Guides · Faisalabad, Lahore, Punjab
Harmonics and Power Quality in Solar-Powered Factories
Short answer
In most factories, harmonics come mainly from variable frequency drives (VFDs), rectifiers, UPS units and furnaces, not from a well-designed solar plant. But solar can make existing harmonics more visible, interact with capacitor banks, and trip inverters on a distorted supply. The answer is to measure power quality before and after solar, and to use detuned capacitors, drive reactors or active filters where the numbers call for them.
This is a practical guide for factory owners and plant engineers. It explains what harmonics are, the warning signs, how solar fits in, and the usual fixes.
What harmonics are
Grid electricity in Pakistan is a 50 Hz sine wave. A simple load such as a heater draws current in the same smooth shape. Many modern loads do not. A VFD, for example, first turns AC into DC through a rectifier, and that rectifier draws current in short pulses.
Those pulses can be described as the normal 50 Hz current plus extra currents at multiples of 50 Hz: 250 Hz (the 5th harmonic), 350 Hz (the 7th), and so on. These extra currents are harmonics. They do no useful work, but they heat cables, transformers and motors, and they distort the voltage for everything else on the same supply.
Two numbers describe the problem:
| Measure | What it means |
|---|---|
| THDi (total harmonic distortion of current) | How distorted the current drawn by a load or a whole site is |
| THDv (total harmonic distortion of voltage) | How distorted the supply voltage has become, which affects every machine on it |
High THDi from your machines, flowing through the impedance of your transformer and cables, creates THDv. THDv is what makes other equipment misbehave.
Harmonics and power quality
Source
VFDs, rectifiers and UPS loads
Effect
Heating, trips and inverter alarms
Common sources of harmonics in Punjab factories
- VFDs on pumps, fans, compressors, spinning and weaving machines. Standard six-pulse drives mainly create 5th and 7th harmonics.
- DC drives and rectifiers in older machinery and electroplating.
- Induction furnaces in steel and foundry plants.
- UPS systems and battery chargers.
- LED lighting drivers and IT loads, which add triplen harmonics (3rd, 9th) that build up in the neutral of three-phase, four-wire systems.
- Welding sets.
Warning signs of a power quality problem
- Capacitors in the power factor panel failing early, bulging, or fuses blowing
- Transformers running hotter than their load would suggest, or humming more
- Neutral conductors running hot, even when phases look balanced
- Breakers tripping without an obvious overload
- Motors running hot or vibrating
- Electronic controllers, PLCs or sensors resetting
- Solar or hybrid inverters disconnecting with grid voltage or frequency faults while the grid seems normal
- Generators hunting or running unstably when drives are on
None of these proves harmonics are the cause. They are reasons to measure.
Where solar fits in
Grid-tied inverters are usually a small source
A modern string inverter delivers current with low distortion when running near its rated output; manufacturers publish a maximum current THD on the datasheet. At low output, early morning, late afternoon or under heavy cloud, the percentage distortion is usually higher, though the harmonic current itself is small. In a well-designed system, the inverters are rarely the main problem.
Solar can raise the measured distortion at the grid
This catches people out. Solar supplies a large share of the factory's 50 Hz current during the day. The VFDs still draw the same harmonic current from the grid. So the harmonic share of the grid current (THDi at the incoming meter) rises, even though nothing got worse in absolute terms. It is the same effect that lowers measured power factor; see power factor and solar.
Inverters are sensitive to a distorted supply
On-grid inverters watch the grid voltage closely and disconnect if it goes outside their settings. In a factory with high THDv, or on a weak feeder, inverters may trip or reduce output more often. If your inverters log grid faults on sunny days, power quality is one thing to check, along with the causes in our inverter faults guide.
Capacitors and resonance
Capacitor banks and the transformer together form a circuit that has a natural resonant frequency. If that frequency lands near the 5th or 7th harmonic, a small harmonic current can be amplified into a large one. Adding solar, adding capacitor steps or changing loads can move the site closer to resonance. Plain capacitors in a factory full of VFDs are a common cause of failures.
Generators and backup mode
A diesel or gas generator has a much weaker supply than the grid, so the same harmonic current causes much more voltage distortion. Hybrid systems that run solar with a generator, or BESS inverters supplying a factory during outages, must be sized with the nonlinear loads in mind. See our guides to solar and generator hybrids and BESS for factories.
How to measure power quality
- Use a power quality analyser, not a plain clamp meter. It records THDv, THDi, individual harmonics, power factor and voltage events over time.
