CASE STUDYDisturbance event comparing PMU and SE voltage angle

difference in ERCOT 

Prakash Shrestha Operations Engineer

Advanced Network Application

PMU and Units Trip Location in ERCOT Grid

State Estimator and Phasor Measurement Unit

• State Estimator (SE) is an application that estimates the power flow based on SCADA telemetry. SE runs every 5 min in EMS system.

• Phasor Measurement Unit (PMU) measures the voltage and current phasors i.e magnitude and angle at 30 samples/sec.

• SE data is static whereas PMU data is dynamic.

Event summary

• On July 30 at about 15:50:59, A Unit loaded to 820 MW tripped with and frequency dropped to approx. 59.85 Hz

• At about 16:03:38, A Unit loaded to 809 MW tripped with and frequency dropped to 59.76 Hz

• At about 16:03:48, A Unit loaded to 815 MW tripped with and frequency dropped to 59.709 Hz.

• The event was a NERC DCS event as the total generation loss was more than 1083 MW.

• The units trip event was captured by the RTDMS application and post analysis of the event were done.

Comparison of the PMU voltage angle difference data with the SE data

• The real-time SE solutions (i.e NETMOM) were used to compare the voltage angle difference with the PMU data. The NETMOM was retrieved in STNET for the analysis.

• Only 7 PMUs out of total 26 PMUs have been selected for the analysis.• The power flow transfer depends primarily on the power angle

between two electrical buses. Since the voltage angle difference (power angle) between the two buses determine the stress in the system, the angle difference between two PMUs are compared with the SE estimates of the angle difference.

• Power Transfer, P = (V1V2*Sin δ)/X

Comparison of the PMU voltage angle difference data with the SE data

The PMU selected for analysis are as: •Matador (Mata)•Tonkawa Switch (TKWSW)•Decordova SES (DCSES)•Odessa Ehv Switch (ODEHV)•Killen Switch (KLNSW)•Jewet•Airline

Comparison of the PMU voltage angle difference data with the SE data

Station to stationPMU voltage angle diff.(δPMU)

SE voltage angle diff.(δSE)

Tkwsw-Mata* 14.3 14.5

Dcses-Tkwsw 6.54 6.3

Odehv-Tkwsw 1.75 1.4

Klnsw-Dcses 7.67 7.4

Jewet-Klnsw 5.5 4.6

Airline-Klnsw 30.2 30

15:47:43 PM

15:52:43 PM

Station to stationPMU voltage angle diff.(δPMU)

SE voltage angle diff.(δSE)

Tkwsw-Mata 14.6 14.5

Dcses-Tkwsw 6.41 6.1

Odehv-Tkwsw 1.61 1.6

Klnsw-Dcses 10.02 9.8

Jewet-Klnsw 3.6 2.5

Airline-Klnsw 33.1 32.9

Comparison of the PMU voltage angle difference data with the SE data

16:02:43 PM

16:07:43 PM

Station to stationPMU voltage angle diff.(δPMU)

SE voltage angle diff.(δSE)

Tkwsw-Mata 14.8 14.9

Dcses-Tkwsw 6.16 5.9

Odehv-Tkwsw 1.75 1.6

Klnsw-Dcses 8.84 8.7

Jewet-Klnsw 4.3 3.3

Airline-Klnsw 32.54 32.8

Station to stationPMU voltage angle diff.(δPMU)

SE voltage angle diff.(δSE)

Tkwsw-Mata 13.7 14.1

Dcses-Tkwsw 5.53 5.2

Odehv-Tkwsw 2.49 2.3

Klnsw-Dcses 12.31 12.1

Jewet-Klnsw 1.7 0.6

Airline-Klnsw 37.1 37.3

Conclusion

• It was noticed that the voltage angle difference measured by the PMU is very close to the angle difference data obtained from the SE solution (NETMOM).

• It can also be concluded from the above analysis that the State Estimator (SE) solution in ERCOT is good since the angle difference measured by the PMUs is nearly equal to the SE.

• Therefore at instances when the real-rime EMS application has failed or is down in worst scenario, PMU voltage angle difference data can be used as the backup tool for the SE.

• PMUs can also help in troubleshooting the state estimator and improve its quality if there are sufficient PMUs to monitor the electric grid.

• In the case that critical telemetry becomes suspect or lost, then the PMU data can be used to verify/identify the wrong telemetry and be used as the backup option.

• Both the PMU and SE may also be used for the cross validation.

Use Synchrophasor data to improve online stability monitoring

 Jian Chen

Operations Support Engineering

Monitoring voltage stability

The high resolution data from PMUs can provide a system-wide voltage magnitude visualization. •Voltage Magnitude Contour Plot

Monitoring voltage sensitivity

The voltage sensitivity is defined as change in voltage magnitude with power flow through a transfer interface. It can provide an early warning of lack of voltage support.

A example of tracking real-time voltage sensitivity using PMU data ( capability provided by RTDMS), which request PMUs are installed on bus next to transfer interface and also inside load area.

With the load area getting close to voltage collapse point, the voltage declines as power transfer increases, and its rate ∆V/∆P also increases

Monitoring angle stability

As the angle differences among buses are tracked with high-resolution PMU data, the inter-area oscillation is visible and can be directly monitored.

Oscillation Mode Analysis

During an oscillation event, PMU data can capture the dominant mode of oscillations, and its damping and energy.

HAMILTON.BS.1131.ANLG.KV139.60KV

7/14/2012 3:35:00 PM7/14/2012 3:33:00 PM 2.00 minutes

Plot-0

139.4

139.9

140.4

140.9

141.4

Frequency Damping energy

Normal SCADA data on a generator during an oscillation event High resolution PMU data during the same event

Model is Important

For dynamic stability analysis, the model is important to provide accurate results. PMU data can help to tune the existing dynamic model.

Simulation results based on original dynamic models

Simulation results after one exciter model tuned

System is stable

System is unstable

Summary

• With the data sampled at high speed, proposed synchrophasor applications are capable of monitoring the dynamics of power system.

• With PMUs installed at desired locations, they can be used to monitor the voltage stability or angle stability on certain areas.

• PMU data can also benefit the existing online stability analysis tools by verifying and tuning the dynamic models.