EMO:Constraint Overview

References:

1. System Security Forecast (SSF), Part B, Section 7
2. Security constraints overview

The SSF states that "Security Constraints may be applied [in SPD] to manage circuit loading:

• to ensure post-event circuit loading does not exceed short term ratings (15 minute off-load times);
• to ensure system stability is maintained following a contingent event;
• to ensure sufficient reactive power is available to maintain voltage levels following a contingent event."

The "15 minute off-load time" is the internationally recognised time required for the SO to redispatch after a contingency. Shorter off-load times may be in place with Special Protection Schemes (SPS) in some cases.

The SSF states that "thermal current ratings (thermal ratings) are determined from the maximum permissible temperature of the conductors to avoid excessive sag or long-term annealing of the conductors. The calculation does not consider environmental factors that may cool the conductor, therefore it is recognised that these static ratings are typically conservative and at times may result in under utilisation of circuits." The SO uses seasonal default ratings or variable line ratings for each line.

There three basic types of constraint: maunal constraints, SFT constraints and Special Protection Schemes.

SFT Constraints

From 28-Mar-11 the majority of constraints are formulated by the SO's Simultaneous Feasibility Test (SFT) software tool. SFT formuates constraints that protect circuits in the event of an N-1 failure of the grid. The constraint equation has the form

A x Protected Branch Flow + B x Contingent Branch Flow ≤ RHS

A and B are coefficients calculated by SFT. The RHS limit ensures that if the contingent branch fails, then the loading on the protected branch remains safe during the 15 off-load time, where 'safe' refers to the amount of sag in the line post-contingency.

Manual Constraints

Constraints that are not automatically generated by SFT may relate to Special Protection Schemes (see below) or may be created manually based on studies undertaken offline. Typical examples are the four voltge stability constraints that are there to ensure voltage is stable after a contingency in one of the main load centres, or over long lines.

The Manual Constraints can be permanent or only apply during outages, and are updated at http://www.systemoperator.co.nz/system-operations/security-management/security-constraints (see mid way down this page under Security Constraints and "Manual constraints under SFT ....xls".

Special Protection Schemes

Automatic SPS' are also known as "operational intertripping schemes" and the SPS activates "within the period of voltage recovery following fault clearance". In most cases, SPS' are designed to operate to avoid security violations without the need for additional constraints in SPD. According to the SSF, the following constraint equations relate specifically to SPS':

• Te Mihi Special Protection Scheme, constrain transfer from WRK bus.

1.24 * RPO_TNG1.1 + 0.50 * THI_WKM1.1 <= 315 MW

• Special Protection Schemes at KAW: Generation at KAW bus may need to be constrained in the event of high generation at KAW bus or sudden loss of interruptible loads. Security constraints on KAW are highly dependent on system configuration and conditions.

There are several types of SPS.

Overload Protection Schemes

A.k.a. intertripping schemes, these SPS open circuits that would otherwise overload. They trip the circuit if overloading continues for a specfied minimum time, allowing time for auto-reclose to work.

SPS that involve the action of busbar section circuit breakers to regulate power flows on the system are referred to as Auto Busbar Splitting Schemes (ABSS).

An intertrip that results in a system split is referred to as a System Split Intertrip Scheme (SSIS).

Planned Automatic Load Shedding

Typical examples are a load shedding scheme for Automatic Under Voltage Load Shedding (AUVLS) using voltage-sensitive relays and occuring within an 8 second time period. The load shedding scheme trips feeders in a sequence to bring load below an overload trip setpoint, or to bring voltage to within allowable limits.

Generation Run Back

These SPS automatically reduce generation to a pre-arranged value to remove circuit overloads or generator instability. If the circuit loading is not reduced within an inverse time-dependent period after a runback scheme operation has been initiated, the circuit will be tripped. Currenty Transpower has automatic runback schemes installed at Arapuni, Cobb, Maraetai and Te Apiti. Runback schemes may allow operation to 5 or 10 minute off-load times on monitored circuits rather than the standard 15 minute criteria.

