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Home   |  Mobile Substations

Mobile Substations


DESIGN AND DEVELOPMENT OF 110KV MOBILE SUBSTATIONS

Instead of purchasing a completed Mobile Substation, Transpower designed and built one from scratch, drawing on local expertise and experience in the New Zealand power industry. The result was a Mobile Substation that can be deployed at 16 small single transformer substations nationwide, is suited to NZ roads, contains multiple safety features and interlocks, and is completely self-contained.

By Graeme Winthrop, Transpower NZ Ltd

A Mobile Substation is a complete Substation on wheels. Specifically, the mobile substation will connect up to the incoming lines at a host substation, bypass the host substation, and connect directly to the customer feeders (outgoing lines), as depicted in Figure 1. In this way, the host substation can be taken completely out of service.

Figure 1: Typical Mobile Substation Connection
Figure 1: Typical Mobile Substation Connection

The use of a mobile substation allows for maintenance outages at the host substation to be planned with flexibility and opportunity for efficiency. For example, instead of an annual shutdown, Transpower can move to longer shut downs every four years. It will also enable construction work that has been put off due to lack of outages to be carried out, and this work can be carried out over a longer, safer, timeframe. By enabling the entire host substation to be de-livened, maintenance work can be carried out more safely without having to work near live equipment.

The justification for Transpower's mobile substation was based around savings in generation costs and the value of lost load for planned maintenance outages. The safety, emergency supply, reduction in customer disruption and flexibility are important benefits over and above the economic benefits.

PRIMARY ITEMS ON A MOBILE SUBSTATION

The main primary items on a Mobile Substation are the HV circuit breaker, a step down transformer (otherwise known as a supply transformer), LV (incomer) circuit breaker, feeder circuit breakers, current transformers, voltage transformers and surge arrestors. See Figure 2 for a typical single line diagram.

Figure 2 Single Line Diagram of Typical Mobile Substation Equipment and Connection
Figure 2: Single Line Diagram of Typical Mobile Substation Equipment and Connection

BUILD/BUY DECISION

The development of Transpower's first mobile substation was an iterative process, trading off desired features and capacity versus size, weight, cost and technical feasibility. The decision was made early on that this integration process would be better carried out locally to enable closer control and easier input from Transpower, rather than offshore. A close working relationship with suppliers and designers enabled an iterative and collaborative approach rather than attempting to specify all requirements and having the Mobile Substation built offshore.

This collaborative approach enabled more features to be incorporated during the design and construction, helping ensure the end result fully met Transpower's operational, maintenance and safety requirements.

Further advantages of designing and building the Mobile Substation locally include:

  • The equipment on the Mobile Substation is similar to other equipment on the Transpower network, enabling consistent operation and maintenance practices.
  • A secure communication system was developed, tested and installed on the Mobile Substation, enabling remote control from Transpower operating centres.
  • The contractor that constructed the Mobile Substation is also the maintainer, and is intimately aware of the design and features.
  • As Transpower owns the IP, it will be easier and cheaper to build a second Mobile Substation.

 MOBILE SUBSTATION FEATURES

As mentioned in the previous section, the collaborative development approach enabled additional features to be incorporated that made the Mobile Substation more fit for purpose.

Some of the features include:

  1. The ability to supply at 33kV, 22kV or 11kV, enabling the mobile substation to operate at all 16 designated sites.
  2. Associated with this multi-voltage functionality is a system of interlocks to prevent mobile substation equipment or feeders being livened at an incorrect voltage.
  3. The transformer phasing can be reversed to enable the Mobile Substation to be parked under a transmission line in either direction yet able to provide correct phase rotation to the customer.
  4. A specialised 110kV CB has bushings that can rotate out so that the phases have correct phase to phase clearance when in service, but rotate together to keep within the allowable width for transport on NZ roads.
  5. A hydraulically operated security fence was developed that can swing out when the CB bushings are rotated out, and moves into the vertical position when in transit.
  6. An automatic SF6 filling mechanism will fill the 110kV SF6 CB prior to livening, and depressurises the CB prior to transportation. Secure communications was developed and interconnected with the Transpower SCADA system, to enable remote control of the Mobile Substation, and integration into the Transpower grid.
  7. Oil interception is provided via a shallow tray on the Mobile substation that pipes directly in to the host substation oil interception facilities.
  8. All cables are stored on the Mobile substation on cable reels that are motorised, able to be remotely controlled and also ableto be lifted off the Mobile substation and used at ground level.

The mobile substation was built on two trailers in order to incorporate all of the required equipment and features. The heavier trailer (MS1), with the supply transformer, is 76 tonnes and the lighter trailer (MS2), with the 33kV switchgear, is 36 tonnes, not including the tractor units.

MS2 Trailer with MS1 in the background
MS2 Trailer with MS1 in the background

CONNECTION AND SITE MAKE READY

To successfully use the Mobile Substation at a site, the site needs to be made ready to receive it. The connections at the site and preparation required are outlined below.

