THD or TDD: Correctly Interpreting Harmonic Values Under Varying Load

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A frequency converter is operating at low load. The power quality analyzer suddenly shows:

THD-I = 42 %

At full load, however, it was only:

THD-I = 18 %

At first glance, it appears that the harmonic impact on the power network has deteriorated significantly at partial load. However, this does not necessarily have to be the case.

The reason lies in the reference value. Current THD relates the existing harmonic currents to the currently measured fundamental current. If this decreases significantly at low load, THD-I can increase considerably even though the actual harmonic currents in amperes have become smaller.

This is exactly where Total Demand Distortion (TDD) comes into play. It does not relate the harmonic currents to the instantaneous fundamental current, but to a defined maximum load or demand current.

THD and TDD therefore do not describe the same thing. Especially with frequency converters, UPS systems, switched-mode power supplies and other nonlinear loads, a high THD-I at low load can lead to an incorrect assessment if the load condition and reference value are not taken into account.

For corresponding measurements, ICS Schneider offers, among other products, the CA 8345 Power and Power Quality Analyzer from Chauvin Arnoux. The Class A instrument measures power quality parameters, harmonics and interharmonics and is suitable for long-term recording in industrial and commercial installations. Further instruments can be found under Electrical Measuring and Test Instruments at ICS Schneider.

What are harmonics?

An ideal AC power system has a sinusoidal voltage and sinusoidal current at the fundamental grid frequency.

In Europe, this is typically:

f1 = 50 Hz

Nonlinear loads, however, do not draw sinusoidal current.

Typical sources include

• frequency converters,
• switched-mode power supplies,
• UPS systems,
• LED lighting,
• rectifiers,
• chargers,
• power electronics.

The distorted current waveform can mathematically be broken down into a fundamental component and additional frequency components.

In a 50 Hz system, for example

3rd harmonic = 150 Hz

5th harmonic = 250 Hz

7th harmonic = 350 Hz

11th harmonic = 550 Hz

These components are superimposed on the fundamental and thereby change the waveform.

What does THD mean?

THD stands for:

Total Harmonic Distortion

and therefore describes the total harmonic content relative to a reference value.

For THD referenced to the fundamental component, the basic relationship is:

THD = √(X2² + X3² + X4² + ... + Xn²) / X1 × 100 %

Where:

• X1 = RMS value of the fundamental component,
• X2 … Xn = RMS values of the individual harmonics.

Depending on whether voltage or current is being considered, the terms used include:

THD-U

or:

THD-I

THD-U for voltage

For voltage:

THD-U = √(U2² + U3² + ... + Un²) / U1 × 100 %

THD-U describes distortion of the supply voltage

Increased voltage distortion can affect, among other things:

• motors,
• transformers,
• capacitors,
• electronic power supplies,
• protection and control devices.

Voltage THD is significantly influenced by the interaction of:

harmonic current × grid impedance

.

A load can therefore draw a highly distorted current without necessarily causing an equally high voltage distortion at the same point.

THD-I for current

For current:

THD-I = √(I2² + I3² + ... + In²) / I1 × 100 %

The denominator is decisive:

I1 = currently present fundamental current

This makes THD-I strongly load-dependent

If the fundamental current decreases significantly at partial load while the harmonic currents do not decrease by the same proportion, THD-I increases.

This does not automatically mean that the installation is now injecting higher absolute harmonic currents into the power network.

What does TDD mean?

TDD stands for:

Total Demand Distortion

Here as well, the RMS values of the harmonic currents are combined quadratically.

The difference lies in the reference value:

TDD = √(I2² + I3² + ... + In²) / IL × 100 %

Where:

IL = defined maximum load or demand current at the point under consideration

This means the reference value remains largely independent of the instantaneous load condition

This is precisely why TDD is better suited to evaluating varying load conditions.

A load is not assigned an extremely high distortion value at low load simply because its instantaneous fundamental current is small.

Direct comparison of THD-I and TDD

Characteristic	THD-I	TDD
Numerator	Total harmonic currents	Total harmonic currents
Denominator	current fundamental current I1	defined maximum demand current IL
Dependence on instantaneous load	high	significantly lower
Suitable for assessing the current waveform	yes	only to a limited extent
Suitable for grid assessment according to IEEE 519	not the primary reference value	yes, at the defined PCC
Typical effect at partial load	can increase sharply	remains referenced to a fixed demand current

THD-I essentially answers the question: How strongly is the currently flowing current distorted relative to its fundamental component? TDD, on the other hand, answers: How large is the harmonic current relative to the relevant maximum installation load?

