harmonics indices

►Harmonic currents produced by nonlinear loads can interact adversely with the utility supply system. ►The interaction often gives rise to voltage and current harmonic distortion observed in many places in the system. ►Therefore, to limit both voltage and current harmonic distortion, IEEE Standard 519-1992 proposes to limit harmonic current injection from end users so that harmonic voltage levels on the overall power system will be acceptable ► This approach requires participation from both end users and utilities.

Harmonic Distortion Evaluations

1

Harmonic Indices

►The two most commonly used indices for measuring the harmonic content of a waveform are: the total harmonic distortion the total demand distortion.

►Both are measures of the effective value of a waveform and may be applied to either voltage or current.

2

Harmonic Distortion Evaluations

Thus, in principle, end users and utilities share responsibility for limiting harmonic current injections and voltage distortion at the PCC.

Since there are two parties involved in limiting harmonic distortions, the evaluation of harmonic distortion is divided into two parts:

Measurements of the currents being injected by the load.

Calculations of the frequency response of the system impedance.

3

Concept of Point of Common Coupling (PCC)

4

For industrial and commercial end users, the PCC is usually at the primary side of a service transformer supplying the facility.

Harmonic Distortion Evaluations - The Utility.

►

Since the harmonic voltage distortion on the utility system arises from the interaction between distorted load currents and the utility system impedance, the utility is mainly responsible for limiting the voltage distortion at the PCC.

►

The limits are given for the maximum individual harmonic components and for the total harmonic distortion (THD). These values are expressed as the percentage of the fundamental voltage. For systems below 69 kV, the THD should be less than 5 percent.

5

Total Harmonic Distortion (THD)

►The THD is a measure of the effective value of the harmonic components of a distorted waveform. That is, it is the potential

heating value of the harmonics relative to the fundamental. This index can be calculated for either voltage or current:

%1001

2

2max

M

MTHD

h

hh

21

1

2 1 THDMMrmsmaxh

hh

.

Mh : is the rms value of voltage (current) harmonic component h

The rms value of a distorted waveform is the square root of the sum of the squares. THD is related to the rms value of the waveform

6

Total Harmonic Distortion (THD)

►Voltage THDV Index

%1001

2

2max

V

VTHD

h

hh

V

21

1

2 1.max

V

h

hhrms THDVVV

7

%1001

2

2max

I

ITHD

h

hh

I

21

1

2 1.max

I

h

hhrms THDIII

►Current THDI Index

The power quality industry most often uses the THD index to describe voltage harmonic distortion and always references harmonic voltages to the fundamental value of the waveform at the time of measurement

Voltage Total Harmonic Distortion Index

►Example on Calculation of Voltage THDV Index

%48.3210011055.0

1000.1

)04.0()05.0()08.0()10.0()15.0()25.0( 222222

VTHD

pu

VVh

hhrms

051.1

)04.0()25.0()1( 222

1

2max

8

Table 1. Harmonic voltage spectrum example

puVrms

051.1)3248.0(11 2

IEEE Voltage Distortion Limits in % of V1

The THD value in this table is expressed as a function of the nominal system rms voltage rather than of the fundamental frequency voltage magnitude at the time of the measurement

9

Harmonic Distortion Evaluations - End Users.

►

Current distortion levels can be characterize with a THDI value, but this can be misleading as a small current can have a high THDI but not be a significant threat.

►

Harmonic problems are more common at end-user facilities than on the utility supply system. Most nonlinear loads are located within end-user facilities, and the highest voltage distortion levels occur close to harmonic sources.

►

For individual end users, IEEE Standard 519-1992 limits the level of harmonic current injection at the point of common coupling (PCC).

►

Recommended limits are provided for both individual harmonic components and the total demand distortion.

►

These limits are expressed in terms of a percentage of the end user’s maximum demand current level (peak demand), rather than as a percentage of the fundamental.

10

►The total demand distortion (TDD) is expressed in terms of the maximum demand load current, i.e.,

Harmonic Evaluation for end-user Facilities

%100%100

2max

23

222

2max

L

h

L

h

hh

IIII

I

ITDD

►In computing the short-circuit current at the PCC, the normal system conditions that result in minimum short-

circuit capacity at the PCC should be used since this condition results in the most severe system impacts.

►A procedure to determine the short-circuit ratio is as follows:

11

►►

IIhh is the magnitude of individual harmonic components (rms

amps).

►►

IIscsc is the short-circuit current at the PCC.

►►

IILL is the fundamental component of the maximum demand load current at the PCC. It can be calculated as the average of the maximum monthly demand currents for the previous 12 months.


12

IEEE Current Distortion Limits in % IL

13

120120--69,000 V69,000 V

69,00069,000--161,000 V161,000 V

> 161,000 V> 161,000 V

IEEE Current Distortion Limits in %

14

1.

Determine the three-phase short-circuit duty IIscsc at the PCC. This value may be obtained directly from the utility and expressed in amperes. If the short-circuit duty is given in megavoltamperes, convert it to an amperage value using the following expression:

where MVA and kV represent the three-phase short-circuit capacity in megavoltamperes

and the line-to-line voltage at

the PCC in kV, respectively.


AkV

MVAILL

sc 31000

15

2.

Find the load average kilowatt demand PD over the most recent 12 months. This can be found from billing information.

3.

Convert the average kilowatt demand to the average demand current in amperes using the following expression:

AkVPF

kWILL

L 3


16

4.

The short-circuit ratio is now determined by:

L

sc

IIRatioCircuitShort

This is the short-circuit ratio used to determine the limits on harmonic currents in IEEE Standard 519-1992.

Example: Calculation of TDD limitsA 3100 kVA, 12.5 kV/480 V single-phase transformers with impedance of 2.6%supplying a 200 hp VFD. Fault current Isc on the primary side is 900 A and 8700 A on the secondary. The utility was not specific to the location of the PCC. Determine whether the current distortion limits are met.

For PCC at Primary side of the transformerIsc = 900 A, IL = 7 A (full load 60 Hz current)Isc /IL = 900/7 = 128.6

128

From the Table TDDI < 15%


17

For PCC at Secondary side of the transformerIsc = 8700 A, IL = 180 A (full load 60 Hz current)Isc /IL = 8700/180 = 48.3

48

From the Table TDDI < 8%

harmonics indices

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