Reactive Devices

The SO dispatches reactive devices so that dynamic reactive reserves are available to provide voltage support for Contingent Events and Extended Contingent Events.

'Current SPS (Feb-16)

Data from System Security Forecast - Transpower Dec-15 http://www.systemoperator.co.nz/documents/reports/system-security-forecast

Information page is the location of further information on the SPS found in Transpower System Security Forecast 2014 Part D - Power System Security Analysis, for that Grid Zone

Transpower Grid Zone GZ1	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ1	Jun-15	Hepburn-Roskill 110 kV Overload Protection Scheme	Normally Enabled	Line protection from Outage	Pg4, section 4.1
GZ1	Jun-15	Penrose Reactor Bypass Scheme	Normally Enabled	Line protection from overloading	Pg4, section 4.2

Transpower Grid Zone GZ2	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ2	Nov-15	Penrose Circuit Overload Protection Scheme	Normally Disabled	Line protection from Outage	Pg3, section 4.1
GZ2	Nov-15	Hepburn-Roskill 110 kV Overload Protection Scheme	Normally Enabled	Line protection from Outage	Pg3, section 4.2
GZ2	Nov-15	Bombay 110 kV System Split Intertrip Scheme	Normally Disabled	Line protection from Outage	Pg3, section 4.3
GZ2	Nov-15	Bombay_Otahuhu Overload Protecton Scheme	Normally Disabled	Line protection from Bus Split	Pg4, section 4.4
GZ2	Nov-15	Penrose Reactor Bypass Scheme	Normally Enabled	Line protection from overloading	Pg4, section 4.5
GZ2	Nov-15	Otahuhu 110 kV Bus Split	Normally Enabled	Line protection from overloading	Pg4, section 4.6

Transpower Grid Zone GZ3	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ3	Jun-15	Arapuni Runback Scheme	Normally Enabled	Line protection from overloading	Pg4, section 4.1	Manages Generation constraints
GZ3	Jun-15	Arapuni Bus Split	Normally Enabled	Line protection from overloading	Pg4, section 4.2	Manages Generation constraints
GZ3	Jun-15	Bombay 110 kV System Split Intertrip Scheme	Normally Disabled	Line protection from Outage	Pg5, section 4.3
GZ3	Jun-15	Arapuni - Kinleith 110 kV Overload Protection Scheme	Normally Disabled	Line protection from Outage	Pg5, section 4.4

Transpower Grid Zone GZ4	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ4	Jun-15	Arapuni - Kinleith 110 kV Overload Protection Scheme	Normally Disabled	Line protection from Outage	Pg5, section 4.1
GZ4	Jun-15	Kaitimaki Intertrip Scheme	Normally Enabled	Overload & line outage protection	Pg5, section 4.2	Manages Circuit overload
GZ4	Jun-15	Tokannu 128 Intertrip Scheme	Normally Enabled	Line protection from Outage	Pg5, section 4.3
GZ4	Jun-15	Te Mihi Intertrip Scheme	Line outage Criteria enabled	Line protection from Outage	Pg6, section 4.4
GZ4	Jun-15	Edgecumbe-Kawerau 1 & 2 Overload Protection Scheme	Normally Enabled	Line protection from Outage	Pg6, section 4.5
GZ4	Jun-15	Edgecumbe-Owhata 2 Overload Proteciton Scheme	Normally Enabled	Overload Protection	Pg6, section 4.6	Manages Generation constraints
GZ4	Jun-15	Kawerau T13 Overload Proteciton Scheme	Normally Disabled	not required	Pg6, section 4.7
GZ4	Jun-15	Maraetai Runback	Normally enabled		Pg6, section 4.8	Manages Generation constraints

Transpower Grid Zone GZ5	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ5	Jun-15	Redclyffe interconnecting Transformers Overload Protection Scheme	Commission 2016	Transformer protection	Pg3, section 4.1

Transpower Grid Zone GZ6	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ6	Dec-14	Hawera Reactor and Automatic Bus Splitting Scheme (ABSS)	Not required	Line protection from Outage	Pg3, section 4.1

