Mitigation of Cascading Outages and Prevention of Blackouts:System-Wide Corrective Control

Transcription

1 10th Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion 6-9 November 2016 Belgrade, Serbia Mitigation of Cascading Outages and Prevention of Blackouts:System-Wide Corrective Control Lei Ding Shandong University

2 Cascading Outages and Blackouts Power System Blackout: the loss of supply to the load in the entire power system, or parts of the system Cascading Outage: a sequence of events in which an initial disturbance, or a set of disturbances, causes a sequence of one or more dependent component outages Each blackout has its own specific nature large number of scenarios and blackout paths

3 Cascading Outages and Blackouts Electric power systems have become extremely complex and difficult to operate and protect It is impractical and uneconomical to design power systems to be stable for every possible disturbance To maximize profit and utilization of all available generation and transmission assets, power systems operate relatively close to their stability limits There is always a risk that cascading outages or blackouts happen. The economic and social implications of blackouts can be disastrous and difficult to quantify in advance

4 New Challenge 1 Integration of RES H P w D P opt B C E A V w 0 ω t1 ω t2 ω t3 ω t MPPT

5 New Challenge 2 AC/DC Hybrid System Modern power systems take more cascading risks in AC/DC hybrid system due to: Tightly-coupled interaction between HVDC and AC systems Large global impact subject to failure in HVDC system HVDC delivers a large amount of active power to receiving AC system. Commutation failure and even cascading commutation failure may be caused by faults in AC system. Once HVDC is blocked, the loss of large power transmitted will create a large disturbance to AC systems connected. Such disturbance is huge for AC system to be withstood in a limited area. Larger disturbance, low inertia system and weakened regulation ability

6 New Challenge 2 AC/DC Hybrid System Two risky accidences, which may result in cascading events, were recorded although no severe blackout was triggered Power grids in China feature typical AC/DC hybrid interconnections Central Grid transmits large power to East Grid by five HVDCs one of which is Ultra HVDC with power of 6400MW On Aug. 8, 2012 simultaneous commutation failures in four HVDCs On Jul. 5, 2013 simultaneous commutation failures and forced blocking in two HVDCs From Prof. Xiaoxin Zhou

7 New Challenge 2 AC/DC Hybrid System Procedure during 7.5 (2013) event A 500kV AC line in East Grid was tripped by temporary single-phase fault Two HVDC i.e. Fulong-Fengxian and Tuanlin-Fengjing suffered commutation failure successively AC faulted line was reclosed successfully Single pole at Fengjing inverter and bipolar at Fulong converter were blocked Power transmitted by HVDC reduced 4,530 MW Frequency of East Grid decreased to 49.82HZ Power through AC tie line from other grid increased to 700MW From Prof. Xiaoxin Zhou

8 Power System Collapse - Blackouts A generic path to a blackout: Initiating Event Uncontrolled System Separation Precondition Cascading Events Restoration Restoration can be very time consuming Steady-State Progression High Speed Cascade Partial or total blackouts Triggering Event

9 Power System Collapse - Blackouts A generic path to a blackout: Initiating Event Special System Protection Emergency control Corrective control Uncontrolled System Separation Precondition Cascading Events Restoration Steady-State Progression High Speed Cascade Triggering Event

10 Power System Collapse - Blackouts A generic path to a blackout (HVDC system, with RES integration): Initiating Event Uncontrolled System Separation Precondition Cascading Events Restoration High Speed Cascade Initiating event, such as HVDC blocking, will be a huge disturbance. System inertia and regulation ability are weakened. Whole cascading process will be highly accelerated.

11 Power System Collapse - Blackouts A generic path to a blackout (HVDC system, with RES integration): Initiating Event Uncontrolled System Separation Precondition Cascading Events Restoration High Speed Cascade System-wide emergency control: alleviate or limit the impacts of the large disturbance using all possible system-wide measures: regulation capacity of RES, active load control, storage, etc. System-wide corrective control: mitigate the cascading outages and prevent blackouts: controlled islanding, system-wide coordinated load shedding

12 Role of Controlled Islanding Controlled islanding is a typical system-wide corrective control measure Avoid uncontrolled system separation (blackout) by initiating a controlled system separation Separation can prevent mechanisms that would, without islanding, cause the system to collapse: e.g. unstable oscillations or voltage collapse Separate the system into isolated islands

13 Role of Controlled Islanding Initiating Event Remedial Actions Taken Remedial Actions Fail Uncontrolled System Separation Healthy System Steady State Progression High Speed Cascade Blackout Restoration Healthy System Triggering Event Pre-Islanding Actions Verify Feasibility of Islanding Solution Post-Islanding Corrective Actions Stable Islanded Operation Healthy System Determine Necessity of Islanding Plan Islanding Separate Islands Resynchronise Islands Islanding can avoid a blackout: limiting the load unsupplied and accelerating the return to healthy operation

14 Successful Controlled Islanding will require extensive supporting infrastructure Wide area monitoring Algorithms for detecting imminent collapse Algorithms for creating the islanding solution Centralised processing to implement algorithms Automated, wide area control of: Breakers for separation Supporting Infrastructure Control elements for pre/post islanding actions (e.g. shedding relays) Protection settings (settings must be changed post islanding)

