Measurement

Real-time voltage stability indicators

Two indices that recognise long-term voltage instability from local measurements, without needing a full state estimate of the network.

NLI

New LIVES Index

The NLI recognises long-term voltage instability early enough for corrective action, from local voltage and current measurements taken by PMUs at Extra High Voltage transmission buses.

It performs particularly well when placed on the receiving buses of a stressed network corridor feeding a weak area — which is where long-term instability tends to appear first.

Computed from synchronised phasor measurements
Requires only a sparse set of PMUs, not full observability
Crosses zero at the onset of instability

Plot of the New LIVES Index against time for three EHV buses.
All three traces start above zero and cross below it at around
eighty seconds, marking the onset of long-term voltage
instability. — Simulated case — NLI at three EHV buses, crossing zero at instability

RLI

Relay-based LIVES Index

The RLI can be calculated by the numerical distance relays that already protect transmission lines, using voltage and current measurements taken from instrument transformers at one end of the line.

Numerical relays have sampling capabilities similar to those of PMUs, so the signal sampling and filtering procedure can be common to that of the NLI implementation. In practice this means an operator can gain a stability indicator from protection equipment that is already installed.

Computed inside existing numerical distance relays
Single-ended measurement, from instrument transformers
Shares its sampling and filtering chain with the NLI

Granted patent

GR 1009503 B

Early detection of voltage instability using transmission line protection relays

The method behind the RLI. It computes an instability index inside the numerical distance relays already installed on transmission lines, using only local voltage and current measurements — requiring no additional equipment and no communication infrastructure.

Filed 25 May 2018 Published 4 April 2019 Vournas · Mandoulidis · Nikolaidis

Read the patent Hellenic Industrial Property Organisation

How it works

Watching a collapse arrive

A corridor is loaded a little harder, minute by minute, and its voltage declines slowly. The index falls as the operating point approaches the nose of the curve, the point of maximum loadability with no stable operating point beyond it, and it crosses zero there. In service, warning and alarm thresholds set above zero sound earlier, minutes before the voltage collapses, which is what gives an operator time to act.

Operating pointMoving down the PV curve to the nose

Bus voltageDeclining slowly, then collapsing

Stability indexFalling to zero at the nose

Stable

An illustration, not a measurement. The axes carry no values and nothing here identifies a real network.

Would these indices work on your system?

It depends on your corridors, your PMU placement and your relay estate. Tell us about them and we will give you a straight answer.

Discuss your network