

200

Max: 170 Min: -170 Avg: 109.055 Abs: 170 RMS: 120.727 CF : 1.40814 FF : 1.10703

Voltage (V)

150 100 50 0 -50 -100 -150 -200 0

10

20

30

40

50

Time (mS)

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Magnetic Ballast

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Harmonic Currents in Typical Building

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DC Drive

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Three-Phase Rectifier

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PWM drive, no choke

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PWM drive with choke

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Six Pulse Bridge Six pulse bridge - harmonic current 25

20

15 % 10

5

0 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Harmonic number

Harmonics

28

Input Current Harmonics • The input current has no dc component nor even order harmonics nor multiple three order harmonics. • The existing harmonic order is n  6h  1 • The displacement power-factor is unity but the true power-factor is not unity.

Harmonics

29

What is Power Factor? • Power factor is a measure of how effectively equipment converts or transmits electrical active power at a given rms current and voltage. • Traditionally, the power factor has been defined as the cosine of phase angle difference between voltage and current (displacement power factor). • Apparent power is the maximum active power that can be delivered at a given rms current and voltage. • Power factor is the ratio between the active power to the apparent power (true power factor). • When the harmonics are present, the PF is different to DPF. Harmonics

30

Power Factor

 I

V Ip

Iq

0



2

Power Factor POWER

Ip

0



2

Power Factor POWER Iq

0



2

Power Factor



V

IL I1

I5

0

I7



2

Single-Phase Power Concept • Sinusoidal voltage and current

v  2V cos t 

i  2 I cost   

• Instantaneous power

      p  vi  VI cos  1  cos 2  t  VI sin  sin 2  t              resistive part

Harmonics

reactive part

35

Single-Phase Power Concept • Active or Average Power

1 P T • Reactive Power



t o T

to

pdt VI cos 

Q  VI sin 

• Apparent Power

S  VI Harmonics

36

Single-Phase Power Concept • Power triangle 2

2

S  P Q

2

• Power factor

P PF   cos  S Harmonics

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Single-Phase Power Concept E 2  V  V  2  V 2

V 2  V  V  2 Thus ΔV  E-V  RI cos φ  XI sin φ RP  XQ XQ   V V Losses  RI 2  R S / V  2



 R  P /V  2  Q /V  2 Harmonics

 38

Single-Phase Power Concept E 2  V  V  2  V 2

V 2  V  V  2 Thus ΔV  E-V  RI cos φ  XI sin φ RP  XQ XQ   V V Losses  RI 2  R S / V  2



 R  P /V  2  Q /V  2 Harmonics

 39

Balanced Three-Phase Power • Instantaneous power

p  va ia  vb ib  vc ic p  3VI cos  • Instantaneous power is a constant that is equal to average power Harmonics

40

Balanced Three-Phase Power • Reactive power is defined as

Q  3VI sin  • Apparent power is defined as

S  3VI Harmonics

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Three-Phase Power System

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Three-Phase Power System

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Three-Phase Four-Wire Systems Three - Phase Three - Wire System

Series Losses Considerat ion :



P  rs I a2  I b2  I c2  I n2 P



 3rs I e2

(unbalanced )

Ie 

(balanced)

Ve 

I a2  I b2  I c2  I n2 Thus, I e  3 Shunt Losses Considerat ion :



2 2 2 2 Vao  Vbo  Vco  Vno P  Rsh

(unbalance d)

Ve2 P  3 Rsh

(balanced)

2 2 2 2 Vao  Vbo  Vco  Vno Thus, Ve  3

or Ve 

V

2 an

I V V

2 a



 I b2  I c2 / 3



2 an

2 2  Vbn  Vcn /3

2 ab

2 2  Vbc  Vca /9





2 2 2 2 2  Vbn  Vcn  Vab  Vbc  Vca / 12

Apparent Power S e  3Ve I e Power Factor  P / S e Harmonics

44

Power concept under nonsinusoidal waveforms

• Voltage



v  Vo  2

V

n

cos nt   n 

n

cos nt   n 

n 1

• Current



i  Io  2

I n 1

• Instantaneous power

p  vi Harmonics

45

Power concept under nonsinusoidal waveforms

• Average power 

P  Vo I o 

V I

n n

cos n   n 

n 1

• Apparent power

S  Vrms I rms

• Power factor 

P PF   S

Vo I o 

V I

n n

cos n   n 

n 1

Vrms I rms Harmonics

46

Reactive Power • What is reactive power under nonsinusoidal conditions? • Some definitions: 

