CASE STUDY Disturbance 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 Hz At about 16:03:38, A Unit loaded to 809 MW tripped with and frequency dropped to Hz At about 16:03:48, A Unit loaded to 815 MW tripped with and frequency dropped to 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 = (V 1 V 2 *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 station PMU voltage angle diff.(δ PMU ) SE voltage angle diff.(δ SE ) Tkwsw-Mata* Dcses-Tkwsw Odehv-Tkwsw Klnsw-Dcses Jewet-Klnsw Airline-Klnsw :47:43 PM 15:52:43 PM Station to station PMU voltage angle diff.(δ PMU ) SE voltage angle diff.(δ SE ) Tkwsw-Mata Dcses-Tkwsw Odehv-Tkwsw Klnsw-Dcses Jewet-Klnsw Airline-Klnsw

Comparison of the PMU voltage angle difference data with the SE data 16:02:43 PM 16:07:43 PM Station to station PMU voltage angle diff.(δ PMU ) SE voltage angle diff.(δ SE ) Tkwsw-Mata Dcses-Tkwsw Odehv-Tkwsw Klnsw-Dcses Jewet-Klnsw Airline-Klnsw Station to station PMU voltage angle diff.(δ PMU ) SE voltage angle diff.(δ SE ) Tkwsw-Mata Dcses-Tkwsw Odehv-Tkwsw Klnsw-Dcses Jewet-Klnsw Airline-Klnsw

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. 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.