Security Constraints

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SPD will normally dispatch the lowest-cost generation to meet demand (allowing for co-optimisation of energy and reserves).

About Security Constraints

When unconstrained dispatch does not allow the System Operator's security policies to be met (for example, if transmission assets are expected to be operated beyond rated short-term capability after any defined contingent event) then a security constraint is applied in SPD.

Security constraints may be applied as:

• a temporary constraint – to deal with an outage situation when some assets are not available
• a permanent constraint – when the normal integrated power system capability and expected generation offers and demand may not result in secure operation.

GL-OC-208 Constraints Overview [ pdf 377.58 KB ]

On 28 March 2011, we implemented the Simultaneous Feasibility Test (SFT) software which automatically creates security constraints that are applied to SPD.

The information on the constraints generated by SFT and which trading periods the constraint applies to is now published through schedules on WITS. The information will be retained for a rolling 12 month period so that a record of constraint history is built up over time. Participants can look up a trading day of interest and see the constraints that were applied for the grid topology and conditions on that day. This will provide an indication of what constraints may be generated by SFT for future trading days that are expected to have a similar grid topology and conditions.

The Excel spreadsheet below provides a list of manual constraints that were used prior to implementation of SFT. Note that this list will not be modified to reflect any constraint changes that occur after the date of SFT implementation. The historical information may be useful to participants as an indication of the branches that may be included in constraints generated by SFT.

The need for this historical list will be superseded over time as information about SFT constraints is made available.

List of Manual Constraints prior to implementation of SFT [ xls 310 KB ] (25 March 2011)

In addition to the above historical information, the spreadsheet below has constraints that the System Operator develops using non-automated processes (manual constraints). This list will be updated as and when the constraints in the list are revised.

Where practicable this information will be published two weeks prior to the date the constraints are intended to be first used. Note this may not always be possible where manual constraints have been identified as being required due to the automatic scheduling process not being able to mitigate a security issue which runs one week out.

Manual Constraints [ xlsx 106.78 KB ] (25 October 2019)

SFT Constraint Assessments

Participants may request an SFT constraint assessment to be performed and posted to the industry via POCP or the System Operator website. The purpose of such assessments is to provide an indication of what the limiting security constraints may be during a particular outage or grid configuration. To request a SFT constraint assessment email:  OPS.Planner@transpower.co.nz.

A set of generation scenarios to be used for SFT constraint assessments has been agreed by the constraint information working group; this is a living document and will be updated based on experience. The aim of the list is to ensure scenarios which will identify unique constraints are studied, while keeping scenarios to a minimum so that studies are performed efficiently and turn around for requests is as quick as possible.

Generation Scenarios [ pdf 586.29 KB ]

SFT Calculation Methodology

The System Operator is required, at least two months prior to the introduction of SFT, to advise:

• an explanation of the methodology and procedures it will employ in formulating constraints, including how information about stability and transmission limits are used to define constraint parameters
• when the information can be published
• automated processes and systems used in formulating constraints.

The System Operator is also required to advise participants of the constraint percentage threshold, being the threshold at which SFT constraints are created. The initial threshold for SFT constraints is 90% and 0% for manual constraints. The above is best illustrated in the linked process flowcharts below and the  SFT FAQs sheet [ pdf 130.91 KB ].

Name/Link to Diagram	Description
Process Overview [ pdf 80.28 KB ]	This diagram shows the SFT Security Constraint development process used by the SFT application. Additional information about inputs/parameters/ outputs can been seen by hovering the cursor over the boxes with yellow icons in them.
Security Constraint creation process with SFT [ pdf 44.43 KB ]	Page 1 of this diagram shows the process used by the SFT Constraint Builder to create Security Constraints, page 2 is the same diagram with additional details added

Procedures for Managing Security Constraints

The System Operator uses a number of procedures to manage the creation and maintenance of security constraints. We have published a combination of procedures and summary flow charts that may be of interest to market participants:

PR-OC-204 Security Constraints Process Overview [ pdf 713.64 KB ]

PR-OC-229 Update Registers, Email Notification CANs Updating SO Website [ pdf 867.63 KB ]