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 Manaual 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 avaoid security violations without the need for additional constraints in SPD. aAccording to the SSF, the following constraint equations relate specifically to SPS':

• Te Mihi special protection scheme, constraints on lines north from WRK 220 kV (Before new WRK-WKM C 220 kV circuit commissioned)

Winter: -1.23 * ATI_OHK.1 + 0.41 * THI_WKM1.1 <= 431 MW

Summer: -1.23 * OHK_WRK.1 + 0.39 * THI_WKM1.1 <= 428 MW

-1.23 * ATI_OHK.1 + 0.41 * THI_WKM1.1 <= 431 MW

Winter: -1.23 * ATI_OHK.1 + 0.40 * THI_WKM1.1 <= 431 MW

Summer: -1.23 * OHK_WRK.1 + 0.39 * THI_WKM1.1 <= 428 MW

• 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 detailed in Section 2.1. Generation at KAW bus may need to be constrained in the even of high generation at KAW bus or sudden loss of interruptible loads. Security constraints on KAW highly dependent on system configuration and conditions

There are sevarl types of SPS.

Overload Protection Schemes

A.k.a. overload protection schemes or 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 involves 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 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.