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Harmonic load flow

Harmonic load flow: get to grips with invisible risks in your electrical installation

Modern electrical installations are becoming increasingly complex due to the increase in power electronics and renewable generation. A standard load flow calculation (50 Hz) only tells you half the story. Do you want to avoid resonance, unexplained breakdowns and overheating? Then harmonic load flow is indispensable. On this page, you will learn how to use advanced simulations to predict and control the impact of harmonic pollution. This way, you guarantee operational reliability and continuity, even before the first cable is connected or the new machine starts running.

In brief: What you need to know about harmonic load flow

What is it: A simulation method that calculates how harmonic currents (caused by non-linear loads) propagate through the installation and cause voltage distortion.

The risk: Without this analysis, you overlook resonance points, leading to component failure (such as capacitor banks) and unexpected tripping of protective devices.

When needed: In new construction, expansions with high power electronics (such as VSDs or EV chargers) or when installing capacitor banks.

The solution: Software modelling of the installation allows you to identify bottlenecks in advance and dimension filter measures.

For whom is this analysis crucial?

Harmonic load flow analyses are not an everyday occurrence for every electrical engineer, but are essential for professionals responsible for the integrity of heavy or complex installations:

  • Electrical Engineers & Consultants: who produce designs for industrial grids, data centres or hospitals and need to demonstrate that the design complies with the standard (such as EN 50160).
  • Installation managers (IVs): Who deal with unexplained breakdowns or components that break down too quickly.
  • Technical Managers: Who need to justify investment decisions for expansions or the purchase of Power Quality equipment.

The mechanism: From dirty current to dirty voltage

In a standard load flow calculation, you look purely at the load at 50 Hz. You calculate whether the cables are thick enough for the power demanded. A harmonic load flow looks deeper. This involves calculating how high-frequency currents (e.g. 150Hz, 250Hz, 350Hz) move through the installation.

Cause and effect:

THDi versus THDu: To understand the results of a simulation, the distinction between current and voltage distortion is essential.

  • THDi (Total Harmonic Distortion Current): This is the cause. Non-linear loads (such as LED drivers or AC drives) do not demand a neat sinusoidal current, but pulses.
  • THDu (Total Harmonic Distortion Voltage): This is the result. When contaminated current (THDi) flows through the impedance of cables and transformers, a voltage drop occurs across those frequencies. This distorts the voltage sine wave.

Calculate THD:

Total Harmonic Distortion is expressed as a percentage, indicating how much of the fundamental (50Hz) voltage consists of harmonic voltage. It is calculated according to the formula below:

Total Harmonic Distortion Formula

To which applies:

  • U1 The amplitude of the fundamental 50Hz voltage or current is.
  • Uh The amplitude of the fundamental voltage or current of order.
  • H The maximum harmonic voltage or current considered (usually 40 or 50).

Why simulate?

Many engineers make the mistake of adding harmonic currents arithmetically, but this gives a distorted picture. 1 + 1 is rarely 2 here. Due to phase shifts, currents can amplify or extinguish each other. Moreover, the resistance (impedance) of your installation is different at each frequency. A simulation model calculates this complex vectorial interplay and predicts exactly where in the installation resonance or excessive THDu occurs.

Voltage wave

Why understanding harmonics is necessary

Ignoring harmonic currents in the design or management process carries risks that directly undermine plant reliability.

  1. Resonance: the biggest danger Every installation contains inductance (transformers, motors) and capacitance (cables, capacitor banks). At a specific frequency, these can form a parallel resonance. The impedance of the grid becomes extremely high there. If harmonic currents are also present at that frequency, a huge voltage swing occurs. Result: faulty electronics or exploding capacitors.
  2. Overloading the Zero (Neutral): In 3-phase systems, the 50 Hz currents cancel each other out in the neutral conductor (with balanced loads). The so-called 'triple harmonics' (3rd, 9th, 15th) do not do this; they add up. As a result, the current in the neutral conductor can become greater than in the phase conductors, resulting in a fire hazard.
  3. Unnecessary costs: Consider the cost of restarting processes, loss of man-hours and the potential damage to reputation in case of supply problems.
  4. Accelerated ageing: Harmonic currents cause additional losses (Skin effect and Proximity effect) in cables and transformers. This leads to additional heat generation. A transformer that is 80% loaded on paper can become thermally overloaded due to harmonic contamination and fail years earlier.

