Distributed Power Demands Safe, Reliable Controls

This article contains an Online Extra

AT A GLANCE

Bring power close to users

Synch with the utility grid

Ensure DP safety, reliability

Monitor vital variables

Export extra power

In the wake of the recent power outage in the Northeastern U.S. comes a fresh reminder of how we take uninterrupted supply of electricity for granted. The need to protect and make power supplies more reliable likewise comes to the forefront. One element along the route to reliability is a nascent trend toward smaller-scale, modular distributed power systems.

Distributed power (DP) locates small electric generating systems close to users and their facilities—in contrast to large central power-generating plants. Typical size of DP sources ranges widely from 1 kW to 10 MW (or more). DP is implemented in several modes: On-site generation, in parallel with the utility power grid, or fed wholly into the grid. Distributed power has particular appeal to users in remote locations.

With current higher costs for distributed power and slow adoption of all new technologies, the DP trend is just starting to emerge. However, substantial generation projects are in place, involving microturbines, fuel cells, wind-powered generators, battery-storage systems, smaller power plants, and others. (Find more about alternative power sources in an Online Extra to this article at www.controleng.com.) Also included under DP are standby generators that can be tapped during peak demand periods to defray utility costs at some sites.

And the trend is growing. Industry experts say 10-30% of new power will come from distributed generation by 2010. An Electric Power Research Institute(EPRI) study indicates that 25% of new generation facilities will be distributed by then.

Synching with the grid

Control hardware and software play a central role in implementing distributed power. Controls must accommodate stand-alone as well as tough grid-connected requirements, along with ensuring safe switching to and from the grid.

Anti-islanding provision is needed to shutdown a distributed resource (DR) on loss of grid (circuit breaker CB2 open) during grid-parallel operation and matched-load condition of DR and local load. Otherwise, an “island” is created in the Area EPS, which is dangerous to maintenance workers and equipment.

Karl Kersey, P.E., senior product specialist at Schneider Electric, Power Management Operations, notes transition issues to consider when synchronizing a DP system with a utility grid. He mentions several common utility-to-generator transfer schemes that “provide a variety of control scenarios for a facility” intended to eliminate faults.

Open transition transfer serves as standard emergency operation in case of loss of utility. “[It] allows five seconds outage time during which utility connection is broken before transition-to-generator power is made,” says Kersey. Open transition synchronized transfer is similar but offers &150 ms outage for quicker transfer between utility and generator power. Mechanical loads can ride through a 150-ms outage, allowing motors in a plant to remain in synch, for example.

Other transfer modes— closed-transition synchronized (CTS) and soft load —provide increasingly capable control for smoother synchronizing to the grid. CTS transfer has &150-ms outage, with momentary parallel operation of utility and generator when connecting back to utility power, explains Kersey. Soft-load transfer has the most-advanced controls and options for distributed generation (DG). It supplies 25-90 seconds of parallel power for synchronizing the generator to the utility and for up/down ramping of load. It also permits flexibility for peak shaving, load following, and the potential to sell excess generation to the utility.

The latter two methods require utility/government regulation, including testing and safety features to show no harmful effects are sent to the power grid, adds Kersey.

Safety, reliability

Eaton Electrical likewise emphasizes reliability and safety aspects of distributed power controls. David Loucks, solution manager at Performance Power Solutions unit of Eaton, notes the significance of grid-parallel operation for DP, where a local generator can export power to the grid in excess of onsite load requirements. However, proper controls need to be provided. Voltage and current feedback must be monitored with numerous samples taken per cycle to match events on the utility and generator side, he explains. This includes power factor (PF) and reactive power (VAR) monitoring.

Control logic must protect the utility connection and engine/generator. On the utility side, protection involves under/over-voltage, under/over-frequency, over-current, unbalanced current and voltage, etc., according to Loucks. All the above protections apply to the generator in addition to loss of field.

Loucks sees analogies between a basic control system and DP controls. In a gas-powered internal-combustion-based generation unit, for example, an engine/generator’s throttle acts like a PI controller. It adjusts speed (Hz) and phase-angle difference between the utility and generator in the synchronizing mode, while generator speed becomes the process variable and desired preset frequency (50 or 60 Hz), the setpoint . In the controller’s synchronized mode, the throttle regulates output (real power, kW) and the process variable is the computed real power (Vrms x Irms x =3 x cosU).

