THD or TDD? Which Parameter Is Important in Harmonic Measurement?

In modern facilities, speed control devices (VFDs), UPSs, rectifiers, LED drivers, and switched-mode power supplies are found in nearly every panel.

In modern facilities, speed control devices (VFDs), UPSs, rectifiers, LED drivers, and switched-mode power supplies are found in nearly every panel. These power electronics loads draw non-sinusoidal current from the grid and generate harmonic distortion. When an electrical engineer in the field connects a power quality analyzer, they typically see two different percentage values on the screen: THD and TDD.

Many technical teams use these two values interchangeably. However, THD and TDD are separate metrics that describe the same harmonic phenomenon from different reference points. When confused, the result is often the same: either an unnecessary investment in filters is made for a nonexistent problem, or a real incompatibility is overlooked. Furthermore, the international standard IEEE 519 evaluates current-side compliance based on TDD rather than THD; yet, the majority of field devices display only THD by default.

In this article, we discuss what THD and TDD mean, their formulas, their differences, the IEEE 519 limits, what to look for in which devices, and common mistakes—all in a technically accurate yet straightforward language.

What Is THD? (Total Harmonic Distortion)

THD (Total Harmonic Distortion) is the ratio of the RMS sum of the harmonic components in a waveform to the RMS value of the fundamental component. It is expressed as a percentage and comes in two types: voltage THD (THDv) and current THD (THDi).

THDv — Voltage THD

It indicates how much the voltage waveform deviates from the ideal sine wave:

THDv = √(V₂² + V₃² + ... + Vₕ²) / V₁ × 100%

Here, V₁ is the RMS value of the fundamental frequency (50 Hz) voltage, and Vₕ is the RMS value of the hth harmonic component. THDv is a first-class indicator for evaluating the quality of the power grid’s voltage.

THDi — Current THD

It provides the distortion ratio in the current waveform:

THDi = √(I₂² + I₃² + ... + Iₕ²) / I₁ × 100%

The critical point is this: The denominator of THDi is the fundamental current I₁ at that moment. In other words, when a facility is operating at a low load, the fundamental current decreases, so the same amount of harmonic current appears as a much higher THDi percentage.

Practical Example

Suppose that during the night shift at a facility, I₁ = 80 A and the total harmonic current is measured at 32 A:

THDi = 32 / 80 × 100 = 40%

When we see “40% THDi” on the screen, our first instinct is to sound the alarm. However, this value reflects not the facility’s actual harmonic load, but rather a proportional representation of the current low-load level. This is where the concept of TDD comes into play.

What Is TDD? (Total Demand Distortion)

TDD (Total Demand Distortion) is the ratio of the harmonic current content to the maximum demand load current (IL):

TDD = √(I₂² + I₃² + ... + Iₕ²) / IL × 100%

The numerator of the formula is the same as that of THDi; the denominator is different. While THDi is divided by the instantaneous I₁, TDD is divided by a fixed reference—namely, IL.

What Is IL?

IL is the maximum demand load current at the PCC (Point of Common Connection). IEEE 519 typically defines this value as the average of the maximum demand currents over the previous 12 months. In new facilities where actual 12-month data is unavailable, the highest demand current during the measurement period can be used as a practical approximation for IL.

Since IL is a fixed reference, the TDD value is minimally affected by load fluctuations, whether the facility is operating at low or high load. This makes it a much more reliable metric at the system level.

Let’s Evaluate the Same Example Using TDD

Suppose the average of the maximum demand over the past 12 months for the same facility is IL = 400 A. For the measurement during the night shift:

TDD = 32 / 400 × 100 = 8%

So, even though the “40% THDi” displayed on the screen may seem alarming, the actual load at the system level is 8%. Decisions should be based on TDD, not THDi.

A mathematical result: Since IL ≥ I₁, TDD is always equal to or less than THDi. When the facility is operating at full load, IL ≈ I₁, and the two values converge.

THD vs. TDD: Differences and When to Use Which?

In short:

·         THDv → to evaluate grid voltage quality and the utility’s responsibility.

·         THDi → to measure the harmonic behavior of an individual device (e.g., a drive) and identify the source.

·         TDD → to evaluate, in a load-independent manner, the actual impact of the harmonic current injected into the grid by the facility at the PCC.

In other words, the answer to the question “Am I compliant?” is provided by TDD on the current side and by THDv on the voltage side. THDi alone is not sufficient to determine compliance.

IEEE 519 Standard: Voltage and Current Limits

IEEE 519-2022 evaluates harmonic compliance at the point of common connection (PCC), which is the common connection point in the utility system where other customers can also be connected. Measurements taken at an individual equipment terminal do not prove system compliance.

