Customer Focus

Milestone PDL drive helps assure power security

Those who suffered the trauma of the central Auckland power supply crisis in 1998 will be pleased to know that PDL Electronics Ultradrive Elite AC variable speed drives will enhance the security of the new permanent power supply link to the downtown area. The Ultradrive contribution to a secure power supply is a milestone for PDL Electronics, too - it includes the 100,000th motor drive to be produced since the pioneer unit was introduced by PDL in 1973.

Even before the failure of old feeder cables in February 1998 forced the Auckland CBD to endure rotating power blackouts, construction had started on a 9.2 km, three metre diameter power tunnel to carry 110 kV cables into the downtown area. The cables in the tunnel will supply most of Auckland’s downtown power demand. The power tunnel is owned by electricity network operator Vector. The tunnel avoids encroaching below private property by following the Southern Motorway and other roads. It is well below ground - some 20 to 100 metres down - and can only be reached through four vertical shafts. The shafts are at the point of supply at Transpower’s Penrose substation, at Vector’s Liverpool St and Hobson St substations, and the main ventilation and maintenance access shaft at Newmarket.

"The Ultradrive contribution to a secure power
supply is a milestone for PDL Electronics, too
- it includes the 100,000th motor drive
to be produced since the pioneer
unit was introduced by PDL in 1973."

The link is due to start feeding the CBD through two 110 kV main circuits shortly. The temporary overhead line installed at the time of the crisis will then be dismantled. The tunnel has a design life of 200 years, and provision has been made for the future addition of extra cables.

Exacting Reliablility Requirements

As well as the HV cables, the tunnel is also fitted with communications, access security, drainage, fire sprinkler and ventilation systems monitored and controlled from programmable logic controllers (PLCs). Because of the exacting reliability requirements, all control systems are fully redundant. Two separate fibre-optic cable loops connect the seven PLCs to each other and to Vector’s control room. Detailed design and the installation of the tunnel services, including communications, ventilation and drainage systems, has been the responsibility of Gooder Ltd.

The ventilation system is a critical reliability feature of the whole project. The 1998 crisis was caused by the failure of older direct-buried cables due to overheating. Special attention has been paid to dissipating heat in the tunnel to keep the cables at an optimum operating temperature, even when fully loaded. Maintaining adequate air flow through the concrete-lined tunnel is essential, to enable the 205 MVA transmission capacity rating of each circuit to be maintained under all conditions. If the ventilation was to fail, heat buildup would force a reduction of load, and possible power cuts, within three hours. Efficient ventilation will also clear fumes arising from maintenance work.

"The Ultradrive Elite was chosen for its suitability
for controlling high power motors
in a rugged industrial environment..."

Air enters through remote-controlled dampers at Penrose and Hobson St, and is sucked up through the main access shaft at Newmarket, near the midpoint of the tunnel. The tunnel is sealed, and maintenance workers enter through airlocks. The two centrifugal exhaust fans are at ground level. One fan is sufficient to create a seven kpa air pressure difference inside the tunnel, to pull air four kilometres from the Hobson St end and five kilometres from Penrose.

A simple 5-button keypad enables complete local control.

Maintaining airflow, while adjusting for fluctuating power loads and ambient temperatures, requires powerful fans with sophisticated controls. The Newmarket facility is equipped with a 355 kW UE660 motor for each fan, to produce a maximum airflow of 5 m3/sec. Electronic control of each motor enables the fan speed to be closely matched to the heat being generated in the tunnel, saving energy by reducing airflow during periods of low load. Normally only one fan will run, with the other on standby. This will maintain an average flow around 2 m3/sec to keep the tunnel temperature between 40 and 50 deg C. Personnel access for maintenance and inspection will occur during periods when the electrical load is low, and tunnel temperatures can be reduced by over-riding the normal settings.

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Reliable Fan Motor Control

Control of each fan motor is entrusted to a PDL Ultradrive Elite variable speed drive. The Ultradrive Elite was chosen for its suitability for controlling high power motors in a rugged industrial environment, and conforms to the IP54 enclosure protection standard. Each drive is mounted on a PDL plinth and tied back to the wall.

The complete Vector Tunnel fan drive setup: 355 kW motor,
PDL Ultradrive enclosure, and the dynamic braking resistors at upper right.

The Ultradrive receives a 4 to 20 milliamp analogue speed regulation signal from the PLC. This signal is stored in an internal register every 15 seconds. If the analogue signal drops below 4 milliamps, the VYSTA software assumes a fault with the PLC, accesses the register, and sets the motor speed to the last register value. The fault is signalled back to the control room, and an operator can be dispatched to take over manual control until the fault is fixed. As Kelvin Burch, Gooder’s Project Supervisor, explains "the problem (if the analogue signal fails) is purely overcome by VYSTA - there is no other way to do it, really".

Tunnel services, including ventilation, are supervised through graphical control room displays.

In the event of a fire in the tunnel, the flow of air must be stopped - fast. A fire alarm or sprinkler activation in the tunnel is communicated via the PLC to the Ultradrive unit. This initiates dynamic braking, producing regenerated power which is dumped through resistors located on the wall above the Ultradrive cabinet. DC-hold braking will then keep the fan impellers stationary until the dampers are fully closed. Each fan can be stopped from maximum speed within 45 seconds, minimising the risk of a fire getting out of control.

The Ultradrive Elite was introduced to the PDL Electronics product range in June 1998, and quickly established itself in the high-power motor control market. Available in three versions according to power requirements, it now accounts for 40% of all PDL variable speed drives sold. There are five other drive models in the PDL Electronics range, to enable the full spectrum of industrial variable speed drive requirements to be met.

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The Key to Success...

The key to the success of the Ultradrive Elite is not only the rugged reliability of the hardware, but also the easy adaptation to specific environments made possible by the VYSTA software which each drive uses. VYSTA is a standard software platform which is customised to the customer’s requirements by PDL’s team of application engineers, installed and thoroughly tested on-site.

" rugged reliability...
& easy adaptation to specific environments..."

VYSTA, and the communications hardware interface built into each Ultradrive Elite, receives and processes analogue and digital signals from a variety of fibre optic and copper wire input interface options. Local manual control is also provided, using a simple and compact five-button membrane keypad and LCD mounted on the cabinet. The functionality of this local keypad can be customised through VYSTA.

Steve Gallagher, of PDL Electronics, demonstrates the easy access
to the rugged IP54-standard equipment enclosure.

Information, including detailed technical specifications of the full range of PDL Electronics motor control systems, can be found on the web at:

http://www.pdlelectronics.com/products/ultra_elite.html