1. Department of Electrical Engineering
Distributed State Estimation And A Knowledge Based Data Exchange Design for Mega-RTO Operations
Dr. G. M. Huang
Mr. J. Lei
Department of Electrical Engineering
Texas A&M University
PSERC

2. New Issue (1) State Estimation on Mega-RTO
A totally new estimator over the whole grid (One State Estimation)
Existing local state estimators distributed in different entities are wasted.
Investment and maintenance cost is enormous.
The size of system is extremely large and the system matrix becomes more ill-conditioned. Therefore, the computation speed becomes slower and convergence performance becomes poor.
A concurrent non-recursive textured algorithm (Distributed Sate Estimation, DSE)
Currently existing state estimators are fully utilized without a new estimator.
Additional cost is only some extra communication for data exchanges
The size of system in each existing estimator is much smaller.
February 16, 2019

3. Our Newly Proposed Textured Algorithm for DSE
Select a set of real time instrumentation data and estimated data to be exchanged between neighboring entities.
Taking the exchanged instrumentation data into account, the multiple local estimators distributed in different entities are executed simultaneously and asynchronously until they converge individually to the desired tolerance.
In view of the exchanged estimated data, modify the estimation result of local estimators accordingly and re-run bad data analysis.
Based on the modified results of local estimators, finally determine the state of whole system according to the different accuracy and reliability of estimators.
February 16, 2019

4. Advantage of new algorithm (1)
Bad data detection ability is higher than existing DSE algorithm, especially when bad data appear close to the boundary of individual estimators.
The estimation accuracy on boundary buses is much higher than existing DSE and is comparable to OSE. Our approach decreases the discrepancy on the boundary buses and makes the whole result more consistent.
The concurrent textured algorithm is asynchronous without a central controlling node. As a consequence, the new algorithm becomes very fast and practical.
February 16, 2019

5. Advantage of new Algorithm (2)
No need of recursion process, which reduce the computation time further.
The multiple local estimators can use different SE algorithms or even different convergence tolerance based on different quality of local measurement system. Accordingly, our new algorithm becomes very flexible in which current existing estimators can be included easily.
The performance of bad data detection and estimation accuracy in individual existing estimators improves as well, which benefits individual companies/ISOs/RTOs. Therefore, they are more willing to share the information for their own benefits.
February 16, 2019

6. New Issue (2) Data Exchange Design
How to exchange instrumentation or estimated data with neighboring entities in power market?
Critical to the newly proposed textured distributed state estimation algorithm.
Selected data exchange improves the quality of estimators in individual entities, on both bad data detection ability and estimation accuracy.
After the introduction of data exchange, the traditional measurement placement methodology will be modified to fully utilize the benefit of data exchange.
Not necessarily all data exchanges are beneficial.
February 16, 2019

7. Bus Credibility Index BCI(b,S)
State estimation credibility probability on bus b with respect to a specified system S.
data exchanges modify the original system S to S’, and the incremental difference of BCI from (b,S) to (b,S’) stands for the benefit of such a data exchange on bus b.
BCI is a more accurate criterion compared with local or global bus redundancy level
February 16, 2019

8. Knowledge Base Raw facts BCI(b, S) Variance of State Estimation Errors
The configuration, parameters and ownership of current power system network and measurement system;
The failure probability and accuracy of measurements;
The cost of instrumentation and estimated data exchange;
BCI(b, S)
Variance of State Estimation Errors
Accuracy on bus b with respect to a specific system S
February 16, 2019

9. A Reasoning Machine (1)
An IEEE-14 Bus system is used to illustrate how the reasoning machine works
Note that the algorithm and principles are applicable to all systems.
RTO B
Two RTOs merge into one Mega-RTO
RTO A
February 16, 2019

10. Step2: Ignore the boundary bus whose maximum possible benefit is small
A Reasoning Machine (2)
Step1: Determine maximum possible benefit on bad data detection ability
Remark: Only boundary buses are concerned because in most cases BCI of internal buses also improves with a much smaller rate when BCI of boundary buses improve.
Step2: Ignore the boundary bus whose maximum possible benefit is small
February 16, 2019

11. A Reasoning Machine (3)
Step3.1: Some principles to search for beneficial Instrumentation data exchange:
For boundary bus bA in A, instrumentation data exchange should extend to boundary bus bB in B given the condition:
For example, it is reasonable for b2 and b4 in B to extends to include b1 and b5 in A, while it does not follow the principle that b9 in B extends to include b10 or b14 in A.
Avoid forming a radial structure; instead, a loop is preferred.
For example, b9 in B extend only to b10 in A will form a new radial branch b9-b10, which violates this principle.
February 16, 2019