Q   Vh I h sin  h h 1

Q

Vrms I rms 

2

P

2

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Sinusoidal voltage case Average power : Power factor :

P  V1 I1 cos1  1 

I1 I1 P PF   cos1  1   cos 1 1 / 2 S I rms  2  2 In   I1    n2 1  1  1



where :

Relationship between power factor and THD:

PF 

cos 1 1  THD 2 Harmonics

48

Balanced nonsinusoidal quantities Let assume: 

va 



Vn cos n t  

n 1

For n=1: va1  V1 cost  For n=2: va 2  V2 cos 2t  For n=3: va 3  V3 cos 3t 



vb 





Vn cos n t 

n 1



vb1  V1 cos t  23





vb 2  V2 cos 2t  23 vb3  V3 cos 3t  Harmonics

2 3





vc 

 n 1



Vn cos n t  23



vc1  V1 cos t  23







vc 2  V2 cos 2t  23



vc 3  V3 cos 3t  49



Balanced nonsinusoidal quantities For n=3k-2, The harmonics are similar to positive sequence quantities. For n=3k-1, the harmonics are similar to negative sequence quantities. For n=3k, the harmonics are similar to zero quantities.

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50

Three-Phase Four-Wire System

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Switched mode power supply currents Phase A (50 Amps)

Neutral (82 Amps)

Phase B (50 Amps)

Phase C (57 Amps)

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Neutral Current Problem

Neutral Current Problem

Neutral Current Problem

Three-Phase Four-Wire System Phase currents :



iR   I h sin  ht   h  h 1 

iS   I h sin  h t  23    h  h 1 

iT   I h sin  h t  23    h  h 1

Neutral current:



iN  iR  iS  iT  3  I n sin  3ht   h  n 3 h

Though the phase currents are balanced, the neutral current is not zero if the waveform is nonsinusoidal. The maximum value of neutral current is 1.73 time of phase current. Harmonics

56

Neutral current problems • Neutral current can be excessive • The zero sequence current cannot be detected by overcurrent protection that is located on the primary side. • The transformer losses can be excessive. • The neutral voltage to ground can be excessive. • The size of neutral cable cannot smaller than the phase cables. • Each phase circuit must be provided by separate neutral cable. Harmonics

57

Neutral conductor sizing Neutral currents can easily approach twice the phase currents - sometimes in a half-sized conductor. IEEE 1100-1992 recommends that neutral busbars feeding non-linear loads should have a crosssectional area not less than 173% that of the phase bars. Neutral cables should have a cross-sectional area that is 200% that of the phases, e.g. by using twin single core cables.

Sizing the neutral conductor For three phase circuits using single core cables: • Use a neutral conductor sized for the actual neutral current • If the neutral current is not known, use a double sized neutral cable • Provide overcurrent protection • But take account of the grouping factors! • Take into account voltage drop

Sizing the neutral conductor For multi-core cables : • Multi-core cables are rated for only three loaded cores - applies to both 4 and 5 core cables • When harmonics are present the neutral is also a current carrying conductor • Cable rated for three units of current is carrying more - three phases plus the neutral current • It must be de-rated to avoid overheating • Neutral must have overcurrent protection • Grouping factor must be taken into account

Sizing the neutral conductor - IEC

Neutral conductor protection Neutral conductors should be appropriately sized and provided with over-current protection. The protective device must break all the phases, but does not necessarily need to break the neutral itself. This implies a future need for 4 pole breakers with double rated neutral poles.

