Analysis and Control of Electric Drives. Ned Mohan

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СКАЧАТЬ can be converted to electricity; in this example, coal is primary energy, synthetic gas is secondary energy, and electricity is tertiary energy.” Often, the primary energy and the savings in the primary energy are expressed in quads, where a quad equals 10¹⁵ BTUs and 10 000 BTUs equal approximately 2.93 kWh.

1‐2 ENERGY SAVINGS IN GENERATION OF ELECTRICITY

Nearly 99% of electricity is produced through electric machines. This percentage was nearly the same, approximately 98.6%, in the United States in 2018. According to the US Energy Information Administration [3], about 4171 billion kWh (or 4.17 trillion kWh) of electricity was generated at utility‐scale electricity generation facilities in the United States in 2018. About 64% of this electricity generation was from fossil fuels (coal, natural gas, petroleum, and other gases). About 19% was from nuclear energy, and approximately 17% was from renewable energy sources. Out of the renewable energy sources, only 1.4% of the total electricity generated was by photovoltaic systems (PVs) that do not use electric machines, whereas all other sources of electricity generation use electric machines. Therefore, any improvement in increasing the efficiency of machines and electric drives will be very consequential.

      1‐2‐1 Energy‐Saving Potential in Harnessing of Wind Energy

One of the significant roles of electric drives is in harnessing wind energy. The block diagram for a wind‐electric system is shown in Fig. 1-2, where the variable‐frequency ac produced by the wind‐turbine‐driven generator is interfaced with the utility system through a power electronic converter (PPU). By letting the turbine speed vary with the wind speed, it is possible to recover a higher amount of energy in the wind compared to systems where the turbine essentially rotates at a constant speed due to the generator output being directly connected to the utility grid. The harnessing of wind energy involving ac drives is crucial for generating carbon‐free electricity [3], and this application is sure to grow rapidly.

Fig. 1-2 Electric drive for wind generators.

1‐3 ENERGY‐SAVING POTENTIAL IN THE END‐USE OF ELECTRICITY

According to [4], the United States consumed approximately 96 quadrillions BTU (quads) of primary energy in 2013 (it was nearly 100 quads in 2018), as shown in Fig. 1-3. Out of the total, 32% was consumed in the industrial sector and 39% in the residential and the commercial sectors combined.

Fig. 1-3 Primary energy consumption by end‐use sector in the United States in 2013.

      1‐3‐1 Energy‐Saving Potential in the Process Industry

Traditionally, motors were operated uncontrolled, running at constant speeds, even in applications where efficient control over their speed could be very advantageous. For example, consider the process industry (e.g. oil refineries and chemical factories) where the flow rates of gases and fluids often need to be controlled. As Fig. 1-4a illustrates, in a pump driven at a constant speed, a throttling valve controls the flow rate. Mechanisms such as throttling valves are generally more complicated to implement in automated processes and waste large amounts of energy. In the process industry today, electronically controlled adjustable‐speed drives (ASDs), shown in Fig. 1-4b, control the pump speed to match the flow requirement. Systems with ASDs are much easier to automate and offer much higher energy efficiency and lower maintenance than the traditional systems with throttling valves.

Fig. 1-4 Traditional and ASD‐based flow control systems.

      According to [1], the US industrial motor systems of all sizes and in all applications have the potential energy‐saving opportunity, as a percentage of the US end‐use electricity load, from 3.3 to 8.9%.

      These improvements are not limited to the process industry. Electric drives for speed and position control are increasingly being used in a variety of manufacturing, heating, ventilating, and air conditioning (HVAC), and transportation systems, as we will see in the subsequent sections.

      1‐3‐2 Energy‐Saving Potential in the Residential and Commercial Sectors

Out of the total, the residential and commercial end‐uses represent 39% of the total energy consumed, as depicted in Fig. 1-3. In the residential sector (Fig. 1-5a), the primary energy consumption of electric motor‐driven systems and components is 4.73 quads. In the commercial sector (Fig. 1-5b), it is 4.87 quads. Fig. 1-5a and b provide a breakdown of motor‐driven energy consumption by end‐use for the residential and commercial sectors, respectively. Thus, approximately 10% of the total primary energy consumed can be attributed to electric motor‐driven systems in the residential and commercial sectors.

Fig. 1-5 Energy usage in (a) residential sector and (b) commercial sector.

      According to [4], the technical energy‐saving potential achievable through motor upgrades and variable speed technology is estimated to be 536 trillion BTU (0.54 quads) of the primary energy in the residential sector.

      In the commercial sector, technical potential due to motor upgrades alone is 0.46 quads of the primary energy, whereas the potential savings resulting from the use of variable‐speed drives alone is 0.53 quads of the primary energy.

      Therefore, the primary energy‐saving potential in the residential and the commercial sectors combined is approximately 1.53 quads. This, as a percentage of the total primary energy consumed, is approximately 1.5%. Assuming the efficiency by which the primary energy is converted to electricity to be 35%, the savings of 1.53 quads of the primary energy equals approximately 157 billion kWh of saved electricity. As a percentage of the total electricity generated in 2018 in the United States, this represents savings of 3.76%.

Therefore, in the United States, industrial motor systems of all sizes and in all applications, combined with the motor systems in residential and commercial sectors, have the potential energy‐saving opportunity, as a percentage of the total US end‐use electricity is from approximately 7 to 12%.

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