However, this BMS was under-performing and Siemens Building Technologies was commissioned to upgrade the system with open BACnet to increase reliability and cut energy use. Fortunately, four ABB standard drives for HVAC, featuring native BACnet as standard, were already installed on the three air handling units (AHUs). The drive was the first variable speed drive to conform to the BACnet standard, and to receive BACnet Testing Laboratories (BTL) approval by BACnet International and BACnet certificate by WSPCert. As such the drives could migrate seamlessly to the new BMS without any extra engineering or cabling.
A BACnet MS/TP (master-slave/token passing) subnetwork was created using cost-efficient ‘daisychain’ cabling between the drives, along with the installation of a BACnet outer from ABB, within one of the drives. Only one new Ethernet cable was needed to connect the router to the direct digital controller (DDC), avoiding changes to the existing Ethernet backbone. The drive’s built-in serial communication assistant ensures new parameters are entered rapidly, making commissioning of the BACnet protocol straightforward. The BACnet router acts as a media converter, transferring messages between the drives’ BACnet MS/TP subnetwork and the BACnet/IP network. The router can be easily configured using a standard PC and web browser. The router enables operators to access all the drive parameters from the workstation. It also gives the commissioning engineer the option to monitor and control the drive while replacing the controllers. Installation was carried out during office hours and the drives were manually operated without the automation system, thereby ensuring the AHUs operated within the buildings comfort zone at all times.
Reliability is enhanced by using the drives predictive maintenance function via the BMS. If a problem occurs, the system triggers an alarm to alert the relevant personnel. The system is remotely accessible over the Internet via a secure virtual private network (VPN) tunnel. Maintenance personnel can access the system at any time for troubleshooting, as can the building manager for configuring schedule programs. Through BACnet, energy consumption data and other system information is readily available from any workstation.
A planned enhancement to the system is to connect the field devices, such as dampers and pressure sensors, direct to the drives – taking advantage of the pass-through access to the drives’ inputs and outputs – rather than via the DDC. This will provide even greater reliability, as the drives are able to continue operating as stand-alone units even if the operation of the DDC is disrupted. This will also reduce cabling, as the field devices are located closer to the drives than the DDC.