AS/NZS 61000.3.6 covers harmonic emission limits for installations connected to MV, HV and EHV networks, and AS/NZS 61000.3.7 does the same for flicker and voltage fluctuations. Both work the same way: the network service provider (NSP) sets a planning level for the network, then gives each customer a share of it, roughly in proportion to the capacity that customer is allowed to use.
That share is the emission limit you see in a connection offer or a generator performance standard. It is not a property of your inverters. It is a slice of a finite network resource, and how it was calculated decides how hard your plant has to work to meet it.
Compatibility, Planning and Emission Levels in Plain Terms
The standards use three levels.
| Level | Who sets it | What it means |
|---|---|---|
| Compatibility level | International EMC standards | The disturbance level equipment is designed to tolerate. Exceeding it often means equipment problems somewhere on the network |
| Planning level | The NSP, for its own network | An internal target, set below the compatibility level, that the NSP plans its network to stay within. It leaves a margin for uncertainty and future growth |
| Emission limit | The NSP, for each customer | The share of the planning level a single installation may use at its connection point |
Planning levels are network policy, not physics. Two NSPs can adopt different planning levels for the same voltage, and many publish their own values in their technical requirements.
Compliance is assessed statistically, not against a single worst reading. Harmonic levels are measured as short interval values over a period of a week or more, and a high percentile of those values is compared with the limit.
How Allocation Works
The standards describe a staged approach. We describe it here in words. The tables and equations are in the standards themselves.
Stage 1: Simplified Acceptance
Small installations relative to the network fault level can be connected without detailed assessment. If the agreed power is a small fraction of the short-circuit power at the connection point, or the disturbing load is a small share of the installation, the NSP can accept it on that basis alone. Most utility-scale solar, BESS and large industrial connections are too large for Stage 1.
Stage 2: Allocation by Agreed Power
This is where most projects sit. The NSP works out how much of the planning level is available to customers on that part of the network, after allowing for distortion transferred down from the upstream network. It then divides that available headroom among customers in proportion to their agreed power, measured against the total capacity the network at that location is planned to supply.
Three details make this more than simple arithmetic:
- Summation. Harmonics from different sources do not add linearly, because their phase angles differ. The standards use a summation law with an exponent that increases with harmonic order. Low-order harmonics are treated as close to additive. Higher orders are assumed to partially cancel.
- Future customers. The total capacity used in the calculation includes load and generation the NSP expects to connect later. A first connector does not get the whole planning level just because it arrived first.
- Voltage to current. Allocations are first worked out as voltage distortion at the connection point. They are converted to harmonic current limits using the network harmonic impedance, which varies with frequency and network configuration. Because impedance is uncertain, some NSPs issue voltage limits only, some current limits only, and some both.
Stage 3: Conditional Acceptance Above the Allocation
Where a network has genuine headroom, for example because expected customers have not connected or actual distortion is well below planning level, the NSP can accept emissions above the Stage 2 allocation. This is normally conditional. The NSP may reserve the right to revisit the limit, or to require mitigation, as the network fills up. Treat a Stage 3 acceptance as a commercial and technical risk, not a permanent entitlement.
What the NSP Gives You and What You Must Prove
A well-specified connection gives you enough data to run a credible study. In practice, the data set usually includes:
Typical NSP Inputs
- Allocated voltage harmonic limits by order, and often a THD limit, at the connection point
- Harmonic current limits, if the NSP converts the allocation to current
- Network harmonic impedance, often as an envelope or loci across system conditions
- Existing background distortion, measured or assumed
- Minimum and maximum fault levels
- Flicker allocation as Pst and Plt values
Your side is to show that the installation stays within its allocation across its full operating range and the range of network conditions. For an inverter-based plant, that means modelling the inverter fleet, the collector cables, transformers, any capacitor banks or filters, and the network impedance together. Part load cases matter, and so do network outages that shift resonances. This work feeds directly into the solar farm electrical design, because filter space, transformer impedance and cable lengths all move the result.
For registered generators, the harmonic and flicker requirements become part of the generator performance standards, under the quality of electricity generated access standard (S5.2.5.2 of the National Electricity Rules). After energisation, compliance is usually confirmed by monitoring at the connection point, compared with background measurements taken before the plant connected.
Flicker Under 61000.3.7: Pst and Plt Allocation
AS/NZS 61000.3.7 applies the same logic to voltage fluctuations. Flicker severity is expressed as two numbers:
- Pst, short-term severity, evaluated over 10 minutes. A Pst of 1 corresponds to the level at which a typical observer begins to find lamp flicker irritating.