Why THD-I can increase significantly at partial load

This effect occurs particularly frequently with frequency converters.

Assume that at high load:

I1 = 100 A

and the total RMS value of the harmonic currents is:

IH = 20 A

Then:

THD-I = 20 / 100 × 100 % = 20 %

At low load

the fundamental current is reduced, for example, to:

I1 = 20 A

The harmonic currents also decrease, but only to:

IH = 8 A

This results in:

THD-I = 8 / 20 × 100 % = 40 %

THD-I has therefore doubled.

At the same time, the absolute harmonic current has decreased from:

20 A

to:

8 A

.

A higher THD-I at partial load can therefore occur simultaneously with a lower actual harmonic load on the power network.

Practical example with varying load

Assume the defined maximum demand current of an installation is:

IL = 100 A

Operating condition	I1	Harmonic current IH	THD-I	TDD
Full load	100 A	20 A	20 %	20 %
50 % load	50 A	12 A	24 %	12 %
20 % load	20 A	8 A	40 %	8 %

The table shows this very clearly

As the load decreases, THD-I rises:

20 % → 24 % → 40 %

TDD, by contrast, decreases:

20 % → 12 % → 8 %

because the actual harmonic current is decreasing.

Both values are mathematically correct.

They simply answer different questions.

Why harmonic currents in amperes remain important

A percentage value alone can easily be misinterpreted.

Compare two installations

Installation A:

I1 = 10 A

IH = 5 A

THD-I = 50 %

Installation B:

I1 = 500 A

IH = 100 A

THD-I = 20 %

Although installation A has the significantly higher THD-I

only:

5 A harmonic current

flows there, compared with:

100 A

in installation B.

For:

• transformer loading,
• neutral conductor loading,
• voltage distortion,
• losses,
• filter design

the absolute currents of the individual harmonics are therefore also relevant.

Understanding the PCC measuring point correctly

When evaluating according to IEEE 519, the:

Point of Common Coupling (PCC)

plays a central role.

This refers to the electrical coupling point between the supply network and the connected installation, or the common connection point defined for normative assessment.

This is important

because a measured value directly at the input of a single frequency converter must not automatically be compared with a limit value for the PCC.

Within an installation, individual loads can have significantly higher current harmonics than the entire installation at the point of connection to the supply network.

Possible reasons include

• different load conditions,
• phase shifts of individual harmonics,
• partial mutual cancellation,
• additional linear loads.

Before evaluating any limit value, it must therefore first be clarified where the measurement was taken and to which point the applicable standard refers.

Correctly interpreting TDD and IEEE 519

IEEE 519 uses Total Demand Distortion for evaluating current harmonics at the PCC, rather than simply the currently measured THD-I.

The current reference value:

IL

is the maximum demand load current at the PCC under normal operating conditions.

The permissible current distortion also depends on the ratio:

ISC / IL

Where:

• ISC = available short-circuit current at the PCC,
• IL = maximum demand load current.

Why?

A power system with high short-circuit power has lower grid impedance and reacts less sensitively to injected harmonic currents in terms of voltage distortion.

A weak power system with lower short-circuit power can be more strongly affected by the same harmonic currents.

For a formal assessment, general statements such as:

THD-I must be below 5 %

must therefore not simply be used.

The specific assessment depends on the measuring point, system voltage, short-circuit ratio, demand current and applicable standard.

Considering grid impedance and short-circuit power

Harmonic currents cause harmonic voltages across the grid impedance.

In simplified form:

Uh = Ih × Zh

Where:

• Ih = current of the respective harmonic,
• Zh = grid impedance at this frequency,
• Uh = resulting voltage component.

A strong power system

has comparatively low impedance.

A given harmonic current therefore causes only relatively small voltage distortion.

A weak power system

has higher impedance.

The same harmonic current can therefore cause significantly higher voltage distortion.

For critical installations, the following should therefore be considered together:

THD-U + TDD + harmonic currents + grid impedance

Correctly evaluating frequency converters

Frequency converters are among the most common sources of current harmonics in industrial installations.

With a conventional converter using a line-side rectifier, the input current is not sinusoidal.

For power quality assessment, measurements are taken on the supply side

that is, upstream of the frequency converter.

A measurement at the:

PWM motor output

addresses a different issue.

At that point, pulsed output voltages with high switching-frequency components are present and must not be confused with conventional harmonic assessment of the supply network.

