Special Protection Systems

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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

Hangatiki Te Awamutu Information Published on 15 Jul 2016 [ pdf 237.66 KB ] 

Special Protection Scheme Outlines 

MTN-WGN Circuit Overload Protection Scheme

Oamaru-Waitaki Circuit Overload Protection Scheme
Arapuni South Generation Runback Scheme Bunnythorpe-Mataroa Circuit Overload Protection Scheme
Timaru 110kV T5 –T8 Overload Protection Scheme RDF T3/T4 Transformer Overload Protection Scheme

MTN-WGN Circuit Overload Protection Scheme

The addition of Waipipi (WPP) 130 MW wind farm connected to the WVY 110 kV bus via a single-circuit line has created the potential to overload either of the BPE_MTN_WGN circuits, one for the loss of the other.  This will occur only for some generation dispatch and load offtake scenarios.

An SPS is needed to avoid pre‑contingency generation constraints being imposed to prevent post‑contingency overloading of the MTN‑WGN circuits directionally from WGN to MTN only.

The scheme is designed to measure directionally the current flowing through WGN CB92 & CB62 (BPE_MTN_WGN_1 & 2 circuits) to MTN.

The scheme has 2 independent sets of components that work in parallel. The Overcurrent portion of scheme progresses from a runback to tripping (first at WPP, then at WVY) at 95% of circuit rating to correct post-contingent overloading while the Governor Action component only runs back generation when circuit rating exceeds only 90% of circuit rating to correct pre-contingent overloads.

The Overcurrent portion of scheme has a 3 stage approach to alleviating an overload on the circuit:

  • Stage 1 runs back WPP generation with a runback signal after 4 seconds of circuit load (WGN to MTN) exceeding 95% of rating.
  • Stage 2 sequentially trips 4 strings of wind turbine generators at 0.25 Second intervals (approximately half of the WPP generation)  via internal customer breakers after a full 20 seconds of  exceeding 95% circuit rating.
  • Stage 3 trips off WPP generation at WVY 110 kV bus CB112 after a full 21 seconds of exceeding 95% circuit rating.

The Governor Action portion of  scheme has a single stage approach to alleviating an overload on the circuit:

  • Stage 1 runs back WPP generation with a runback signal sent immediately for circuit load (WGN to MTN) exceeding 90% of seasonal rating.

For an outage on BPE_MTN_WGN_1 or related protection, disable the BPE_MTN_WGN_1 portion of the scheme is disabled. Similar for circuit 2. This prevents a potential inadvertent scheme operation.

Oamaru-Waitaki Circuit 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.

Arapuni South Generation Runback Scheme

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).

Bunnythorpe-Mataroa Circuit Overload Protection Scheme

The purpose of the Bunnythorpe–Mataroa Circuit Overload Protection Scheme (BPE–MTR COPS) is to allow a higher level of north transmission capacity from the Taranaki and Wellington regions into Whakamaru.  The scheme removes the need to apply pre-contingency generation constraints in these regions to manage post contingent overloading of the 110 kV Bunnythorpe–Mataroa–1 circuit.

A series reactor installed on the Bunnythorpe-Mataroa-1 circuit at the Mataroa end will also alleviate these constraints.

The scheme has a two stage approach to alleviate an overload on the circuit

  • first tripping the reactor bypass (if not already bypassed), with the reactor acting to limit any overload of the Bunnythorpe–Mataroa circuit
  • after a suitable delay, Mataroa-Ohakune is tripped to remove any remaining overload. Mataroa load would then be supplied from the south, while Ohakune, National Park and Ongarue will be supplied from the north.

Timaru 110kV T5 –T8 Overload Protection Scheme

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.


RDF T3/T4 Transformer Overload Protection Scheme (TOPS)

Electricity flows into the Hawke’s Bay can be constrained by the Redclyffe interconnecting transformers (RDF ICTs).  
A special protection scheme, the Redclyffe T3 & T4 Transformer Overload Protection Scheme (RDF T3/T4 TOPS), has been put in place to help manage this. 
The scheme detects overloading on the Redclyffe T3 and T4 220/110 kV  interconnectors for power flow from the 220 kV to the 110 kV.  If an overload is detected, the scheme will reduce load to remove the transformer overloading.  
On detecting a transformer overload for power flow into the Hawkes Bay 110 kV system, the scheme will initiate the following:

  • After 5 seconds, send a signal to Unison Networks and Eastland Energy requesting automatic load management.  
  • If the overload remains and the other transformer has “just opened”, after 4 minutes and 50 seconds of detecting the overload, send a ‘Critical Action Required’ signal to Unison Networks to reduce load.  
  • If the overload remains and the other transformer has “just opened”, after 5 minutes of detecting the overload trip all RDF 33 kV feeders simultaneously.

The scheme reduces the risk of total loss of supply to the Hawke’s Bay regional Grid Exit Points at Redclyffe, Fernhill and Tuai, but is not designed to guarantee there will be no total loss of supply.
The scheme also has the ability to manage a de-rating of a transformer caused by oil pump failure. In this instance maximum capacity into the Hawkes Bay will be 75 MVA.