Figure 1: Single Line Diagram of Asfordby Test Track 132/25 kV Substation and Asfordby 132/11 kV Substation

Figure 2: Asfordby Test Track Feeder Station

Table 1: Interface Responsibilities

Design Concept

By the time we were invited to participate as advisors and design checkers, the contractor, Alstom Transmission and Distribution Ltd. (AT&D) had already provided the preliminary draft design diagram and equipment support information to the client, ATL, as a proposal. The traction power transformers were already on order. That meant the overall design needed to be actually ongoing and developed during the tight program schedule.

For this unorthodox approach, a co-operative and co-ordinated team effort would be required from all partners—from the simulation and modelling specialists through to the various contractors operating on site. Our team managed the co-ordination of the design development, producing interface responsibilities and chairing technical meetings with the parties involved to that end.

The single line diagram in Figure 1 shows how the 25 kV power supply is derived from the 132 kV tower at the RJB intake substation and delivered via a 1.1-km (0.66-mile) overhead transmission line to the trackside feeder station (Figure 2). The interface matrix provided in Table 1 on the following page gives an idea of the companies involved and their respective responsibilities, as co-ordinated by PB.

Development and Stages of the Work

Time was of the essence in terms of providing a supply to the test track in a few months, so it was necessary to work on the basis of implementing various installation stages in line with equipment availability and EME/RJB permitted outages. Stage 1 had to involve a direct bus connection to the primary side of RJB’s 132 kV/25 kV distribution transformer that temporarily relied on protection through an existing fault thrower backed up by the EME network. Other stages were planned to connect the standby traction power transformer and its associated equipment, as well as the developments described below.

132 kV Metering. It became apparent during design/co-ordination meetings that commercial metering of the supplies could not take effect at 25 kV, as this would involve a second user. RJB owned the site and already had an agreement with EME, so only one or sole user would be legally acceptable. Metering would have to take place at 132 kV, meaning additional switchgear and protection would be necessary, as shown in Figure 1. This development placed added pressure on the design and installation phases and constituted a second installation stage. Furthermore, check metering was also included to assist in the monitoring of consumption relative to the test track.

Problems Due to Negative Phase Sequence. Studies and measurements by EME showed existing high, out-of-phase, negative phase sequence phenomena on their network. The introduction of the traction loads from the test site would exceed the recommended requirements of Engineering Recommendation (ER) P24 when added to the existing background levels. The fault level at the Asfordby main intake substation was significantly low enough to maintain a negative phase sequence that was above the requirements.

One option was to install a new 132 kV grid site in the vicinity to increase the fault level. This solution would take at least five years in terms of planning, design and installation, however, so did not provide a remedy in the short term. Phase balancers, although less expensive in relation to a new grid site but known for their effectiveness, were considered as an alternative. There was sufficient space adjacent to the RJB mining 132 kV/11 kV substation site. Therefore, another future but final installation stage would need consideration.

Introduction of Phase Balancing. The design principle and objective of a phase balancing system is to eliminate completely the negative phase sequence currents by adding one or more other single-phase loads. The most orthodox or obvious way of achieving this and correcting the unbalance that results from a single-phase load is to equalise the three phases by adding appropriate single-phase loads into the other two phases. This is not easy to achieve practically, especially for a variable or dynamic load. The real and reactive power of the traction load must be monitored and then the appropriate control signals generated to the thyristor controlled reactors. This will then provide the required value of reactive power output in each phase. Independent control of each phase of the balancer is needed to obtain the necessary correction of the single-phase load.

Phase balancers generally take about 18 months to design and manufacture from when the various technical parameters are known. The size and layout of the equipment is quite formidable. Fortunately, there was knowledge of an existing balancer that could be modified and adapted, thereby having the advantage of reducing some of the lead-time. Connections for the phase balancer are shown in Figure 1.

