Comparison of SCADA and WAMS real-time data in dispatch center

Beijing Sifang Automation Co., Ltd.，CHINA

Qi Wenbin qiwenbin@sf-auto.com

Introduction

PMU has been widely used in power systems for more than ten years. Sifang Automation Co., Ltd. has over 50% of the WAMS and PMU market share in China. How is the PMU data quality? What is the relationship between the millisecond level

PMU data and the second level RTU data? whether PMU measurement data is accurate enough to

support better power system analysis and calculation?

Diff =|(Vp-Vr)/Vr|

Comparison Method of PMU & SCADA Data

We have developed software to compare SCADA and WAMS real-time data in dispatching center.

Comparison Method of PMU & SCADA Data

Take real-time measuring point with both PMU and RTU measurements, whose measurement status and state estimation error are all in normal state, as statistical objects, calculate the difference between PMU and RTU data, the equation as follows: Diff =|(Vp-Vr)/Vr| Where: Vr is real-time RTU data; Vp is real-time PMU data with the same time stamp from a physical point .

Implementation of the Comparison Method

Get real-time measurement sample every second and compute the difference, get average difference every 5 minutes.

Abnormally large or small value (larger than 104 or smaller than 10-3) of PMU or RTU data is excluded from calculation.

Measurements with error greater than 10% after state estimation are excluded from calculation.

Comparison of PMU & RTU Real Power Data Curve

Comparison of PMU & RTU Voltage Data Curve (Average Difference 0.0036%)

Comparison of PMU & RTU Real Power Data Curve

Difference Between PMU & SCADA Real Power Measurements

1%<diff<5%

Difference Between PMU & SCADA Real Power Measurements

1%<diff<3%

Comparison of PMU & RTU Reactive Power Data Curve

Comparison of PMU & RTU Real Power Data Curve

Huge Difference Between PMU & SCADA Reactive Power Measurements

1%<diff<70%

Huge Difference Between PMU & SCADA Reactive Power Measurements

10%<diff<18%

Difference between PMU & SCADA real power measurement is small;

Difference between PMU & SCADA reactive power measurement is large.

Conclusions

Difference Statistical Results (in 2011)

Measurement Type

No. of Measuring Points Diff > 1% Diff > 2% Diff > 5% Diff> 10%

Frequency 17 0 0 0 0

Voltage 33 1 0 0 0

Current 2 0 0 0 0

Real Power 164 95 48 15 5

Reactive Power 152 134 112 74 37

Basic Conclusions

For a straight telemetered physical measurement, the time delay between RTU and PMU are less than 1 second, and the difference between RTU & PMU measurement curves is small.

For 70% of the active power measurements, difference between PMU and RTU data is less than 2%.

For reactive power measurements, difference between PMU and RTU data ranges from 5% to 50%; 50% of the measuring points have a difference larger than 5%, 24% of them have a difference larger than 10%. The reactive power meets a great measurement difference.

For the same measuring point, difference between RTU and PMU data remains almost the same for a period of time.

Great Measurement Difference of Reactive Power

Q=

The reactive power measurement difference between RTU and PMU reflects the relative measurement error of current and voltage angle in PMU device.

Power calculation in PMU device： P = IUcosφ Q = IUsin φ At certain φ0, the measurement error of φ will

lead to the following measurement errors of real power P0 and reactive power Q0:

⊿P/P0 = ⊿φ sinφ0/ cosφ0 ⊿Q/Q0 = ⊿φ cosφ0/ sinφ0

Great Measurement Difference of Reactive Power

Φ0 is a decimal value close to 0. sinφ0 0，cosφ0 1 sinφ0/cosφ0 0 cosφ0/sinφ0 Max When error ⊿φ is fixed: ⊿P/P0 0 ⊿Q/Q0 Max

Great Measurement Difference of Reactive Power

Difference between reactive power measurement of RTU and PMU reflects the relative measurement error of current and voltage angle in PMU device.

To obtain the same accuracy, PMU is more difficult.

Conclusion

Effects of Advanced Algorithms Using PMU Data

State Estimation Transmission Line Parameter Identification

State Estimation

There is no significant improvement on the accuracy of state estimation after adding PMU measurements. Measurements

Voltage

Real Power Reactive Power Total

Pass Rate No. Pass Rate No. Pass Rate No. Pass Rate

No.

SCADA data 79.38% 1411 81.57% 3488 89.64% 3542 84.59% 8441

PMU and SCADA 74.43% 1408 69.76% 2976 87.10% 3131 77.86% 7515

Transmission Line Parameter Identification

Reliable parameters could be reached for some transmission lines.

Parameters of some transmission lines could not be identified reliably.

Transmission Line Π-Model

Parameter Identification Results of Line Taishun-2

Parameter Identification Results of Line Wanshun-3 with Shunt Reactor at One End

Transformer Parameter Identification

Reliable parameters could be reached for some transformers.

Parameters of some transformers could not be identified reliably.

Transformer Parameter Identification

Transformer Parameter Identification

Conclusions

According to the PMU measurement quality of field data: the accuracy of active power, current magnitude and

voltage magnitude measurements of PMUs is equal to that of RTUs, indicating that these measurements can be used in power system analysis.

the reactive power, current angle and voltage angle

measurements of PMUs should not be used directly without selection. The accuracy of PMU phase angle measurements can be improved through periodically checking and

adjusting of the phase angle compensation parameters.

Suggestions for Improving PMU Measurement Accuracy

According to the checking method for metering device, establish regular validation system for PMU data and check the angle compensation parameter of PMU device regularly, in order to decrease field measurement error to allowed level for standard products.

Improve calculation algorithm of reactive power in PMU

device. Reactive power calculation method used in RTUs , phase shifting method, could be applied to PMUs. Phase shifting method can avoid reactive power error caused by angle error. Real-time compensation algorithm for angle error could also be implemented in PMUs.

Thank you!