12. A Reasoning Machine (4)
Step3.2: Principle to search for beneficial estimation data exchange:
If BCI(b,A)>BCI(b,B)
where bus b is in the common part of A and B
Then estimation result exchange from A to B on this bus will improve BCI(b,B) to the magnitude of BCI(b,A) .
February 16, 2019

13. A Reasoning Machine (5)
Step4.1 System A or B are modified accordingly based on the data exchange newly found.
BCI, estimation accuracy and the economic cost are evaluated on the ‘new’ system S’ to verify the benefit.
If BCI(b,S’) are already close to BCI(b,Whole), then there is no need to search for new data exchange for bus b.
Step4.2 Searching process is iterated on all boundary buses.
February 16, 2019

14. Economic Factor (1)
Hardware/software cost on data exchange implementation should be minimized given the condition that performance is satisfied.
Even if scheme D1 is slightly better than scheme D2 in performance, but it is still possible for industry to select D1 when D1 is much more economical than D2.
The benefit of different data exchange schemes may differ greatly. The benefit may saturate after some data exchange, which implies no major benefit can be obtained for even much more data exchange.
February 16, 2019

15. Economic Factor (2)
Price tag reflects not only installation cost but also market value.
It is possible for system A to attach a rather high price tag to a measurement that is especially useful to system B.
The proposed expert system is critical for the companies to determine the market price based on the benefit of data exchange.
February 16, 2019

16. Economic Factor (3) New measurements can be sold to other companies
Data exchange will have some impact on measurement placement decision.
Proposed expert system is useful for the new measurement placement decision
February 16, 2019

17. Case1:Harmful Data Exchange
B before data exchange
Average BCI on the buses of B
Original B
Modified B
Whole System
0.9647
0.9643
0.9662
Average Estimation Error on the buses of B
Original B
Modified B
Whole System
7.7314e-007
8.1738e-007
2.6326e-007
B after harmful data exchange
Data Exchange
Not following our principles
Bad data detection ability decreases
SE Accuracy decreases
Wasted investment
February 16, 2019

18. Case2: Efficiency of Beneficial Data Exchange
Local estimators after beneficial data exchange
Estimator A
Estimator B
Overlapping Areas
Average BCI on the buses of B
Original B
Modified B
Whole
0.9647
0.9662
Average Estimation Error on the buses of B
Original B
Modified B
Whole
7.7314e-007
2.6471e-007
2.6326e-007
Following our principles
Bad data detection ability is as good as the whole system
Estimation Accuracy is almost as good as the whole system
Efficient investment
February 16, 2019

19. Case3: Impact on New Measurement Placement (1)
Suppose the probability of accidents in the SCADA on station of b1 is extremely high
System becomes unobservable and traditionally at least one new measurement has to be installed.
With data exchange, such a new measurement is not necessarily needed because:
When we follow the data exchange scheme suggested in Case 2, state estimation in A can be run normally because the estimation result on b1 and b5 is exchanged from B to A (B is always observable under such an accident).
February 16, 2019

20. Case4: Impact on New Measurement Placement (2)
Suppose A wants to improve the estimation accuracy on b5.
From a traditional measurement placement viewpoint, there are basically two alternatives: improve the accuracy on measurement 5-1 or 5-6.
With data exchange, it is better for A to invest on measurement 5-1 instead of on measurement 5-6:
If the accuracy of 5-1 improves, the accuracy of B also improves with data exchange in Case2.
It makes sense for B to share part of the cost with A.
February 16, 2019

21. Conclusions (1)
Instead of starting a totally new estimator over Mega-RTO, a distributed textured algorithm is developed to determine the state of whole grid in Mega-RTO:
the currently existing estimators are fully utilized only with some extra communication for data exchange.
non-recursive, asynchronous, no central controlling node, fast and practical
Improvement on bad data analysis, estimation accuracy and elimination of discrepancy on boundary buses compared with existing distributed SE algorithms
February 16, 2019

22. Conclusions (2)
Selected data exchange improves the estimator quality of individual entities on both bad data analysis and estimation accuracy.
Data exchange has an impact on new measurement design
Benefit of different data exchange can be quite different:
Properly selected data exchanges will enable the local distributed estimator perform as well as the one estimator for the whole system in both bad data detection capability and precision.
Poorly designed data exchanges, which did not follow our design principles, may be harmful to local estimators.
February 16, 2019

23. Proposed expert system is useful for:
Conclusions (3)
Proposed expert system is useful for:
Newly proposed distributed state estimation algorithm
Design of the data exchange scheme
New measurement placement decision
Determination of the market price for data exchange
February 16, 2019