Parsons Brinckerhoff Associates OTAHUHU SUBSTATION DIVERSITY PROJECT: REVIEW OF THE CIRCUIT BREAKER CONFIGURATION PROPOSED BY TRANSPOWER A report prepared for 23 March 2007
Disclaimer Notice Report for the Benefit of Electricity Commission This Report has been prepared exclusively for the benefit of the Electricity Commission. Parsons Brinckerhoff Associates Ltd () will not be liable to any other persons or organisation and assumes no responsibility to any other person or organisation for or in relation to any matter dealt with or conclusions expressed in the Report, or for any loss or damage suffered by any other persons or organisations arising from matters dealt with or conclusions expressed in the report (including without limitation matters arising from any negligent act or omission of or for any loss or damage suffered by any other party relying upon the matters dealt with or conclusions expressed in the report). No person or organisation other than the Electricity Commission is entitled to reply upon the Report or the accuracy or completeness of any conclusion and such other parties should make their own enquiries and obtain independent advice in relation to such matters. Reliance on Data In preparing this Report, has relied on information supplied by Transpower and gathered from a number of sources including public domain and proprietary data services, internet sites, news services as well as parties involved in the industry. Some of the pricing information used is deemed confidential and may compromise suppliers if made public. In addition, it may also adversely influence future tender processes if used to set precedents or benchmarks for item prices. This information therefore has to be excluded from public versions of this document. Furthermore. any projections are estimates only and may not be realised in the future. No blame or responsibility should be attached to any of these sources of information for any factual errors or misinterpretation of data in this Report. has not independently verified the accuracy of the information with which it has been provided and has not audited any financial information presented in this Report. Limitations This Report covers analysis and capital cost estimates relating to transmission lines and substations and is based on the facts known to at the time of preparation. This report does not purport to contain all relevant information for all plant discussed. has made a number of assumptive statements throughout the Report, and the Report is accordingly subject to and qualified by those assumptions. The estimates were prepared at a desktop level only and did not involve any site surveys, geotechnical analyses or detailed design work. The uncertainties inherent to capital cost estimation for large construction projects, especially where such estimates are prepared at a desktop level only should therefore be taken into account by any party relying on this report. Parsons Brinckerhoff Associates Ltd. Level 3, Revera House 48-54 Mulgrave Street Wellington Phone : 04 499 1000 Fax : 04 916 6514
1. INTRODUCTION The Electricity Commission requested to review section 1.1 and section 5 of Transpower s document titled Response to the Electricity Commission Queries on Grid Upgrade Plan, dated 07 March 2007. In particular we were requested to provide an opinion of the statements made by Transpower regarding existing back-to-back connections at the OTA switchyard, and the switchyard configurations. All references in this review report to specific sections of a document, refers to this Transpower report. 2. REVIEW 2.1 SECTION 1.1: BACK-TO-BACK CONNECTIONS With reference to the Transpower SLD TX25609 Rev AD 1 for Otahuhu substation, it can be confirmed that there are three back-to-back connections at the 220 kv switchyard: 1. Bay 2 GLN 1 circuit & capacitor bank C29 2. Bay 6 HLY 2 circuit & transformer T5 3. Bay 9 HLY 1 circuit & PEN 5 circuit Following our analysis of the issues raised in section 1.1 of the Transpower document with regard to back-back connections 1, we conclude as follows: First Issue: Transpower notes that any failure within the zone between the circuit breakers and disconnectors will trip out both connected circuits. PBA response : It is indeed not possible to isolate a fault in this zone without tripping out both circuits. therefore agrees with Transpower on this matter. Second Issue: Transpower notes that one circuit breaker cannot be taken out of service unless it is properly isolated from the system, and in order to do this with a back-to-back connection, both circuits have to be taken out of service. PBA response : agrees with Transpower. 1 Transpower SLD: TX25609 Rev AD: Otahuhu Substation 220/22/11 kv Single Line Diagram (Operating) 152299 - CB configuration final.doc March 2007 2