Case study: The unexplained fuse trap

Situation: An industrial company expands with a new production line full of variable speed drives. The main connection seems ample. Yet after commissioning, circuit breakers regularly trip and the control system fails, even though the current rating (RMS) seems well within standards.

Analysis: A measurement and subsequent harmonic load flow simulation revealed resonance around 350 Hz (7th harmonic). The present reactive current compensation together with the power transformer forms a circuit that resonates exactly at this frequency.

Solution: Based on the simulation, the capacitor bank was fitted with chokes (detuning). This shifts the resonance point to a safe frequency (e.g. 189 Hz) where no harmonic currents are present. The disturbances disappear immediately.

This is how a professional simulation works

A reliable simulation depends on the quality of the data. We usually follow these steps:

  1. Data collection: We inventory the single line diagram, cable lengths, types, transformer data and load specifications.
  2. Measurement (if possible): For existing construction, we first measure the current Power Quality (measurement of THDu, THDi and background distortion, among others). This serves as calibration for the model.
  3. Modelling: We recreate the installation in specialised software (such as Vision or PowerFactory). Here, it is crucial to model cables frequency-dependently.
  4. Scenario analysis: We simulate different operating situations. What happens when emergency power is running? What if we are running at 50% capacity?
  5. Reporting & Advice: The results are checked against the standard(EN 50160 or IEC 61000). If standards are exceeded, we directly simulate the effectiveness of possible solutions, such as an Active Harmonic Filter.

Pitfalls when analysing harmonics

Completely preventing dips in the public grid is impossible; after all, the grid operator cannot influence weather or excavation damage. You can, however, make your installation resilient against it. We distinguish three levels of solutions:

  • Looking only at THD: Total Harmonic Distortion is an average. It says nothing about which specific frequency is causing problems.
  • Ignoring blind current compensation: Placing standard capacitor banks in a grid with harmonics is asking for resonance problems.
  • Wrong cable models: The resistance of a cable is higher at 500 Hz than at 50 Hz. Those who calculate with fixed values get unreliable results.
  • Consider the source voltage as 'clean': Pollution also enters from the public grid (background distortion). You have to include this in the simulation.
  • Single-moment focus: Loads fluctuate. A simulation should take into account worst-case scenarios, not just the average load.

Checklist: Is my installation 'Harmonics Proof'?

Use this checklist to determine whether harmonic analysis is relevant to you:

  • [ ] Does a large part of my load consist of AC drives, LED lighting or EV chargers?
  • [ ] Am I going to install or replace a capacitor bank?
  • [ ] Am I dealing with equipment that inexplicably fails or resets?
  • [ ] Are transformers or cables getting hotter than would be expected based on current (RMS)?
  • [ ] Is there a weak grid (high grid impedance) or islanding (generators)?

Can you answer 'yes' to one or more questions? Then an in-depth analysis via measurement or simulation is recommended.

From simulation to certainty

Simulation software is powerful, but interpreting the graphs requires experience. When is an overshoot incidental and acceptable, and when is it an acute risk to your business security?

HyTEPS engineers combine theoretical knowledge of simulation models with years of practical experience in the field. We do not deliver a pile of graphs, but a clear diagnosis with a concrete improvement plan. Whether sizing a filter or redesigning a distribution: we ensure that your installation meets the standards and is ready for the future.

Engineering Services

Want to read more about Power Quality?

Harmonics

Resonance in electrical grids

Grid analysis and measurements

Blind current compensation

Standards: EN 50160 and IEC 61000

In doubt about the quality of your voltage?

Don't take unnecessary risks with your installation. Talk to an engineer from HyTEPS about your specific situation. We will be happy to help you think about a suitable measurement or simulation setup.

Call: 0492 37 12 12
Mail: office@hyteps.nl

HyTEPS

Beemdstraat 3

5653 MA Eindhoven