“Reactive power is separately managed as a PI loop,” continues Loucks. It’s part of a voltage PI controller that regulates dc current going to the generator’s rotor. There are two operating modes: Constant power factor mode , where PF = (real power output/Vrms x Irms x =3) = cosU is the process variable, and desired preset PF is the setpoint. Constant reactive power mode has Vrms x Irms x =3 x (1-PF) as the process variable, while internally preset VAR magnitude is the setpoint.

Typical communication ports found on engine generators include Modbus, DeviceNet, and Lonworks (widely used in building automation). Transfer switches and generator switchgear typically have interfaces to major PLC networks, adds Loucks.

ABB Automation is active in various areas of distributed generation and control. One product sector is its Power Electric Building Block (PEBB) technology, made up of modular power converters that target microturbine, traction, power quality, and other applications. Specifically, PowerPak3 converters provide up to 2,500 kW output, based on low-voltage (LV) insulated-gate bipolar transistors (IGBTs). Sensors for current, voltage, and temperature monitoring are integrated into PowerPak3. A control interface and a 24 V dc power supply connection for the gate-driver boards are included. Medium voltage (MV) PEBB units up to 15 MW nominal power are also available.

Higher speed applications need more efficient Power Electronic Controllers (such as PEC 800), mentions Tor-Eivind Moen, business development manager for Power Electronics at ABB. PEC 800 features programmability with high-level software, including the Mathworks’ Simulink and Matlab. These controllers come in LV units (IGBT-based), as well as MV units that use integrated gate-commutated thyristor (IGCT) power-switching devices.

Power conversion and controls technology at ABB also extends to energy-storage applications. Moen cites the company’s notable involvement in the “world’s largest battery energy storage system” (BESS) to date, which recently went online for testing in Fairbanks, Alaska. Operated by generation co-operative Golden Valley Electric Association (GVEA), the BESS is designed to stabilize electricity networks in the Fairbanks area. ABB supplied a power-conversion system; metering, protection, and control devices; and service equipment to this BESS installation, which will comprise 13,760 special, sealed nickel-cadmium batteries (first photo). BESS will deliver 27 MW power for 15 minutes—allowing GVEA plenty of time to start local backup generation. Alternatively, BESS supplies higher output for shorter time periods.

ABB’s Moen has a positive perspective on distributed power generation and new energy sources, tempered by practical “early use issues” and cost considerations. “Distributed power generation suffers from issues of sales volume and overall acceptance expected with all new technologies,” he says.

Products for distributed power conversion made by Ballard Power Systems derive from the company’s experience since 1992 with converters designed for electrical vehicle applications. However, key control techniques have been modified to suit DG requirements. The product line, named Ecostar, achieves accurate grid voltage and frequency sensing by the addition of features, such as isolated voltage sensing, signal processing, and software phase-locked loop-based frequency measurement, explains Ross Witschonke, VP of marketing.

To meet interconnect standards, grid voltage, current, and power factor are monitored at the point of connection rather than at the inverter output terminals. “The grid interconnect was simplified by avoiding monitoring at the point of common coupling,” says Witschonke. “This was achieved by incorporating the anti-islanding function in the controls and meeting the interconnect protection requirements (over/under frequency, voltage trip setpoints, and time to trip).”

Ballard’s Ecostar Power Converter (UL 1741 listed) supplies reliable grid-parallel operation using current-mode control, with protection provided to the inverter and utility grid under fault conditions. PF is “controlled closely to unity” at the point of connection to ensure stable operation. Converter design and manufacturing incorporate automotive disciplines to add product quality. “An end-of-line test is done on all units with a grid simulator to verify protective functions,” he states.

Photovoltaic (PV) power converters for grid-tied applications draw the current focus at Ballard. Its 75 kW Ecostar PV power converter is commercially available. Future targets for these controls are in UPS systems with multiple units in parallel and multiple inverters for large PV applications. “Ecostar power converter for microturbine applications is designed and ready to be customized,” adds Witschonke.