2.2 CONCLUSION Overall, supports the view that to add further back-to-back connections to the existing switchyard is not an acceptable solution. Reliability and availability of this substation are key requirements for its ongoing development and using back-to-back configurations compromises this. 3. TRANSPOWER SECTION 5 : ADDITIONAL INFORMATION SWITCHGEAR CONFIGURATIONS 3.1 SINGLE BREAKER DOUBLE BUS (SBDB) CONFIGURATION (SECTION 5.1) We note the following key points on SBDB features from Fink & Beaty 2 : The configuration allows transfer of a feeder from one bus to the other without de-energising the feeder circuit. A bus-tie circuit breaker connects the two buses. A bus fault, or maintenance on a bus, will require disconnection of all the feeder circuits connected to that bus. Operating with a closed bus-tie breaker requires a very selective bus protective relaying scheme. Disconnect switch operation becomes quite involved, with a high possibility of operator error or injury, and possible shutdown. This scheme has been a standard installation in New Zealand for many years and most existing major substations are of this configuration. It provides a reasonable trade-off between cost and reliability and has generally provided adequate service levels (bus faults are relatively rare). However, given the major impact of a bus fault, for key substations this is not a preferred option. Fink & Beaty notes that internationally this configuration is not considered good practice for important substations. Costs Based on recent cost estimates for SBDB schemes for assessment of Transpower s Otahuhu proposal, the indicative cost estimate for ONE bay includes the following equipment: Structures Busbar Circuit breaker (1) Disconnectors (3) Earth switches (1) Surge arrestor (1) CT (1) 2 Standard Handbook for Electrical Engineers, Fink & Beaty, 12 th Edition, McGraw-Hill, 1987 - Refer to section 17: Substation Design. 152299 - CB configuration final.doc March 2007 3
VT (1) LV cables AC/DC systems Protection and metering Controls Bay set-up costs Based on this the total cost for one bay, with one circuit per bay is $1, 438,000. 3.2 BREAKER AND A HALF (1½ CB) CONFIGURATION (SECTION 5.2) We note the following key points on 1.5CB features from Fink & Beaty 3 : Two circuits are protected by three circuit breakers, where each circuit connects between a pair of breakers. Normally all circuit breakers are closed and both buses are energised. A circuit is tripped by opening the two associated circuit breakers. It is possible to operate with both buses out of service with a source connected opposite a load across the bay. One bay accommodates two circuits. Breaker maintenance can be performed with no loss of service to adjacent circuits. Either main bus can be taken out of service for maintenance and (single) bus failure does not require any feeder circuit to be removed from service. This scheme is generally more expensive than the SBDB configuration but is superior in flexibility, reliability and safety. However, for substations with a small number of bays, as demonstrated below, it may actually be more cost-effective to use a 1½ CB arrangement, given that the need for a bus-tie and bus-section circuit breakers is avoided. (For substations with larger numbers of bays, the effect of these fixed costs diminishes as a proportion of the additional cost per 1½ CB arrangement vs. the single CB SBDB bays.) Costs Based on recent cost estimates for SBDB schemes for assessment of Transpower s Otahuhu proposal, the indicative cost estimate for ONE bay (which is defined here to include two feeders), includes the following equipment: Structures Busbar Circuit breaker (3) Disconnectors (6) Earth switches (2) Surge arrestor (2) 3 Standard Handbook for Electrical Engineers, Fink & Beaty, 12 th Edition, McGraw-Hill, 1987 - Refer to section 17: Substation Design. 152299 - CB configuration final.doc March 2007 4