For a THD/TDD analysis of the power network

the following should therefore be measured on the supply side:

• voltages L1/L2/L3,
• converter input currents,
• fundamental currents,
• individual harmonics,
• THD-I,
• THD-U,
• TDD or the corresponding reference value.

Typical harmonic orders

Which harmonics occur depends strongly on the respective load.

With conventional three-phase rectifier circuits

pronounced:

• 5th harmonic,
• 7th harmonic,
• 11th harmonic,
• 13th harmonic

are often observed.

However, the overall THD value does not indicate which individual frequency components dominate.

The spectrum should therefore also be evaluated

For example, two measuring points may each have:

THD-I = 25 %

but completely different harmonic spectra.

For fault analysis and filter design, analysis of the individual harmonics is therefore significantly more informative.

Do not confuse THD-R with TDD

In addition to THD referenced to the fundamental, various power quality analyzers also provide values such as:

THD-F

or:

THD-R

or manufacturer-specific designations.

These abbreviations must not automatically be equated with TDD

With THD-F, the reference value is typically the fundamental component.

With THD-R, the reference value may, depending on the instrument and definition, be the total RMS value of the signal.

TDD, on the other hand, fundamentally uses a demand-based reference value.

Therefore, for every measuring instrument

the mathematical reference value behind the displayed parameter must be checked before carrying out a normative assessment.

Chauvin Arnoux explicitly specifies TDD functionality for IEEE 519 applications for the Qualistar CA 8336 and CA 8345 instruments. Nevertheless, the displayed parameter or instrument designation should be documented using the current operating manual and the selected instrument configuration.

Why a snapshot is often not sufficient

A single measured value can be misleading when loads vary.

Especially in installations with:

• frequency converters,
• compressors,
• pumps,
• welding systems,
• charging stations,
• production machines

the load condition can change within seconds or minutes.

A long-term recording shows

• when high harmonics occur,
• which load condition exists at the same time,
• whether THD-I rises only at partial load,
• whether THD-U also rises,
• which harmonics dominate,
• whether the problem is continuous or only sporadic.

Trend recordings are therefore often more important for a reliable assessment than a single screen value.

Planning the measurement setup correctly

1. Define the measurement objective

First, clarify whether the objective is to evaluate:

• a single load,
• a machine feeder,
• a subdistribution board,
• the complete installation at the PCC.

2. Connect voltages correctly

In a three-phase system, the voltage leads are connected according to the system configuration to:

L1 / L2 / L3 / N if applicable

3. Orient current sensors correctly

The current sensors must:

• match the expected current range,
• be assigned to the correct phase conductor,
• be installed with the correct current direction.

4. Record the load condition as well

THD-I without the simultaneously flowing current is only of limited significance.

At minimum, the following should be recorded in parallel:

• TRMS current,
• fundamental current,
• THD-I,
• individual harmonics.

5. Record for a sufficiently long period

With varying operating conditions, the measurement should cover several representative load conditions.

Which values should be evaluated together?

For a meaningful harmonic analysis, one single THD value is usually not sufficient.

Measured quantity	Why it is important
THD-U	shows distortion of the supply voltage
THD-I	shows distortion of the current relative to the fundamental component
TDD	relates current harmonics to the defined demand current
I1	shows the current fundamental current
Ih in A	shows the actual magnitude of individual harmonic currents
Individual harmonics	help with fault analysis and filter design
Active power	provides correlation with the load condition
PF / power factor	shows additional effects of distorted current waveforms
Trend over time	makes load dependence and sporadic events visible

Typical misinterpretations

Observation	Common misinterpretation	Correct check
THD-I rises sharply at partial load	converter now generates more harmonics	compare I1 and absolute harmonic currents
THD-I = 40 %	installation automatically violates a 5 % limit	check standard, measuring point and TDD reference
converter has high THD-I	IEEE 519 limit exceeded at the converter	perform assessment at the defined PCC
THD-U low, THD-I high	measurement must be incorrect	consider grid impedance and short-circuit power
THD-I decreases at full load	harmonic currents are automatically lower	check harmonic currents in amperes
THD value from one instrument differs from another	one of the instruments is measuring incorrectly	compare reference definition, harmonic order range and aggregation
measurement directly at the motor output of a VFD	assessment of supply-network harmonics	measure on the input side for network impact
TDD is calculated from instantaneous current	correct demand assessment	check IL as the defined reference value
only total THD is stored	cause can be clearly identified	also record individual harmonics and load condition
high THD at very low current	acute high loading of the power network	evaluate absolute harmonic currents and TDD

