Distributed Power Poised to Surge into the Grid

by Chris Kambouris,
Manager, Distributed Power Generation,
Ecostar Electric Power Train and Power Conversion Systems
Dan Jones, Incremotion Associates, Inc.

The dwindling power reserves in California will further compel the development of alternative energy sources from both renewable and nonrenewable sources. Rolling blackouts and power interruptions place a premium on developing and employing responsive distributive sources for electric power. Many energy experts believe that electricity generated by on-site manufacturing plants will supply the power to such businesses as supermarkets, fast food restaurants, office buildings, hospitals, medium-sized factories and other facilities with lower power requirements. These plants could also be used to compliment the utility grids during peak demand periods when electric power costs soar. The need for rolling blackouts to conserve power would be mitigated or eliminated by use of these distributed power generators.

Micro turbines

Natural gas is today's best choice for large amounts of inexpensive energy. Gas-fired turbines are safe, clean and more efficient than any competing energy technology and less controversial than other energy sources. The range of 30 kW to 80 kW Industrial TurboGenerators (ITGs) -- referred to in the industry as micro-turbines -- lead this emerging market. Larger size 100 kW to 350 kW mini-turbines are expected in the next few years to become part of the on-site scene for the distributed power generation. As already applied in the aircraft industry for the last 40 years, micro-turbines provide clean, reliable power for use in non-linear load conditions.

The Honeywell Parallon 75 Industrial Power Generator

An example of current design in an ITG would be the Honeywell Parallon 75. The overall machine size is approximately 92-in. long by 48-in. wide by 85-in. high, weighing approximately 2850 pounds (not including transformer and battery). The ITG can be unloaded and installed at most locations by a forklift truck. It is designed for a life of 40,000 hours (almost five years under 24/7 operation), and operating for a few hours at peak grid load conditions, defined as peak load shaving operation, extends the ITG operational life beyond 10 years. Emissions are low, and the heat of the exhaust can be reapplied (or co-generated) for heating water or air for use in facilities, or as a second source of energy, thus providing more than twice the overall system efficiency. These units can be clustered to deliver more than 350 kilowatts of electric power. Nominal output voltage from individual or clustered ITGs is 275 VAC, 120 to 575V with transformer using a 4-wire auto-transformer or isolation transformer.

The Ecostar Power Converter

The turbine, compressor and generator are all housed in a single unit, with a common shaft as the only moving component. The gas-driven micro-turbine uses a lubrication-free air bearing system to support the rare earth permanent magnet generator as it turns at speeds approaching 65,000 rpm, yet it runs quietly with a maximum noise level of 65 dBa at a distance of 10 meters.

Electronic power converters

The Honeywell ITG uses stationary power converter electronics developed by Ecostar Electric Power Train and Power Conversion Systems, Dearborn, MI, that are based on a voltage source inverter. The package is based on electric vehicle technologies from the mid 1990s. The power section employs insulated-gate bipolar transistors, which possess high voltage capability, low on-resistance, ease of drive and fast switching rates and are packaged in a special module and integrated with a fluid jacket (cold plate). The cooled power converter, associated electronics and filtering fits into a 40-in. long by 30-in. wide by 8.63-in. high package weighing approximately 250 pounds, and is scalable to meet future requirements.

The converter controls the ITG as it operates through power generating modes that include; standalone mode supply up to 75 kW (then full power to the loads); parallel with the electric power grid (single unit); auto-transfer between grid and standalone; multi-units in parallel with the grid; or multi-unit standalone. Multi-unit operation requires a fast, accurate and responsive control to keep all generator units synchronized in both amplitude and phase and share the loads evenly during light or heavy load usage.

The dual inverters

The start inverter drives the generator as an electric motor, spinning the micro-turbine during startup. It develops a 25 kW output and drives the permanent magnet generator (PMG) from zero to 550 Hz -- approximately 33,000 rpm -- before the gas-driven turbine switches on to provide rotary power to the generator. It also rectifies the turbo generator outputs. The voltage source inverter uses an advanced pulse-width modulation (PWM) scheme for improved power efficiency and lower voltage and current space harmonics. Power efficiency levels exceed 95% in all normal operating conditions.

The gas-driven turbine increases shaft speed until it reaches the top generator speed of 65,000 rpm (about 1100 Hz). The PMG develops 3-phase, 410 Vrms line-to-line with phase currents reaching approximately 160 amps. This PMG output is fed into a rectifier which is an integral part of the inverter, converting the AC to DC. The DC bus structure includes laminated bus bars that provide for low eddy currents and leakage currents, minimizing AC inductance and power losses, and allows inverter performance to exceed 95% efficiency.

The main inverter converts the DC power from the rectifier into an electric grid-usable 3-phase 75 kW output at 50 or 60 Hz. The nominal main inverter outputs reach 275 Vrms line-to-line. Inverter currents will vary from 170A at unity power factor (the difference in phase between the phase voltage and phase current) to 195A at 0.75 power factor. The portion of the current in phase with the voltage is real power, while the out of phase portion is the reactive (wasted) power. A unity power factor represents the optimum condition when the phase voltage and current are in phase with each other. Large inductive loads from electric motors, transformers and power supplies switching on and off in an electric power utility grid cause huge problems in power factor values. Active control methods used in the Ecostar distributed power converter controls and alleviates these reactance effects.

Operation, control and safety

The ITG operates in current-control mode when connected in parallel with the utility power grid, tracking the grid frequency variations up to 5% of nominal and providing a sinusoidal current waveform with less than 5% harmonic distortion. The system operates in voltage-control mode when operated as a standalone power system, with a harmonics content not exceeding 5% at rated power output levels, and voltage levels remaining within 5% of nominal output. The Ecostar power converter maintains individual harmonic components and the total demand distortion values to IEEE 519 requirements.

A microprocessor in the Ecostar unit, functioning like a digital signal processor, provides the hardware platform for field-oriented control algorithms employing a vector space modulator needed to achieve the critical power management and power quality requirements. This allows the control unit to monitor and change voltage and current waveform amplitudes and phase angles on-the-fly; continually smooth and reduce voltage and current harmonics; correct for load transients; adapt the power factor to within 0.75 lead to 0.75 lag condition continuously; and recover from transient voltage surges. Monitoring and controlling these parameters requires high performance voltage and current closed loop bandwidths of greater than 200 Hz.

The control system also incorporates a number of fundamental protection features and on-board diagnostics to ensure the system operates safely. When required to disconnect from the power grid, a shut down sequence is initiated to control conditions such as over- and under-frequency, over- and under-grid voltage, over-current limit, over-temperature (at power block heatsink), and DC bus over- and under-voltages.

The power converter system can be disconnected from the electric grid by an output contactor. Additionally, fuse protection is provided for shutting the system down for internal faults. The control system has a number of reconnect and retry strategies to power up the ITG.

Along with remote control and monitoring capabilities, the ITG can monitor natural gas and electricity consumption via an RS232C or Ethernet TCP/IP (Internet) port. An RS485 port is used to connect to other ITGs for multiple unit operation.

Power grids of the future

New electronic components with continually improving performance characteristics will provide the basis for faster start-up cycles in back-up power generators as these units close the response gap with uninterruptible power supplies (UPS). Distributed power systems may become as plentiful as air conditioners by 2010. A mixture of free-standing fuel cells, micro turbine power systems, flywheel power systems, solar and wind power, and other environmentally sound technologies could ease today's electric grid problems and provide individual power to each home.

For more information:

Circle 420 - Honeywell Power Systems, or connect directly to their website via the
Online Reader Service Program at http://www.OneRS.net/104df-420