Figure 1. Soft load power transfer switch (SLTS).


Power transfer switching devices are a key element of the modern emergency power supply system (EPSS). Unlike open transition (break before make), delayed transition (center off), or closed transition (make before break), the soft-load power transfer switch combines the best features of each in a very special design.

A soft-load power transfer switch provides closed transition or overlapping transfer between two energized sources (normal and emergency). A traditional closed transition power transfer switch does the same; however, the overlap time is intentionally limited, a passive sync check function is employed, and the entire load shifts in three steps (detect coincidental synchronism, parallel both sources, disconnect from the source you are leaving).

A soft-load power transfer switch has an active synchronizer that controls or drives the emergency source prime mover into synchronism and a load controller to gradually “walk” the load from one source to the other “softly” until the source is unloaded to a pre-determined threshold and disconnected. When transferring from a hot normal source to a hot emergency source, the load controller will drive the prime mover of the emergency source to assume more load until the normal source load is reduced to a specified value, say 200 kilowatts. At this point, the normal source is disconnected and the entire load is supplied by the emergency source.

Conversely, when transferring from a hot emergency source back to a hot normal source, the load controller will throttle back the emergency source prime mover, thereby walking the load off the emergency source softly. At a predetermined load set point, the emergency source is disconnected.

Figure 1 shows a block diagram of a soft load power transfer switch. Note the interface between the synchronizer, load controller, and the unit control panel. These may be separate components, but for clarity simply know that the function is required and interconnected accordingly.

The key components of these type units include;

  • A dual-operator closed-transition transfer switching mechanism designed to UL 1008
  • A power measurement device to provide precise information to a specially designed controller
  • A specially designed controller to regard all power measurement information and precisely control the connected engine generator set in all modes of operation
  • Utility-grade protective devices as required by the utility

Let’s discuss the various operating modes and flexibility of the soft-load concept. Power outages are the main threat to a mission critical environment. As with any other automatic power-transfer switching device, the soft-load device provides an automated engine start, power quality verification, and transfer in less than 10 seconds.

The direct and indirect cost of power is very much in the forefront today as demand for power outpaces supply and more emphasis is placed on environmentally friendly ways to support business-critical infrastructure. Sometimes, augmenting the utility power supply with distributed local generation capacity is desirable. Soft-loading features the ability to maintain the essential base load of a facility during peak demand periods. Results? Lower power consumption from the grid plus reduced demand peaks and their associated charges.

Utility export programs may help the local utility supply other customers and significantly reduce your monthly charges. The same argument can be made for curtailment on demand programs offered by some utility suppliers.

Mission-critical facilities demand reliable power. The soft-load power transfer switch provides the flexibility to place a facility on the emergency power source in anticipation of a power interruption caused by weather, grid overload, etc.

Whenever we connect a local power source to a commercial utility supply, we must be sure to protect the grid. In order to operate safely, a variety of utility-grade protective devices may be required.

Soft-load design has its roots in the gas landfill plants intended to use sustainable methane gas to generate power for landfill operations or utility export. Gas engines in general are “softer” than other fuels used in electrical generating sets. Methane is softer yet and requires compression and pressurization in order to be utilized efficiently. The ability to “softly” walk loads from one source to another is highly desirable in many applications. In general, this type of unit offers flexibility to any EPSS regardless of the type and size of the emergency source.

One size does not fit all. Over the past four columns, we reviewed the four major types of power transfer switches on the market today. With these tools, your design consultant can provide a highly reliable EPSS to meet the challenges of operating mission critical facilities in this ever-changing environment.

The term mission critical has been exclusively used in reference to the data center. That’s changing as many industries become more data-centric. The hospital of today, for example, is virtually connected end to end. With new regulations, increasing liability and pressure to operate more profitably and efficiently, hospitals are literally becoming data centers with patients.

Choosing the correct design and equipment for your mission-critical application is directly related to how long your plant remains viable.