Role of Synchronized Measurements In Operation of Smart Grids Ali Abur Electrical and Computer Engineering Department Northeastern University Boston, Massachusetts Boston University CISE Seminar November 12, 2010
Operating States of a Power System NORMAL STATE SECURE or INSECURE RESTORATIVE STATE EMERGENCY STATE PARTIAL OR TOTAL BLACKOUT OPERATIONAL LIMITS ARE VIOLATED
Recommendation 12: Install Additional Time-Synchronized Recording Devices as Needed A valuable lesson from the August 14 blackout is the importance of having timesynchronized system data recorders. NERC investigators labored over thousands of data items to synchronize the sequence of events, much like putting together small pieces of a very large puzzle. That process would have been significantly improved and sped up if there had been a sufficient number of synchronized data recording devices. NERC Planning Standard I.F Disturbance Monitoring does require location of recording devices for disturbance analysis. Often time, recorders are available, but they are not synchronized to a time standard. All digital fault recorders, digital event recorders, and power system disturbance recorders should be time stamped at the point of observation with a precise Global Positioning Satellite (GPS) synchronizing signal. Recording and timesynchronization equipment should be monitored and calibrated to assure accuracy and reliability. Time-synchronized devices, such as phasor measurement units, can also be beneficial for monitoring a wide-area view of power system conditions in real-time, such as demonstrated in WECC with their Wide-Area Monitoring System (WAMS).
Recommendation 12a: The reliability regions, coordinated through the NERC Planning Committee, shall within one year define regional criteria for the application of synchronized recording devices in power plants and substations. Regions are requested to facilitate the installation of an appropriate number, type, and location of devices within the region as soon as practical to allow accurate recording of future system disturbances and to facilitate benchmarking of simulation studies by comparison to actual disturbances. Recommendation 12b: Facilities owners shall, in accordance with regional criteria, upgrade existing dynamic recorders to include GPS time synchronization and, as necessary, install additional dynamic recorders.
Tenets of Smart Grid Operation (Department of Energy) Self-healing from power disturbance events Enabling active participation by consumers in demand response Operating resiliently against physical and cyber attacks Providing power quality for 21st century needs Accommodating all generation and storage options Enabling new products, services, and markets Optimizing assets and operating efficiently
Role of State Estimation Facilitating Smart Grid Operation Self-healing from power disturbance events Faster (scan rate) state estimation, wide-area observability, multi-level monitoring capability Enabling active participation by consumers in demand response Monitoring at lower voltage levels Operating resiliently against physical and cyber attack Redundancy and strategic placement of measuring devices/sensors Detection, identification and elimination of errors in data and models Providing power quality for 21st century needs Accommodating all generation and storage options Enabling new products, services, and markets Wide-area monitoring to facilitate the feedback to address congestion, avoid spilling renewable energy, active market participation Optimizing assets and operating efficiently Monitoring dynamic line loading, equipment operating limits
State Estimation Analog Measurements P i, Q i, P f, Q f, V, I, θ k, δ ki Topology Processor State Estimator (WLS) V, θ Bad Data Processor Load Forecasts Generation Schedules Network Observability Analysis Pseudo Measurements [ injections: P i, Q i ] Circuit Breaker Status Assumed or Monitored
Measurement Model Given a set of measurements, [z] and the correct network topology/parameters: [z] = [h ([x]) ] + [e] Measurements System State Errors
Network Observability [z] = [h ([x]) ] + [e] Given [z], can [x] be estimated? Which branch flows can be estimated? Unobservable branches separate observable islands. How to merge observable islands? Optimal measurement placement. Role of pseudo measurements.
Observable Islands ISLAND 1 ISLAND 2 ISLAND 3
Loss of Observability due to Line Switching CLOSED CB OBSERVABLE
Loss of Observability due to Line Switching OPEN CB UNOBSERVABLE BRANCHES IN RED
Robustness Against Topology Changes Base Case Observable Observable Base Case Observable Unobservable
Meter Placement Given a set of measurements z = h(x) + e Must be able to estimate x using z if any one measurement is missing, or any branch is disconnected Accomplish the above with least cost metering upgrade
Three stage solution Make the network minimally observable. Identify essential measurements Find a set of candidate measurements. To fix each contingency ( a branch outage or loss of a measurement). Determine the optimal selection. To provide secure state estimation under single contingencies.
Candidate Matrix [A] CANDIDATES Contingencies
Optimal Selection 0/1 Integer Programming minimize C T X Subject to A X b A ij X 1 0 i 1 0 b T =[1 1 1 1] If meas. j is a candidate for contingency i otherwise If meas. i is selected otherwise C i c i 0 cost of installing meas. i if meas. i already exist
30-Bus System Example
PMU Measurements: Several channels measuring: Phase angles or magnitudes of bus voltages Phase angles or magnitudes of branch currents Use of synchronization signals from the global positioning satellite (GPS) systems Minimal time skew between measurements
Algorithm CONSIDERATIONS Could it be modified / improved? Measurement Design Where to place new PMUs? Impact Will the grid be better monitored?
