System Ratings, Limits and Real-Time Monitoring Presented to: Operating Committee April 21, 2016 Thermal Facility Ratings • In New England • Summer ratings (April 1 to October 31) • Winter ratings (November 1 to March 31) • Thermal capacity ratings for transmission facilities • ISO-NE procedure OP-16 • ISO-NE database NX-9 • Normal and Emergency ratings are applied • Normal – continuous 24 hour rating • *Long Time Emergency (LTE) – 12 hour summer and 4 hour winter “load cycle” ratings • Short Time Emergency (STE) – 15 minute rating • Drastic Action Limit (DAL) – 5 minute rating *The use of these ratings requires the pre-contingent circuit loading to be at or below the Normal rating and that the circuit loading be returned to or below the Normal rating after the daily load cycle 2 2 Voltage Limits • Voltage limits required with NERC standard changes • Full implementation in summer of 2015 • No seasonal limit changes for voltage • Transmission owners supply limits based on their equipment ratings • Voltage limits for transmission facilities • ISO-NE procedures OP-19 and M/LCC-15 • M/LCC-15, attachment H lists LCC operating area limits • M/LCC-15, attachment E lists all exceptions • Normal and Emergency limits are applied, “high and low” • Normal Voltage Limit (NORMVL) – continuous limit • Long Time Emergency Voltage Limit (LTEVL ) – minimum of 30 minute limit • Short Time Emergency Voltage Limit (STEVL) – 15 minute limit • Drastic Action Limit Voltage Limit (DALVL) – 5 minute limit 3 3 Voltage Limits (M/LCC-15H) 4 4 Transmission Voltage Limits Used in Operations • Operation at voltages and time durations outside of the shaded area may result in an SOL exceedance 5 5 SCADA Transmission Voltage Alarms 6 6 SCADA Subtransmission Voltage Alarms 7 7 Transmission Voltage Limits Used in Planning Company Other New England Utilities National Grid VELCO kV Level 115 230 345 115 230 345 115 230 345 Pre Contingency Post Contingency Pre Switching Post Switching High Voltage Low Voltage High Voltage Low Voltage High Voltage Low Voltage 1.05 0.95 1.05 0.90 1.05 0.95 1.05 0.95 1.05 0.90 1.05 0.95 1.05 0.95 1.05 0.90 1.05 0.95 1.05 0.95 1.05 0.90 1.05 0.90 1.05 0.98 1.05 0.90 1.05 0.95 1.05 0.98 1.05 0.90 1.05 0.95 1.05 0.95 1.05 0.95 1.05 0.95 1.05 0.98 1.05 0.95 1.05 0.95 1.05 0.98 1.05 0.95 1.05 0.95 • Subtransmission 34.5 kV, 46 kV and 69 kV limits • • • • 8 8 0.95 pu to 1.05 pu pre-contingency 0.90 pu to 1.1 pu post-contingency What is the post-contingency pre switching low voltage limit? What is the lowest voltage allowed at the customer service meter? Real Time Monitoring • Real-Time Contingency Analysis (RTCA) is a predictive tool required for Transmission Operators • Uses a power-flow model ingesting real time data • In the VELCO control room RTCA runs every 5 minutes • RTCA application outages 521 transmission and subtransmission contingencies (element outages) within VT and with our neighboring utilities • Ranks thermal and voltage limit violations by severity for the Operators to monitor • The Operator must know the risk to the system at all times with this application, have mitigation plans and if necessary take pre-contingency actions to prevent a transmission limit violation 9 9 Real Time Monitoring • Both VELCO and ISO-NE monitor the VELCO transmission system and run RTCA • Both VELCO and ISO-NE have what NERC requires, a “wide-area view” within our geographic areas • VELCO has better detail of the VT system in the EMS model • ISO-NE looks to VELCO for ensuring local area reliability • ISO-NE Transmission Operating Guides (TOGs) are used for monitoring interface limits • Studies performed by ISO-NE and VELCO determine the thermal, voltage and stability operating “interface” limits for the VT and NE areas 1010 Real Time Monitoring • The PSSE load-flow model used by ISO-NE for studies completely models the VT subtransmission system and distribution loads • ISO-NE ensures that for all transmission system contingencies, the area customer loads and transfers can