Current vs. Voltage Harmonics Distorted Voltage +

Pure Sinusoid

(Voltage Drop)

-

Distorted Load Current

Harmonic currents flowing through the system impedance results in harmonic voltages at the load Harmonics

63

Voltage distortion

Why bother about Harmonics?  Important aspect of power quality  Damaging Effects to Consumer Loads and Power System  Problems may be incipient  Non-Linear Loads are Increasing  Power Factor Correction Capacitors

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Capacitors • Shunt capacitor has a significant effect on harmonic levels. • Capacitors do not generate harmonics, but provide network loops for possible resonant conditions. • Resonant frequency:

f res  50 MVAsc / MVAR

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66

Series Resonance

Harmonics

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Parallel Resonance

Harmonics

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Harmonic Resonance TO POWER SYSTEM

LV

CONVERTOR

M

HARMONICS

AMPLIFIED HARMONICS

Guidelines • • • •

If the KVA of the harmonic producing load is less than 10% of the transformer kVA rating, capacitors can be applied without concern for resonance If the kVA of the harmonic producing load is less than 30% of the transformer kVA rating and the kVAR is less than 20% of the transformer kVA rating, capacitors can be applied without concern for resonance If the kVA of the harmonic producing load is more than 30% of the transformer kVA rating, capacitors should be applied as filters. These guidelines are applicable when transformers with a 5-7% impedance are used and the system impedance behind the transformer is less than 1% of the transformer base.

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Capacitor standard • • • •

110% of rated rms voltage 120% of rated peak voltage 180% of rated rms current 135% of rated reactive power Including the harmonics

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Actions must be taken If the limits are exceeded: • Relocation of the capacitors to other parts of the circuit. The harmonic generating loads and the capacitor bank should not share the same transformer. • For wye connected utility transformer banks, the neutral connection to ground may be removed to prevent third harmonics from flowing through the capacitors. • If the above remedies fail, it may be necessary to add a tuning reactor. Harmonics

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Example (1)

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Measurement data of example (1)

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Impedance Seen by The Harmonics

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Circuit Breakers and Fuses • Currents with 50% distortion factor limited the breaker blowout coil’s ability to force the arc into the arc chute. Vaccum interrupters are less sensitive to harmonic current. • Harmonic distortion affects the current sensing ability of thermal magnetic breakers. • Fuses are not affected by the harmonic content. It should be noted, fuses respond to rms current.

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Lighting • The incandescent lamp will have a definite loss of life when operated with distorted voltage.

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Meters • Modern rms meters are relatively immune to the influence of waveform distortion. • Induction disk watthour meter is affected by waveform distortion. The errors vary from 5 to 20%, depending on the harmonic content. This type of meter must be avoided when the harmonic content is high.

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Digital Meters

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Digital kWh Meters • For best results, a digital kWh meters must be accurate at least up to 1000 Hz. • The ADC must be at least 12 bit. • By using this specification, it has been shown that the error is less than 1% for current harmonic up to 88 % and voltage harmonic up to 5%.

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Protective Relaying • In most cases, the waveform distortion of the load current has little effect of the fault current. • Every relay performs differently in the presence of waveform distortion.

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Rotating Machines • Nonsinusoidal voltages applied to electric machines may cause overheating, pulsating torques, or noise. • Rotoroverheating is the main problem. • For generators, zero sequence current is very dangerous to the rotor.

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Motor de-rating curve for harmonic voltages 1

De-rating Factor

0.95

0.9

0.85

0.8

0.75

0.7 0

2

4

6 Harmonic Voltage Factor (HVF)

8

10

12

Transformers • The primary effect of power system harmonics on transformers is the additional heat generated by the losses caused by the harmonic content of the load current. • The additional heating caused by harmonics requires load capability derating to remain within the temperature rating of the transformer. • The loading of a delta connected transformer may be misleading because of circulating triplen harmonic currents. Harmonics

84

Transformer Losses • No-load losses or iron losses. These losses can be divided into hysteresis and eddy current losses. These losses almost constant if the voltage is almost sinusoidal. • Load losses. These losses can be divided into I2R loss, eddy current in conductor, and stray losses due to magnetic leakages into the tank, iron core, etc. • The eddy current loss in the conductor is almost proportional to the harmonic frequency. • The stray load losses are usually proportional to fx , where x = 0.8 – 2. Harmonics

85

K-Rating of Transformers Two rating or de-rating systems for transformers:• American system, established by UL and manufacturers, specifies harmonic capability of transformer - known as K factor. • European system, developed by IEC, defines de-rating factor for standard transformers known as factor K.

