Challenges of Phasor Measurement Unit (PMU)

BySarasij Das

Content

• What is PMU?

• Why PMU?

• Applications of PMU

• Challenges of PMU

What is PMU?

• Power system parameters represented as ‘complex numbers’

• Phasors contain magnitudes and angles• Conventional instruments measure only magnitudes

of phasors• Local clocks used for time tagging of conventional

digital instruments• Inconsistent local clocks make it difficult to measure

phase angles

• PMU measures magnitudes and angles of phasors at higher rates (10-60 samples/s) with accurate time tag

• PMUs can be found as standalone device (N60 of GE) or integrated with power system protection relay (L60, D60 of GE)

Fig.1. GE N60 PMU [1]

Fig.2. Block diagram of a PMU [2]

Fig.3. Phasor representation

Fig.4. Architecture of PMU based WAMS [3]

Why PMU?

• Limitations of SCADA- Measurements obtained at slower rate (1 sample/1-4s)- Measurements are not time synchronised- Does not provide dynamic behaviour of system- Limited situational awareness conveyed to the operator

- SCADA is ‘X-ray’ of power grid where PMU is ‘MRI’

• August 14, 2003 blackout: - Problems developed hours before the system collapse - System operators were unaware of overall worsening system conditions

Applications [4]

Challenges

• Consistent performance required for a multi vendor PMU system

• Diverse requirements from all utilities - different application requirements - difference in infrastructure

• WAMS architecture - present architecture not suitable for large system

• High investment - initial high investment requirement acts as an deterrent - clear roadmap is needed

Challenges

• Lack of related products- not enough related products (PDC, application software)

• PMU placements - non-linear optimization problem

- currently one can get sub-optimal solution

• Visualization of PMU data - difficult to visualise the voluminous data

• Communication of PMU data- expensive communication network required

Challenges

• Communication delays - leads to delay in generating proper control signals

• Low frequency oscillation monitoring

- algorithms are computationally heavy - all modes may not be captured - distorted power system waveforms make it difficult

Challenges

• State estimation - deal with the hybrid system of PMUs and SCADA

- bad data estimation

• On line voltage instability prediction - higher computational requirement - lack of system models are the main challenges for this of application.

Reference

[1] http://www.gedigitalenergy.com/multilin/catalog/n60.htm[2] “Phasor Measurement Unit (PMU) Implementation and Applications”, EPRI

report, October 2007[3] “Synchronized phasor measurements and their applications”, by Arun G.

Phadke, John Samuel Thorp [4] Chakrabarti, S.; Kyriakides, E.; Tianshu Bi; Deyu Cai; Terzija, V.;

"Measurements get together" Power and Energy Magazine, IEEE issue January-February 2009, Volume 7, Issue:1, page(s): 41 - 49