Ensuring safe control of backup power

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There is a more effective way of managing power than using PLCs, argues Deif’s René Kristensen. He builds the case for multi-master controlled power management systems. 

Mission critical sites must be able to trust their backup power systems, and a reliable control solution for detecting and acting on power issues is crucial. While power line communication (PLC) solutions have traditionally been used to control such systems, multi-master controlled power management systems (MM PMS) are claimed to provide several advantages, according to a new white paper. 

Published by Deif, a global supplier of power control solutions for decentralised power generation, the paper says that the multi-master architecture of MM PMS means that these systems are extremely reliable – even if a controller fails – providing uninterrupted control for applications where power failure is not an option (see deif.com/whitepapers/multi-master-power-management-system).

Traditionally, PLC solutions have been used to control critical power systems, with one PLC monitoring the entire power system and switching from grid power to backup power sources, such as a genset or battery, in case of grid power failure. 

While this solution is widespread, Deif claims there are some disadvantages: 

  • If the controlling PLC fails, there is no power control until the PLC is repaired or replaced 
  • Troubleshooting and repairing PLCs requires in-depth technical knowledge of the PLC solution used, making the installation vulnerable to prolonged shutdowns 
  • Complete PLC solutions including hardware, special dongles or connectors, software licenses, and training are often comparatively expensive 

“Critical sites using PLCs often experience problems if one of the controllers break down, or they want to add gensets and expand the system. The PLC program may have been created by one software programmer – if this person is no longer available, this can create issues, as the system is not standardised. 

“It is not easy for a new software programmer to come in and change an existing PLC system when adding new sources such as transformers and gensets,” explains Deif Solutions’ sales manager, René Kristensen. 

A key difference between a power management system controlled using a PLC and one controlled with a MM PMS is the number of controllers used. In a PLC-based system, one PLC monitors and controls the entire installation. With a MM PMS, there is one controller for every power source and associated breaker, and optionally one controller for every additional important switch such as the bus tie breaker (BTB). Each controller is capable of acting as master controller in the system, as all controllers are interconnected over a communication network such as a CAN bus. 

An MM PMS setup provides three major benefits: 

  • Improved security and reliability through redundancy and features such as close before excitation 
  • Simpler system design and serviceability, with a standardised software platform and features such as system emulation 
  • Financial savings through lower purchase and opex

“Compared to a PLC system, a multi-master controlled power management system is much safer,” argues Kristensen. 

He explains that a multi-master controlled power management system with intelligent controllers offers controller redundancy, improving the security and reliability of the critical power system. Intelligent controllers offer features that enable sites to get full backup power online quickly. In one respect, an MM PMS critical power system performs just like one controlled by a PLC: if a power source fails, the controller will quickly change to a backup power source. 

The real safety and reliability benefit in using a PMS becomes apparent if a controller fails. In an MM PMS, the multi-master design philosophy means that even if one controller breaks down, the control system will not fail. It is only the individual controller and associated breaker that does not work. 

When controllers are interconnected over a CAN bus, the master controller role is automatically assigned to the available controller with the lowest CAN ID on the network (for example, 01). This controller is assigned a so-called token. All controllers constantly exchange availability information over the CAN bus. If the master controller fails and no longer responds over the CAN bus, the token is instantly and automatically moved to the available controller with the next-lowest CAN ID (in this case, 02) which then becomes the new master controller. 

This solution ensures that there are no power or control interruptions, and the multi-master setup therefore provides much greater security and reliability than a single-master PLC solution. (Redundant PLCs are possible but it is a very costly solution). When a controller is down, an alarm message is generated to alert the operator that the faulty controller needs to be repaired or replaced. 

The CAN bus setup is very robust. Two independent communication channels (A and B) ensure that even if one channel becomes unavailable (because of a cable break, for example), the system is able to use the other channel for communication. 

In order to ensure continued communication in case of an incident on one channel, the cabling for the two channels must be physically separate. 

In many critical power applications, the time it takes to restore full power from an emergency backup source is a key consideration. For hospital applications, for example, EU regulations require full backup power to be available within 15 seconds of a grid blackout while the US FPA stipulates a 10 second limit. In data centre applications using rotary UPS solutions, backup power needs to be restored before the UPS stops spinning, and the faster you get backup power online, the less power you need to restore in the system. 

It therefore makes a great difference how quickly full power is restored, and this is another area where MM PMS solutions with intelligent controllers can offer additional advantages. The CBE (Close Before Excitation) feature found in Deif’s AGC-4 controller, for example, reduces the time it takes to get full backup power with several gensets. 

With a flexible hardware platform, designing an MM PMS system is comparatively easy: the system designer only needs to specify one controller per power source and breaker, and connect the controllers over a communication network such as the CAN bus. Once everything is connected, the designer can set up all system parameters, often with user-friendly PC software. The designer or installer can then connect the PC to any controller in the network through Ethernet or USB and upload the system setup to the controllers. 

The built-in intelligence of an MM PMS can also include the load side of the system. Traditionally, PLCs have been used to carry out load side power management, for example disconnecting and reconnecting non-critical loads as needed to ensure sufficient power capacity. By using intelligent controllers on the load side, it is possible to design an integrated system in which power and load side controllers communicate without the need for complex and costly PLC programming. 

Controllers such as the ALC-4 from Deif can be used to provide a configurable load control logic where the designer only needs to set load levels for disconnection and reconnection. The integrated solution means that the designer often does not need separate measurement devices. 

One additional financial advantage of using an MM PMS is that intelligent controllers often include built-in features that optimise power consumption. These features go beyond ensuring constant power, adding value to the customer’s business. The AGC-4, for example, offers peak shaving that adds additional power for peak load situations. The peak shaving feature detects when installation power consumption approaches the upper limit set by the grid power provider. Exceeding this limit usually means incurring a much higher fee from the provider. To avoid this, the controller activates and readies one or more backup power sources such as gensets. All power consumption in excess of the limit set by the grid power provider is handled by the backup power source; usually a much more economical solution than purchasing grid power at higher rates.

“The industrial sector in Sweden particularly likes this functionality. If there is a black-out, however, the system is still able to continue with the gensets in a peak situation. The technology also allows facilities to export power to the grid and we are seeing increasing demand for this capability in countries such as Denmark. Sites across Europe are seeing significant financial benefits,” comments Kristensen. 

The real financial saving with an MM PMS lies in avoiding unplanned downtime. However, Kristensen concludes: “Ultimately, critical sites can avoid the failures that can occur with PLC systems. Standardised software means that many different engineers can programme the controllers; they aren’t locked into one person’s unique idea and approach. The technology is easy to use, flexible and scalable, which means you can easily add gensets and transformers (or remove assets), while the system is operational.”

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