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Combined-Cycle Unit Modeling in the Nodal Design
Bill Blevins
03-26-07
Date
Nodal Integration Design Authority
Table of Contents
Table of Contents 2
Executive summary 4
Background: 11
Discussion of alternative models of Combined Cycle plants 12
Aggregate CCP Modeling 12
Configuration-Based Modeling 12
Physical unit-based modeling 13
IDA Recommendations 13
High Level Approach: 14
Registration: 14
Metering: 14
NMMS: 14
EMS: 15
CRR: 15
MMS: 15
Outage schedules and Derating: 16
Power Augmentation: 16
Data Requirements: 17
SCED: 17
Deployments: 18
Settlements: 18
Appendix A 20
Appendix B 25
18
IDA003 Combined Cycle Whitepaper v.91 .doc 5/31/2007
Nodal Integration Design Authority
Combined-Cycle Plant modeling5/25/2007 / Executive summary
Nodal Market systems (DAM, SASM, RUC, SCED and LFC) shall support modeling of each configuration for a power block of Combined-Cycle Resources as considered as a single Resource unless multiple generators are connected to the ERCOT Transmission Grid at different voltage levels (Nodal protocol 6.5.5.2 (8) (a)). Each QSE shall provide ERCOT with the elements comprising each Combined-Cycle Configuration for a Resource through the Registration system, through Real-Time telemetry, and by appropriate entries in the COP. The configuration based model is provided to MMS and EMS by the NMMS system and includes attributes and interdependencies to represent the configurations available from the Combined-Cycle Resource.
Physical modeling of the individual generators within Combined-Cycle facility is important in the Network Security Analysis related to contingencies, transient/dynamic stability and voltage collapse. In addition to telemetry of Combined-Cycle facility, according to Nodal protocol 6.5.5.2 (8) (c), each QSE shall provide individual telemetered generator output (MW and Mvar). This telemetry is used by the State Estimator as input to the Network Security Analysis system.
Day Ahead|ONE
The DAM uses all three parts of the Three-Part Supply Offer or uses Only Energy Offer Curves submitted without a Startup Offer and without a Minimum-Energy Offer. The Three-Part Supply Offer and Only Energy Offer Curves can be submitted for each configuration of Combined Cycle plant. Additionally, the transition array is provided as well to specify allowed transitions from one configuration to another. DAM will solve for the configuration schedules using the available configuration information and provided Three Part Offers. Exclusivity shall be incorporated to ensure that only one configuration at a time can be procured from the Combined-Cycle configuration and transition array. DAM energy settlements use DAM Settlement Point Prices that are calculated for Resource Nodes, Load Zones, and Hubs for a one-hour Settlement Interval using the LMPs from DAM. In contrast, the Real-Time energy settlements use Real-Time Settlement Point Prices that are calculated for Resource Nodes, Load Zones, and Hubs for a 15-minute Settlement Interval.
DAM will allow an energy and Ancillary Service offer for each logical configuration.
Figure 1: Configuration 1 offer
Figure 2: Configuration 2 offer
Figure 3: Configuration 3 offer
Nodal Protocol 4.4.9.2.1 Startup Offer and Minimum-Energy Offer Criteria (1) (d) and (4) specifies the Resource’s hot, intermediate, and cold Startup Offer in dollars; equal to or less than the Resource Category Generic Minimum-Energy Cost for that type of Resource listed in Section 4.4.9.2.3, Startup Offer and Minimum-Energy Offer Generic Caps, unless ERCOT has approved verifiable Resource-specific minimum-energy costs for that Resource, under Section 4.4.9.2.4.
Figure 4 Generic Startup Offer
· Combined-Cycle greater than 90 MW with 5+ HRS offLine 6,810
· Combined-Cycle greater than 90 MW with less than 5 HRS off line 5,310
· Combined-Cycle less than or equal to 90 MW with 5+ HRS off line`6,810
· Combined-Cycle less than or equal to 90 MW with less than 5 HRS off line`5,310
Nodal Protocol 4.4.9.2.1 Startup Offer and Minimum-Energy Offer Criteria (1) (d) and (4) specifies the Resource’s Minimum-Energy Offer in dollars per MWh; equal to or less than the Resource Category Generic Startup Cost for that type of Resource listed in Section 4.4.9.2.3, Startup Offer and Minimum-Energy Offer Generic Caps, unless ERCOT has approved verifiable Resource-specific startup costs for that Resource.