- Log for several days of normal production, including start-up and shift changes. A week is better.
- Measure at the main incoming supply and at large drive panels or MCCs.
- Repeat after solar is commissioned, at the same points, so you can compare like with like.
- Compare with recognised limits. Engineers commonly use the IEEE 519 recommendations as a reference for harmonic limits at the connection point.
Power quality logging fits naturally into an energy audit before industrial solar.
Fixes, from simplest to most involved
| Fix | What it does | When it suits |
|---|---|---|
| AC line reactors or DC chokes on VFDs | Smooth the current drawn by each drive and cut its harmonics | Many drives without reactors; low cost per drive |
| Detuned capacitor bank | Capacitors with series reactors tuned below the 5th harmonic (7% or 14% detuning is common) so they correct power factor without resonating | Any site with VFDs that needs power factor correction |
| Separate feeders | Keep sensitive loads off the same bus as heavy drives | New panels or plant expansions |
| Oversized neutral and correct transformer rating | Handle triplen harmonic currents and extra heating | Sites with many LED, IT and single-phase electronic loads |
| Multi-pulse or active front-end drives | Drives designed to draw low-harmonic current | Large new drives |
| Passive tuned filters | Absorb a specific harmonic | Large, steady harmonic loads |
| Active harmonic filter | Measures distortion and injects opposite current in real time; many also correct power factor | Sites with mixed, changing loads where other fixes are not enough |
The right answer comes from measurement. An active filter fitted without data can be the wrong size; a plain capacitor bank fitted without data can make things worse.
Good design habits for solar in factories
- Connect the solar plant at a point that suits the load and protection design, and check transformer capacity.
- Choose inverters with suitable grid settings and published THD figures, matched to the site; see on-grid inverters for factories.
- Review the capacitor bank in the same design: detuning, step sizes and controller settings.
- Use correctly rated protection and earthing; see breakers and protection and our earthing and surge protection guide.
- Include power quality checks in the operations and maintenance plan, especially after new machinery is added.
How Solar Works Pakistan approaches power quality
Industrial EPC, hybrid systems and BESS are our specialist areas. Solar Works Pakistan was founded in 2021, and our power-generation heritage goes back to the 1980s through our founder, Akram Ullah Khan, whose career includes WAPDA and Saudi Electricity Company. We have served 50+ industrial clients, and design, load analysis and commissioning are handled by our own engineers.
For a factory project, we look at the drives, the capacitor bank and the generator together with the solar design, and we tell you plainly where separate power quality work is needed. See our industrial and commercial solar page.
Get today's price or a free site survey
Send us your single-line diagram if you have one, a list of large drives, details of your capacitor bank and generator, and 12 months of bills.
- WhatsApp or call: 0336 666 4321
- Email: info@solarworks.pk
- Head office: Main Boulevard, Sargodha Rd, Block F FDA City, Faisalabad (Faisalabad page)
- Lahore branch: 2nd Floor, Jeff Heights, Gulberg, Lahore (Lahore page)
- Hours: 9:00 AM to 6:00 PM, daily
We serve factories and businesses throughout Punjab, and site surveys are normally free. You can also contact us online.
Frequently asked questions
Do solar inverters cause harmonics?
They produce a small amount, usually low when running near rated output and published on the datasheet. In most factories, VFDs, rectifiers, UPS units and furnaces are much larger sources.
Why did harmonics look worse after we installed solar?
Solar supplies much of the clean 50 Hz current, while the drives still draw the same harmonic current from the grid. The harmonic share of the grid current therefore rises, even if absolute distortion has not changed.
Can harmonics make my solar inverter trip?
Yes. High voltage distortion or a weak supply can push the grid outside the inverter's settings, so it disconnects. Measuring power quality helps separate this from other grid faults.
What is a detuned capacitor bank?
It is a power factor correction bank with a reactor in series with each capacitor step, tuned below the 5th harmonic. It corrects power factor without forming a resonance with the transformer, which protects the capacitors in sites with many drives.
Do I need an active harmonic filter?
Only if measurement shows distortion that simpler fixes, such as drive reactors and detuned capacitors, cannot bring within limits. Active filters suit sites with mixed, fast-changing loads.
Do harmonics matter when running on a generator?
They matter more. A generator's supply is weaker than the grid, so the same harmonic current causes more voltage distortion and can upset the generator's regulator. Hybrid systems must be designed for this.