ELECTRICAL: Power Play

At the new Royal Bank Centre in Mississauga, Ontario, on-site generators work in tandem with the utility and can handle 100% of the load.

Completed this spring, the $200 million Royal Bank Centre is the flagship of the 25-acre Meadowvale Business Park located at the intersection of highway 401 and Mississauga Road west of Toronto. The project consists of two nine-storey office towers totalling 76,178 square metres (820,000 s.f.), as well as a parking structure for 3,000 cars.

While the building may be considered state of the art in many respects, two innovations make the project unusual: namely, a significant on-site electrical power generation plant, as well as a system to mitigate the effects of electro-magnetic fields.

The initial program called for 50 per cent of the building’s electrical load to be powered by emergency standby generators to support “mission critical” processes. Certain business processes of the bank had to be continuously free of any power interruption — 24 hours per day/7 days per week/365 days per year.

This requirement presented several challenges. Firstly, the nature and specific location of the power loads could not be accurately defined in the early stages of the project. Mergers had taken place at the bank leaving the new organization in somewhat of a state of flux. Furthermore, given the long lead times required for design and construction of a building of this type, the users could not accurately predict the specific nature of their business in the future, nor could they accurately assign the activities or number of personnel.

Secondly, the design had to allow for flexibility so that in the future the power loads could be relocated anywhere within the complex. With these criteria, access to the standby system had to be at any point within the building.

The traditional method of providing a standby system would entail a virtual duplication of the electrical distribution system within the building — one system powered by the utility, the other powered by the standby generators feeding a series of distributed transfer switches. It became apparent, however, that merging the standby system and the utility system at the point of entry into the building would not only be a more practical solution, but also a less costly one.

Shared distribution

In the system that evolved the standby generators are sized to handle 100% of the building load and can interface with the utility at the point where the utility enters each building. From that point onward the distribution system throughout the buildings is a single system, supported at the head end by either the utility or the on-site generator plant.

Due to pending deregulation of the electricity supply market in Ontario, utility companies are becoming much more flexible in terms of the conditions of supply. As electrical consulting engineers, Mulvey & Banani began to negotiate with Mississauga Hydro to develop a parallel generation scheme — one in which the buildings’ generators would operate in parallel with the utility to satisfy the building load. Four prime power rated 1250KW (1500KVA) diesel-fueled generators were installed. The prime power rating would allow the generators to be used on a continuous basis as opposed to on a standby rating which would limit their time of operation.

The generators are connected to the primary bus through a synchronizing switchboard at the same point in the distribution system as the utility power transformers. The generators can assume any portion of the load up to 100%. Since they are fully synchronized with the utility, the source of power is completely transparent to the load, i.e. the load continues to operate in a normal fashion regardless of the source. The two sources are combined as one feeding into the common distribution system. This approach offers the building operators numerous advantages.

In the event of a utility power failure, the building and its mission-critical facilities are fully supported by the standby plant. The costly duplication of power distribution components is eliminated. In fact, 100% of the building can be supported at virtually the same cost as a distributed system, which would support only 50%.

The operator may engage the standby plant as a peak-shaving device to reduce the demand on the utility. This may become extremely advantageous should power be sold on the spot market at some time in the future.

The operator may engage the standby plant during a critical operation or if an electrical storm is anticipated to ensure that power will remain undisturbed.

The cost and maintenance of distributed transfer switches have been eliminated.

Should the utility market continue to evolve as envisioned, excess power capacity can be sold back to the utility and transmitted into the utility grid.

Life safety system

The introduction of the on-site generator plant introduced a complication. Since the generators are supporting 100% of the load, they would also satisfy the building code requirements to support the life safety systems. The code mandates that life safety systems must be fire rated for two hours. In this case, since the life safety systems and other building loads are being fed from a common system, all the main feeders have to be fire rated.

The synchronization point occurs at the main service entry to each building. For convenience, their main mechanical and electrical services are located at the perimeter in the basement level. The building load above grade, however, is fed via the cores at the centre of each tower’s footprint. Consequently, the main feeders need to traverse the buildings horizontally for a considerable distance, about 90 metres, in order to reach the riser points at the cores. The entire feeder length consists of four runs of three single conductor 500 MCM cables for each tower.

The obvious choice was to use mineral insulated cable due to its inherent fire rated properties. The magnitude and physical size of the combined feeder runs made encasement in concrete or drywall impractical.

Mitigating electro-magnetic effects

The feeder route from the main electrical room at the perimeter of each building to the cores occurs in the basement level at the underside of the ground floor. All apparatus that carries current creates an electro-magnetic field (EMF) in its immediate vicinity. These electro-magnetic fields may cause interference with sensitive electronic components. There is also some suggestion that prolonged exposure to electro-magnetic fields may be hazardous to human health.

In many buildings, ground floor space is largely devoted to lobbies and circulation spaces that do not contain sensitive equipment and are not occupied by personnel on a continuous basis. In this project, however, the ground floors have a significant office component. Consequently steps needed to be taken to mitigate the effects of the electro-magnetic field.

Mulvey & Banani has worked extensively in the past with specialist firm C-Intech, who were engaged to assist in the EMF mitigation. To shield the feeders in the traditional way would entail completely wrapping the entire length of feeder with a specially designed high-grade alloy metallic material. This method was dismissed quickly as being too costly and impractical given the magnitude of the cabling involved. Rather, C-Intech designed an EMF mitigation compensator installed at the supply side of the feeder. This compensator mirrors the conductor current in the cable sheath by creating an equivalent circuit.

In a normal circuit for most cables, inductance and resistance are of the same order of magnitude resulting in an inductive current creating an external flux field around the conductor. The use of a mineral insulated cable reduces the sheath resistance substantially, thereby partially reducing the external flux field. Introducing the compensator eliminates the cable inductance and replaces it with a large compensator inductance, which drives the current through the sheath. The result is a reduction in external EMI of approximately 350:1, which is the order of mag
nitude necessary to reduce the magnetic field to less than 5 milligauss at 3 feet from the cable. Working with Pyrotenax, the supplier of the mineral insulated cable, the specialists reduced the magnetic field virtually to zero, which allowed the building operator to have complete flexibility in configuring the ground floor.

We incorporated many other notable electrical features, namely 2-750KVA parallel redundant uninterruptible power supplies (UPS); parabolic louvre lighting to meet the Illuminating Engineering Society recommended practice for lighting spaces containing computer video terminal screens; and raised access flooring throughout for distributing the power and communications cable to work stations. However, the on-site parallel generation scheme and the fire rating and EMF mitigation of the main feeders set the project apart from most others.CCE

Project: Royal Bank Tower, Meadowvale Industrial Park, Mississauga, Ontario

Electrical consultant: Mulvey & Banani International (Myron Washchyshyn, P.Eng., Joe Berardi, Anne Francoise-Hayman, P.Eng.); C-Intech (EMF mitigation)

Mechanical: The Mitchell Partnership

Structural: Stephenson Engineering

Project developer: Penreal Capital Management

Architect: Adamson Associates

Electrical contractor: Plan Group/ Bell Canada