7.2.1 Superconducting Magnetic Energy Storage

Although superconductivity was discovered in 1911, it was not until the 1970s that SMES was first proposed as an energy storage technology for power systems. SMES systems have attracted the attention of both electric utilities and the military due to their fast response and high efficiency; they have a charge–discharge efficiency over 95%. Possible applications include load leveling, dynamic stability, transient stability, voltage stability, frequency regulation, transmission capability enhancement, and power quality improvement [1].

When compared with other energy storage technologies, today’s SMES systems are still costly. However, the integration of an SMES coil into existing FACTS devices eliminates what is typically the largest cost for the entire SMES system—the inverter unit. Some studies have shown that micro (0.1 MWh) and midsize (0.1–100 MWh) SMES systems could potentially be more economical for power transmission and distribution applications. The use of high-temperature superconductors should also make SMES cost-effective due to reductions in refrigeration needs. There are a number of ongoing SMES projects currently installed or in development throughout the world.

An SMES unit is a device that stores energy in the magnetic field generated by the direct current (DC) current flowing through a super-conducting coil. The inductively stored energy (E in joules) and the rated power (P in watts) are commonly given specifications for SMES devices, and they can be expressed as follows:

where L is the inductance of the coil, I is the DC current flowing through the coil, and V is the voltage across the coil. Since energy is stored as circulating current, energy can be drawn from an SMES unit with almost instantaneous response with energy stored or delivered over periods ranging from a fraction of a second to several hours.

An SMES unit consists of a large superconducting coil at the cryogenic temperature. This temperature is maintained by a cryostat or dewar that contains helium or nitrogen liquid vessels. A power conversion/conditioning system (PCS) connects the SMES unit to an AC power system, and it is used to charge and discharge the coil. Two types of power conversion systems are commonly used. One option uses a current source converter (CSC) to both interface to the AC system and charge and discharge the coil. The second option uses a voltage source converter (VSC) to interface to the AC system and a DC-to-DC chopper to charge and discharge the coil. The VSC and DC-to-DC chopper share a common DC bus. The components of an SMES system are shown in Figure 7.2. The modes of charge–discharge–standby are obtained by controlling the voltage across the SMES coil (V_coil). The SMES coil is charged or discharged by applying a positive or negative voltage, V_coil, across the superconducting coil. The SMES system enters a standby mode operation when the average V_coil is zero, resulting in a constant average coil current, I_coil.

Several factors are taken into account in the design of the coil to achieve the best possible performance of an SMES system at the least cost. These factors may include coil configuration, energy capability, structure, and operating temperature. A compromise is made between each factor by considering the parameters of energy/mass ratio, Lorentz forces, and stray magnetic field and by minimizing the losses for a reliable, stable, and economic SMES system. The coil can be configured as a solenoid or a toroid. The solenoid type has been used widely due to its simplicity and cost-effectiveness, though the toroid coil designs were also incorporated by a number of small-scale SMES projects. Coil inductance (L) or PCS maximum voltage (V_max) and current (I_max) ratings determine the maximum energy/power that can be drawn or injected by an SMES coil. The ratings of these parameters depend on the application type of SMES. The operating temperature used for a superconducting device is a compromise between cost and the operational requirements. Low-temperature superconductor devices (LTSs) are available now, whereas high-temperature superconductor devices are currently in the development stage. The efficiency and fast response capability (milliwatts/millisecond) of SMES systems have been and can be further exploited in applications at all levels of electric power systems. The potential utility applications have been studied since the 1970s. SMES systems have been considered for the following: (1) load leveling; (2) frequency support (spinning reserve) during loss of generation; (3) enhancing transient and dynamic stability; (4) dynamic voltage support (VAR compensation); (5) improving power quality; and (6) increasing transmission line capacity, thus enhancing overall reliability of power systems. Further development continues in power conversion systems and control schemes, evaluation of design and cost factors, and analyses for various SMES system applications. The energy and power characteristics for potential SMES applications for generation, transmission, and distribution are depicted in Figure 7.3. The square area in the figure represents the applications that are currently economical. Therefore, the SMES technology has a unique advantage in two types of application: power system transmission control; and stabilization and power quality.

The cost of an SMES system can be separated into two independent components: (1) the cost of the energy storage capacity; and (2) the cost of the power handling capability. Storage-related cost includes the capital and construction costs of the conductor, coil structure components, cryogenic vessel, refrigeration, protection, and control equipment. Power-related cost involves the capital and construction costs of the power conditioning system. While the power-related cost is lower than the energy-related cost for large-scale applications, it is more dominant for small-scale applications.

7.2.2 Battery Energy Storage Systems

Batteries are one of the most cost-effective energy storage technologies available, with energy stored electrochemically. A battery system is made up of a set of low-voltage/power battery modules connected in parallel and series to achieve a desired electrical characteristic. Batteries are “charged” when they undergo an internal chemical reaction under a potential applied to the terminals. They deliver the absorbed energy, or “discharge,” when they reverse the chemical reaction. Key factors of batteries for storage applications include: high-energy density, high-energy capability, round-trip efficiency, cycling capability, life span, and initial cost.

