Special Protection Schemes are means by which circuit loadings, voltage or frequency can be controlled after an event on the power system which might otherwise cause an overload of transmission equipment (eg circuit or transformer).
The following is information that we have published on special protection schemes
|Management of Redclyffe Interconnecting Transformers [ pdf 143.78 KB ]|
|Update on management of Redclyffe T3 and T4 rating, updated 23 Dec 2015 [ pdf 314.82 KB ]|
|Hangatiki Te Awamutu Information Published on 15 Jul 2016 [ pdf 237.66 KB ]|
Special Protection Scheme Outlines
|Arapuni South Generation Runback Scheme||Oamaru-Waitaki Circuit Overload Protection Scheme||Timaru 110kV T5 –T8 Overload Protection Scheme|
The Oamaru-Waitaki circuit overload scheme is designed to reduce loading on the 110 kV circuits between Bells Pond, Black Point and Waitaki following a contingency. This scheme replaces the former Bell’s Pond-Waitaki overload protection scheme.
The Black Point-Waitaki circuit has reached its summer thermal capacity for an Oamaru-Waitaki 2 contingent event. Loading on this circuit is impacted by loads at Oamaru and Black Point and the voltage set point at Waitaki.
The pumping station supplied from Black Point is expanding, increasing offtake at Black Point in 2 MW steps up to 18.5 MW, from the existing 10.5 MW load. As regional loads increase there is potential for the Black Point-Waitaki circuit to overload for an Oamaru-Waitaki contingent event. This depends on a number of factors, in particular, Oamaru loads.
The Bells Pond-Waitaki circuit has an existing Special Protection Scheme on it. This is designed to detect an overload on the Bells Pond-Waitaki circuit, determine if it was the Oamaru-Waitaki 1 circuit that tripped, and open an appropriate circuit breaker at Studholme to stop through transmission to Timaru.
The dairy factory at Bells Pond is expanding and a large irrigation scheme was commissioned last summer. There are plans to expand the scheme. This will significantly worsen the Bells Pond-Waitaki overload. This is a particular problem following an Oamaru-Waitaki 1 contingent event. In that case the current scheme function (removing the connection to Studholme and Timaru) is not sufficient to remove the overload. Another stage of action is required to reduce load at Bells Pond if the Bells Pond-Waitaki circuit is still overloaded following the initial SPS action.
The upgrade means the load shedding functionality is added to the original grid reconfiguration scheme. Consequently, the scheme will reduce load at either Bells Pond or Black Point, depending on the circuit overloaded.
The Arapuni South Generation Runback scheme will reduce loading on either of the two 110 kV circuits between Arapuni and Kinleith to their circuit ratings, following a contingency.
Following the commissioning of a 8 MW milk drier at Lichfield in the 2016/17 summer, the Kinleith–Tarukenga circuits can exceed their n-1 capacity when there is low generation at Kinleith and/or Arapuni.
A solution for that contingency is the Arapuni–Kinleith generation runback scheme which will automatically reduce generation on the 110 kV Arapuni South bus post contingency. The scheme will allow more generation to be dispatched on the Arapuni south bus pre-contingency, to prevent the Kinleith–Tarukenga circuits from exceeding their n-1 capacity.
This scheme is similar to the existing Arapuni Runback scheme (which protects the Arapuni–Hamilton circuits).
Current loading on the Timaru interconnecting transformers has exceeded their n-1 thermal capacity limit.
An outage of a 220 kV Ashburton-Timaru-Twizel circuit will result in the Timaru interconnecting transformers overloading at times of high demand at Timaru and Temuka. The overload is much greater if Tekapo A generation is unavailable. Managing such an outage will require the System Operator to manage load pre-contingently at Timaru and/or Temuka to manage the post contingency loading on the Timaru interconnecting transformer banks.
A special protection scheme is already in place at Studholme to prevent overloads on the Bells Pond-Waitaki circuit for the same contingency. This scheme ensures Studholme is supplied from Waitaki. However, it also removes the infeed to Timaru from Waitaki, resulting in a higher loading on the Timaru interconnecting transformers. This scheme is being updated in another Transpower Grid Owner project (the Bells Pond-Waitaki overload protection scheme).
The Timaru 110kV overload protection scheme project involves installing a special protection scheme at Timaru to avoid the need for pre-contingent load management by the System Operator by automatically reducing load post contingency at Timaru. This post contingent load management scheme will be designed with a delay to allow customers to perform targeted load management.
The special protection scheme at Timaru will detect overloads on the Timaru interconnecting transformers and reduce load at Timaru by:
- first sending a signal to Alpine Energy to allow Alpine Energy to perform targeted load management using ripple signalling, and
- after a suitable delay, sequentially open feeders (to an Alpine Energy- nominated order) to reduce load as needed to remove the overload.