Earthing and EMC Issues Within the Area of the Light Maintenance Depot. The 25 kV overhead line catenary power supply was originally going to be wired up to a distance remote from the light maintenance depot. The building’s internal services would be in accordance with and signed off as compliant with the IEE regulations. Later requirements dictated that the 25 kV traction power supply would be brought into the building, however. The signalling and telecommunications engineers then raised concern over this change of requirement because their equipment, which was originally designed to be housed in rooms within the light maintenance depot, might now be subjected to potential traction faults in terms of touch voltages and EMI/EMC issues.

Power Up and Tests. A test procedure document and test plan was conceived by PB in advance of the power-on date and made available to the client and the relevant disciplines. Stage one of the power supply design was completed on time, prior to delivery of the first vehicle for ceremonial use. Power was switched on in a controlled manner late at night via an approved switching procedure involving PB, EME Border (overhead catenary installation specialists) and AT&D, the main mechanical and electrical power supply equipment designer and installer.

Load Tests. Although one of the Class 390 vehicles was delivered and thus perceived available for initial load tests, sufficient steady load in terms of continuous current could not be successfully drawn. A contributory factor was the not-fully-commissioned lighting, air conditioning and heating that could have provided the appropriate minimum “pick up” values and thus prove the protection relays. This problem was rectified and verified at a later date along with other measurements to satisfy the EME and ATL.

Figure 3: Principle of Current Distribution - No Booster Transformers

Figure 4: Principle of Current Distribution - Main Line and Supply Booster Transformers

Figure 5: Distribution of Percentage Traction Current Prior to Insertion of Mains Booster Transformer

Mains Supply Booster Transformer. Initial current measurements revealed that a larger than normal percentage of return current was travelling back to the substation via 11 kV cable sheath earths in the vicinity of the light maintenance depot and RJB’s coal preparation plants. Earth return currents may share up to 15 percent to 20 percent of the total current delivered by the substation site. Although load currents were low due to the use of a non-fully-commissioned train’s auxiliaries static load, a 54 percent share of the return current via earth was ascertained by current measurement transformers, with only 46 percent flowing back in the return conductor path (Figure 3). A means to overcome this phenomenon had to be developed and applied in the medium term before the loads became significant.

A supply booster transformer has the capability of encouraging the return currents to flow in the return current system (Figures 4 and 5). Although other power supply isolation methods were considered, this option proved to be more economic and less disruptive. A supply booster transformer was thus acquired and installed inside the trackside feeder station immediately after the completion of stage one. This application was successful in performing the desired effect and encouraged the currents to flow through the orthodox return current conductor system route.

Technology Impact

Strictly speaking, the technology used in the design applications does not have an impact, as such, on PB because it is generally included within the human resource and expertise of its personnel. One could argue, however, that there is an impact in the sense that working with this technology broadens or bolsters our experience base. The impact of the technology to the project is, for example, in the introduction of a mains booster transformer, paramount in finalising a successful design whilst being cognisant of safety and economics. Moreover, such elements of this technological know-how contributed “added value” to the project.

There were no “new” technical developments to be perceived as such, but the introduction of the mains supply booster transformer and the phase balancer, the resolution of the earthing issues and how they came about were innovative on the part of our key team members. Particularly unique was the fact that all this phenomena occurred on the same project and under very tight time scales for resolving such issues.

The Future

At the time of writing, there are still the remaining design stages yet to take effect. Stage 2 involves the installation of the second traction transformer, 2-phase 132 kV circuit breakers and 3-phase 132 kV circuit breakers with their metering facilities. A final stage will include the installation of the phase balancer. These remaining stages are currently ongoing and will run on until the end of the year, thereafter to provide what is perceived to be a reliable and secure power supply, not only for the testing of tilting trains but also for the variety of other vehicles that come off the production line. Moreover, a British test site will be available to put them through their paces.

End Note
Hopefully, this article is helpful and interesting for those working not only in the area of traction power supply, but also for those involved in the realms of technical project/engineering management. Our team was interfacing with a variety of contractors and designers who all worked extremely hard to satisfy the client’s requirements.