There are a number of battery technologies under consideration for large-scale energy storage. Lead acid batteries represent an established, mature technology. Lead acid batteries can be designed for bulk energy storage or for rapid charge and discharge. Improvements in energy density and charging characteristics are still an active research area, with different additives under consideration. Lead acid batteries still represent a low-cost option for most applications requiring large storage capabilities; low-energy density and limited cycle life are the chief disadvantages. Mobile applications favor sealed lead acid battery technologies for safety and ease of maintenance. Valve-regulated lead acid (VRLA) batteries have better cost and performance characteristics for stationary applications. Several other battery technologies also show promise for stationary energy storage applications. All have higher energy density capabilities than lead acid batteries, but at present they are not yet cost-effective for higher-power applications. Leading technologies include nickel–metal– hydride batteries, nickel–cadmium batteries, and lithium–ion batteries. The last two technologies are both being pushed for electric vehicle applications where high-energy density can offset higher cost to some degree.

Due to the chemical kinetics involved, batteries cannot operate at high power levels for long time periods. In addition, rapid, deep discharges may lead to early replacement of the battery, since heating resulting in this kind of operation reduces battery lifetime. There are also environmental concerns related to battery storage due to toxic gas generation during battery charge and discharge. The disposal of hazardous materials presents some battery disposal problems. The disposal problem varies with battery technology. For example, the recycling and disposal of lead acid batteries is well established for automotive batteries. Batteries store DC charge, so power conversion is required to interface a battery with an AC system. Small, modular batteries with power electronic converters can provide four-quadrant operation (bidirectional current flow and bidirectional voltage polarity) with rapid response. Advances in battery technologies offer increased energy storage densities, greater cycling capabilities, higher reliability, and lower cost. Battery energy storage systems (BESSs) have recently emerged as one of the more promising near-term storage technologies for power applications, offering a wide range of power system applications such as area regulation, area protection, spinning reserve, and power factor correction. Several BESS units have been designed and installed in existing systems for the purposes of load leveling, stabilizing, and load frequency control. Optimal installation site and capacity of BESSs can be determined depending on their application. This has been done for load-leveling applications. Also, the integration of battery energy storage with a FACTS power flow controller can improve the power system operation and control.

7.2.3 Advanced Capacitors

Capacitors store electric energy by accumulating positive and negative charges (often on parallel plates) separated by an insulating dielectric. The capacitance, C, represents the relationship between the stored charge, q, and the voltage between the plates, V, as shown in (7.2). The capacitance depends on the permittivity of the dielectric, e, the area of the plates, A, and the distance between the plates, d, as shown in (7.3). Equation (7.4) shows that the energy stored on the capacitor depends on the capacitance and on the square of the voltage.

The amount of energy a capacitor is capable of storing can be increased by either increasing the capacitance or the voltage stored on the capacitor. The stored voltage is limited by the voltage-withstand-strength of the dielectric (which impacts the distance between the plates). Capacitance can be increased by increasing the area of the plates, increasing the permittivity, or decreasing the distance between the plates. As with batteries, the turnaround efficiency when charging and discharging capacitors is also an important consideration, as is response time. The effective series resistance (ESR) of the capacitor has a significant impact on both. The total voltage change when charging or discharging capacitors is shown in (7.5). Note that C_tot and R_tot are the result from a combined series/parallel configuration of capacitor cells to increase the total capacitance and the voltage level. The product C_totR_tot determines the response time of the capacitor for charging or discharging.

Capacitors are used in many AC and DC applications in power systems. DC storage capacitors can be used for energy storage for power applications. They have long seen use in pulsed power applications for high-energy physics and weapons applications. However, the present generation of DC storage capacitors sees limited use as large-scale energy storage devices for power systems. Capacitors are often used for very short-term storage in power converters. Additional capacitance can be added to the DC bus of motor drives and consumer electronics to provide added ability to ride voltage sags and momentary interruptions. The main transmission or distribution system application where conventional DC capacitors are used as large-scale energy storage is in the distribution dynamic voltage restorer (DVR), a custom power device that compensates for temporary voltage sags on distribution systems. The power converter in the DVR injects sufficient voltage to compensate for the voltage sag, such that loads connected to the system are isolated from the sag. The DVR uses energy stored in DC capacitors to supply a component of the real power needed by the load.

Several varieties of advanced capacitors are in development, with several available commercially for low power applications. These capacitors have significant improvements in one or more of the following characteristics: higher permittivities, higher surface areas, or higher voltage-withstand capabilities. Ceramic hypercapacitors have both a fairly high voltage-withstand (about 1 kV) and a high dielectric strength, making them good candidates for future storage applications. At present, they are largely used in low-power applications. In addition, hypercapacitors have low effective-series-resistance values. Cryogenic operation appears to offer significant performance improvements. The combination of higher voltage-withstand and low effective-series-resistance will make it easier to use hypercapacitors in high-power applications with simpler configurations possible.