K-Factor  K-Factor is ratio of eddy current losses due to distorted current compared to the losses for the same rms fundamental frequency current  Example:  Eddy Current Losses with 100 A rms with harmonics = 270 Watts  Eddy Current Losses with 100 A rms 60 Hz sine wave = 27 Watts

 K - Factor = 270/27 = 10 Harmonics

87

K-Factor

h=

K =

 I (pu)

h1

h

Harmonics

2

h

2

88

K-Factor  Assumes eddy current losses are proportional to f 2 - OK for small conductor sizes and low harmonics  At higher frequencies, eddy current loses are proportional to f  Transition frequency depends on winding configuration, material  Al - 2200 Hz, Cu - 700 Hz  K-factor over estimates harmonics effect at higher frequencies

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THD and K-Factor Calculation EXAMPLE Fundamental = 10 A rms = 0.96 pu 5th Harm0nic = 2.0 A rms = 0.19 pu 7th Harm0nic = 1.5 A rms = 0.14 pu 11th Harm0nic = 1.0 A rms = 0.096 pu 13th Harm0nic = 1.0 A rms = 0.096 pu Irms = Sqrt (102 + 22 + 1.52 +1.02 + 1.02) = 10.4 A THD = Sqrt (22 + 1.52 + 1.02 +1.02)/10 = 2.87/10 = 28.7 % K = (0.962 + 0.192 * 52 + 0.142 * 72 + 0.0962 *112 + 0.0962*132) = 5.55

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K-Factor

For this typical PC load, the K factor is 11.6 (See IEE 1100-1992 for a worked example) Harmonics

91

K-Rating of Transformers - US System Next, select a transformer with a higher K rating: standard ratings are 4, 9, 13, 20, 30, 40 and 50.

NB - for non K-rated transformers: The K factor describes the increase in eddy current losses, not total losses.

Transformer Derating  Non K-rated transformers have to be derated when load current has harmonics  IEEE C57.110 “Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents”

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K-rating  K-rated transformers can handle nonsinusoidal load current up to the full load rating with k-factor up to the k-rating of the transformer  K-rated transformers are designed to have lower eddy current losses

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K-Rating of Transformers - European System In Europe, the transformer de-rating factor is calculated according to the formulae in BS 7821 Part 4. The factor K is given by:

 e  I1   K  1   1 e  I   

  I    n q  n      I1   n2  

2 n N

2

0.5

e is ratio of eddy current loss (50 Hz) to resistive loss n is the harmonic order q is dependent on winding type and frequency, typically 1.5 to 1.7

K-Rating of Transformers - European System

For the same PC load, the de-rating factor is 78%

K-Rating or De-rating? ‘K-rated’ transformers are designed to supply harmonic loads by : • using stranded conductors to reduce eddy current losses • bringing secondary winding star point connections out separately to provide a 300% neutral rating

K-Rating or De-rating? ‘De-rating’ a standard transformer has some disadvantages: primary over-current protection may be too high to protect the secondary and too low to survive the in-rush current  the neutral star point is likely to be rated at only 100% of the phase current  it is less efficient  future increases in loading must take the de-rating fully into account

Effect of triple-n harmonics in transformers Triple-n harmonic currents circulate in delta windings they do not propagate back onto the supply network. - but the transformer must be specified and rated to cope with the additional losses.