· Combined-Cycle greater than 90 MW = 10 MMBtu/MWh * FIP or FOP, as specified in Minimum-Energy Offer;
· Combined-Cycle less than or equal to 90 MW = 10 MMBtu/MWh * FIP or FOP, as specified in Minimum-Energy Offer;
Nodal Protocol 4.4.9.3.1 Energy Offer Curve Criteria specifies each Energy Offer Curve must be reported by a QSE and must be a monotonically increasing offer curve for both price (in $/MWh) and quantity (in MW) with no more than 10 price/quantity pair. This curve should be provided for each configuration offered in the DAM.
Figure 5 Typical Energy Offer Curve
Reliability Unit commitment|Two
The Three-Part Energy Offers along with transition array are submitted for each configuration of Combined Cycle facility. The RUC uses the Startup Offer and the Minimum-Energy Offer components for determining RUC commitments and Proxy Energy Offer Curves derived from DAM Energy Offer Curves to schedule capacities.
The Energy Offer Curve may be used in settlement to claw back some or all of a RUC-committed Resource’s energy payments.
A QSE that submits an Energy Offer Curve without also submitting a Startup Offer and a Minimum-Energy Offer is considered not to be offering the Resource into the RUC, but that does not prevent the Resource from being committed in the RUC process like any other Resource that does not submit an offer in the RUC.
RUC will solve for the configuration schedules using the available configuration information and provided the Startup Offer and the Minimum-Energy Offers. Exclusivity shall be incorporated to ensure that only one configuration at a time can be procured from the Combined-Cycle configuration and transition array.
Real-time|Three
In the real time QSEs representing a Combined-Cycle Resources will telemeter the Combined-Cycle Resource’s current configuration to ERCOT over ICCP. This will be passed to SCED by the EMS SCADA so that SCED will select the correct Energy Offer Curve for the current configuration. SCED will then calculate the new base point for the Combined-Cycle Resource. Note: SCED will be using only the current telemtered configuration and will not be considering any additional configurations for a particular Combined-Cycle Resource when it computes the new base point. SCED uses HDL and LDL of Combined-Cycle Resource dispatch range calculated by Resource Limit Calculator to compute the Base Point for Combined-Cycle Resource. The Combined-Cycle Resource Base Points will then be passed to the EMS SCADA and Generation Subsystem LFC for distribution to the QSE.
Figure 6 Current Configuration Energy Offer Curve
LFC will send Updated Desired Base Point for Combined-Cycle Resource. Each time LFC detects new SCED base points for Combined-Cycle Resource are available, it will begin to ramp the Combined-Cycle facility to the Updated Desired Base Point. Calculation of the Updated Desired Base Point will be by linear interpolation from the current Output MW to the new target base point over a four (?) minute period. LFC will continuously monitor the system frequency deviation against a pre-set ERCOT Operator-entered threshold. Whenever the magnitude of the system frequency deviation is above this threshold, LFC will temporary suspend ramping of the Updated Desired Base Points if that ramping is in a direction that will worsen ACE. During emergency (EECP is in effect or Operator initiated emergency because of SCED failure), the Updated Desired Base Points are immediately set equal to the LFC calculated emergency Base Point values ignoring the ramping.
The Desired Base Point for Combined-Cycle Resource is sent to QSE. The plant operator wil distribute the Desired Base Point to currently available generating units within Combined-Cycle facility.
Regulation is deployed to the QSE and the QSE distributes the Regulation to its Resources including Combined-Cycle facilities according to Regulation Participation factors. LFC will deploy
Responsive reserve and the QSE will divide up the resulting responsive reserve deployment signal among the Resources providing responsive reserve, accounting for the amount of Responsive Reserve Ancillary Service Responsibility for each Resource. The QSE will telemeter back via ICCP the Responsive Reserve Ancillary Service Responsibility and the Responsive Reserve Ancillary Service Schedule for each Resource. The difference between the Responsive Reserve Ancillary Service Responsibility and the Responsive Reserve Ancillary Service Schedule is the amount of Responsive Reserve deployed for each Resource.
Non-Spin deployments are made by the operator using the MMS displays. For all Resources affected by the Non-Spin deployment, LFC will monitor that the Non-Spin Ancillary Service Schedule is updated to zero to reflect the 100% deployment of the Non-Spin Ancillary Service Responsibility within a specified time after the deployment.
Figure 7 Generation Subsystem
Physical modeling|Four
3.10.7.2 Modeling of Resources and Transmission Loads (1) Each Resource Entity shall provide ERCOT and TSPs with information describing each of its Generation Resources and Load Resources connected to the transmission system. All Resources greater than 10 MW, Generation Resources less than 10 MW but providing Ancillary Service, Split Generation Resources, DC Tie Resources, and the non-TSP owned step-up transformers greater than 10 MVA, must be modeled to provide equivalent generation injections to the ERCOT Transmission Grid. ERCOT shall coordinate the modeling of Generation Resources, DC Tie Resources and Load Resources with their owners to ensure consistency between TSP models and ERCOT models.