Transpower Grid Zone GZ7	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ7	Dec-14	Tokannu 128 Intertrip Scheme	Normally Enabled	Line protection from Outage	Pg4, section 4.1
GZ7	Dec-14	Bunnythorpe - Woodville 1 and 2 Overload Protection Scheme	Normally enabled	Line protection from overloading	Pg4, section 4.2	Manages Circuit overload
GZ7	Dec-14	Woodville 110 kV Bus Overload Protection Scheme	usually Disabled	Bus protection from overloading	Pg4, section 4.3

Transpower Grid Zone GZ8	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ8	Jun-15	Bunnythorpe - Woodville 1 and 2 Overload Protection Scheme	Normally enabled	Line protection from overloading	Pg4, section 4.1	Manages Circuit overload
GZ8	Jun-15	Bunnythorpe - Woodville 1 and 2 Overload Protection Scheme	Normally enabled	Line protection from overloading	Pg4, section 4.1	Manages Circuit overload
GZ8	Jun-15	Central Park Transformer Overload Protection Scheme	usually Disabled	Transformer Protection from overloading	Pg5, section 4.2
GZ8	Jun-15	Mangamaire Overload Trip and Autochange Over Scheme (MGM OTAS)	usually Disabled	Line protection from overloading	Pg5, section 4.3

Transpower Grid Zone GZ9	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ9	Dec-14	Blenheim_Kikawa 110kV Circuit Overload Protection Scheme	Normally enabled	Line protection from Outage	Pg4, section 4.1
GZ9	Dec-14	Kikiwa-Stoke 220 kV Outage Intertrip Scheme	Normally Disabled	Line protection from Outage	Pg5, section 4.2
GZ9	Dec-14	Stoke T7 Outage interip Scheme	Normally Disabled	Transformer protection low generation	Pg5, section 4.3

Transpower Grid Zone GZ10 & GZ11	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ10&GZ11	Jun-15	Bells Pond Waitaki Overload Protection Scheme	Normally Enabled	Line protection from overloading	Pg6, section 4.1	Manages Circuit overload
GZ10&GZ11	Jun-15	Timaru Transformer Overloading Scheme	Normally Enabled	Transformer protection low generation	Pg6, section 4.2

Transpower Grid Zone GZ12	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ12	Dec-14	Coleridge Intertrip Scheme	Normally Enabled	Line protection from Outage	Pg3, section 4.1
GZ12	Dec-14	Hororata Automatic Under Voltage	Normally Enabled	Load scheding from Line outage	Pg3, section 4.2

Transpower Grid Zone GZ13	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ13	Jun-15	Bells Pond Waitaki Overload Protection Scheme	Normally Enabled	Line protection from overloading	Pg6, section 4.1	Manages Circuit overload
GZ13	Jun-15	Roxburgh Export Overload Protection Scheme (REOLP)	Normally Enabled	Line protection from overloading	Pg6, section 4.2	Manages Circuit overload
GZ13	Jun-15	Roxburgh Import Overload Protection Scheme	Enabled Southward Flow	Line protection from overloading (increase Manapouri Generation)	Pg7, section 4.4	Manages Circuit overload

Transpower Grid Zone GZ14	Updated	Scheme Name	Status	Reason	Information page	Comments
GZ14	Nov-15	Manapouri Intertrip Scheme	Normally Enabled	Line protection from Outage	Pg6, section 4.1	Manages Generation constraints
GZ14	Nov-15	Halfway Bush Reverse power Protection Scheme	Normally Enabled	Line protection from Outage	Pg6, section 4.2
GZ14	Nov-15	Roxburgh Import Overload Protection Scheme	Enabled Southward Flow	Line protection from overloading (increase Manapouri Generation)	Pg7, section 4.4	Manages Circuit overload
GZ14	Nov-15	Roxburgh Export Overload Protection Scheme (REOLP)	Normally enabled	Overload Protection	Pg6, section 4.3	Manages Circuit overload