Skin effect Alternating current tends to flow on the outer surface of a conductor - skin effect - and is more pronounced at high frequencies.  At the seventh harmonic and above, skin effect will become significant, causing additional loss and heating.  Where harmonic currents are present, cables should be de-rated accordingly. Multiple cable cores or laminated busbars can be used.

Conductors and Cables



Ploss   I h2 Rh h 1

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101

Conductors and Cables 

Ploss  

h 1

2 I h Rh

If the skin effect is neglected Ploss 

2 RI rms



2 RI1

Harmonics

1  THD

2

102

When Should You be Concerned About Harmonics 20 % of total load is power electronic load  If service transformer is loaded near rating  When PF correction capacitors are planned Neutral to ground voltage in 120 V supply exceeds 1 to 2 volts 

 Harmonics

103

Harmonic Standards  Several Countries have developed Standards to limit harmonics  IEEE 519-1992  IEEE 519A-2004?  IEC 61000-3-2, 61000-3-4, 61000-3-12

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104

IEEE 519 IEEE 519 “Recommended Practices and Requirements for Harmonic Control in Electric Power Systems” Specifies load current harmonic limits at PCC Specifies supply voltage harmonic limits at PCC IEEE 519A “Guide for Applying Harmonic Limits on Power Systems”

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105

IEEE 519 Standard Limits HARMONIC CURRENT DISTORTION LIMITS IN % OF IL V  69 kV ISC / IL 20 20-50 50-100 100-1000 1000

h < 11

11  h  17

17  h  23

23  h  35

35  h

TDD

4.0 7.0 10.0 12.0 15.0

2.0 3.5 4.5 5.5 7.0

1.5 2.5 4.0 5.0 6.0

0.6 1.0 1.5 2.0 2.5

0.3 0.5 0.7 1.0 1.4

5.0 8.0 12.0 15.0 20.0

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Application Concerns • • • • • •

Selecting PCC Calculating ISC and IL What is TDD ? Measurement Problems Time Varying Harmonics General Procedure for Applying Harmonic Limits • Cost of Problems vs. Cost of Solutions • Distributed Generation Limits Harmonics

107

What is PCC ?  “Point in the public network which is closest to the consumer concerned and to which other customers are or may be connected” IEC 61000-34:1998

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PCC

Harmonics

109

PCC

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110

IEEE 519 Standard Limits (Utility) HARMONIC VOLTAGE DISTORTION LIMITS (in % of Nominal Fundamental Frequency Voltage) Bus Voltage at PCC

Individual Harmonic Voltage Distortion

V  69 kV

3.0

69 kV  V  161 kV

1.5

2.5

V  161 kV

1.0

1.5

Harmonics

Total Voltage Harmonic Distortion (THDV)

5.0

111

IEEE 519 Standard  Limits apply for the “worst case” for normal operation (lasting longer than one hour)  For shorter periods, during start-ups limits may be exceeded by 50%  Even harmonics are limited to 25% of odd harmonic limits  Co-gen - use Isc / IL < 20, irrespective of actual value

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112

Harmonic Current Measurements • Calculate harmonics as % of a fixed (average max. demand) current, not as % of fundamental • Limits in the Table Apply only to Odd harmonics – Even Harmonics are limited to 25% of those limits • CT Characteristics are important – usually good (should be less than 3 dB) • How long to monitor? – Very stable, One day may be adequate – one week – for most cases – Permanent monitoring in some cases

Harmonics

113

Presentation of Results – snap shots

Harmonics

114

Presentation of Results – Time Trends

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115

Harmonic Voltage Measurements • Measure at PCC • Low Voltage – measure with direct connection • Higher Voltages – Connect with PT – frequency response is good to 3 k Hz (50th harmonic) • Avoid CCVTs – frequency response is not good

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116

Evaluation Procedure • Non-linear load is less than 10 - 20% of total plant load – No harmonic evaluation necessary • Weighted Disturbing Power

SDw 

 (S

Di

 Wi )

i

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117

T ype of L oad

T y p ic a l W a v e fo rm

C u rre n t D is to rtio n

W e ig h tin g F a c to r (W i)