Systematically performing a harmonic measurement

1. Define the objective: Investigate a single device, distribution board or PCC.
2. Determine the system configuration: Correctly configure single-phase, three-phase without neutral or three-phase with neutral.
3. Select the measuring point: For normative assessment, use the actually relevant connection point.
4. Connect voltage leads: Check phase sequence and neutral assignment.
5. Select current sensors: Match the measuring range to the load current.
6. Orient current sensors: Observe correct phase assignment and current direction.
7. Configure the instrument: Check grid frequency, current sensor and measurement mode.
8. Measure THD-U: Compare voltage distortion on all phases.
9. Measure THD-I: Evaluate current distortion together with the current fundamental current.
10. Measure individual harmonics: Identify particularly dominant orders.
11. Store absolute currents: Do not evaluate harmonics only as percentages.
12. Determine the demand reference current: Define the correct I_L for TDD.
13. Document the load condition: Record power, machine condition or converter load in parallel.
14. Perform trend recording: Capture representative operating cycles completely.
15. Compare measured values by load condition: Evaluate full, partial and minimum load separately.
16. Determine the PCC for IEEE 519 assessment: Do not confuse individual loads with PCC limits.
17. Consider short-circuit ratio: Use I_SC/I_L to determine permissible TDD.
18. Check THD-U in parallel: Evaluate the actual effects on the supply voltage.
19. Analyze the spectrum if abnormalities occur: Narrow down the cause using individual harmonics.
20. Document the measurement setup: Record measuring point, sensors, instrument settings and reference values.

CA 8345 for power quality and harmonic analysis

CA 8345 Power and Power Quality Analyzer

The CA 8345 from Chauvin Arnoux is a Class A power and power quality analyzer for industrial and commercial installations.

ICS and Chauvin Arnoux specify, among other things:

• full compliance with IEC 61000-4-30 Class A Edition 3,
• analysis of harmonics and interharmonics according to IEC 61000-4-7,
• trend recording of a large number of power network parameters,
• power and energy measurement,
• alarm functions,
• transient capture,
• TrueInrush capture,
• USB, Ethernet and Wi-Fi communication,
• analysis using Power Analyzer Transfer software.

TDD with the CA 8345

Chauvin Arnoux confirms that the CA 8345 can determine a TDD parameter for applications related to IEEE 519.

When evaluating the measurement, however, it is important to know the display and reference definition used by the instrument.

Instrument designations such as:

THD-%f

or:

THD-%r

should therefore not be interpreted solely on the basis of the abbreviation.

For a documented normative assessment, the following must be clearly recorded:

• measuring point,
• reference current,
• instrument setting,
• measurement duration,
• applied standard.

Why the CA 8345 is useful for this task

Long-term recording is particularly important when loads vary.

This makes it possible to track, for example:

current + power + THD + harmonics + supply voltage

simultaneously throughout the complete production cycle.

This makes it possible to determine whether a high THD-I value is actually caused by higher harmonic currents or merely by a strongly reduced fundamental current at partial load.

Further instruments can be found under Power and Energy Analyzers at ICS Schneider and under Electrical Measuring and Test Instruments.

Conclusion

THD and TDD are two different parameters for evaluating harmonics.

THD-I is referenced to the current fundamental component

The relationship is:

THD-I = IH / I1

If the fundamental current decreases at partial load, THD-I can increase sharply.

TDD uses a fixed demand reference current

The relationship is:

TDD = IH / IL

This makes the value much better suited to comparisons between different load conditions.

A high THD-I does not automatically mean high absolute harmonic currents

Especially at low load, a small harmonic current can generate a high percentage value.

Absolute currents therefore remain important

THD, TDD and the individual harmonic currents in amperes should be evaluated together.

IEEE 519 evaluates current distortion at the PCC using TDD

THD-I measured directly at an individual frequency converter must therefore not simply be compared with a TDD limit at the point of connection to the power network.

THD-U shows the actual voltage distortion

Whether a harmonic current becomes problematic for the power network depends significantly on grid impedance and short-circuit power.

Snapshots are often insufficient with varying load

A trend recording across several typical operating conditions provides much more reliable results.

