1. Cascading simulation techniques in Europe: the PRACTICE experience E. Ciapessoni, D. Cirio, A. Pitto 2013 IEEE PES General Meeting Vancouver, British Columbia, Canada July 21-25, 2013 1

2. PRACTICE operational risk Tool for probabilistic assessment of operational risk in power systems Based on risk concept – Combination of probability and severity of a disturbance (contingency) Assessing cascading evolution … – important task Cascading engine has two operation modes: – Single path – Multi path 2

3. Cascading engine: general features Robust power flow program Robust power flow program enhanced with steady-state models of: – frequency regulation (5% default droop) – main protection and defence systems, e.g. line and transformer overcurrent, minimum impedance for lines, minimum and maximum voltage for generators and loads, under-frequency load-shedding (pumps and loads). load-flow divergence Able to assess an impact for the contingencies which cause load-flow divergence – adopting suitable load reduction techniques 3

4. Single path mode Uncertainty only related to initiating event Check violations of currents/voltages One element tripped at a time – The element with highest violation Does not take into account the uncertainty on protection systems response Fast algorithm 4

5. Multi-path mode Considers also uncertainty in protection systems response Probabilistic models are defined for: – Hidden failures – Hidden failures (HF) of protections exposed by the initial contingency or by overloads overcurrent relays – Correct operation of the overcurrent relays 5

6. Benchmark for cascading engine Italian EHV transmission grid with foreign equivalents: – 1400 electrical nodes – 1000 lines – 700 transformers – 300 generators Peak and off peak load early 2000s Goal: comparing time sequence of events given by T-D simulator with the sequence of trippings by the single path cascading 6

7. Benchmarking Dynamic model: – Prime movers & AVRs – Automatic load shedding – Overcurrent protections for branches (120% Imax) – No secondary frequency control – Standard model for loads (50% dyn 50% static) Quasi static model (in PRACTICE): – Primary control – Automatic load shedding for power deficits – Overcurrent protections for branches (set to 120% Imax) – Constant power model for loads – Under/over voltage for loads and generators 7

8. Benchmarking results (I) 8 Loss of an important 400 kV line in the North East Time domain simulator [tripping time, s] – line ID Quasi static approach Tripping cause / line ID 28 - VV2196-UDNV21Overloading / BUIV211-UDNV211 42 - OV2215-LNZO21Overloading / SOVV212-LNZO211 Tripping of BUIV-UDNV 220 kV line Tripping of SOVV-LNZO 220 kV line at 42 s eliminates violation on this line!

9. Benchmarking results (I) 9 Loss of an important 400 kV line in the North East Time domain simulator [tripping time, s] – line ID Quasi static approach Tripping cause / line ID 28 - VV2196-UDNV21Overloading / BUIV211-UDNV211 42 - OV2215-LNZO21Overloading / SOVV212-LNZO211 Tripping of BUIV-UDNV 220 kV line Tripping of SOVV-LNZO 220 kV line at 42 s eliminates violation on this line! Mutual relief mechanisms taken into account in quasi static approach

10. Benchmarking results (II) Three most probable cascading paths identified by multi-path cascading engine (future time interval=5 minutes) 10 Cascading Path Probability of occurrence [over future 5-min interval] (1) BUIV211-UDNV211 --> SOVV212-LNZO211 8.6 10 -7 (2) No cascading 7.9 10 -7 (3) BUIV211-UDNV211 3.9 10 -7 Loss of the first line BUIV211-UDNV211 implies a very high overloading (140%) on branch SOVV212-LNZO211 Prob. of tripping of both lines (path # 1) >> prob. of tripping only of the first line (path # 3)

11. Benchmarking results (III) 11 Quasi static toolTime domain simulator Step nr.Tripped branchDue to...Time [s]Tripped branch 1 Soverzene-Vellai overload51.5 Soverzene-Vellai 2 Musignano-Lavorgo overload55.0 Musignano- Lavorgo 3 Bulciago-Soazza overload62.5 Bulciago-Soazza 4 Sondrio-Cislago overload64.0 Sondrio-Cislago 5 Baggio-Castelnuovo overload 67-68 Many EHV lines, due to instability 6 Loadflow diverges Cascading trippings well caught by PRACTICE Loss of a large thermal power plant

12. Benchmarking results (III) 12 Loss of a large thermal power plant Cascading trippings well caught by PRACTICE (angle, voltage) instability mechanisms Quasi static toolTime domain simulator Step nr.Tripped branchDue to...Time [s]Tripped branch 1 Soverzene-Vellai overload51.5 Soverzene-Vellai 2 Musignano-Lavorgo overload55.0 Musignano- Lavorgo 3 Bulciago-Soazza overload62.5 Bulciago-Soazza 4 Sondrio-Cislago overload64.0 Sondrio-Cislago 5 Baggio-Castelnuovo overload 67-68 Many EHV lines, due to instability 6Loadflow diverges

13. Remarks Proposed a benchmark Proposed a benchmark for cascading tools – A model of the Italian EHV transmission system with foreign equivalents (early 2000s) cascading engine Quasi static «single path» cascading engine tested against time domain simulator – Very good matching with the sequence of events by time domain simulation at least in the early stages of cascading Multi-path cascading Multi-path cascading engine provides probability of different sequences of trippings – Taking into account hidden failures and uncertainties on protection relay settings 13 Contact: Dr. Andrea Pitto, PhD e-mail: andrea.pitto@rse-web.itandrea.pitto@rse-web.it