Line Flow

The purpose of the transmission network is to transmit power from generators to loads. But power doesn’t travel from a generator to a customer because a contract says so, it spreads across every available path in proportion to the electrical impedance of each, and the flow on any given line is the net result of every generator and every load on the system at that moment.

Line Flow is the measured power passing through a piece of transmission equipment (a line, a cable, or a transformer) as reported by the operational telemetry AEMO uses to run the market.

What units are used for Line Flow?

There are two units you might see for Line Flow: MW (megawatts) and MVA (megavolt-amperes). MW represents real power: the part of the flow that does work at the far end. MVA is apparent power: the total, including the reactive component that tags along and fills up the line without delivering usable energy at the far end. The relationship is:

MVA = sqrt( MW² + MVAr² )

so MVA is always at least as large as MW.

For thermal purposes, MVA is the number that matters. What heats a conductor is current, and current is proportional to apparent power, not real power. A line’s rating will be expressed in MVA to reflect that.

Which one you get depends on what the equipment’s measurement point actually reports. Some report apparent power, some report real power, and some report both.

Direction reverses, routinely

Flow on a line has a direction associated with it: at any instant in time power is flowing in one way OR the other. A radial line into a region with a large solar farm can run one way during the day and the other when local demand takes over. Around the shoulders of the day, flow can reverse within a single half hour.

This direction is typically represented by a sign, i.e. + means one direction, - is the other. But if you average a signed flow that spent 15 minutes at +200 MVA and 15 minutes at −200 MVA, you get zero, a number that implies an idle line when in fact it was fully loaded the whole half hour. We avoid that by keeping the two directions as separate series, each averaged in its own right, so the magnitude of flow in each direction survives the averaging. Adding the two back together gives the average absolute flow, how hard the equipment worked, regardless of which way, and that’s the figure headroom calculations use.

Where you see it in RenewMap

A measurement point is a single monitored quantity on a single piece of equipment. One transmission line in RenewMap can map to several flow measurements. Typically there is a measurement at each end, but sometimes only one end.

How it’s calculated

Line flow is measured as an instantaneous power reading at either end of a line at a meter. The telemetry reports a single signed value every 5 minutes, where the sign carries the direction. We split into positive and negative flow series, for each of MVA and MW measurements.

We assign a direction to the line based on the primary flow direction over the last two Australian Financial Years. If there are two readings for the same line, we align the signs and take the larger absolute value of the measurements.

Taking the maximum rather than the average across measurement points is intentional: if one end of a line is more heavily loaded than the other, the constrained end is the one that matters.

Special notes

Coverage doesn’t follow the network map. There is no public feed of transmission telemetry in Australia; measurements are published for some equipment and not others. Line flows are always published where they are involved in market constraints, but there are some lines with no flow data. The covered set is large and includes most of the transmission network. See Data sources & coverage.

MVA-only, MW-only, and both. Roughly speaking, line and transformer loading points tend to report MVA, and some points report MW only. Charts and headroom calculations prefer MVA and fall back to MW, so a line whose measurement points report only MW will show its flow in MW.

Two ends rarely agree exactly. Line losses, reactive compensation, telemetry timing skew and instrument accuracy all introduce differences between the ends of the same line. Taking the largest magnitude is the current way to determine flow ‘on’ the line.

Where the data comes from

Flow figures are AEMO’s own published measurements for each dispatch interval. See Data sources & coverage.

Common questions

Why do both flow directions have values in the same half hour?

Because the flow reversed during the half hour. Each 5-minute reading goes into one series or the other, so a period containing a reversal contributes to both. Add the two together for the net average; use them separately to see how hard the line worked in each direction.

Why is flow in MW on some lines and MVA on others?

Because that’s what the measurement point measures. We don’t convert between them, since the conversion needs reactive power that generally isn’t published for the same point. Compare like with like: MVA flow against MVA rating.

Why does that line have no operational data at all?

Because no operational telemetry is published for it. Equipment-level measurement isn’t available across the whole transmission network in Australia, and where it doesn’t exist we leave the line blank rather than estimating a flow for it. See Data sources & coverage.

Related pages

• Ratings & Thermal Headroom: what the equipment can carry, and the simple gap
• Constrained Headroom: the gap the market is actually enforcing
• Data sources & coverage: which equipment is covered, and why
• Operational metrics overview: how all the metrics fit together