DISTRIBUTED GENERATION

Finding Surplus Electric Power in Traditional sources

Not too terribly long ago, electric utilities appeared capable of producing increasing amounts of electrical energy to feed the apparently insatiable demands of North American consumers. Today, the entire question as to whether or not sufficient generation exists to fulfil existing demands is placed in doubt. The prospect of additional generating plants being constructed in time to provide interruption free service for all consumers is questionable. A notable example being that of the State of California, where demand now exceeds generation and a series of 'rolling blackouts' became inevitable.

Emerging Technologies
The burden of providing increased generation is being transferred to the consumer. 'Distributed Power', meaning generation that is relatively small in size and located at its point of use, is no longer merely a buzzword. A couple of new technologies are poised to participate in the burgeoning need for more power. Fuel cells, which produce electricity from natural gas by means of a catalytic process, are one example. Fuel cells typically are sized from about 45 kW up to about 300 kW. Provided that fuel and air are supplied to the unit, electricity is generated instantaneously. The fuel cell is able to cope well with dynamic load changes up to its maximum capacity.

Another example is the so-called micro-turbine, a high-speed miniature gas turbine that converts natural gas, sour gas or even bio-mass waste gas into electrical energy, by driving a generator. Power outputs of these devices are currently in the range of about 25-75 kW. The turbine system does have a couple of shortcomings. First, the time from being provided with a start signal to actually achieving power output may be as long as two minutes. Second, turbines are intolerant of overloads. Even a slight overload will cause the system to shut down to protect itself from over temperature damage.

Both of the new technology solutions are relatively environmentally friendly and adapt themselves well to many residential and small business energy needs. Both systems also produce considerable heat that may be recovered and used in other processes. On a less positive note, volatility in the cost of and supply of natural gas, will play a major role in the success of either of these emerging systems.

Finding Surplus Power In Traditional Sources
Other, more traditional untapped sources of electrical power generation, are the standby generator sets that are installed in many commercial and institutional buildings. These machines generally use diesel engines as their prime movers and may use diesel grade oil or natural gas as a fuel source. Other standby units may use large gas turbines as the motive source, running on a wide variety of fuels. In general, emissions from standby generator sets are higher than the emerging alternatives.

At the same time, the power output available from individual diesel generator sets may be much greater, ranging from a few kiloWatts to several megaWatts. Diesel powered units are relatively tolerant of short time overloads. They are capable of accepting full load within some 10 seconds after being commanded to start. Larger gas turbines, as in the case of micro turbines, are essentially incapable of accepting overloads and suffer the same long time to start limitation.

Codes and standards for the construction of new buildings often include a minimum requirement for standby power for life safety reasons. Many end users actually choose to exceed the minimum standard in order that they may continue to carry on business during periods of utility power outages.

The Use of 'Standby' Generators
So it may be, that in the short term, the large and growing installed base of standby diesel generator sets will afford the greatest opportunity to provide the on-site generation needed to supplement essentially fixed utility capabilities. Let us look at some possibilities that would allow for the efficient use of these underutilized machines.

The first and simplest use of a standby generator is to use it to simply shed load by using its associated automatic transfer switch to start the generator and transfer the already connected emergency loads to it. Starting may be programmed on a timeof-day basis or as a function of facility load demand. Using the generator in this way can trim peak demands and lower utility demand charges for the user. This is likely not the most efficient use of the generator because emergency requirements do not usually load the generator to its most efficient capacity. The use of a standard automatic transfer switch will result in a brief (from about 40 milliseconds to as long as 10 seconds) power outage when transferring to and from the engine generator. The actual time of outage is purely dependent on the design of the transfer switch as well as the nature of the loads connected to it.

A second possibility is to use a Closed Transition Transfer Switch (CTTS) instead of the standard, open transition automatic transfer switch (ATS). In Canada, at the present time, the Canadian Electrical Code, Part 1 and the Automatic Transfer Switch Standard CSA C22.2 No. 178, do not permit the use of closed transition devices. Therefore approval will have to be granted by the utility having jurisdiction to connect a CTTS to its network. An alternative is to use a Solid State Transition Transfer Switch (SSTTS). The SSTTS does not physically interconnect utility and emergency power sources and is actually a 'load uninterrupted' device, switching in less than 1/2 cycle. Either of these alternatives will provide transfer without any apparent interruption, provided both sources are live. This operation is otherwise known as 'Seamless Transfer' due to the lack of disturbance to electrical loads. Properly designed, no speed adjustment is required to be made at the engine generator, permitting the use of less sophisticated governor controls.