State Estimation with PMUs State Estimation with PMUs Mathematical model Mathematical model r x h x h x h x h x h z z z z z I I V trad I I V trad ) ( ) ( ) ( ) ( ) ( ) Im( ) Re( ) Im( ) Re( r x h z to Subject Wr r x J Minimize T ) ( ) (
Impact on Problem Solution: Impact on Problem Solution: iterative Non Z R H R H X e X H Z Measurements Phasor Iterative Z R H R H X e X h Z Measurements Conventional T T 1 1 1 1 1 1 ) ( ˆ ) ( ˆ ) (
Removal of Slack (Ref) Bus Eliminate the reference phase angle from the SE formulation. Bad data in conventional as well as PMU measurements can be detected and identified with sufficiently redundant measurement sets. PMU Meas. Other Meas. State Estimator & Bad Data Processor Estimated/Corrected PMU Meas. Estimated State
Merging Observable Islands PMU PMU PMU
3 Critical PMUs Error in PMU at Bus 12 Measurement p 7 8 p 8 q 2 V 1 q 3 Test A Normalized residual 0.0113 0.0112 0.0082 0.0076 0.0064 NO BAD DATA DETECTED
Adding 2 Current PMUs PMU Cur Cur PMU PMU
3 Voltage, 2 Current PMUs Measurement 12 12 6 q 6 13 12 13 q 6 12 Test B Normalized residual 3.26 3.21 2.89 2.76 1.22 BAD PHASE ANGLE MEASUREMENT AT BUS 12 IS DETECTED
A Practical Advantage PMUs can be placed at any bus in the observable island. Pseudo-measurements can merge observable islands only if they are incident to the boundary buses.
PMUs with Multiple Channels: PMU with multiple channels will measure: V_phasor at the bus I_phasors for all incident branches I phasors PMU V phasor
Simple Illustration for Full Observability Only 3 PMUs make the entire system observable. Note that bus 7 is a zero injection bus.
PMUs with Two Channels: PMU with two channels will measure: V_phasor at the bus I_phasor along a single branch I phasor PMU V phasor
Simple Illustration of PMU Placement for Full Observability Only 7 PMUs make the entire system observable.
PMU Placement Problem Can be formulated and solved efficiently using Integer Programming even for very large systems. PMUs can also be placed to make one or more geographical zones or voltage levels observable. PMUs placement can take advantage of any existing zero injection buses.
Effects of considering zero injections Systems No. of zero injections Number of PMUs Ignoring zero Injections Using zero injections 14-bus 1 4 3 57-bus 15 17 12 118-bus 10 32 29
Impact of Channel Limits on PMU Placement How does the optimal placement change as a function of available number of channels? Assume that one channel corresponds to one positive sequence measurement. Number of neighbors of a bus usually has a small upper limit for typical power systems due to sparse interconnections.
6 13 6 12 6 11 6 10 6 9 6 8 6 7 7 10 6 7 10 5 7 10 4 8 10 3 9 11 2 14 15 1 5 30-bus 4 4 5 4 4 4 4 4 3 5 5 2 7 7 1 1 14-bus Consider Zero Injections Ignore Zero Injections Channels Zero Injections System Simulation Results
28 32 9 28 32 8 28 32 7 28 32 6 28 32 5 28 32 4 31 33 3 39 42 2 57 61 1 10 118-bus 11 8 11 7 11 17 6 11 17 5 11 17 4 12 17 3 14 19 2 23 29 1 15 57-bus Consider Zero Injections Ignore Zero Injections Channels Zero Injections System Simulation Results
How can PMUs help to improve Bad Data Detection? A measurement is said to be critical if the system becomes unobservable upon its removal. Bad data appearing in critical measurements can NOT be detected. Adding new measurements at strategic locations will transform them, allowing detection of bad data which would otherwise have been missed.
Illustrative Example Two Critical Measurements P NO Critical Measurements
Number of Critical Meas. Number of PMU needed 13 2 P P IEEE 57-bus system Critical Meas. Type 1 F41-43 43 2 F36-35 35 3 F42-41 41 4 F40-56 5 I-11 6 I-24 7 I-39 8 I-37 9 I-46 10 I-48 11 I-56 12 I-57 13 I-34
Number of Critical Meas. Number of PMU needed 29 13 IEEE 118-bus system P P P P P P P P P P P P P
Impact on Covariance of the State (G -1 ) 10% 20%PMU PMU 30%PMU 40%PMU Only PMUs 38 38 38 38 38 0 No PMU Power Injections 38 Power Flows 56 56 56 56 56 0 Voltage magnitude 1 1 1 1 1 0 PMU ( 1 v, 3 currents ) 0 3 6 9 12 10 -PMU Example- PMU 1PMU Measurement =1Voltage + 3 Currents --Measurement Measurement-- Power Injection Power Flow
Simulation Results for IEEE 30Bus-System:
Simulation Results for IEEE 30Bus-System:
Impact of PMUs on Parameter Error Identification When a network parameter is wrong: Can it be identified? How? Are there cases where errors in certain parameters can not be identified without synchronized phasor measurements?
Current Practice Augment the state vector with the suspected set of parameters Simultaneously estimate the states and parameters v x x x 1, 2,..., n p AUGMENTED STATE VECTOR
Multiple Solutions What happens when more than one set of parameters satisfies all measurements? Multiplicity of solutions for [p].
Illustration of Multiple Solutions Illustration of Multiple Solutions ),, ( ),, ( 2 1 2 1 p p x J p p x J k l kl kl P x lm m l lm x P ' ' kl k l kl k l x x ' ' lm l m lm l m x x l m lm lm P x kl l k kl x P
Identifying Parameter Errors by Phasor Measurements TEST A: NO PMUs TEST B: WITH PMUs : Parameter Error : PMU
Simulation Results Test A Test B Meas /Par. R N / N Meas /Par. R N / N X 10-11 27.5040 X 13-14 14 X 13-14 27.3629 X 9-14 X 6-11 27.9763 X 10-11 11 X 9-14 24.4732 X 6-11 X 9-10 20.2291 X 9-10 29.0162 26.0467 20.1385 20.1349 17.0387
Final Remarks SE performance have significant impact on all other application functions related to smart grid operation PMUs can improve SE performance in the following areas: Bad data detection Reduced variance of estimated state Merging observable islands Identifying network parameter errors
For further information: http://www.ece.neu.edu/~abur To download the software: http://www.ece.neu.edu/~abur/ pet.html