Avoid ‘islanding’

All respondents to this article stress the need for safe, reliable interconnection of DP sources and the grid. The diagram (“Need for Anti-Islanding”) shows how an undesirable power “island” can result for a distributed resource (DR) with a local load connected to the utility grid. When power output from the DR equals the local load, the grid provides only reference voltage and frequency (circuit breakers CB1 and CB2 closed). Under this matched-load condition, if CB2 opens, a power balance results and voltage and frequency remain undisturbed. However, this condition can form an “island” within the Area Electric Power System (EPS) as the DR, now isolated from the grid, inadvertently continues to power the local load—unless special provision is made for disconnection. Protective relays installed to detect under/over voltage and under/over frequency at the point of interconnection will not respond to the loss of grid, says Ballard Power.

Islanding is dangerous. It poses a safety hazard to utility maintenance workers unaware that the DR still powers a local area. Islanding can also damage the DR if loss of grid is, for example, due to an upstream “recloser.” According to Ballard’s Witschonke, the recloser automatically closes after a few cycles, but the interconnect voltage (controlled by the island) is not necessarily synchronized with the Area EPS voltage.

Utilities require “anti-islanding” measures for DP generation. This safety feature disconnects the DR from the grid when grid power is lost. Requirements for anti-islanding are specified by standards, such as UL 1741 and IEEE 1547 and 929 (more online at www.controleng.com/issues).

Besides the overall controls, distributed power demands a gamut of metering/monitoring equipment, interconnect components, and software tools. Appropriate controls will need to keep pace with the coming expansion of distributed, onsite power sources.

For more products, visit www.controleng.com/buyersguide. For integrators go to www.controleng.com/integrators .

ABB Automation

www.abb.com/powerelectronics

Ballard Power Systems

www.ballard.com

Eaton Electrical

www.eatonelectrical.com

EPRI

www.epri.com

Schneider Electric

www.squared.com

Online Extra to November 2003 Control Engineering article on‘Distributed Power Controls’

Frank J.Bartos, Control Engineering

The trend toward distributed power (DP)—consisting of small-scale, modular electric generating systems located close to users and their facilities—is just starting to grow. Most experts agree about the growth; however, the pace of that growth is unknown and will be shaped by several developments: availability of traditional power generation, changes in the cost of various power sources, government regulation, and public opinion, among others. Control devices and systems for DP are key to realizing that growth.

Here is a bit of background on some alternate power sources, which are projected to have a major role in delivering distributed power.

Microturbines are power generation systems made up of a small gas turbine and a directly driven high-speed generator. An exhaust gas recuperator is often included to improve system efficiency. The system’s power converter (part of the controls) changes generated electricity into useful voltage and frequency. Typically, microturbines are sized in the 30-80 kW range, with units in the 100-350 kW range sometimes referred to as miniturbines. They work on the same principle as a jet engine, but microturbines offer wider, more economical fuel options s, such as natural gas, diesel, ethanol, and biogas.

Microturbine systems can be applied in various modes: for continuous power production; to improve a weak grid’s power capacity, quality, or reliability; and for stand-by power. The latter mode also is used to reduce high-demand electric loads (peak shaving), thereby cutting power costs. These turbines reduce harmful emissions compared to the standard motor-generator set.

Fuel cells are electrochemical devices that produce electricity (and heat) as a result of direct reaction of a fuel and an oxidant (typically hydrogen and oxygen). Electric output of the cell is direct current (dc) voltage. Several types of fuel cells exist, characterized by the kind of electrolyte used in the power conversion process. Proton exchange membrane (PEM) fuel cell is one of the simplest and most common technologies. It consists of four basic elements: anode (or negative element) that conducts electrons freed from the hydrogen molecules and disperses hydrogen gas over the surface of the catalyst; cathode (or positive element) that distributes oxygen to the catalyst and conducts electrons back from the external circuit to the catalyst for recombination with hydrogen and oxygen to form water; electrolyte (proton exchange membrane), a specially treated material that conducts only positively charged ions and blocks electrons; and a catalyst, another special material that speeds the reaction of oxygen and hydrogenPEM fuel cells are used in automotive and mobile applications. Other fuel cells types such as solid oxide and molten carbonate operate at higher temperatures and find application in power generation.

