Providing reliable power to the people

An automated load dispatch system provides reliable power and improved quality of life to a remote Alaskan village.

11/10/2011


Providing power to communities is no simple matter, especially when the communities are remote villages in rural Alaska (see Figure 1). Since 2003, Kohler Power Systems has designed and supplied paralleling switchgear to the Alaska Village Electric Cooperative (AVEC), a nonprofit electric utility powering 54 isolated communities in interior and western Alaska. Kohler and AVEC worked together to develop a control scheme that improved reliability and fuel efficiency for AVEC’s new power plants.

Figure 1: Power plant modules are built to withstand the harsh Alaska winters. Courtesy: Alaska Village Electric Cooperative

Each village that AVEC serves typically has three generators of different sizes. For example, Elim is a village of about 300 people located on the northwest shore of Norton Bay on the Seward Peninsula, 96 miles east of Nome. Elim’s three gensets are 200 kW, 350 kW, and 500 kW units (see Figure 2). Because most of the villages that AVEC serves are accessible only by water or airplane, a critical-design requirement for the prime power applications is to minimize the fuel use.

The villagers’ electricity rates directly reflect the high fuel costs. For a residential customer receiving the State of Alaska Power Cost Equalization credit, the current rates vary from 20.36 cents to 21.60 cents per kWh, depending upon the village, for the first 500 kWh. Elim’s current residential rates are 20.87 cents per kWh for the first 500 kWh. From 501 to 700 kWh, the current residential rates vary from 47.99 cents to 72.73 cents per kWh, with Elim’s current residential rate at 58.07 cents per kWh. This rate poses quite an incentive to use less than 500 kWh per month. Nonresidential consumers do not receive Power Cost Equalization, so they pay the higher rate on all the electricity they use.

Advanced solution to advanced requirements

It was discovered early in the design process that the standard generator management schemes commonly used in standby applications would not provide the granularity required to optimize the online gensets. Something more than a simple scheme based on actual load and some online/offline kW setpoints with timers would be required. This situation required a system that minimizes fuel use, while ensuring there is enough spinning reserve to supply the power demand.

The new power plant has an automated dispatch system that matches the online gensets to the load demand—plus a spinning reserve margin. The dispatch system optimizes the online gensets. The dispatch logic calculates a dispatch kW level.

System operation

The system runs the genset—or best combination of gensets—to match their combined kW capacity with the load demand value that the dispatch logic calculates. Gensets are soft loaded onto the paralleling bus and soft unloaded off the paralleling bus. Before a generator is taken offline, it remains running in parallel with the new online gensets for an overlap delay to ensure a smooth transition.

Figure 2: The generator modules for the Elim, Alaska power plant are the three on the right. Courtesy: Alaska Village Electric Cooperative

The dispatch kW level, or the required kW online capacity, is constantly trying to match the kW request value. If the load is steady, the dispatch kW level will eventually equal the total kW request (see “Dispatch kW level calculation”).

The dispatch logic is located in the master PLC and is monitored and controlled using a local touchscreen human machine interface (HMI). A trend chart allows the operator to monitor the actual load kW, the calculated dispatch kW level, and the online capacity. This allows the operator to monitor and fine-tune the automated system. The operator also has the option to turn off the automatic dispatch system and operate the generators manually.

Each genset has its own PLC. The logic for the synchronizer and load sharing is located in this PLC along with the other genset control logic. Analog outputs control the genset’s speed and voltage. The genset PLCs have additional I/O to accommodate gensets with less sophisticated controllers. This provides maximum flexibility and simplifies the use of gensets from different manufacturers with different types of genset controllers. Each genset has its own HMI for monitoring, control, and setup.

It’s about time

The PLCs’ flexibility proved to be useful several months after the initial system start-up. Villagers were adjusting their clocks every few weeks. It was suspected that gensets cycling online and offline was causing the system to eventually deviate slightly from 60 Hz. By adding a frequency trimmer to the PLC logic, the system successfully maintains the frequency at 60 Hz.

The five-section paralleling switchboard consists of a section for each of the three generators, a master control section, and a section containing the distribution breakers (see Figure 3). In order to minimize time on-site, the power plant consists of modular enclosures that are interconnected on the job site. Each generator is in its own enclosure, the switchboard is in its own enclosure, and there is an enclosure for spare parts. There is even an enclosure that serves as living space—complete with a kitchen, bathroom, and a place to sleep—for technicians while they are on the job site for maintenance.

The collaboration between Kohler Power Systems and AVEC produced a power system control scheme that provides flexibility, reliability, and most of all, an improved quality of life to a remote village in Alaska.

Dispatch kW level calculation

 

 

Dispatch kW level calculation

 

Actual bus kW

 

 kW

 

Actual load on the bus in kW

 

Overload margin

 

+ kW

 

Fixed level of spinning reserve in kW

 

Stability margin

 

+ kW

 

Preset kW value added for each genset start, due to increasing dispatch kW level

 

Time-of-day anticipation

 

+ kW

 

Preset spinning reserve kW level, based on time-of-day and day-of-week

 

 

 

=Total kW request

 

The four variables listed above are used to calculate the total kW request

 

 

 

 

 

 

 

Adjusted total kW request

 

Ramp to total kW request

 

Slows down the system’s reaction to load changes

 

Anticipated feeder pick-up load

 

+ kW

 

Extra fixed level of spinning reserve kW added if a feeder breaker is open

 

Under-frequency safety margin

 

+ kW

 

Extra fixed level of spinning reserve kW added if the system experiences an under-frequency condition

 

Dispatch kW level

 

= Dispatch kW level

 

 

 

 

This table shows how the dispatch level kW is calculated. The required variables are explained within the right-most column. However, some of the variables in the table are clarified further by the following:

Stability margin: Whereas kW is added when a genset starts, this value slowly ramps to zero. Stability margin prevents a newly online generator from quickly going back offline.

Time-of-day anticipation: This value is added to the spinning reserve during weekday mornings in anticipation of loads being added.

Adjusted total kW request: Instead of immediately jumping, the adjusted total kW request is always ramping from its current level to match the total kW request. In steady state, the adjusted total kW would equal the total kW request. For example, if the total kW request is 100 kW and the system is in steady state, the adjusted total kW will be 100 kW. If the total kW request changes to 300 kW due to the addition of a large load, the adjusted total kW will ramp from 100 kW to 300 kW. If the total kW request drops back to 100 kW, the adjusted total kW will ramp from its current value toward 100 kW.

Anticipated feeder pick-up load: After the feeder breaker closes, the anticipated feeder pick-up load is an extra fixed value of load that remains for a certain time delay, which can be adjusted by the user.

Under-frequency safety margin: This fixed kW value remains in effect for a certain time delay, which can be adjusted by the user.

(Right) Figure 3: The five-section paralleling switchboard includes—from left to right—three generator sections, the master control section, and the distribution section. Courtesy: Alaska Village Electric Cooperative


Pincus is manager of sales, switchgear systems at Kohler Power Systems. He has worked at Kohler since 1995 and has held various positions in the switchgear department. He has a BSEE from UW Madison and an MBA from UW Milwaukee. Pincus is also a registered professional engineer in the state of Wisconsin.



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