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