Power quality analysis : THD, thd, TDD

[sesa53355_bridg]

Hi,

Do we have document where detailed explanation of THD, thd, TDD is available with its End user benefits.

How do we take advantage of this feature compared to competition.

Regards,

Chetan Dhake.

[sesa252887_brid]

I am also interested in this.

[Charles_Murison]

The PM8 user guide has some information: Page 43 in the PDF, page 37 in the guide.

THD. Total Harmonic Distortion (THD) is a quick measure of the total distortion present in a waveform and is the ratio of harmonic content to the fundamental. It provides a general indication of the “quality” of a waveform. THD is calculated for both voltage and current. The power meter uses the following equation to calculate THD, where H is the harmonic distortion (See guide)

thd. An alternate method for calculating Total Harmonic Distortion, used widely in Europe. It considers the total harmonic current and the total rms content rather than fundamental content in the calculation. The power meter calculates thd for both voltage and current. The power meter uses the following equation to calculate THD, where H is the harmonic distortion: (See guide)

http://download.powerlogic.schneider-electric.com/C12579F100161A32/All/FAF46C3369CDFEB385257A11005B4...

for TDD have a look at http://www.powerlogic.com/kbarticle.cfm?myrequest=RESL205832 for the calculations.

As for the end user benefits I am not sure.

[sesa53355_bridg]

Thanks for the explanation of how to calculate THD, thd & TDD.

I am interested to understand, to End Customer what are the benefit of these 3 measurements.....how do they utilize these values ?

Regards,

Chetan Dhake.

[sesa55514_bridg]

The end-user benefit between THD and thd is really to align with regional expectations (i.e. is it expected to create the ratio based on total RMS or fundamental RMS).

The end-user benefit of TDD is a bit more obvious.  It uses the Demand value to establish a fixed number to use in the ratio calculation.  This creates a more realistic value of the harmful impact of current harmonics, as they are expressed as a ratio against the peak demand rather than whatever the load current happens to be at that moment.  As an example, let's assume there is 250 mA of harmonic content on the 5A nominal CT input of a meter, and the circuit loaded at 2.5A fundamental current.  THD would say the harmonic percentage is 250mA/2.5A, or 10%.  TDD would say the harmonic percentage is 250mA/5A, or 5%.  If the customer is concerned about things like harmonics summing in the neutral, the TDD value gives a better representation of the magnitude of the problem.

[mrsrinivas · Accepted solution]

Hi Chetan,

1) About THD, TDD:

• TDD                           =     Total Demand Distortion of current.
• THDI% (or thdi%)      =     Total Harmonic distortion of current.

The reason for above two terminologies as below:

THDI is the ratio of Harmonic current magnitude to Fundamental Current.

1. i.e. THDI %      =      Ih / I1 x 100.

As the load (or I1)  varies, the ratio (THDI%) will also vary. This makes things complicated to arrive at specific limit of harmonic current. Hence TDD is used as a limiting factor for harmonic current distortion.

TDD             =     Ih / Imax x 100.

Where,

Imax     =     the current at Maximum Demand measured (or calculated) at the Point of Common coupling (PCC).

(PCC can be either Utility metering point or secondary of transformer.)

2) Measurement of THD, TDD:

THDI% can be measured by directly connecting harmonic analyzer or any THD measurable instrument.

But, in most of the situation it may be difficult to measure TDD, as TDD has to be measured at Maximum Demand scenario, hence TDD will be calculated based on THDI% measurement and Maximum Demand data from customer.

3) Why TDD?:

As per IEEE 519, the limit on harmonic current distortion is specified in TDD. Hence, TDD plays key role to compare the harmonic distortion as per IEEE 519.

Best regards,