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 (PB Associates) 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 PB Associates 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, PB Associates 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. PB Associates 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 PB Associates at the time of preparation. This report does not purport to contain all
relevant information for all plant discussed. PB Associates 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
Otahuhu Substation Diversity Project : CB configurations
PB Associates
1.
INTRODUCTION
The Electricity Commission requested PB Associates 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 AD1 for Otahuhu
substation, it can be confirmed that there are three back-to-back connections at
the 220 kV switchyard:
1.
2.
3.
Bay 2 – GLN 1 circuit & capacitor bank C29
Bay 6 – HLY 2 circuit & transformer T5
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 connections1, 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. PB Associates 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 : PB Associates 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
Otahuhu Substation Diversity Project : CB configurations
PB Associates
2.2
CONCLUSION
Overall, PB Associates 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 & Beaty2:
•
•
•
•
•
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 PB Associates’ 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
Disconnectors
Earth switches
Surge arrestor
CT
(1)
(3)
(1)
(1)
(1)
2
Standard Handbook for Electrical Engineers, Fink & Beaty, 12th Edition, McGraw-Hill, 1987 - Refer to section 17: Substation
Design.
152299 - CB configuration final.doc
March 2007
3
Otahuhu Substation Diversity Project : CB configurations
PB Associates
•
•
•
•
•
•
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 & Beaty3:
•
•
•
•
•
•
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 PB Associates’ 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
Disconnectors
Earth switches
Surge arrestor
(3)
(6)
(2)
(2)
3
“Standard Handbook for Electrical Engineers”, Fink & Beaty, 12th Edition, McGraw-Hill, 1987 - Refer to section 17: Substation
Design.
152299 - CB configuration final.doc
March 2007
4
Otahuhu Substation Diversity Project : CB configurations
PB Associates
•
•
•
•
•
•
•
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. PB Associates’ 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 m2
•
SBDB:
6000 m2
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
Otahuhu Substation Diversity Project : CB configurations
PB Associates
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
Transpower
Cost – based
on previous
Transpower
submittals4
Transpower
Cost
ODV Cost
PBA Cost
estimate
SBDB Scheme
single bay cost
Number of bays required
to
accommodate
8
circuits
Additional equipment
required
$985,220
$1,000,000
$1,330,000
$1,438,000
8
8
8
8
Bus-tie and bus
section circuit
breakers – three
additional CBs
Bus-tie and bus
section circuit
breakers – three
additional CBs
Additional bays required
for this equipment
Additional
associated
cost
Full number of bays
necessary
$1,398,220
TOTAL COST
2
Bus-tie and bus
section circuit
breakers – three
additional CBs
Bus-tie and bus
section circuit
breakers – three
additional CBs
2
2
2
5
$1,824,000
$1,580,000
10
10
10
10
$9,279,980
$10,200,000
$12,464,000
$13,084,000
$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
$8,544,680
$9,100,000
$11,388,000
$11,872,000
$2,200,000
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
TOTAL COST
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 PB Associates 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
Otahuhu Substation Diversity Project : CB configurations
PB Associates
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.
PB Associates 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