Voltage THD Limits (Absolute Thresholds)

The limits on the voltage side depend on the bus voltage at the PCC and are independent of the Isc/IL ratio:

Current TDD Limits (Dependent on Isc/IL)

Current limits, on the other hand, vary depending on the Isc/IL ratio, which defines the system’s “stiffness.” A robust system (high Isc/IL) can absorb more harmonic current:

Critical warning: Statements taken out of context, such as “The TDD limit is 5%” or “It is 8%,” are misleading. The correct limit can only be determined after calculating the project-specific Isc/IL ratio. While the limit may be 5% in a weak system, it can rise to as high as 20% in a very powerful system.

Additionally, IEEE 519-2022 evaluates compliance using a statistical method: the daily 99th percentile of 3-second intervals and the weekly 95th/99th percentiles of 10-minute intervals are compared to the standard limits. In other words, a single snapshot measurement does not constitute proof of compliance.

Practical Application: What to Look for on Which Device?

Use this checklist to correctly interpret every percentage displayed on your analyzer’s screen when you’re out in the field:

1.       Is it THDi or THDv? These two values are displayed differently at the same time. Make sure you know which one you’re looking at on the screen.

2.       Is TDD displayed? Most analyzers display only THDi by default. The IL value must be manually entered into the device to display TDD.

3.       What is the fundamental component (I₁)? A low I₁ can cause high THDi; verify the load level first.

4.       Is the CT ratio correct? An incorrect CT ratio distorts all current-side derived values (THDi, TDD).

5.       Was the connection type (3Ø WYE, 3Ø DELTA, 1Ø) selected correctly? An incorrect configuration directly affects power and harmonic calculations.

6.       Is the recording duration sufficient? Since IEEE 519 requires statistical evaluation, at least one week of continuous recording is necessary. An instantaneous snapshot is not sufficient.

7.       Is the measurement point the PCC? Equipment terminal ≠ PCC. Compliance decisions are based on data from the PCC point.

Class A analyzers that can display both THDi and TDD simultaneously in the field, offer IEC 61000-4-7-compliant harmonic measurement (up to the 100th harmonic for 50/60 Hz), and provide continuous recording capabilities significantly reduce the workload during this process. For example, the AHA AH2222 power quality analyzer can simultaneously record THD, TDD, phase angle, and power/energy parameters for four phases at user-defined intervals ranging from 1 second to 60 minutes; with 32 GB microSD support, it provides a recording duration ranging from 2 hours to 1 year. A device of this type can single-handedly provide the data infrastructure required for the weekly statistical evaluation mandated by IEEE 519.

However, please note: the device’s capabilities do not automatically guarantee compliance with the standard. Compliance is achieved by interpreting the collected data against the IEEE 519 tables using the correct reference values (Isc, IL, PCC position, busbar voltage).

Common Mistakes

1.       “If THDi is high, the system is non-compliant.” Incorrect. Even if THDi is high, there is no non-compliance under IEEE 519 as long as it remains within the TDD limit. The determination is based on TDD.

2.       “The TDD limit is the same for every facility.” Incorrect. The limit depends on the project-specific Isc/IL ratio and varies between 5% and 20%. A single limit cannot be specified without calculating Isc and IL.

3.       “IEEE 519 limits apply to equipment terminals.” Incorrect. IEEE 519 is a system standard; the limits apply at the point of common connection (PCC). A measurement at the input of a single drive does not prove system compliance.

4.       “A single snapshot measurement is sufficient for compliance.” Incorrect. The harmonic profile varies depending on shifts and production conditions. At least weekly continuous recording and a 95th/99th percentile statistical evaluation are required.

5.       “If voltage THD is low, everything is fine.” Incorrect. THDv and TDD are two independent metrics; a good result for one does not guarantee the same for the other.

6.       “A K-factor transformer ensures IEEE 519 compliance.” False. K-factor transformers are designed to manage harmonic heating; they do not reduce upstream harmonic current injection and therefore do not directly contribute to TDD compliance at the PCC.

7.       “At full load, THDi and TDD are always equal.” They are equal only when IL = I₁. Since IL represents the maximum demand over the past 12 months, TDD remains less than THDi as long as the current load does not exceed that maximum.

8.       “The PCC is always on the secondary side of the transformer.” This is not necessarily the case. The PCC is the point in the utility system where other customers are connected or can be connected, and its location varies depending on the facility.

Conclusions and Recommendations

THD and TDD are two different ways of interpreting the same phenomenon, and you must choose the correct one for your decision:

·         Since THDi depends on the instantaneous load condition, it is a valuable indicator for diagnosing the source of harmonics and performing equipment-based evaluations. However, it should not be used on its own to determine compliance.

·         TDD is not affected by load fluctuations because it is referenced to the maximum demand current (IL) and is the primary metric for the current-side compliance assessment in IEEE 519.

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