Providing Premium Power through Distributed Resources

The modern industrial facility depends on sensitive electronic equipment that can be shut down suddenly by severe power system disturbances. A large number of these disturbances on the power system are a result of line faults which can cause momentary voltage sags. This results in equipment malfunctioning and high restart cost. This papers describes the control of distributed resources as a solution to such problems. In particular the focus is on systems of distributed resources that can switch from grid connection to island operation without causing problems for critical loads. Introduction Premium power is a concept, based on use of power electronics equipment (such as custom power devices and active filters), multi utility feeders and uninterruptible power supplies to provide power to users having sensitive loads. This power must have a higher level of reliability and power quality than normally supplied by the utility. With the age of automation, computers, process controls, drives and robotics the quality of electrical power become a major issue. The number of outages, voltages dips and duration is an important issue. In the manufacture of computer chips alone, losses from sags amount to $1 million to $4 million per occurrence, accordance to Central Hudson Gas & Electrical Corp. Small distributed resources (DR) can increase reliability and power quality by allowing them to be placed near the load. This provides for a stiffer voltage at the load and uninterruptible power supply functions during loss of grid power. The power electronics interfaces found on most small DRs can also control voltage dips and unbalances. Currently, systems for controlling volt disturbances use a voltage sources inverter which injects reactive power into the system to achieve voltage correction. One method is to inject shunt reactive current, the other is to inject series voltage. These systems are effective in protecting against single phase voltage drops (or swells) due to distant faults or unbalanced loads. These systems are costly, complex and are needed only during voltage events. An alternative to these systems are microsources with a more robust control system to protection against single phase voltage drops and swells. 0-7695-0493-0/00 Distributed resources (DR) include a variety of energy sources, such as micro-turbines, photovoltaics, fuel cells, and storage devices, with capacities in the 1 kW to 10 MW range. Deployment of DR on distribution networks could potentially increase their reliability and lower the cost of power delivery by placing energy sources nearer to the demand centers. By providing a way to by-pass conventional power delivery systems, DR could also offer additional supply flexibility. Emerging Technologies The trends in technology points toward smallness, under the 500kW level. An excellent example are the small gas fired micro-turbines in the 25-100 kW range that can be mass produced at low cost. They are designed to combine the reliability of on board commercial aircraft generators with the low cost of automotive turbochargers. These systems are high speed turbines (50,000-90,000 rpm) with air foil bearings. They are small and use power electronic to interface with the load. Examples include AlliedSignal's 75-kW Turbogenerator, Allison Engine Co's. 50-kW generator and Capstone's 30 kW system. Fuel cells are also well suited for distributed generation applications. They offer high efficiency and low emissions, but today's costs are high. Phosphoric acid cell are commercially available in the 200-kW range, while solid oxide and molten carbonate cell have been demonstrated. In October 1997 the U.S. Depart of Energy and Arthur D. Little unveiled the “first-ever on-board gasoline powered fuel cell for the automobile”. The possibility of using gasoline as a fuel for cells has resulted in a major development effort by the automotive companies. This work is focused towards the polymer electrolyte membrane (PEM) fuel cells. By 2002, Ballard Power Systems expects to be selling a 250kW PEM fuel-cell based generator at prices competitive with the grid to shopping malls and large commercial buildings. Cinergy Corporation recently placed a commercial order with Ballard for a 250-kW PEM fuel cell for stationary power. Mixed fuel cell and micro-turbine systems will also be available as distributed generation. In a joint DOE West$10.00 (c) 2000 IEEE 1 Proceedings of the 33rd Hawaii International Conference on System Sciences 2000 inghouse project a solid oxide fuel cell has been combined with a gas turbine creating a combined cycle power plant. It has expected electrical efficiency of greater than 70 percent with low and and virtually zero . The expected power levels range from 250-kW to 2.5-MW Distributed resources include more than small generators and fuel cells. Storage technologies such as batteries, ultracapacitors and flywheel play an important role. Combining storage with micro-sources provide peak power and ride-through capabilities during disturbances. Storage systems are far more efficient than five years ago. Flywheel systems can deliver 700-kW for 5 seconds while 28-cell ultracapacitors can provide up to 12.5 kW for a few seconds. Areas of Applications The availability of small, low-cost, power sources provides opportunities for radical change in the structure of power delivery. One of the most basic is to provide firm power. This could be an isolated community, a commercial center or an industrial plant. It is highly probable that gas utilities, and ESCO will