80% (h ig h 3 rd )

2 .5

1.0

S in g le P h a s e P o w e r S u p p ly

C u r e r n t

0.5

0.0

-0.5

-1.0

0

10

20

30

40

Time (mS) 1.0

0.5

C u r e r n t

S e m ic o n v e rte r

0.0

-0.5

-1.0

0

10

20

30

h ig h 2 n d ,3 r d , 4 th a t p a r tia l lo a d s

2 .5

80%

2 .0

40%

1 .0

28%

0 .8

15%

0 .5

v a rie s w ith firin g a n g le

0 .7

17%

0 .5

40

Time (mS) 1.0

6 P u ls e C o n v e r te r, c a p a c itiv e s m o o th in g , n o s e rie s in d u c ta n c e

C u r e r n t

0.5

0.0

-0.5

-1.0

0

10

20

30

40

Time (mS) 1.0

o n v e r te r, s m o o th in g u c ta n c e > 3 % , d riv e

0.5

C u r e r n t

6 P u lse C c a p a c itiv e w ith s e rie s in d or dc

0.0

-0.5

-1.0 0

10

20

30

40

Time (mS) 1.0

0.5

C u r e r n t

6 P u ls e C o n v e rte r w ith la rg e in d u c to r fo r c u rre n t s m o o th in g

0.0

-0.5

-1.0

0

10

20

30

40

Time (mS) 1.0

1 2 P u ls e C o n v e rte r

C u r e r n t

0.5

0.0

-0.5

-1.0

0

10

20

30

40

Time (mS) 1.0

0.5

C u r e r n t

a c V o lta g e R e g u la to r

0.0

-0.5

-1.0

0

10

20

30

40

Time (mS)

F lu o re s c e n t L ig h tin g

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118

Evaluation Procedure • If SDw / Ssc < 0.1%, then automatic acceptance • SDw is weighted disturbing power • Ssc is short circuit capacity at PCC • If customer has or considering PF Correction Capacitors, harmonic evaluation is always necessary

Harmonics

119

UTILITY

CUSTOMER

Choose PCC

Estimate Weighted Disturbing Power (S ) or % Nonlinear DWLoad

Calculate Short Circuit Capacity (I ) SC Is Power Factor Correction Existing or Planned?

Yes

No

Calculate Average Maximum Demand Load Current (I ) L

Stage 1: Is Detailed Evaluation Necessary?

Yes

No

Calculate Short Circuit Ratio (SCR=I /I ) SC L

Characterize Harmonic Levels (Measurements, Analysis)

Stage 2: Does facility meet harmonic limits?

Yes

No

Design Power Factor Correction and/or Harmonic Control Equipment (include resonance and interaction concerns)

Verification Measurements and Calculations (if necessary)

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120

Applying Harmonic Limits For Industrial Facilities 1. 2. 3. 4. 5. 6.

Usually supplied by dedicated transformer Several nonliner loads – ASDs, Rectifiers, DC drives, Induction furnaces Loads are relatively low PF - Power factor correction capacitors are installed Industrial loads like motors do not provide much damping for resonance conditions Problems inside the facility before causing problems in utility system Limit Voltage distortion to 5% at PCC – provide some margin for distortion within facility

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121

Applying Harmonic Limits For Industrial Facilities 1. 2. 3. 4.

Choose PCC Characterize Harmonic Loads Determine if PF Correction Needed Calculate Expected Current Harmonics at PCC 5. Design Harmonic Control Equipment, if necessary 6. Verify performance with measurements

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122

Applying Harmonic Limits For Commercial Customers • Significant percentage of Load is Electronic Equipment and Switch mode Power Supplies • High Efficiency Fluorescent Lighting • HVAC Load is ASD drives • Significant harmonic cancellation -Meeting IEEE 519 at SES is rarely a problem • Internal Harmonic Problems – neutral overheating, transformer overloading, communication interference

