Transend advises interested parties that for the 2010 winter period between May 2010 and October 2010 Transend intends to extend its Network Support arrangements with a Tasmanian generator. Transend will also seek an option to further extend this arrangement for a period in 2011. Transend considers that demand side options to address the identified issues would not be cost effective given the duration and magnitude of the network support requirements.
Transend’s southern transmission system, in particular the Liapootah - Chapel Street transmission line, may become constrained during the winter periods under certain generation scenarios and transmission network configurations. The constraint may potentially lead to transmission overloading and risk of voltage collapse of the southern system. Non-market ancillary services in the form of generation network support have been contracted by Transend since 2005 to assist in the maintenance of power system security in the southern region. The network support services also provide de-icing of the Gordon – Chapel Street 220kV transmission line. The previous network support arrangements ended in October 2009.
Transend invites comments from interested parties by 26th February 2010 regarding the proposed extension of network support arrangements for the winter period of 2010 and the option of further extending the arrangements for the winter period of 2011.
Please forward any comments by email to Mr Ajay Maharaj, Manager Network Operations, at ajay.maharaj@transend.com.au.
Further information about the network support requirements is outlined below.
NETWORK SUPPORT REQUIREMENTS
Transend is in the process of augmenting the transmission network supplying the southern region of Tasmania; until that work is complete the load demand of southern Tasmania can exceed the firm limits of the transmission network. At such times of high southern network load, predominately over winter periods, the electricity network is at real risk of voltage collapse in the event of loss of either Liapootah – Chapel St 220 kV transmission lines. These two transmission lines are the only 220 kV feeds into the southern region of Tasmania, and the loss of either line during periods of high southern load will result in the southern voltage reducing to a level from which voltage collapse and overloading of the 110 kV lines feeding into the southern region will occur.
Network support is used when the power transfer on the Liapootah–Chapel Street 220 kV transmission lines is constrained due to voltage or thermal limits during winter. Generally this happens at times when load in the southern region exceeds 547 MW and Gordon Power Station is not running. During the period between 1st May 2009 and 15 October 2009 the southern region load exceeded 547 MW for 907 hours.
The Gordon-Chapel St 220 kV transmission lines are also at risk of “icing up” during snow storms. The resultant asymmetrical build up of ice results in additional weight on the lines causing unacceptable sag and reduction in clearance between the phase conductors. Issues also arise when the conductors de-ice causing conductor clashing.
To mitigate these risks to supply, Transend entered into a Network Support Arrangement with a Tasmanian generator, whereby the generator would:
- Provide sufficient generation to prevent voltage collapse in the Tasmanian Southern Region during times of high southern load, and
- Provide generation from Gordon Power Station when requested to prevent ice from forming on the Chapel St – Gordon 220 kV transmission lines (referred to as de‑icing), so the lines remain available for service.
NETWORK SUPPORT AGREEMENT
The agreement that expired in October 2009 required the contracted Generator to provide sufficient generation support at all times during the support period. A constraint dispatch service was deemed to have been provided by the generator when the southern load exceeded 547 MW (up to an agreed maximum level of 760 MW). There was a fixed daily charge applied for this service.
The agreement noted that if southern region capacitor banks were unavailable, the upper limit was reduced.
AEMO manages southern Tasmanian generation to ensure that voltage and thermal constraints relevant to the southern region do not violate. Under the network support agreement, a penalty clause applied to the generator if a relevant constraint violated for more than three consecutive dispatch intervals.
Transend also requested the generator to provide the de-icing service on the Chapel St – Gordon 220 kV transmission lines. The agreement provided for up to 4 GWh of energy for the de-icing service before any additional payment was required.
Transend intends to extend the network support arrangements to cover the 2010 winter period between May 2010 and October 2010, with an option to further extend this arrangement for a period in 2011. The agreement will be renegotiated on similar terms to the previous agreement.