be approaching commercial centers and industries to provide firm power using distributed sources. For utility the two prime uses for microsources are for peak shaving at the distribution level and to deter or avoid the cost of increasing the distribution infrastructure. Micro-turbines enable distribution to shave peaks through generation rather than demand side management (DSM) techniques. In addition to shaving peaks they also provide capacity for emergencies. Local generation not only increases overall system efficiency but also reduces investments in traditional generation, bulk transmission and distribution facilities. The utility can also serve incremental load growth in areas where there is a shortage of substation and/or distribution feeder capacity. For this to happen, a method for control and dispatch of l0s-to-100s of units is needed. Major commercial and industrial users of electrical power pay demand charges to the utility. Micro-turbines could be used to reduce demand charges. In addition to saving in demand charges the turbines could be connected to the more critical loads to provide emergency power. Since all the micro-sources must have a power electronics interface they can all provide the quality of power provided by “Custom Power Devices,” such as active filtering, and voltage support during single and three phase disturbances. CO2 NOx SOx 0-7695-0493-0/00 Power Electronics & Inertia-less Generation These technologies require power electronics to interface with the power network and its loads. In all cases there is a D.C. voltage source which must be converted to an ac voltage or current source at the required frequency, magnitude and phase angle. In most cases the conversion will be performed using a voltage source converter with a possibility of phase width modulation to provide fast control of voltage magnitude. Power electronic interfaces introduces new control issues and new possibilities. A system with clusters of microgenerators and storage could be designed to operate in both an island mode and connected to the power grid. One large class of problems are related the fact that micro-turbines and fuel cell have slow response and are inertia-less. It must be remembered that the current power systems have storage in generators’ inertia. When a new load comes on line the initial energy balance is satisfied by the system’s inertia. This results in a slight reduction in system frequency. A system with clusters of micro-generators could be designed to operate in an island mode provide some form of storage to provide the initial energy balance. The control of inverters used to supply power to an AC system in a distributed environment should be based on information available locally at the inverter. In a system with many micro-sources, communication of information between systems is impractical. Communication of information may be used to enhance system performance, but must not be critical for system operation. Essentially this implies that the inverter control should be based on terminal quantities. It is essential to have good control of the power angle and the voltage level by means of the inverter. Control of the inverter's frequency dynamically controls the power angle, and the flow of the real power. To prevent overloading the inverter and the micro sources, it is important to ensure that load changes are taken up by the inverter in a predetermined manner, without communication. Basic Component Models There are two basic classes of micro-source systems; one is a D.C. source, such as fuel cells, photovoltaics, and battery storage, the other is a high frequency ac source such as the micro-turbine which needs to be rectified. In both cases the D.C. voltage needs to be interfaced to the ac network using a voltage source inverter. The time constants for changes in power output for the micro-turbines and fuel cell range from 10 to 200 seconds. This slow $10.00 (c) 2000 IEEE 2 Proceedings of the 33rd Hawaii International Conference on System Sciences 2000 response requires that either the D.C. bus or ac system has storage to insure load tracking. For systems studied it this work, it is assumed that there is battery storage on the D.C. bus to provide fast response to load demands. The general model for a micro-source coupled to the ac system through an inverter is shown in. Figure 1. Figure 1. Interface Inverter System As a minimum the inverter needs to control the flow of real and reactive power (P & Q) between the microsource and the power system. The P & Q are coupled with P predominantly dependent on the power angle, δp, while Q is dependent on the magnitude of the converter's output voltage V. It is also possible to independently control P and voltage E. The equations below indicate that for small values of δp and small difference in V & E, the real power P is proportional to δp and the reactive power Q depends on voltage diffference. The inverter output voltage is not sinusoidal. The inverter output voltage are one of seven vector values, six active and one zero voltages. The inverter output voltage can only switch discretely between these values. These seven vectors transformed to the stationary d-q reference frame are shown in Figure 2. It is convenient to define a continuous quantity as the time-integral of the inverter output voltage often called the inverter flux vector : The choice of which vector to apply comes from comparisons of measured and with the desired values. If the angle is larger than its reference the zero vector is chosen, since this is the only choice that can reduce the a fi o