Harmonics

123

Overview of Proposed Revisions to IEEE 519 • Immediate – Increased voltage limits for buses < 1 kV – Limits for time-varying harmonics – Revised notch and ringing limits and definitions • Near-term – Measurements • Limits for Single-Phase Equipment – Dropped

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124

Voltage Distortion Limits (% of nominal fundamental frequency voltage)   B us V o ltag e at P C C  (V n ) 

In d iv id u a l H ar m o n ic  V o ltag e D is to rtio n (% ) 

T o tal V o ltag e  D isto rtio n ­ T H D V n  (% ) 

V n  6 9 kV  

3 .0  

5 .0  

6 9 kV  V n  1 6 1 kV  

1 .5  

2 .5  

V n  1 6 1 kV  

1 .0  

1 .5  

 

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125

Harmonic Voltage Limits • Add a new voltage limit category for buses less than 1 kV – 5% limit for individual harmonics – 8% limit for voltage THD

• Main concern is associated with multiple zero crossings – Research has shown that concern has merit Harmonics

126

Time-Varying Harmonics • Limits must be based on factual cause/effect – Thermal effects occur over time – Burst distortion effects can be instantaneous – Startup/abnormal conditions should be tolerated • The facts suggest providing – Significant limit increases for short periods – Some limit increases for intermediate periods – No increases for the majority of the time • Some statistical techniques may be needed

Harmonics

127

Time Varying Harmonics (24 hour period) Total Duration of Harmonic Bursts

Maximum Duration of a Single Harmonic Burst

Acceptable Harmonic Distortion Level

<15 minutes

< 15 seconds

3.0 x design limit

>15 minutes and < 1.2 hours

>15 sec and < 30 minutes and

2.0 x design limit

>1.2 hours and

> 30 minutes

design limit

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128

Measurements • Define measurement specification – Many commercial meters exist • 8, 12, and 16 cycle windows • 128 and 256 samples/cycle • Filtering

– IEC 61000-4-30 offers potential • Specific requirements • Captures 3s, 10m, and 2hr values

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129

IEC Standards  Apply at equipment level, 240 V or less, 1-ph, 690 V or less, 3-ph, 50 or 60 Hz  61000-3-2: loads with input current < 16 A  61000-3-12: loads with input current >16A and <75A (published in 2004)  61000-3-4: loads with input current > 75 A  Use varies from country to country, mandatory in EC  UL certification available in US Harmonics

130

IEC 61000-3-2  Class A - General Purpose loads, 3-ph balanced equipment (plus any eqpt not in B,C,D)  Class B - Portable tools  Class C - Lighting equipment  Class D - Equipment with “special wave shape” (conduction angle < 600), P < 600W Harmonics

131

Class A (Balanced 3-ph Equipment) Harmonic Order 3 5 7 9 11 13 15-39 2 4 6 8-40

Max. Permissible Harmonic Current (Amps) 2.3 1.14 0.77 0.4 0.33 0.21 0.15 x 15/n 1.08 0.43 0.30 0.23 x 8/n Harmonics

132

Class C Equipment (Lighting >25W) Harmonic Order

Max. Harmonic Current (% of Fund.)

2 3 5 7 9 11-39

2 30 x PF 10 7 5 3

(odd harmonics only)

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133

Class D Equipment (Special Waveshape) Harmonic Order

Relative Limit (mA/W)

3 5 7 9 11 13-39

3.4 1.9 1.0 0.5 0.35 3.85/n

Max. Harmonic Current (Amps) 2.30 1.14 0.77 0.40 0.33 0.15 x 15/n

(odd harmonics only)

Harmonics

134

IEC 3-12 (for Equpt >16 A and < 75 A)

Harmonics

135

IEC 61000-3-4  Loads with rated current > 75 A

 Stage 1: SC KVA/EQ. KVA > 33  Stage 2: SC KVA/EQ. KVA 66, 120, 175, 250, 350, 450, 600  Stage 3: Local Utility Requirements apply. Harmonics

