There is significant interest among electric utilities in distributed generation resources for deregulated and regulated service territories in the United States. In fact, many industry experts believe that the most aggressive competition will likely come from new technologies capable of meeting customer energy needs on-site. One electric utility, Chugach Electric Association Inc., is a good example of this movement. It recently teamed up with the U.S. Postal Service to install one of the largest fuel-cell projects in the world.
Chugach Electric is Alaska's largest utility. In addition to serving its own retail load, Chugach sells power to five other utilities under various wholesale contracts. Alaska is by far the largest of the 50 states, with a land mass equal to approximately 20% of the entire lower 48 states. There are more than 90 utilities in Alaska, with the vast majority consisting of isolated electric systems served by small diesel-generating plants. Most of Chugach's plants consist of natural gas-fired combustion turbines.
Under a special contract with the U.S. Postal Service, Chugach Electric provides electricity on a 24/7 basis to an international sorting center in Anchorage, via a 1MW fuel-cell system. The fuel-cell output provides full electrical service to the center with excess electrical output flowing back to the Chugach electric distribution grid. Heat from the fuel cells is connected via glycol loops to the building boiler plant and used to provide hot water to the center year-round.
The project consists of five phosphoric acid fuel cells, a pump house (to forward hot water to the post office heating system), a nitrogen system (for the safe shutdown of fuel cells when necessary), and a site management system (SMS)(photo). The SMS contains the switchgear, static transfer switch, and four control modules. The control modules include the load share module (LSM), site supervisory controller (SSC), load shed controller (LSC), and the static switch controller itself.
The historic peak of the post office's electric load is 800kW. Each of the five fuel cells is rated for 200kW. This configuration allows one fuel cell to be out of service for maintenance while still having enough capacity to meet the full load requirements of the facility.
While electric grid outages in this particular area of the Chugach Electric system are rare, the area is experiencing significant growth and construction. This has led to more frequent electric feeder “dig-ins,” resulting in voltage transients that can cause complex post office sorting equipment to misoperate.
For the postal center, reliability and power quality are nearly synonymous. The static switch controller, working in coordination with the LSM in the SSC, detects grid transients and seamlessly transfers to fuel-cell power in just 4 ms.
High-speed transfer ensures uninterrupted operation of post office equipment during even minor grid transients. Should the electric grid go outside preset limits (either voltage or frequency), the system will transfer from grid parallel to grid independent in ¼ cycle. The post office's engineering staff originally requested this seamless transfer capability, and it became one of the chief design objectives of the project.
Interconnection and Monitoring
Each of the five fuel cells is a prepackaged, self-contained unit, which operates unattended using pipeline natural gas fuel. The 480V, 3-phase electric output of the fuel cells connects to the SMS switchgear through a 1.7kVA K-rated, delta-wye isolation transformer.
The SMS switchgear consists of an SCR static switch controller rated at 2000A continuous. It also consists of 480V metal-clad circuit breakers that allow for maintenance bypass and, when necessary, provide a visible electrical opening for isolating the fuel-cell system from the electric grid (Fig. 2, on page 48).
The static switch controller functions in coordination with the LSM. This ensures the fuel-cell power plant, when commanded to transfer from grid parallel to grid independent, goes from 1MW output to load share/load following in less than 4 ms.
The SSC (a PLC-based control) achieves on-site control of the plant by capturing data and forwarding it to a human machine interface (HMI) and Chugach's SCADA system. The HMI resides in the SMS building on a desktop computer. The facility operator uses it to monitor systems, trend various plant functions, and issue local control commands. For example, the operator can command the fuel-cell plant from grid parallel to grid independent and back, or from one kW set point to another.
The fuel-cell manufacturer also has a monitoring software package that allows the operator to monitor the subsystems of each individual fuel cell, as well as troubleshoot alarm conditions, change set points, and anticipate maintenance needs. Similar in concept to other power plant control rooms, the HMI and software provide the operator with information to make decisions and develop operating strategies.
At the postal sorting center, the project typically operates unattended, so a remote HMI also was developed. The remote HMI resides on a laptop computer, which the operator can carry to other locations. It's equipped with special programming that permits it to function as the site HMI via a standard modem from any location with an analog telephone line.
Chugach Electric's Power Control Center monitors and controls the entire project using its SCADA system. Using a leased copper line from the control center to the SMC, Chugach monitors alarms and controls the sorting center like any other power plant in the electric supply system. The fuel-cell plant can be dispatched through either automatic generation control or given a set point to hold for both kW and power factor.
While in grid-parallel mode, the project and load are subject to the same electrical system transients that occur on any electric grid. However, with the tight operating settings of the control systems, the power supplied to the postal facility is expected to approximate IEEE 519 power quality standards. This feature can only be measured over time, so a power monitor was installed in the SMS switchgear to allow for power quality monitoring by both post office officials and Chugach Electric.
Commissioning and Testing
Initial testing involved simulating grid transients on the distribution system and transferring the fuel-cell plant output from grid parallel to grid independent using a load bank that replicated the postal center load. Following the completion of these separate tests, new tests were conducted with the system connected to the postal center load through the SMS switchgear.
With the postal center's sorting systems operating, the static switch controller was commanded to transfer from grid parallel to grid independent and back. The sensitive loads continued running through the transfers without interruption. This verified a seamless transfer and demonstrated that the system functioned within specification (Fig. 1, on page 44).
Next, post office loads were intentionally manipulated while the equipment operated in grid independent. This was done to measure the response of the fuel-cell plant output. Frequency and voltage remained within limits and grid independent operation met project specifications. As a whole, project testing demonstrated successful shielding of the equipment from normal and abnormal grid transients.
There are undoubtedly other locations throughout the U.S. that could benefit from the lessons learned during this project. Depending on the application, some facilities may not require a high-speed static switch controller to assure continuity of electric service, whereas others will. In any case, projects can be developed on a modular basis and configured to meet site-specific needs.
This project is relatively new, having been commissioned last August. Long-term reliability assessments at this point are difficult to determine, but designs for the project include a “6 nine's” plus level of reliability.
As of this writing, it has operated 36 times — all in response to electric grid transients or disturbances. There have been static switch operations due to construction contractors accidentally digging into underground feeders and at least two lightning-related operations. But the vast majority of the static switch operations were due to normal transients associated with an active dynamic electric grid.
The project goals included several firsts in fuel-cell distributed generation deployment:
Operate 5 IFC/ONSI PC25TM model C phosphoric acid fuel cells as a single-power plant in parallel with an operating electric grid.
Prove that a multi-unit, fuel-cell project can be operated as a utility distributed generation resource via a SCADA system.
Confirm that a high-speed static transfer scheme can separate the fuel-cell plant from the electric grid, carry the customer load during grid transients, and resynchronize after the grid restabilizes.