Synchronous Inertial Response Julia Matevosyan

Synchronous Inertial Response SIR is stored kinetic energy that is extracted from the rotating mass of synchronous machines following a disturbance in a power system SIR can be provided by synchronous machines, whenever in operation. SIR is independent of machine’s operating point Quantity of inertia contribution is determined as kinetic energy that can be provided by a synchronous machine during system imbalance: H·MVA where H is machine inertia constant in seconds, MVA is machine’s rated power (RARF data)

Frequency Control and SIR Need Provide sufficient time from Point A to Point C, for Fast Frequency Response and Primary Frequency Response

Why SIR is becoming a concern? Significant share of non-synchronous generation (WGRs) leads to displacement of synchronous generation in unit commitment, during low load/high wind periods (night time in spring). With low SIR, the rate of change of frequency (RoCoF) is high, leaving less time for PFR and/or FFR to deploy and arrest the system frequency excursion.

Wind Generation Current Status Wind generation capacity is over 12.6 GW (Apr 2015) Wind generation record 11,154 MW (Feb 2015), Instantaneous wind penetration record is 40.58% (10.3 GW wind, 25.4 GW load Mar 29, 2015) Previous Instantaneous wind penetration record was 39.84% (9.9 GW wind, 25 GW load, Nov 3, 2014) Higher penetration levels of wind and solar generation capacity are expected in the future.

ERCOT Wind Capacity

Historic SIR and Future Projections SIR, MWs Each box represents one year of historic SIR data. Red line is the median, the edges of the box are the 25th and 75th percentiles, the whiskers are +/- 2.7 sigma, red crosses are the outliers. Blue dots show SIR during maximum wind penetration hour in each year. Blue stars show projected SIR during maximum wind penetration hour in future years based on planned wind projects with SGIA and Financial Commitments. 7

Supporting data for maximum penetration hours   2010 2011 2012 2013 2014 2015 2016 2017 Installed Capacity, MW 9,116 9,452 10,034 10,570 11,066 16545 17849 18738 Max Pwind/Pload 25.5% 27.4% 29.8% 35.8% 39.4% 55.3% 59.7% 62.7% Pwind, MW 6,483 6,772 7,247 8,773 9,699 13,732 14,838 18,738 Pwind/Pwind_inst 71% 72% 83% 88% Pload, MW 25,427 24,745 24,328 24,488 24,617 24,800 SIR, GWs 161.7 147 133.7 120 119.6 96.4 91.1 87.6

SIR v.s. Net Load High load low wind High wind low load Data from Jan, 2014 to May, 2014

Basis for minimum SIR need NERC BAL-003, for the largest N-2 event, (ERCOT: 2,750 MW) prevent first step under frequency load shedding (at 59.3 Hz) Load Resources with under frequency relays, providing RRS (or FFR2 in FAST), responding within 0.5 s of frequency being at or below 59.7 Hz. Some minimal SIR is needed to keep frequency from reaching 59.4 Hz (assuming 0.1 Hz margin above 59.3 Hz) within fist 0.5 seconds of 2750 MW generation trip.

System frequency after 2750 MW trip (first 0.5 s) Calculated frequency following 2750 MW trip at minimum SIR conditions

Basis for minimum SIR Requirement, cont. There might be other considerations: Potential need to maintain minimum FFR requirement at low SIR; too frequent deployment FFR for a smaller generation trip; insufficient synchronizing torque can be an issue if too many (or at critical locations) synchronous generators are de-committed in some areas; RoCoF protection at the generators?

Real Time SIR Calculator Based on earlier discussions at FAST meetings ERCOT set up a real time SIR calculator; Calculates system SIR as sum of H*MVA for all online generators (with output above certain threshold); As well as contributions by generation type; The calculator simplifies the analysis of historic SIR data and trends.

Spring, 2014 System inertia is calculated as H*MVA for all online machines (i.e. independent of the machine output)

Inertia Provided by Resource Type (MW-s) Spring, 2014 Inertia Provided by Resource Type (MW-s)   Min Max Average Nuke 17,219 23,749 23,109 Coal 16,287 24,435 20,651 CT-CC 43,556 114,099 83,419 ST-CC 19,420 46,142 34,375 CT-SC 13,988 27,944 18,254 Gas ST 9,267 38,025 13,966 Hydro 200 29 Total 119,899 272,318 193,804

Summer, 2014 System inertia is calculated as H*MVA for all online machines (i.e. independent of the machine output)

Inertia Provided by Resource Type (MW-s) Summer, 2014 Inertia Provided by Resource Type (MW-s)   Min Max Average Nuke 23,749 Coal 27,807 34,974 33,377 CT-CC 73,164 130,737 106,306 ST-CC 26,438 52,873 43,383 CT-SC 12,280 37,024 18,977 Gas ST 22,862 64,441 37,792 Hydro 441 78 Total 190,458 343,693 263,663

Fall, 2014 System inertia is calculated as H*MVA for all online machines (i.e. independent of the machine output)

Inertia Provided by Resource Type (MW-s) Fall, 2014 Inertia Provided by Resource Type (MW-s)   Min Max Average Nuke 18,406 23,749 20,590 Coal 25,036 34,974 30,232 CT-CC 42,416 129,566 85,469 ST-CC 17,751 51,567 36,803 CT-SC 3,667 30,725 14,269 Gas ST 14,321 52,158 24,502 Hydro 406 37 Total 122,075 321,862 211,903

Appendix

Frequency deviation after unit loss Frequency deviation after the loss of generation unit or load can be calculated as follows: 𝛥𝑓 𝑡 = −𝛥𝑃 𝐷 (1− 𝑒 −𝑡𝐷 2𝐻 ) where D is load damping, H is total system inertia, ΔP is generation lost, in pu on committed capacity MVA base, Pbase Source: P. Kundur Power Systems Stability and Control

Frequency deviation after unit loss Load damping ( 𝐷 𝐿𝐵 ) usually given as fraction of total load 𝑃 𝑙𝑜𝑎𝑑 per Hz. Expressing in pu on committed capacity base, Pbase. 𝐷= 𝐷 𝐿𝐵 𝑃 𝑙𝑜𝑎𝑑 𝑃 𝑏𝑎𝑠𝑒 H, total system inertia on committed capacity base, Pbase is calculated as:  𝐻= 𝑖 𝑃 𝑀𝑊,𝑖 𝐻 𝑖 𝑃 𝑏𝑎𝑠𝑒   Substituting expressions for D and H in the equation for 𝛥𝑓 𝑡 𝛥𝑓 𝑡 = − 𝛥𝑃 𝑀𝑊 𝑃 𝑙𝑜𝑎𝑑 (1− 𝑒 −𝑡 𝐷 𝐿𝐵 𝑃 𝑙𝑜𝑎𝑑 2 𝑖 𝑃 𝑀𝑊,𝑖 𝐻 𝑖 )

System inertia need Expressing for system inertia: 𝑖 𝑃 𝑀𝑉𝐴,𝑖 𝐻 𝑖 = −𝑡 𝐷 𝐿𝐵 𝑃 𝑙𝑜𝑎𝑑 2 ln⁡(1+ Δ𝑓 𝑡 𝐷 𝐿𝐵 𝑃 𝑙𝑜𝑎𝑑 Δ 𝑃 𝑀𝑊 ) For given system load conditions, substituting criteria for unit loss, time and permissible frequency deviation (before other frequency control measures become effective), minimum inertia requirement can be estimated from the above expression.