Ultracapacitors (also known as supercapacitors) are double-layer capacitors that increase energy storage capability due to a large increase in surface area through use of a porous electrolyte (they still have relatively low permittivity and voltage-withstand capabilities). Several different combinations of electrode and electrolyte materials have been used in ultracapacitors, with different combinations resulting in varying capacitance, energy density, cycle life, and cost characteristics. At present, ultracapacitors are most applicable for high peak-power, low-energy situations. Capable of floating at full charge for 10 years, an ultracapacitor can provide extended power availability during voltage sags and momentary interruptions. Ultracapacitors can be stored completely discharged and installed easily, are compact in size, and can operate effectively in diverse (hot, cold, and moist) environments. Ultracapacitors are now available commercially at lower power levels.

As with BESSs, application of capacitors for power applications will be influenced by the ability to charge and discharge the storage device. At present, ultracapacitors and hypercapacitors have seen initial application in low-energy applications with much of the development for higher-energy applications geared toward electric vehicles. Near-term applications will most likely use these capacitors in power quality applications. For example, ultracapacitors can be added to the DC bus of motor drives to improve ride-through times during voltage sags. Ultracapacitors can also be added to a DVR or be interfaced to the DC bus of a distribution static compensator (DStatCom).

7.2.4 Flywheel Energy Storage (FES)

Flywheels can be used to store energy for power systems when the fly-wheel is coupled to an electric machine. In most cases, a power converter is used to drive the electric machine to provide a wider operating range. Stored energy depends on the moment of inertia of the rotor and the square of the rotational velocity of the flywheel, as shown in (7.6). The moment of inertia (I) depends on the radius, mass, and height (length) of the rotor, as shown in (7.7). Energy is transferred to the flywheel when the machine operates as a motor (the flywheel accelerates), charging the energy storage device. The flywheel is discharged when the electric machine regenerates through the drive (slowing the flywheel).

The energy storage capability of flywheels can be improved by increasing the moment of inertia of the flywheel or by turning it at higher rotational velocities or both. Some designs use hollow cylinders for the rotor allowing the mass to be concentrated at the outer radius of the fly-wheel, improving storage capability with a smaller weight increase.

Two strategies have been used in the development of flywheels for power applications. One option is to increase the inertia by using a steel mass with a large radius, with rotational velocities up to approximately 10,000 rpm. A fairly standard motor and power electronic drive can be used as the power conversion interface for this type of flywheel. Several restorer flywheels using this type of design are available commercially as uninterruptible power supplies (UPSs). This design results in relatively large and heavy flywheel systems. Rotational energy losses will also limit the long-term storage ability of this type of flywheel.

The second design strategy is to produce flywheels with a lightweight rotor turning at very high rotational velocities (up to 100,000 rpm). This approach results in compact, lightweight energy storage devices. Modular designs are possible, with a large number of small flywheels possible as an alternative to a few large flywheels. However, rotational losses due to drag from air and bearing losses result in significant self-discharge, which poses problems for long-term energy storage. High-velocity flywheels are therefore operated in vacuum vessels to eliminate air resistance. The use of magnetic bearings helps improve the problems with bearing losses. Several projects are developing superconducting magnetic bearings for high-velocity flywheels. The near elimination of rotational losses will provide flywheels with high charge and discharge efficiency. The peak power transfer ratings depend on the power ratings in the power electronic converter and the electric machine. Flywheel applications under consideration include automobiles, buses, high-speed rail locomotives, and energy storage for electromagnetic catapults on next-generation aircraft carriers. The high rotational velocity also results in the need for some form of containment vessel around the flywheel in case the rotor fails mechanically. The added weight of the containment can be especially important in mobile applications. However, some form of containment is necessary for stationary systems as well. The largest commercially available flywheel system is about 5 MJ/1.6 MVA weighing approximately 10,000 kg. Flywheel energy storage can be implemented in several power system applications. If an FES system is included with a FACTS or custom power device with a DC bus, an inverter is added to couple the flywheel motor or generator to the DC bus. For example, a flywheel based on an AC machine could have an inverter interface to the DC bus of the custom power device, as shown in Figure 7.4. Flywheel energy storage has been considered for several power system applications, including power quality applications as well as peak shaving and stability enhancement.

7.2.5 Pumped Hydroelectric Energy Storage

Hydroelectric storage is a process that converts electrical energy to potential energy by pumping water to a higher elevation, where it can be stored indefinitely and then released to pass through hydraulic turbines and generate electrical energy. A typical pumped-storage development is composed of two reservoirs of equal volume situated to maximize the difference in their levels. These reservoirs are connected by a system of waterways along which a pumping generating station is located. Under favorable geological conditions, the station will be located underground; otherwise, it will be situated on the lower reservoir. The principal equipment of the station is the pumping-generating unit, which is generally reversible and used for both pumping and generating, functioning as a motor and pump in one direction of rotation and as a turbine and generator in opposite rotation.