CT (3) VT (2) LV cables AC/DC systems Protection and metering Controls Bay set-up costs Based on this, the total cost for one bay, two circuits per bay, is $2, 968,000. 3.3 COMPARISON OF THE CONFIGURATIONS In section 5.3 of Transpower s reply to the Commission, a comparison is made between the SBDB and 1½ CB schemes. view on the comparison is presented below. 3.3.1 Space With reference to Transpower s figures in section 5.3, it is easy to see the space impact of using a 1½ CB scheme as opposed to a SDBD scheme. Overall we agree that the 1½ CB scheme requires less substation space than the equivalent SDBD scheme. For eight circuits, because the 1½ CB scheme accommodates two circuits per bay it requires roughly half the length of the SBDB scheme with the same number of circuits. However the width of the 1.5CB scheme is approximately twice that of a SBDB scheme and requires the switchyard layout to accommodate both incoming and outgoing circuits on both sides, whereas the SBDB scheme only needs circuit access to one side. Also there are height increases in the switchyard when using the 1½ CB scheme over the SBDB scheme for AIS installations. Due to the nature of the arrangement of 1½ CB schemes the incoming/outgoing lines pass over the busbar and circuit breaker below to enable connection at the correct point of the switchyard. This requires additional gantry structures to support the lines making the 1½ CB switchyard approximately double the height of the SBDB scheme. The comparative land area require for an eight circuit switchyard of each type is as follows: 1½ CB : 5300 m 2 SBDB: 6000 m 2 This is based on a bay width of 12.2 m (40 foot) for all bays except for the bus section bay required for the SBDB switchyard, which is 15.3 m (50 foot) across. The bay depth for the 1½ CB switchyard is 107 m and the SBDB is approx 48 m. 152299 - CB configuration final.doc March 2007 5
3.3.2 Costs Table 1 below provides a summary of the total costs associated with an eight circuit switchyard of each type. This comparison does not take into account the cost of the substation site. SBDB Scheme SBDB Scheme ODV Cost Transpower Cost Transpower Cost based on previous Transpower submittals 4 PBA Cost estimate single bay cost $985,220 $1,000,000 $1,330,000 $1,438,000 Number of bays required to accommodate 8 circuits Additional equipment required Additional bays required for this equipment Additional associated cost Full number of bays necessary Bus-tie and bus section circuit breakers three additional CBs 8 8 8 8 Bus-tie and bus section circuit breakers three additional CBs Bus-tie and bus section circuit breakers three additional CBs Bus-tie and bus section circuit breakers three additional CBs 2 2 2 2 $1,398,220 $2,200,000 5 $1,824,000 $1,580,000 10 10 10 10 TOTAL COST $9,279,980 $10,200,000 $12,464,000 $13,084,000 1½ CB Scheme Single bay cost (2 feeders) Number of bays required to accommodate 8 circuits Additional equipment required Additional bays required for this equipment Additional associated cost Full number of bays necessary $2,136,170 $2,300,000 $2,847,000 $2,968,000 4 4 4 4 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 4 4 4 4 TOTAL COST $8,544,680 $9,100,000 $11,388,000 $11,872,000 Table 1: Cost Summary for an eight circuit switchyard of each type. 4 Refer to Transpower letter from Tim George dated 6 July 2006 Auckland Grid Upgrade Proposal Component Costs. This letter contains 220 kv substation component costs which were reviewed and accepted by at the time. The information was used here to build up a substation bay cost for each type of switchyard scheme. 5 Assumed - based on total cost, cost of single bay and number of circuits required. 152299 - CB configuration final.doc March 2007 6
3.4 CONCLUSION There are definite advantages in using the breaker and a half scheme over the single breaker double bus arrangement for large and important substations such as Otahuhu. These benefits include greater flexibility, reliability and safety over the SBDB scheme and for a relatively small switchyard (in terms of the number of circuits) there is not only a saving in space but also a capital cost savings for the 1½ CB scheme. therefore agrees with Transpower s decision to adopt the 1½ circuit breaker configuration in preference to the SBDB configuration for the proposed Otahuhu Substation Diversity Project. 152299 - CB configuration final.doc March 2007 7