Wind-power systems use variable-speed wind turbines with giant aerodynamic blades and geared transmission rotor drive train that spins a generator at higher speed to efficiently convert raw wind energy to electric power. Direct-drive wind turbines without the need for a gearbox are under development. Control of wind turbines encompasses the starting and stopping of the blades at appropriate wind-speed limits; blade-pitch control; emergency braking; and other functions.

Standards view Availability of industry standards will aid new power generation advances. The most recent example is IEEE 1547, “Standard for Interconnecting Distributed Resources with Electric Power Systems,” focusing on technical requirements for linking fuel cells, photovoltaics, microturbines and other local generating technologies into the national grid. Issued by the Institute of Electrical and Electronics Engineers, Standards Association (IEEE-SA, Piscataway, NJ), IEEE 1547 addresses hardware/software aspects of performance, operation, testing, and safety of interconnection products and services for DP control and communication. Further coverage extends to product quality, interoperability, design, engineering, installation, and certification issues.

More than 350 people “from all aspects of the power industry” participated in the standard’s development, including, manufacturers of electric components/devices and alternative power equipment, utilities/energy service companies, universities, government labs, and state and federal government agencies, according to Richard DeBlaso, Working Group chair. IEEE 1547 is actually the first in a family of interconnection standards for distributed resources (DRs).

Further standards are under development:

IEEE P1547.1—will provide detailed test procedures to prove/validate that interconnection specifications and equipment conform to the functional and test requirements of IEEE 1547;

IEEE P1547.2—will provide technical background and application details to simplify use of IEEE 1547, characterizing various DR technologies and their interconnection issues; and

IEEE P1547.3—will aid interoperability via guidelines for monitoring, information exchange, and control among distributed generators (fuel cells, photovoltaics, wind turbines, and others) interconnected with an electrical power system.

Two other documents also apply to the distributed power generation and control arena: Underwriters Laboratories Inc.’s “Standard for Inverters, Converters, and Controllers for Use in Independent Power Systems” (UL 1741) and IEEE Standard 929 “Recommended Practice for Utility Interface of Photovoltaic (PV) Systems.”

UL 1741 combines product safety requirements with the utility interconnection requirements of IEEE 1547 to provide a testing standard to evaluate and certify distributed generation products, says Underwriters Laboratories. Design (type) testing and production testing are included under UL 1741. IEEE 929 offers guidance on equipment and functions needed to ensure compatible operation of PV systems connected in parallel with the electric utility. Factors such as personnel safety, equipment protection, power quality, and utility system operation are addressed. This document also discusses islanding of PV systems when not connected to the utility for voltage and frequency control, and ways to avoid islanding of distributed resources.

However, even with voluntary standards available, the Electric Power Research Institute (EPRI) cautions that performance “as expected and without unforeseen compatibility problems” is not assured. Further issues need to be resolved “before distributed resources become more widespread.” As an example, EPRI cites the basic incompatibility among three effects: fault clearing, reclosure, and islanding.

To round out this overview of distributed power and its associated controls, sidebars on “Companies and products” and “Applications” follow.

Distributed generation company, product sampler

Capstone Turbine Corp. manufactures and supplies microturbine technology for stationary distributed power and other applications. Typical units include a 30-kW model (C30) and a larger model C60, with physical size of a large refrigerator. An air-cooled, digital power controller or inverter—based on insulated-gate bipolar transistor (IGBT) technology and advanced software—provides reliable, accurate regulation and remote operation/monitoring. Micro-turbine accessories include mode controllers to handle automatic switching between stand-alone and grid-connect operation, batteries with digital control, and protocol converters for Internet connectivity.

Swedish company Turbec was founded by Volvo Aero and ABB in 1998 for the business of developing microturbine technology for small-scale power generation. The first commercial product, T100 CHP microturbine, claims “long life and high efficiency necessary for low-cost, low-maintenance power generation.” Fuelled initially by natural gas, T100 targets combined heat and power (CHP) applications. Newer models include biogas as optional fuel for waste management applications along with expanded features.