7.2.6 Flow Batteries

Flow batteries (FBs) are a promising technology that decouples the total stored energy from the rated power. The rated power depends on the reactor size, whereas the stored capacity depends on the auxiliary tank volume. These characteristics make the FB suitable for providing large amounts of power and energy required by electrical utilities. FBs work in a similar way as hydrogen fuel cells (FCs), as they consume two electrolytes that are stored in different tanks (no self-discharge), and there is a microporous membrane that separates both electrolytes but allows selected ions to cross through, creating an electrical current. There are many potential electro-chemical reactions, usually called reduction–oxidation reaction (REDOX), but only a few of them seem to be useful in practice [45].

Figure 7.5 shows a schematic of an FB. The power rating is defined by the flow reactants and the area of the membranes, whereas the electrolyte tank capacity defines the total stored energy. In a classical battery the electrolyte is stored in the cell itself, so there is a strong coupling between the power and energy rating. In the cell (flow reactor), a reversible electrochemical reaction takes place, producing (or consuming) electric DC current. At this time, several large- and small-scale demonstration and commercial products use FB technology.

The main advantages of FB technology are (1) high power and energy capacity; (2) fast recharge by replacing exhaust electrolyte; (3) long life enabled by easy electrolyte replacement; (4) full discharge capability; (5) use of nontoxic materials; and (6) low-temperature operation. The main disadvantage of the system is the need for moving mechanical parts such as pumping systems that make system miniaturization difficult. Therefore, the commercial uptake to date has been limited.

7.2.7 Compressed Air Energy Storage

Compressed air energy storage (CAES) is a technology that stores energy as compressed air for later use. Energy is extracted using a standard gas turbine, where the air compression stage of the turbine is replaced by the CAES, thus eliminating the use of natural gas fuel for air compression. System design is complicated by the fact that air compression and expansion are exothermic and endothermic processes, respectively. With this in mind, three types of systems are considered to manage the heat exchange:

1. Isothermal storage, which compresses the air slowly, thus allowing the temperature to equalize with the surroundings. Such a system works well for small systems where power density is not paramount.

2. Adiabatic systems, which store the released heat during compression and feed it back into the system during air release. Such a system needs a heat-storing device, complicating the system design.

3. Diabatic storage systems, which use external power sources to heat or cool the air to maintain a constant system temperature. Most commercially implemented systems are of this kind due to high power density and great system flexibility, albeit at the expense of cost and efficiency.

CAES systems have been considered for numerous applications, most notably for electric grid support for load leveling applications. In such systems, energy is stored during periods of low demand and then converted back to electricity when the electricity demand is high. Commercial systems use natural caverns as air reservoirs to store large amounts of energy; installed commercial system capacity ranges from 35 to 300 MW.

7.2.8 Thermoelectric Energy Storage

Thermoelectric energy storage (TEES) for solar thermal power plants consists of a synthetic oil or molten salt that stores energy in the form of heat collected by solar thermal power plants to enable smooth power output during daytime cloudy periods and to extend power production for 1–10 h after sunset. End-use TEES stores electricity from off-peak periods through the use of hot or cold storage in underground aquifers, water or ice tanks, or other storage materials and uses this stored energy to reduce the electricity consumption of building heating or air conditioning systems during times of peak demand.

7.2.9 Hybrid Energy Storage Systems

Certain applications require a combination of energy, power density, cost, and life cycle specifications that cannot be met by a single energy storage device. To implement such applications, hybrid energy storage devices (HESDs) have been proposed. HESDs electronically combine the power output of two or more devices with complementary characteristics. HESDs all share a common trait: combining high-power devices (devices with quick response) and high-energy devices (devices with slow response). Proposed HESDs are listed next, with the energy-supplying device listed first followed by the power-supplying device:

1. Battery and electric double-layer capacitor (EDLC)

2. FC and battery or EDLC

3. CAES and battery or EDLC

4. Battery and flywheel

5. Battery and SMES

7.4.1 Method of Calculating Power System Frequency

In this study, the index of the smoothing effect is used in power system frequency analysis. Power system frequency fluctuation occurs due to an imbalance between supply and load power in power system. Then, the frequency fluctuation can be described using two components: the rate of generator output variation, K_G (MW/Hz), and load variation, K_L (MW/ Hz). They are representing the amount of power variation causing 1 Hz frequency fluctuation. When generator output variation, ΔG (MW), and load variation, ΔL (MW), occur, frequency fluctuation of the power system, ΔF (Hz), is expressed as

where K is frequency characteristic constant. In general, frequency characteristic is expressed as percentage K_G (expressed as %K_G) for the total capacity of all generators and percentage K_L (expressed as %K_L) for the total load. In general, it is known that %K_G and %K_L are almost constant and generally take a value of 8–15% MW/Hz and 2–6% MW/Hz, respectively. However, K_L and K_G change greatly during a day because the number of parallel generators changes depending on the amount of load during a day. And when power imbalance ΔP occurs in power system, frequency fluctuation ΔP/K cannot occur immediately due to the governor characteristic and generator inertia. Normally, ΔF converges to a new steady-state value in 2 to 3 sec. In general, when ΔP is changing slowly, the relationship between ΔP and ΔF can be expressed as follows:

where ΔP = ΔG – ΔL. Since changing load is not considered in this study, ΔL is 0. Time constant, T (sec), depending on the setting of generator governor and generator inertia, is generally 3 to 5 sec. In this study, power system capacity is assumed to be 100 MW, and frequency characteristic K (MW/Hz) is selected to 8 MW/Hz. This selection means that adjustability of the system frequency is weak, resulting in a severe situation. Similarly, time constant T is selected to 3 sec. In this study, frequency fluctuation in the power system is evaluated using Equation (7.11). Therefore, frequency fluctuation, ΔF, is obtained as shown in Figure 7.12.

7.4.2 Simulation Results and Discussions

To evaluate the performance of the SMES strategy to minimize system frequency fluctuations, simulations are carried out considering variable wind speed data as shown in Figure 7.13. The specifications of wind speed are given in Table 7.4. The simulation time and time step were chosen as 60 sec and 10 msec, respectively. Figures 7.14 and 7.15 show the responses of active powers of the wind generators, diesel generators and hydraulic generators, and the system frequency without SMES systems with 2 MW and 6 MW loads, respectively. It is seen that without the wind generator the system frequency is maintained at the nominal value of 60 Hz; however, when the wind generator is used in the power system model the system frequency considerably fluctuates, especially for low load (2 MW load). This fact motivates the use of the SMES method to minimize system frequency and power fluctuations resulting from the wind generator system.

7.4.3 SMES Power and Energy Ratings

It is important to know the optimal power and energy ratings of SMES systems so that their cost is minimal. In this work, the relationship between SMES power capacity and the smoothing ability is investigated by evaluating a wind turbine generator output, P_WG, and reference value of transmission line power, PG-ref, with the condition that the LPF time constant is changed from 3 to 300 sec. Since a large number of wind turbine generators are going to be connected to a power system in the near future, a percentage of wind turbine generators to the total power system capacity is assumed fairly large: 10% (10 MW) and 20% (20 MW) in this study. Smoothing of the wind turbine generator output is investigated by using PG-ref instead of an SMES unit in the calculation to estimate a required capacity of the SMES [3].

The model system used in this simulation analysis is shown in Figure 7.22. Table 7.5 shows the parameters for the induction generator shown in Figure 7.22. The wind speed patterns and conditions used in the simulation are shown in Figure 7.23 and Table 7.6. Three kinds of wind speed patterns with relatively large fluctuations are selected. SMES output is assumed to be the difference between P_WG and PG-ref in this simulation, and then a standard deviation of the SMES output, σ, is calculated. In addition, smoothing effect is evaluated by using frequency fluctuation ΔF (ΔF_WG, ΔFG-ref). ΔF is calculated by applying P_WG and PG-ref to the frequency calculation model. The required SMES power capacity for smoothing P_WG and an LPF time constant suitable for the reference value with enough smoothing effect are investigated by using σ and ΔF in this simulation.

Figures 7.24 and 7.25 show the maximum frequency fluctuation with respect to the LPF time constant for two cases of wind generator capacity, 10% and 20%, respectively. Frequency fluctuation decreases as the LPF time constant increases. Therefore, if the transmission line power is compensated according to reference value PG-ref, it is possible to decrease the system frequency fluctuation due to the wind generator output fluctuations. It is clear from Figures 7.24 and 7.25 that the maximum frequency fluctuations converge to almost 0 Hz when the LPF time constant is over 120 sec.

Figure 7.26 shows the standard deviation, σ, of the SMES output with respect to the LPF time constant for the two wind generator capacities (10% and 20%). σ increases as the LPF time constant increases. However, as can be seen from Figure 7.26, the function of σ is not monotonous and saturates where the LPF time constant is about 120 sec. Therefore, if a longer LPF time constant and thus the larger capacity of SMES are adopted, the frequency deviation becomes small; however, the degree of improvement also becomes small. Table 7.7 shows σ and ΔF in each condition. Considering these results, the reference value of transmission line power corresponding to σ for 120 sec, the LPF time constant may be sufficient for the smoothing control. Consequently, it can be said that if a 120 sec LPF time constant is adopted, the suitable reference value with enough smoothing effect can be obtained. If the power capacity of the SMES is determined based on the value of 2σ, approximately 95% of necessary smoothing output can be achieved according to the characteristic of standard deviation. This capacity of SMES is applied for compensating P_WG fluctuations in the next simulation analysis.

