Harmonic Analysis for Power System in New Zealand | Carelabs
Harmonic distortion behaves according to network impedance, and New Zealand's networks have characteristics that make distortion behave differently here than in a dense industrial grid. Understanding that is what makes the difference between a measurement and an explanation.
Why the network matters more than the load
Harmonic currents are injected by non-linear loads. The voltage distortion those currents produce depends on the impedance they flow through. The same variable speed drive installed at two sites produces identical harmonic current and substantially different voltage distortion, purely because of what it is connected to.
This is where New Zealand conditions become relevant. Distribution outside the main centres is characterised by long feeders serving relatively modest loads, which means higher source impedance at the point of connection. A high-impedance supply is a "weak" supply, and a weak supply converts injected harmonic current into voltage distortion far more readily than a strong one.
The practical consequence is that a rural processing plant, a pack house or a remote industrial site can experience distortion problems from a load that would be unremarkable in an urban industrial area.
Resonance is the mechanism that surprises people
The most damaging harmonic problems are not caused by distortion accumulating gradually. They are caused by resonance, which appears suddenly when something changes.
Power factor correction capacitors form a parallel resonant circuit with the supply transformer inductance. That circuit has a resonant frequency, and if it lands near a harmonic the site actually produces — typically the 5th at 250 Hz or the 7th at 350 Hz — the impedance at that frequency rises sharply and a modest injected current produces a large voltage distortion.
Two things then follow. The capacitors absorb current far beyond their rating and fail, often repeatedly, and the elevated distortion affects everything else on the busbar.
What makes this the classic trap is that the resonant frequency depends on both the capacitor size and the source impedance. Change either and the resonant point moves. A lines company reinforcement that strengthens your supply, a transformer replacement, or switching a capacitor stage in or out can all move a previously harmless resonance directly onto a harmonic you produce. Nothing in the plant changed, and the plant now has a serious problem.
What measurement needs to establish
The point of common coupling. Limits apply at the boundary with the lines company. Distortion measured at a drive terminal will look alarming and means little on its own.
A representative window. Distortion tracks load, so a snapshot answers the wrong question. Logging runs across a period covering real operation — shift patterns, start-up, and the low-load periods when capacitor stages switch and behaviour changes. A week is the usual minimum. Seasonal operations such as dairy and horticulture need measurement in the mode that matters, not the quiet one.
Instrumentation to a defined method. AS/NZS 61000.4.30 sets out measurement methods for power quality parameters, which matters because results obtained by different methods are not comparable. The spectrum should be recorded, not just a summary distortion figure.
Both voltage and current. Voltage distortion describes what your equipment experiences. Current distortion describes what you are injecting into the network. They are different questions with different owners, and reporting only one leaves half the picture.
Modelling, and why measurement alone is insufficient
Measurement describes the present state. It cannot tell you where the resonance sits, how close you are to it, or what a planned change will do.
A frequency scan of the modelled network shows impedance against frequency and locates resonant points directly. That is what answers the questions actually being asked: can we add this drive line, what happens if we increase the capacitor bank, and is the lines company's planned reinforcement going to create a problem.
Without the model, every future modification is an experiment conducted on live plant, and harmonic problems characteristically appear after a change rather than developing gradually.
Attribution
Spectrum analysis at individual feeders identifies which loads inject what. Six-pulse drives characteristically produce 5th, 7th, 11th and 13th harmonics; the signature is recognisable, which lets responsibility be assigned on evidence rather than by assumption about which department is at fault.
That matters because mitigation is far cheaper applied at source. Line reactors or DC link chokes at the drives responsible usually cost less than filtering at the main busbar, and they address the problem where it originates.
Related reading: why harmonic studies matter commercially covers emission limits and the connection relationship.
Frequently Asked Questions
Long distribution feeders mean higher source impedance at the point of connection. Harmonic voltage distortion is the product of injected current and that impedance, so a weak supply converts the same harmonic current into substantially more voltage distortion. A load that causes no difficulty on a strong urban supply can cause real problems at the end of a long rural feeder.
It can, which is counter-intuitive. Resonance between power factor correction capacitors and the supply transformer depends on source impedance, so strengthening the supply moves the resonant frequency. If it moves onto a harmonic you produce, distortion is amplified and capacitors begin failing, without anything inside your own installation having changed.
A week is the usual minimum, because distortion varies with load and a snapshot risks capturing either a peak or an idle period. For seasonal operations such as dairy or horticultural processing, measurement should cover the operating mode that matters rather than being extrapolated from a quiet period, since the load mix differs substantially between them.
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