Announced in October 2002 by SatCon Technology Corp. its Power Inverter was “the highest power rated on-site generation inverter to comply with UL 1741.” Rated at 462 kVA, the three-phase Utility Interactive Multi-Mode Inverter design (Model AE-462-60-F-A) successfully completed compliance testing to Underwriters Laboratories Inc.’s “Standard for Inverters, Converters, and Controllers for Use in Independent Power Systems” (UL 1741). According to SatCon, this was a significant step for application of these products in on-site power generation, particularly for gaining approval for installations in California where such compliance is a requirement. David Eisenhaure, SatCon President and CEO, said, “In addition to fuel cell applications, these UL-compliant inverters will be suitable for photovoltaics, microturbines and other on-site power generation systems.” Manufacture and marketing of power systems for distributed power generation, power quality, and factory automation—including inverter electronics from 5 kW to 5 MW—falls under SatCon Power Systems (one of three SatCon Technology business units).

EGCP-2 is an example of a complete generator control and engine management package from Woodward Governor Co. designed for use with an automatic voltage regulator and speed control to automate/protect engine-generator sets (either diesel-engine or gas-engine type). Micro-processor-based EGCP-2 can be configured for stand-alone operation or for utility-paralleled generation. A network of EGCP-2 units can control up to eight autonomous, small- to medium-sized generator sets for base-load, peak shaving, or backup-power generation, according to Woodward. Business of the company’s Industrial Control unit covers controls for traditional generators as well as microturbines and fuel cells.

Growing distributed power applications

The number and variety of distributed power applications, and their attendant controls, is growing.

Caterpillar Inc. and FuelCell Energy Inc . recently announced a “first joint sale of an ultra-low emissions fuel-cell power generation plant in California.” The customer, Los Angeles County Sanitation Districts, expects to use the new 250-kW Direct FuelCell (DFC) power plant to reduce energy costs compared to present utility rates in the LA area. About 530 million gallons of wastewater/day are treated, while recovered biogas and biomass become fuel for generating electricity. The “Districts” expects to install the DFC300A unit in its Palmdale Water Reclamation Plant in NW Los Angeles County in fourth-quarter 2004.

The project is part of an April 2002 alliance between Caterpillar and FuelCell Energy, wherein they agreed to jointly develop fuel-cell power plants in the 250 kW to 3 MW range, incorporating FuelCell Energy’s technology. DFC power plants generate hydrogen internally without the requirement of an external process to obtain hydrogen from hydrocarbon fuel.

Another recent project involving Caterpillar is a generator farm encompassing 26 diesel generator units at the Ford Rouge plant in Dearborm, MI. These Cat Model 3516 engine-generator sets provide a total load peak-shaving capacity of nearly 38 MW. Wireless remote control helps reduce time for start-ups and grid synchronization. (For more detail, see “Wireless control system cuts generator farm start time,” in CE, March 2003, p 18).

On the grid

Reportedly, the first use of a fuel cell system on a U.S. electric utility grid was the 1-MW application for the Postal Service’s Mail Processing Center at Anchorage, AK, which began in August 2000. Five fuel cells, manufactured by United Technology Corp.’s UTC Fuel Cells unit, operate in parallel with the Chugach Electric Association grid, Alaska’s largest electric utility. Each UTC PC25 fuel cell produces 200 kW of electricity and over 700,000 BTU/hr usable heat for space heating, for example.

Excess power from the fuel cells flows into the grid. In case of a grid outage, the fuel cells transition to operate as an independent power source for the facility. Conventional uninterruptible power supplies or stand-by generators are eliminated. The Processing Center is the major distribution point for mail to and from Alaska.

The largest fuel-cell installation in the world was claimed by UTC Fuel Cells in April 2001 for its six-unit PC25 power plant installed as the main power source for the Connecticut Juvenile Training School in Middletown, CT. PC25 fuel cells, producing a total of 1.2 MW of electricity, form the core of a power installation that consists of generators and the local electrical grid. In this project for the Connecticut Dept. of Public Works, UTC Fuel Cells partnered with three subsidiaries of Northeast Utilities.

Battery storage

The battery-energy storage system (BESS) application for stabilizing electricity networks in the Fairbanks, Alaska area (mentioned in the main article), has additional noteworthy details. ABB Automation was the main design and control engineering provider for this largest BESS facility of its kind to date. Each of the facility’s 13,760 liquid electrolyte-filled Ni-Cad batteries is roughly the size of a large PC and weighs 156 lb. Expected life span of the batteries is 20 to 30 years, according to the facility operator Golden Valley Electric Association of Fairbanks.

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