Required energy storage capacity of the SMES is estimated when the latter compensates for the wind generator’s fluctuating power according to reference power PG-ref calculated using the LPF with several time constants including the best one determined in the previous section. Figure 7.27 shows the model for this evaluation. The wind power output is assumed as an ideal periodic sinusoidal wave form or trapezoidal wave form, and then the behavior of the SMES stored energy is investigated. To evaluate the influence of the period of wind power fluctuation on the smoothing effect, five LPF time constants (30, 60, 120, 180, and 300 sec) are used and the period of wind power fluctuation is varied from 10 to 1,200 sec. The required energy capacity of SMES can be determined as a difference of the maximum and the minimum values of the stored energy.

Table 7.8 shows a difference of the maximum and the minimum values of the SMES stored energy with respect to the period of fluctuation for several LPF time constants in two cases. In Table 7.8, the result in the case of an extremely long period (12,000 sec) is also indicated. From these results, (7.12) can be established. Consequently, the required SMES energy storage capacity to compensate fluctuating wind power with relatively long periodic change can be decided approximately by using (7.12), and the energy storage capacity greater than this is not needed [3–4].

where E (MJ) is the saturated value of the range of energy change of the SMES, T (sec) is the LPF time constant, and P_WF (MW) is the rated wind farm output.

Since it is shown that the frequency fluctuation can be decreased sufficiently by the reference value PG-ref, which is determined based on a LPF time constant of 120 sec, this value is used to calculate the reference value using the system shown in Figure 7.11. The model system used in the simulation analysis is shown in Figure 7.28. In this simulation analysis, the power capacity of the SMES is determined based on the value of 2σ, because the necessary smoothing effect can be achieved by an SMES with this capacity as shown in the previous section. When the power capacity of the wind generator is 10% of the entire power system, then the SMES power capacity is 5.5 MW (2σ = 2.7 × 2 = 5.4, where σ = 2.7 MW). Similarly, when the wind generator capacity is 20%, the SMES capacity is 11 MW (σ = 5.4 MW). SMES power capacities in these two cases are both 55% of the wind generator rated capacities (10 and 20% of the entire power system). In the following simulations, two cases using SMES systems with capacities of 55 and 50% of the wind generator rated capacity are examined for the three wind speed patterns.

Figure 7.29 shows responses of wind generator output and transmission line power when the capacity of the wind generator is 10% of the entire power system and the SMES power capacity is 55% and 50% of the wind generator. Figure 7.30 shows responses of frequency fluctuation under the same condition as Figure 7.29. Figure 7.31 shows responses of wind generator output and transmission line power when the capacity of the wind generator is 20% and the SMES power capacity is 55% and 50%. Figure 7.32 shows responses of frequency fluctuation under the same condition as Figure 7.30. Table 7.9 shows values of the maximum frequency fluctuation in each case. As shown in Figures 7.29 to 7.32, since necessary compensating power exceeds the capacity of the SMES system in some instances, large frequency fluctuation occurs, especially in the case of wind-2. It can be seen in Table 7.9 that frequency fluctuations in SMES power capacity at 50% are greater than those in SMES power capacity at 55%. The maximum frequency fluctuation in SMES power capacity at 50% is 0.061 Hz for 10% WG and 0.122 Hz for 20% WG, whereas the maximum frequency fluctuation in SMES power capacity at 55% is 0.006 Hz for 10% WG and 0.012 Hz for 20% WG. The former is approximately 10 times larger than the latter. Therefore, it can be said that the SMES with 50% power capacity of that of wind generator is not sufficient.

Consequently, it can be concluded that if an LPF time constant of 120 sec is selected and an SMES system with 55% power capacity of that of wind generator is adopted, a suitable reference value for the transmission line power can be obtained and then sufficient smoothing effect can be achieved. Moreover, the energy storage capacity of the SMES can be determined to 2,400 MJ according to (7.12).

7.5.1 DC Bus Microgrid System

Among the microgrids that have been researched recently, low-voltage DC (LVDC) bus-based systems have received attention because of advantages such as fast control without need for communication, a variety of DC energy sources and loads, and efficiencies on system size and cost. A block diagram of a typical DC bus microgrid system is shown in Figure 7.33.

The LVDC systems use the voltage droop technique, involving the DC bus voltage as a command signal. DC bus systems do not have the functional issues of AC systems such as synchronization and reactive power compensation. Unlike large-scale distribution systems, the LVDC microgrid system does not have a resistive loss issue because the length of the bus is much shorter. Also, sharing a DC bus has a structural advantage because all the energy sources and loads are connected via DC-to-DC or DC-to-AC voltage source converters that use DC voltage as their medium. Another layer of DC-to-AC converters is necessary for some subsystems to make an AC bus system.

Using the fast-acting power electronics converters, constant voltage can be supplied to the loads regardless of some fluctuations on the bus side, and the renewable energy sources can readily generate maximum power with maximum power point tracking (MPPT) techniques. As a small-scale power system, a microgrid can have relatively higher load fluctuations, especially when it is not operating in grid-connected mode. This is because the inertia of the generators is not as large as that found in the large-scale synchronous generators. Generation of power from the renewable energy sources relies heavily on natural conditions, and they are intermittent. Hence, energy storage devices are required for stable operation of a microgrid system in either grid-connected or islanded operations. A flywheel-based energy storage system is investigated in this work.

