EMO:Constraint Overview

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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.

Contents

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
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