Generator Control Developments -- Soft Load Transfer
An automatic transfer switch development known as a 'Soft Load Transfer Switch' (SLTS) can further enhance load shedding operations. An SLTS essentially uses CTTS technology but actively changes the amount of load accepted by the generator. Again, because of present restrictions imposed by Canadian Electrical Codes and Standards, it is essential to secure utility approval prior to beginning construction.

Almost universally, engine generators are incapable of accepting their optimum load in one step. The use of any non-interactive automatic transfer switch will require that the connected load be within the capability of the machine to accept a block load. Therefore, the generator's peak efficiency cannot be realized. An SLTS solves this situation by gradually increasing the load applied to the generator over a pre-determined time. Gradual acceptance of the load over a short interval avoids problems with block loading and permits the machine to operate at its most efficient level.

An SLTS will operate in any one of several modes. 'Islanding Mode' where transfer between sources is conducted in a closed transition soft loading mode. Once the programmed load level is achieved, the SLTS disconnects the load from the utility. It will also operate in 'Base Load Mode' where the optimum load level for a given generator set is determined, the generator is loaded to that point and run in parallel with the utility. The third mode is 'Import/Export Mode' which is similar to the Base Load Mode, with the added ability to maintain optimum loading of the generator set through its ability to import power from, or export power to, the utility.

SLTS Operational Modes
Islanding Mode satisfies the requirements of those utilities, or users, who wish to transfer loads in a seamless fashion, but then operate in isolation until it is time to retransfer. Actual power generation will be dependent on the demand of loads connected to the emergency power system. The soft loading ability operates when transferring in both directions. This avoids any block loading problems with the generator as well as minimizing utility disturbances. Installations that operate in this mode may require a supplemental switching device for use when utility power is lost.

The Base Load Mode offers more efficient use of the engine generator set. By operating the generator continuously in parallel with the utility, all facility loads are connected to the generator, not merely the emergency loads.

This enables the machine to constantly run at its optimum output level, providing a fixed amount of power while demand on the utility rises and falls to accommodate total facility power demand. When the requirement for on site generation ceases to exist, the SLTS disconnects from the generator, which is then shut down. When facility loads may vary below as well as above the most efficient level of generation, Import/ Export Mode permits a constant power output from the generator. It allows the utility to supply additional power to the facility during periods of high demand as in the case of the Base Load Mode. More importantly however, with the utility's agreement, during periods of low facility demand, the excess capability of the generator may be exported to the utility. This has two benefits. First, the generator is allowed to operate at its highest efficiency and second, revenue is generated by the sale of excess power. Agreements with specific utilities may determine time periods or peak demand loads that on site generation is used to augment utility power. At other times the SLTS connects loads only to the utility.

As is the situation with any standard automatic transfer switch, during such times that on site generation is not being used, the SLTS functions if an unexpected utility outage occurs. The SLTS will signal the generator to start and then transfer emergency loads to it. Once utility power is restored, the SLTS can again function in any of its operational modes.

All three operating modes are completely compatible with currently available generating systems including fuel cell and micro turbine technology. Seamless transfer is inherent in their design and operation. Protection against inadvertent overloading is especially important with the newly developed generation systems as well as traditional gas turbine generators. In line with modern technology, SLTS units should have the ability to interact with building management systems as well as be remotely controlled. Internet access to these devices readies them for immediate remote controlled action during times of utility initiated power curtailment.

Properly designed SLTS equipment contains all of the necessary protective relaying equipment to safeguard the utility and the owner alike. Attention must be given to possible failure modes such as the loss of either source when operating in parallel. The equipment should be designed to operate and communicate with generator control systems supplied by a variety of engine generator OEMs.

Conclusion
Soft Loading Transfer Switch equipment may well prove to be valuable as a first line of defense against the threat of declining electrical power availability. The technology is worthy of consideration to help solve power shortage problems on existing projects. It should also be given very careful consideration for use in new construction. In all likelihood, a generator is a requirement. Using that generator at its most efficient will provide an attractive payback.