To develop a cost-effective system, a squirrel-cage induction machine is selected for the motor or generator of the flywheel energy storage in this work. A per-phase equivalent circuit of an induction machine is shown in Figure 7.34. Easy parallel operation is an important factor in energy storage for higher capacity.

7.5.2 Volt/Hertz Control

Volt/hertz control has been widely used for induction machine speed control. The field-oriented vector control technique has been used for applications that need fast response, but it requires parameter measurements, current feedback, a machine model, and controller tuning. On the DC bus Flywheel contrary, all of the information necessary to run an induction machine in rated condition, such as voltage, speed, and slip, can be found on the nameplate of the machine for volt/hertz control.

Much research on flywheel energy storage systems has used field-oriented controllers because the applications need quite fast energy flow, for example, Uninterruptable Power Supply (UPS) application compensating the voltage dip. However, if an energy storage system absorbs or releases the energy in slow dynamics, volt/hertz control can be a valid candidate due to its simplicity and inherent stability.

7.5.3 Microgrid System Operation

The microgrid system shown in Figure 7.33 uses the DC bus voltage as a control signal. Hence, a prime mover, such as a grid-connected converter or micro turbine unit, does not control the voltage tightly at a fixed reference voltage to reflect the energy flow on the bus voltage if it is in the nominal operating region above the minimum threshold. All of the renewable energy source units are operating in the power mode, which is generating its maximum power when the energy is available. Hence, the bus voltage will rise above the nominal operating range if the generation is larger than the load power consumption. The storage devices detect the excessive energy in the bus and absorb it. If the generated energy is large enough to exceed the maximum threshold, the grid-connected converter can push it back to the grid.

When the DC bus voltage gets lower than the discharge threshold due to the increased load or decreased generation, the energy stored in the storage units is discharged to the bus to maintain the bus voltage at the minimum level. If all the energy storages and generators are not able to hold the bus voltage at the minimum level, load shedding can be initiated by disconnecting some of the power electronics converters supporting lower priority loads. If different kinds of storage or load control units are connected to the bus, the priority can be easily controlled by setting the thresholds differently. The units in the microgrid are autonomously operating using the bus voltage without communicating between units or to a central controller; hence, addition, replacement, or removal of the units can be done easily without any major change in the control configuration unlike the centrally controlled system.

7.5.4 Control of Flywheel Energy Storage System

The control block diagram of the flywheel drive system is shown in Figure 7.35. The controller consists of three parts—mode control, slip control, and voltage control—to generate the proper voltage and frequency for the flywheel induction motor or generator. The low-pass filter filters out the high-frequency voltage fluctuations. The mode control determines the operating mode based on the bus voltage. The operating modes of the proposed flywheel energy storage system, and the state machine of the mode control can be seen in Figure 7.36. Each mode has associated voltage levels predefined in the mode control, as shown in Figure 7.37.

7.5.5 Stability Consideration

It is well known that DC power distribution systems can have stability issues due to the negative impedance of the connected converters, even if the individual subsystems are stable. The input impedance of the converter can be expressed as (7.13). The input impedance of the converters becomes negative when they are operating in constant power mode. where Δ denotes the deviation from the steady-state operating point values.

The power electronics converters can tightly control their output power as almost constant, and the negative impedance affects the DC bus stability adversely. It has also been suggested that the output impedances of the sources Z_o should be smaller than input impedances of the loads Z_i for the overall stability of the DC bus.

Although this is not a direct issue for the proposed energy storage system because it does not operate in constant power mode, its effect on overall system stability needs to be considered. The proposed system controls the power with the slip and constant volt/hertz ratio, which keeps the torque proportional to the slip. When the flywheel energy storage system charges the energy from the bus, the slip is proportional to the bus voltage. Hence, the power that the storage system takes from the bus is proportional to the bus voltage and the impedance of the system is always positive. On the other hand, the slip is inversely proportional to the change of the bus voltage, which lowers the overall source impedance because the DC current the storage system discharges increases as the bus voltage decreases. Therefore, the energy storage system can improve the overall stability of the system in either operating mode.

7.6.1 Control of Individual DFIG Wind Turbines

The control system of individual DFIG wind turbines generally consists of two parts: (1) the electrical control of the DFIG; and (2) the mechanical control of the wind turbine blade pitch angle and yaw system. Control of the DFIG is achieved by controlling the RSC, the GSC, and the ESS as shown in Figure 7.38. The control objective of the RSC is to regulate the stator-side active power P_s and reactive power Q_s independently. The control objective of the GSC is to maintain the DC link voltage V_dc constant and to regulate the reactive power Q_g that the GSC exchanges with the grid. The control objective of the ESS is to regulate the active power P_g that the GSC exchanges with the grid.