136

IEC Standards  IEC Standards are based on European distribution system  3 ph, 3-wire feeder, and 3-ph, 3-wire branches, 11 or 12 kV  3-ph (delta-star), large (500 kVA - 1000 kVA) distribution transformers  400/230V, 3-ph long secondary  USNC - IEC standards in US Harmonics

137

US distribution systems are different  3-ph, 4-wire Feeder, 1-ph, 2-wire branches, most 15 kV class  Small (50 - 100 kVA) transformers serving 6 to 8 residents  120/240 V, 1-ph, short secondaries  No consensus between manufacturers, utilities and users Harmonics

138

Comparison of European and North American Systems European

North American

Feeder

3-ph, 3-wire

3-ph, 4-wire

Branch

3-ph, 3-wire

1-ph, 2-wire

Transformer 500 kVA-1MVA 50 kVA-100kVA Connection

Y/

Secondary

400/230V, 3-ph 120/240V, 1-ph

Length

Long

Gr Y / Gr Y short

Harmonics

139

Harmonic Mitigation Techniques • Harmonic Source Side • Medium side • Equipment side

Harmonics

140

Harmonic Source side • Multipulse rectifiers • PWM rectifiers • Unity power-factor rectifiers

Harmonics

141

12-Pulse Rectifier

Harmonics

142

12-Pulse Bridge Twelve pulse bridge - harmonic current 25

20

15 % 10

5

0 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Harmonic number

Harmonics

143

6N-Pulse Rectifier

Harmonics

144

Multipulse Rectifiers

Harmonics

145

Unity Power Factor Rectifier

Harmonics

146

Three-Phase Unity Power Factor Rectifier

Harmonics

147

Single-Phase PWM Rectifier • Bidirectional power flow • The input power factor is adjustable • The input current waveform is adjustable

Harmonics

148

Three-Phase PWM Rectifier

Harmonics

149

Medium side • Passive filters • Active filters • Combination of active and passive filters

Harmonics

150

Harmonic reduction transformers

Load I3 Interconnected Star Transformer sized for harmonic currents only

Isolating transformers Delta-star isolating transformers reduce propagation of harmonic current into the supply.  Transformers should be adequately rated to cope with the harmonics  Although the transformer effectively establishes a new neutral, don’t use half-sized neutrals  Provide a well rated four wire feed so that the transformer can be isolated for service

Isolating transformers

Isolating transformers

Isolating transformers

Isolating transformers

Parallel Passive Filters

Harmonics

157

Series Passive Filters

Harmonics

158

Passive harmonic filters Filters are useful when  the harmonic profile is well defined – such as motor controllers  the lowest harmonic is well above the fundamental frequency - but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive

Active filters  Where the harmonic profile is unpredictable or contains a high level of lower harmonics, active filters are useful  Active harmonic conditioners operate by injecting a compensating current to cancel the harmonic current

Filters are useful when the harmonic profile is well defined – such as motor controllers the lowest harmonic is well above the fundamental frequency - but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive

Harmonics

161

Series Active Filters

Harmonics

162

Parallel-Passive Parallel-Active

Harmonics

163

Series-Active Series-Passive

Harmonics

164

Series-Active Parallel-Passive

Harmonics

165

Series-Passive Parallel-Active

Harmonics

166

Series Parallel-Passive Parallel-Active

Harmonics

167

Parallel Series-Passive Series-Active

Harmonics

168

Comparison of hybrid filters

Harmonics

169

Equipment side • K rated transformers • Generator derating • Cable derating

Harmonics

170

Reducing Voltage Distortion by Circuit Separation

Harmonic solutions  Keep circuit impedances low  Rate neutrals and multi-core cables generously 1.73 to 2 times normal size  Always use true RMS meters  Provide a large number of separate circuits to isolate problem and sensitive loads  Take harmonics into account when rating transformers  Provide appropriate filtration where required

Conclusions • Harmonics are important aspect of power quality • Application of power electronics is causing increased level of harmonics • Survey the loads and make preliminary evaluation • IEEE and IEC Standards reviewed Harmonics

173