be supported, i.e. both thermal loading and voltage • The looped VT subtransmission system is critical to supporting load and is impacted by the overlying transmission system • VELCO has the responsibility to monitor the subtransmission that ISO-NE does not fully model or monitor in their SCADA/EMS 1111 System Models and Ratings • The VT subtransmission system is completely represented in the regional power flow PSSE models • Annual update process includes review of the subtransmission system • Topology changes provided at least one month prior to change • Needed for EMS modeling, Operations awareness, Engineering and Planning studies • Modeling data is needed for the following • • • • • 1212 Lines Transformers Shunt capacitor banks Generators Others, such as overload protection schemes Modeling Data • Data responsibilities • GMP, VEC, BED, Swanton, WEC, Stowe, VPPSA • Existing process • Send all updates to DUSYSTEM‐DATA@VELCO.COM • GMP-North consistently using process 1313 Thermal Circuit Ratings (GMP-North) • Covers 34.5kV system • Includes other utilities, e.g. Morrisville and Hardwick 1414 Thermal Circuit Ratings (GMP-South) • We follow table based on conductor rating • Could there be a more limiting element? 1515 Thermal Circuit Ratings (VELCO) • Publish 1st and 2nd limiting elements • Device legend to define limiting elements 1616 Recent Operating Events • Several operating events in recent months regarding system thermal ratings used in real-time operation • February 3rd, loss of VELCO K22 line with the K21 open for maintenance • Morrisville #3 to GMPR-Johnson B22 34.5kV circuit was operated up to 111% above its winter limit of 37.5MVA for 22 minutes • Ambiguity with the VELCO and GMP control centers on the winter rating • Morrisville #3 to VELCO-Stowe 3329 34.5kV circuit operated up to 85% of rating • The line conductor sagged making contact with the underbuilt lower voltage distribution 1717 Recent Operating Events • March 22nd, VELCO F206 Granite to Comerford line out of service for maintenance • The GMP Websterville to McIndoes 3311 line approached overload at 17.9 MVA winter rating • 3311 limiting element, Websterville substation switch • The 3311 was opened by SCADA control • Opening of the 3311 line as a pre-planned option added more load on the NGRID Comerford to GMP Marshfield 3316 line • Ambiguity between the VELCO and GMP control centers on the 3316 winter ratings, caused three area generators (Dodge Falls, Ryegate, and McIndoes) to be reduced • 3316 limiting element, WEC Danville Tap switch • Replacement of the Websterville switch has been postponed with the intention to upgrade with rebuild of the substation 1818 Geographical Area 1919 GMP Area One-line 2020 Survey of VT Control Centers • BED – have max alarm limits in SCADA and unclear on seasonal changes • GMP-N – have a list used in the control room, hope to have SCADA upgrade include limits in SCADA and update for Winter and Summer • GMP-S – have alarm limits in SCADA, however they are very conservative and when in alarm the Operators refer to the list in the control room to verify. No seasonal limit changes being made • Swanton – no SCADA indications, the reclosers control (limit) line loading. Rely on experience for operating. • VEC – mainly rely on Engineering, no list in the control room 2121 Possible Solutions • Develop a common VT thermal circuit database for subtransmission and specific distribution (e.g. BED 13.8kV) • VELCO would be responsible for administration • We need consistency in rating methodology • In the interim, send all updates to DUSYSTEM‐DATA@VELCO.COM • Make priority for looped subtransmission that can more easily be overloaded • Publish common ratings in each control center for real time operation • Will require a revision and update process • For control centers with SCADA monitoring, apply seasonal limits with alarm set-points • Provide training to the VT operators on rating methodologies, contingencies, limit exceedance resolution 2222 Questions? 2323