For practical applications

Define the measuring point → connect correctly on the supply side → measure fundamental current → measure THD-I → evaluate absolute harmonic currents → check THD-U in parallel → determine demand current I_L → calculate TDD or configure it correctly in the measuring instrument → for IEEE 519 assessment, consider PCC and I_SC/I_L → record several load conditions → analyze individual harmonics → only then compare the measured values with the applicable limits.

FAQ: Correctly Distinguishing THD and TDD

What does THD mean?

THD stands for Total Harmonic Distortion and describes the total harmonic content relative to a defined reference value.

What does THD-I mean?

THD-I describes the total harmonic current relative to the currently present fundamental current.

What does THD-U mean?

THD-U describes the total harmonic voltage distortion relative to the fundamental component of the voltage.

What does TDD mean?

TDD stands for Total Demand Distortion and describes harmonic currents relative to a defined maximum demand load current.

What is the main difference between THD-I and TDD?

The denominator. THD-I uses the currently measured fundamental current, whereas TDD uses the defined maximum demand current I_L.

Why does THD-I increase at low load?

Because the fundamental current in the denominator decreases. The harmonic currents often do not decrease by the same proportion.

Does a higher THD-I automatically mean more harmonic current?

No. The absolute harmonic current can decrease at the same time.

Can THD-I be 50 % at partial load even though the load on the power network is low?

Yes. With a very small fundamental current, even a relatively small harmonic current can result in a high percentage THD-I.

Why is TDD easier to compare under varying load?

Because the denominator does not continuously change with the instantaneous load current, but is based on a defined demand current.

Is TDD always lower than THD-I?

Not necessarily in every conceivable operating condition. Under typical partial-load conditions, however, TDD is often significantly lower because I_L is greater than the currently flowing fundamental current.

What is I_L in TDD?

I_L is the defined maximum demand load current at the relevant point under normal operating conditions.

What is the PCC?

PCC stands for Point of Common Coupling and refers to the common point of connection between the installation and the utility network or other loads that is relevant for power quality assessment.

Does IEEE 519 apply directly at a frequency converter?

The limits in IEEE 519 fundamentally apply at the defined PCC and not simply at the input of an individual nonlinear load.

Can a frequency converter have 40 % THD-I while the installation still complies with the requirements at the PCC?

Yes. The decisive factors include the overall installation, demand current, PCC, short-circuit ratio and the harmonic currents actually present there.

Why is THD-U measured as well?

THD-U shows how strongly the supply voltage itself is distorted by harmonic components.

Why can THD-I be high while THD-U remains low?

In a strong power system with low impedance, even relatively high harmonic currents may produce only low voltage distortion.

What does I_SC/I_L mean?

The ratio describes the available short-circuit current at the PCC relative to the maximum demand load current and is relevant for evaluating permissible current harmonics.

Are individual harmonics more important than total THD?

For fault analysis and filter design, often yes. THD combines all harmonics into one value and does not show which harmonic order dominates.

Which harmonics are common with frequency converters?

With conventional three-phase rectifier circuits, pronounced 5th, 7th, 11th and 13th harmonics are often present. The exact distribution depends on the converter and grid impedance.

Where should network harmonics be measured on a frequency converter?

On the supply or input side of the frequency converter.

Why should measurements not be taken at the PWM motor output?

The motor output contains pulsed switching voltages and represents a different measurement task from conventional harmonic analysis of the supply network.

What is THD-R?

Depending on the instrument manufacturer and definition, THD-R refers to THD using a different reference value, for example the total RMS value. The exact definition must be taken from the instrument documentation.

Is THD-R the same as TDD?

No, not fundamentally. TDD uses a demand current as its reference value. Instrument designations must therefore be checked against the documentation.

Can the CA 8345 measure TDD?

Chauvin Arnoux confirms corresponding TDD functionality for the CA 8345 for applications related to IEEE 519.

Can the CA 8345 measure harmonics?

Yes. The CA 8345 is designed for analysis of harmonics and interharmonics.

Why is a long-term measurement useful?

Because THD-I, power and harmonic currents can depend strongly on the operating condition. Trend recording shows these relationships much more clearly than a single snapshot.

Which values should I store during a harmonic analysis?

At minimum, voltage, current, fundamental current, THD-U, THD-I, individual harmonics, power and, where required, TDD or the corresponding demand reference value.

Where can I find the CA 8345 at ICS Schneider?

Further information is available under CA 8345 Power and Power Quality Analyzer at ICS Schneider.

Where can I find further power and network analyzers?

An overview is available under Power and Energy Analyzers at ICS Schneider.