7.6.2 Control of the RSC

Figure 7.39 shows the overall vector control scheme of the RSC, in which the independent control of the stator active power P_s and reactive power Q_s is achieved by means of rotor current regulation in a stator-flux-oriented synchronously rotating reference frame. Therefore, the overall RSC control scheme consists of two cascaded control loops. The outer control loop regulates the stator active and reactive powers independently, which generates the reference signals i*_dg and i*_dg of the d- and q-axis current components, respectively, for the inner-loop current regulation. The outputs of the two current controllers are compensated by the corresponding cross-coupling terms v_dg0 and v_dg0, respectively, to form the total voltage signals v_dg and v_qg. They are then used by the PWM module to generate the gate control signals to drive the RSC. The reference signals of the outer-loop power controllers are generated by the high-layer WFSC.

7.6.3 Control of the GSC

Figure 7.40 shows the overall vector control scheme of the GSC, in which the control of the DC link voltage V_dc and the reactive power Q_g exchanged between the GSC and the grid is achieved by means of current regulation in a synchronously rotating reference frame. Again, the overall GSC control scheme consists of two cascaded control loops. The outer control loop regulates the DC link voltage V_dc and the reactive power Q_g, respectively, which generates the reference signals i*_dg and i*_qg of the d- and q-axis current components, respectively, for the inner-loop current regulation. The outputs of the two current controllers are compensated by the corresponding cross-coupling terms v_dg0 and v_qg0, respectively, to form the total voltage signals v_dg and v_qg. They are then used by the PWM module to generate the gate control signals to drive the GSC. The reference signal of the outer-loop reactive power controller is generated by the high-layer WFSC.

7.6.4 Configuration and Control of the ESS

Figure 7.41 shows the configuration and control of the ESS. The ESS consists of a supercapacitor bank and a two-quadrant DC-to-DC converter connected to the DC link of the DFIG. The DC-to-DC converter contains two IGBT switches S₁ and S₂. Their duty ratios are controlled to regulate the active power P_g that the GSC exchanges with the grid. In this configuration, the DC-to-DC converter can operate in two different modes (i.e., buck or boost mode) depending on the status of the two IGBT switches. If S₁ is open, the DC-to-DC converter operates in the boost mode; if S₂ is open, the DC-to-DC converter operates in the buck mode. The duty ratio D₁ of S₁ in the buck mode can be approximately expressed as

and the duty ratio D₂ of S₂ in the boost mode is D₂ = 1 – D₁. In this book, the nominal DC voltage ratio V_SC,_n/V_dc,_n is 0.5, where V_SC,_n and V_dc,_n are the nominal voltages of the supercapacitor bank and the DFIG DC link, respectively. Therefore, the nominal duty ratio D_1,n of S₁ is 0.5.

The operating modes and duty ratios D₁ and D₂ of the DC-to-DC converter are controlled depending on the relationship between the active powers P_r of the RSC and P_g of the GSC. If P_r is greater than P_g, the converter is in buck mode and D₁ is controlled, such that the supercapacitor bank serves as a sink to absorb active power, which results in the increase of its voltage V_SC. On the contrary, if P_g is greater than P_r, the converter is in boost mode and D₂ is controlled, such that the supercapacitor bank serves as a source to supply active power, which results in the decrease of its voltage V_SC. Therefore, by controlling the operating modes and duty ratios of the DC-to-DC converter, the ESS serves as either a source or a sink of active power to control the generated active power of the WTG. In Figure 7.41, the reference signal P^*_g is generated by the high-layer WFSC.

7.6.5 Wind Turbine Blade Pitch Control

Figure 7.42 shows the blade pitch control for the wind turbine, where ω_r and P_e (= P_s + P_g) are the rotating speed and output active power of the DFIG, respectively. When the wind speed is below the rated value and the WTG is required to generate the maximum power, ω_r and P_e are set at their reference values, and the blade pitch control is deactivated. When the wind speed is below the rated value but the WTG is required to generate a constant power less than the maximum power, the active power controller may be activated, where the reference signal P_e^* is generated by the high-layer WFSC and P_e takes the actual measured value. The active power controller adjusts the blade pitch angle to reduce the mechanical power that the turbine extracts from wind. This reduces the imbalance between the turbine mechanical power and the DFIG output active power, thereby reducing the mechanical stress in the WTG and stabilizing the WTG system. Finally, when the wind speed increases above the rated value, both ω_r and P_e take the actual measured values, and both the speed and active power controllers are activated to adjust the blade pitch angle.

7.6.6 Wind Farm Supervisory Control

The objective of the WFSC is to generate the reference signals for the outer-loop power controllers of the RSC and GSC, the controller of the DC-to-DC converter, and the blade pitch controller of each WTG, according to the power demand from or the generation commitment to the grid operator. The implementation of the WFSC is described by the flow chart in Figure 7.43, where P_d is the active power demand from or the generation commitment to the grid operator; v_wi and V_essi are the wind speed in meters per second and the voltage of the supercapacitor bank measured from WTG i (i = 1,. . ., N), respectively; and N is the number of WTGs in the wind farm.