6.5.5.2 Operational Data Requirements (2) A QSE representing a Generation Resource connected to Transmission Facilities or distribution facilities shall provide the following Real-Time data to ERCOT for each Generation Resource.
(a) Net real power (in MW);
(b) Net Reactive Power (in Mvar);
(c) Power to standby transformers serving Plant auxiliary Load;
(d) Status of switching devices in the plant switchyard not monitored by the TSP or DSP affecting flows on the ERCOT Transmission Grid;
(e) Any data mutually agreed to by ERCOT and the QSE to adequately manage
system reliability;
(f) Generation Resource breaker and switch status;
(g) High Sustained Limit;
(h) High Emergency Limit, under Section 6.5.9.2, Failure of the SCED Process;
(i) Low Emergency Limit, under Section 6.5.9.2, Failure of the SCED Process;
(j) Low Sustained Limit;
(k) Ancillary Service Schedule for each quantity of Responsive Reserve and Non-Spin; and
(l) Reg-Up, Reg-Down and Responsive Reserve Services participation factors that represent how a QSE is deploying the Ancillary Service energy on a percentage basis to specific qualified Resource.
The results of the State Estimator (SE), an estimate of the state of the ERCOT network, are used as the input to all other Network Security Analysis programs including Contingency analysis, Security Enhancement, and Dynamic Stability Analysis (DSA): Voltage (VSAT) and Transient Stability Analysis (TSAT). These applications run hourly as part of the Real-Time Sequence and require generator specific information.
VSAT uses a specialized powerflow solver designed to handle large complex systems and it requires:
· Resource AGC
· Governor Response options for contingency solution
TSAT model for conventional transient and extended term dynamic simulations includes:
· Two-axis generator models of up to 6th order
· Standard IEEE models for generator excitation
· Standard IEEE models for generator speed controls
Generic constraints – protect the ERCOT Transmission Grid against transient instability, dynamic instability or voltage collapse. The constraints are considered in the DAM, RUC and SCED through TCM.
In order for ERCOT to comply with Nodal Protocol 6.5.7.1.11(1) the Dynamic Stability Analysis must be accurate which requires telemetry from each generator to properly model the physics of the system and thus derive correct solutions to stability issues.
18
IDA003 Combined Cycle Whitepaper v.91 .doc 5/31/2007
Nodal Integration Design Authority
Note: This is an updated version of the original whitepaper on Combined Cycle modeling. This version reflects discussions between ERCOT, the MMS vendor ABB and the various stakeholders with CC assets in the ERCOT Market.
Combined-Cycle Unit Modeling in the Nodal Design |Five
Background:
A Combined Cycle generating station consists of one or more combustion turbines (CT), each with a heat recovery steam generator (HRSG). Steam produced by each HRSG is used to drive steam turbines (ST). Each steam turbine and each combustion turbine have an electrical generator that produces electric power (CTG and STG). Typical configurations (Appendix A Figure 1-Figure 7) contain one, two, or three combustion turbines each with a HRSG and a single steam turbine. Because of high thermal efficiency, low initial cost, high reliability, relatively low gas prices and low air emissions, combined-cycle gas turbines have been the new resource of choice for bulk power generation for well over a decade.
ERCOT relies on a diverse mix of generation resources to meet the demand needs of the electrical system in Texas and approximately 20% of ERCOT’s total generation capability comes from Combined-Cycle Plants (CCPs). ). There is a variety of CCPs in ERCOT, the most complex consists of four CT and two ST generating units. Some of CCPs are capable of power augmentation. The CCPs create unique challenges for ERCOT because they can operate in a number of different configurations. For example, a “2x1” power block (2 combustion turbines with 1 steam turbine) may have as many as three individual configurations in normal operation and six if the plant has the ability to bypass the HRSG and operate simple-cycle. With the use of various power augmentation methods, the same power block can have upwards of twelve configurations. A “3x1” power block (3 combustion turbines with 1 steam turbine) may have as many as seven individual configurations in normal operation and fourteen if the plant has the ability to bypass the HRSG and operate simple-cycle. With the use of various power augmentation methods, the same power block can have upwards of twenty six configurations. The stations thermal operating and electrical generating characteristics differ from one state to another and the transition from one state to another has own operational limits and costs. ERCOT and the stakeholders must provide mechanisms to solve modeling issues surrounding the flexible configurations and operation of Combined Cycle facilities.
The Market Management System (MMS) requires this information to produce optimum DAM, RUC and SCED solutions and provide dispatch instructions that are operationally feasible; therefore, each operational configuration should be used as inputs into the unit commitment and economic dispatch software.