  • Physical Parking and High Voltage Connections
    The MS1 trailer is first parked under the 110kV transmission line that comes into the host substation. When considering the parking location, the ground stability needs to be considered. Another consideration is access to the rest of the site once the Mobile Substation is in place.
  • Feeder Connections
    Feeder cables are connected from the M52 trailer toconnection points on the lines company network to enable the Mobile Substation to feed the lines company directly.
  • Road
    AccessAlthough the Mobile Substation is suitable for theNew Zealand road system, due to its size and weight, a transportation analysis is still required to ensure that the roads are suitable for such large trailers. Standard switchyard gravel may cause the heavy trailers to get stuck, so mats are carried on the MS2 trailer that can be laid out over the switchyard gravel to enable trailers to drive over the top.
  • Isolation Points
    One of the key advantages of using the mobile substation is that the entire host substation can be de-livened for maintenance or construction purposes. To provide this totally de-livened state, there needs to be an isolation point in the 110kV line away from the host substation and also a break in the feeder away from the host substation. 
  • Typical isolation point in an incoming 110kV Transmission line
    Typical isolation point in an incoming 110kV Transmission line

  • Oil Interception
    The 110/33-22-11 kV 18MVA transformer on the Mobile Substation contains approximately 15,000 litres of mineral oil. For environmental reasons, this must not be allowed to spill out on to the ground or go into the stormwater system. Continued over To accommodate this situation, a bladder or "onion" is transported with the mobile substation, and is connected to the drain pipes along with an oi/water separator. 
  • Connection Arrangement of the Mobile Substation at Murchison
    Connection Arrangement of the Mobile Substation at Murchison
  • Local Service 400V Supply
    The Mobile Substation requires both a connection from and a connection to the host substation 400V system. When the Mobile Substation first arrives, it will require a 400V connection from the host substation in order to keep its systems livened and batteries charged.
  • Communications and Control
    The Mobile Substation connects back to the Transpower operating centres via either cellular or satellite communications. In this way, the Mobile Substation can be remotely controlled much like any other Transpower substation.
  • Lightning Assessment
    A lighting protection analysis needs to be carried out to ensure that the Mobile Substation equipment be protected from lightning strikes.
  • Phasing Diagram
    In particular because of the Mobile Substation's ability to reverse the phase rotation, a phasing diagram is essential to ensure that cables and transformer settings are correct and will result in the correct phasing when the Mobile Substation feeder cables are connected to the lines company network.
  • Protection Settings
    The fault levels and downstream arrangements will vary from site to site and this will affect the settings of certain relays on the Mobile Substation. Therefore, a separate protection setting analysis is required for each site.
  • Earthing
    The trailers need to be earthed and EPR issues need to be assessed to ensure that step and touch potential issues are addressed around the Mobile Substation trailers.
  • DEVELOPMENT AND INSTALLATION

    As with any project, the Mobile Substation development had its fair share of challenges, and I would like to outline a few of these below:

  • Locking Down the Design
    The Mobile Substation development was like a prototype development, with an iterative approach to design and a relatively long design phase. For example, when the project was initiated,there was only to be one trailer and no on board communications. However, as the project progressed, it became apparent that two trailers were required to keep within New Zealand road weight limitations. Once installation was underway, the design needed to be locked down to prevent delays caused by further iterative design modifications.
  • Transformer Failures
    The specialised transformer was originally to use Ester Oil, a type of bio-degradable oil that would not create an environmental incident should there be a catastrophic leak while the Mobile Substation is in transit. However, the transformer failed its partial discharge test five times, delaying the project, and in the end the decision was made to use standard mineral oil.
  • Stoke Access
    Originally, the Mobile Substation was to be commissioned at Stoke Substation, with livening off the 110kV tie line within the substation. A Site Make Ready analysis was therefore prepared for Stoke. The installation contractor decided to double check the access feasibility by having MS1 trailer towed up the steep incline to the 110kV structure. It was found that MS1 trailer was too long and not suitable for the incline, with the wheels leaving the ground at the top.
  • Reduction in Security during Connection
    At the Murchison deployment we encountered a situation where the connection of the Mobile Substation, and associated de-livening of the host substation, reduced security of supply to other parts of the network. In this case, by creating a break between the Mobile Substation and the host substation, a break was formed in one of the two main circuits supplying the West Coast of the South Island. This situation was resolved by adding a temporary bypass around the Mobile Substation.
  • COMMISSIONING AND SUBSEQUENT USE

    The Mobile Substation was commissioned at Murchison Substation, with no loss of supply while taking over supply from the host substation. It has since been deployed at five other Transpower sites enabling transformer replacements, protection replacements and other system projects and maintenance to take place that otherwise would not have been economically feasible. For example, transformer replacements requiring four week outages have taken place at two substations. Use of the Mobile Substation meant there was no need for generation or unnecessary additional works to transfer supply to a second bay.


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