- Plt, long-term severity, combined from twelve consecutive Pst values over two hours.
The NSP sets planning levels for Pst and Plt below the compatibility levels, then allocates them by agreed power using a summation law, in the same staged way as harmonics. Flicker uses a cubic summation law, so combined flicker from several sources is well below their arithmetic sum, reflecting that fluctuating loads rarely peak together.
For solar farms, flicker comes mostly from fast output changes during passing cloud, and from switching events. For industrial customers, it comes from arc furnaces, welders, large motor starts and cyclic loads. The standard also deals with rapid voltage changes, the single step changes caused by energising transformers or tripping large blocks of generation.
Common Disputes: Background, Summation and Measurement
Most power quality disagreements we see on projects come down to one of three things.
Background Distortion
Some networks already run close to, or above, their planning level at certain harmonic orders. If the background is high, a plant that emits very little can still appear to cause an exceedance. The question becomes whose distortion it is. Good practice is to measure background levels before connection, over long enough to capture weekday and weekend patterns, and to agree with the NSP how those measurements will be used in the post-connection assessment.
A related issue is resonance. A plant with long cable runs or capacitor banks can amplify existing background harmonics without injecting much itself. Under the allocation approach, that amplification counts as the installation's emission.
Summation Assumptions
How the inverters in one plant combine is a separate question from how your plant combines with other customers. Manufacturer data is usually for a single unit. Assuming hundreds of units add arithmetically gives a conservative and often unrealistic result. Assuming full cancellation is optimistic. The assumption should be stated, justified and agreed with the NSP before the final study, not argued about at the review stage.
Measurement
Post-connection compliance depends on the instrument and the transducers. Power quality meters should meet the Class A requirements of IEC 61000-4-30. The bigger trap is the voltage transformer. Some magnetic VTs and capacitive voltage transformers do not reproduce higher-order harmonics accurately, so the meter faithfully records a distorted picture. Agree the measurement point, the transducers and the assessment method in advance, including how the plant's contribution will be separated from the background.
A Note on the Standards Themselves
This article explains how the standards work in practice. It does not reproduce their tables, equations or clause text, which are copyright to Standards Australia. The current Australian editions are technical reports published by Standards Australia as TR IEC 61000.3.6:2012 and TR IEC 61000.3.7:2012, adoptions of the corresponding IEC technical reports, although industry and many NSP documents still call them AS/NZS 61000.3.6 and AS/NZS 61000.3.7. NSPs sometimes reference a specific edition. Buy the current editions from Standards Australia, and check which edition your NSP's technical requirements cite before you start a study.
Get Your Emission Limits Assessed
An emission limit in a connection offer is the start of a conversation, not the end of one. Checking how it was allocated, whether the background data is sound and whether Stage 3 headroom is available can change the mitigation a project needs. It also fits naturally into the wider grid connection process for solar farms.
ElectraGlobe's power quality analysis team models harmonic impedance, assesses emissions against the allocation your NSP has issued, and designs filters only where they are genuinely needed. Resonance and network interaction work draws on our PSS/E and PSCAD modelling. If you have an allocation you want checked, get in touch with our team.
Frequently Asked Questions
What does AS/NZS 61000.3.6 cover?
It sets out how harmonic emission limits are worked out for distorting installations connected to MV, HV and EHV networks. It defines compatibility, planning and emission levels, and describes a staged method for sharing the network's harmonic headroom among customers. It does not set limits for individual appliances, which are covered by other parts of the series.
How are harmonic limits allocated between customers?
The NSP works out how much of the planning level is available on that part of the network, then gives each customer a share in proportion to its agreed power relative to the total capacity the network is planned to supply. A summation law accounts for harmonics from different sources partially cancelling. Where the network has spare headroom, the NSP can accept higher emissions on a conditional basis.
What is a planning level?
A planning level is an internal target the NSP sets for disturbance on its own network, below the compatibility level that equipment is designed to tolerate. It leaves a margin for uncertainty and future connections. Emission limits for individual customers are derived from it.
Is AS/NZS 61000.3.7 about flicker?
Yes. It covers voltage fluctuations and flicker for installations on MV, HV and EHV networks, using Pst and Plt as the measures of flicker severity. It allocates flicker headroom among customers in the same staged way that AS/NZS 61000.3.6 allocates harmonic headroom, and it also addresses rapid voltage changes.