15 Supporting Infrastructure System state, dynamics and events Reliable, Fast Communication Network Transparent database technique Centralised Computing Algorithm to Detect Imminent Cascade System-wide Corrective Control Performed within centralised control centre Algorithm to Determine Islanding Solution State estimation Security Evaluation Risk Pre-warning Wide Area Monitoring Power System Automated, remote access to protection settings Centralised, Automated Decision Making Reliable, Fast Communication Network External Inputs Initiate Islanding - Breaker opening schedule - Pre/Post control actions - New protection settings

16 Role of Controlled Islanding Need an islanding solution to island the system Islanding solution must describe: If or whether to split? Where to split: splitting boundary? When to split: timing What order should the islands be created and should other actions be taken before/after this, e.g. load reduction (HOW?) The answers to these may be interdependent and not optimally determined in isolation

17 Where to Split?--SCCI Description Stable islanded operation: Coherent generators and small power-imbalance SCCI: Spectral Clustering based Controlled Islanding Two Steps Construct Dynamic Graph and cluster generator nodes Construct Static Graph and cluster all nodes Spectral Clustering Construct the Laplacian matrix of graphs (dynamic, static) Compute the first two eigenvectors (bisection case) Use K-medoids to cluster nodes

18 SCCI Description First Step: Dynamic graph and generator grouping Generator groups {1}, {2,3}

19 SCCI Description 19 Second Step: Static graph and splitting boundary Splitting boundary {4-5;4-6} (red dash line)

20 When to Split? Disturbance Separation pre-islanding process post-islanding process Pre-disturbance SEP Post-islanding SEPs The post-islanding SEP is determined by the splitting boundary-island topology. Find a proper splitting boundary to make sure post-islanding SEP exists During the whole process, dynamic stability must be satisfied. Given a certain the splitting boundary, the post-islanding dynamic process is determined mainly by the timing of controlled islanding. If the separation is undertaken at the right time, the transition will be smoothly.

21 When to Split?-right time to split Construct a transient energy function of each intended island If the island is separated when the energy is below the critical energy, then the island will retain its stability If the island is separated when the energy of the island is greater than critical energy then the island may lose its stability. 21

22 When to split? 22 Consider two islands: Island 1 and Island 2 The stability of each island (post separation) is determined by the time the separation occurs at A window for stable separation is created Legend stable unstable ISLAND 1 ISLAND 2 Time 22

23 Interdependency 23 No intersection time exists where the islands can be separated and both remain stable? Legend stable unstable ISLAND 1 ISLAND 2 t 1 t 2 Time change the splitting boundary produces a non-empty intersection; take remedial control; split in a sequential way. So, by changing where the islands are created it is possible to create a suitable When for separation

24 Controlled Islanding Example 1 Wind Speed >= Cut Out Speed Disconnection of Wind Farms Initiating Event Time(s)

25 Controlled Islanding Example 2 Line trips due to overload 2.16s Time(s)

26 Controlled Islanding Example 3 Line trips due to overload Time(s) s

27 Controlled Islanding Example 4 Line trips due to overload 2.29 s Time(s)

28 Controlled Islanding Example 5 Generator trips due to out of step operation 2.64 s Time(s)

29 Controlled Islanding Example 6 Line trips due to overload Time(s) s

30 Controlled Islanding Example 7 Line trips due to overload Time(s) s

31 Controlled Islanding Example 7 Time(s) s

32 Controlled Islanding Example 8 Generator trips due to out of step operation 7.10s Time(s)

33 Controlled Islanding Example 9 Generator trips due to out of step operation Time(s) s

34 Controlled Islanding Example 10 Generator trips due to out of step operation Time(s) s

35 Controlled Islanding Example Time(s) s

36 Controlled Islanding Example 1 Wind Speed >= Cut Out Speed Disconnection of Wind Farms Initiating Event Time(s)

37 Controlled Islanding Example 2 Intentional tripping of the lines 0.60s Time(s)

38 Controlled Islanding Example s Time(s)

39 Controlled Islanding Example 3 Generator trips due to out of step 1.24s Time(s)

40 Controlled Islanding Example 3 Time(s) s

41 Controlled Islanding Example 4 Line trips due to overload 2s Time(s)

42 Controlled Islanding Example Comparison Time(s) s Without Controlled Islanding With Controlled Islanding Time(s) 0 2s Element Assets Lost Capacity Lost (%) Element Assets Lost Capacity Lost (%) Tripped Generators 1,8,9, Tripped Generators Unsupplied Loads 1,3,4, Unsupplied Loads Non-Energised Lines 1,2,3,4,11,17, 22, 23, 24, 25, 26, 27, Non-Energised Lines 1, 2, 4, 9,

43 Conclusion System-wide corrective control could play a role in mitigating cascading outage and preventing blackouts, especially in AC/DC hybrid systems with large-scale RES integration. Prevents an uncontrolled separation by performing a controlled separation of the power system System-wide corrective control is very complex and must be found online, during a system collapse Demands high level of confidence in automated, closed loop control with widespread authority over breakers and other control procedures

44 10th Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion 6-9 November 2016 Belgrade, Serbia Mitigation of Cascading Outages and Prevention of Blackouts:System-Wide Corrective Control Lei Ding Shandong University