Chapter 9
Distributed energy resources and power electronics
Masahide Hojo Department of Electrical and Electronic Engineering, Faculty of Engineering, Tokushima University, Japan
Abstract
Most of the distributed energy resources (DERs) generate DC power or alternating current (AC) power with variable frequency but the power system is operated with an almost constant frequency. In order to inject electric power from DERs to a power system, a suitable power conversion has to be required. From the DC power source, the power conversion is provided by an inverter, which converts the electric power from DC to AC. On the other hand, the AC power with variable frequency is often converted to DC first, and then it is converted to the AC power by an inverter, synchronizing with the power system. Therefore, most of DERs have an inverter at the coupling point with the power system. The inverter usually configured by a sinusoidal voltage source, which is emulated by a self-commutated static power converter. In addition, the converter normally equips some functions not only to protect itself from a power system disturbance but also to provide some ancillary services. Therefore, the inverter is called a utility interactive inverter.
Keywords
distributed energy resources
DC
alternating current (AC)
power conversion
inverter
Most of the distributed energy resources (DERs) generate DC or AC power with variable frequency but the power system is operated with an almost constant frequency. In order to inject electric power from DERs to a power system, a suitable power conversion has to be required. From the DC power source, the power conversion is provided by an inverter, which converts the electric power from DC to AC. On the other hand, the AC power with variable frequency is often converted to DC first, and then it is converted to the AC power by an inverter, synchronizing with the power system. Therefore, most of DERs have an inverter at the coupling point with the power system. The inverter usually configured by a sinusoidal voltage source, which is emulated by a self-commutated static power converter. In addition, the converter normally equips some functions not only to protect itself from a power system disturbance but also to provide some ancillary services. Therefore, the inverter is called a utility interactive inverter.
This chapter discusses popular power conversion technology for several DERs. At first, a basic technology is discussed in order to inject the electric power generated by the DERs to the power system with the constant frequency. In the following sections, three types of popular DERs, a photovoltaic (PV) generation system, a wind turbine, and an energy storage system by a battery, are explained separately. And finally, models for considerations of interferences among the DERs are considered.
In a stable power system, a node voltage, which the DER is connected, can be considered as a sinusoidal voltage source with a nominal frequency. Although the amplitude and frequency of the node voltage may actually fluctuate by some disturbance, they stay within a small range in general stable power system. Therefore, the node voltage used to be considered as an ideal sinusoidal voltage source in the research field of the utility interactive inverter.
Fig. 9.1 introduces a one-line diagram with the simplest model of the DER, which can trade an active and a reactive power between the DER and the power system. Vc is the output voltage phasor of the DER, Vs is the node voltage at the coupling point of the DER, and jXc is a reactance, which represents an interconnecting inductor placed between the utility interactive inverter and the power system. The percent reactance of the inductor may be 3–5% of the DER.
Figure 9.1 The simplest model of the DER.
The relations between Vc and Vs can be displayed as shown in Fig. 9.2. In this diagram, the active and reactive power of the DER is determined by the relation between Vc and the current phasor I. On the other hand, practical restrictions are given by the generated power of the primary energy resources such as solar panels or wind turbine, and the rated apparent power of the utility interactive inverter. The former defines the active power uniquely, and the latter restricts the maximum value of the reactive power. Therefore, the controller of the DER has to define the output current phasor of I depending on the voltage phasor of Vs at the coupling point of the power system by a phase locked–loop controller. In general, the current phasor is defined by the two-axis theory based on the reference phase angle given by the phase locked–loop controller. The inverter controller detects its three-phase output current as two-axis components, compares them with their reference values, and defines the appropriate modification of the output voltage. When the reference output voltage phasor is decided, it should be converted to three-phase sinusoidal reference voltage waveforms by decomposition of the two-axis theory.
Figure 9.2 A phasor diagram around the DER.
Generally, the sinusoidal voltage Vc is realized by a self-commutated inverter from an equivalent ideal DC voltage source, which is provided by the DER. Various circuit topologies and converter control techniques can be found, but the most popular one is a full bridge inverter and a pulse width modulation control. When the frequency of its carrier wave is sufficiently higher than the nominal frequency, the utility interactive inverter does not basically cause harmonic problem. It should be high frequency under the restriction by the semiconductor devices because the inverter may cause audible noise depending on the switching frequency. In addition, the utility interactive inverter has some necessary functions for electric safety and protection of the apparatus.
9.1. Power electronics in PV power generation systems
Fig. 9.3 shows a basic configuration of the PV generation system for the three-phase power system. It consists of three components; a solar panel, a DC–DC converter, and a utility interactive inverter. The DC–DC converter is the first stage of power conversion. It regulates the DC voltage across the solar panel so as to derive its maximum power as well as keeps a constant DC voltage at the DC side of the utility interactive inverter. And then, the DC power is converted to AC power by the utility interactive inverter. When the active power output of the utility interactive inverter is controlled to keep the DC voltage constant, this power conversion can work successfully.
Figure 9.3 A basic configuration of the PV generation system.
Fig. 9.4 represents a simple model of the PV generation unit, which consists of solar panels. It has a typical relationship between output current and voltages shown in Fig. 9.5. By this relationship, the power curve by the open-circuit voltage can be derived as shown in Fig. 9.6. As can be seen in Fig. 9.6, the PV generation unit has an optimal operating point at which the unit can generate its maximum power. However, these curves shown in Figs. 9.5 and 9.6 depend on the conditions of solar irradiation and the temperature. In the circumstances, the input voltage of the DC–DC converter in Fig. 9.3 has to be considered as a variable voltage, depending on the conditions around the solar panels. Therefore, the DC–DC converter is required to connect the variable input voltage and the constant output voltage for the utility interactive inverter.
Figure 9.4 A simple model of the PV generation unit.
Figure 9.5 The relationship between output current and voltages of the model.
Figure 9.6 Characteristics of the output power of the model.
Normally, the voltage across the PV generation unit is designed as smaller than the DC voltage of the utility interactive inverter. The power flow through the DC–DC converter is only one direction from the solar panels to the grid. Therefore, the DC–DC converter is realized by a boost chopper in many cases. There are some alternative circuit topologies such as a high-frequency DC–DC converter with an isolating transformer for a small rated power unit.
9.2. Power electronics in wind power generation systems
The most simple wind power generation unit simply consists of an induction motor. If a wind turbine is accelerated by the wind over the nominal rotating frequency of the grid, the induction motor becomes to generate the electric power.
Today, a lot of high power wind turbines are installed. The mechanical torque from the wind turbine is converted to the electric power by a synchronous generator or a variable speed induction generator.
The former needs a utility interactive inverter because the synchronous generator supplies AC power with variable frequency depending on the wind speed. Fig. 9.7 shows a basic configuration of the system based on the synchronous generator. The power conversion consists of two steps. First, the generated AC power with variable frequency is converted to DC power by the rectifier, and then it is sent to a utility interactive inverter.
Figure 9.7 A basic configuration of the wind power system based on a synchronous generator.
Fig. 9.8 shows a typical wind power system by variable speed induction generator. The generated main power does not flow through a utility interactive inverter. The back-to-back converter applies magnetizing current to the rotor of the induction generator. As a result, the wind turbine can rotate with frequency different from the nominal frequency of the grid.
Figure 9.8 A basic configuration of high power variable speed wind power system.
In addition, the wind power generation system is connected or combined with high- or medium-voltage DC transmission system in such as offshore windfarms.
9.3. Power electronics in battery energy storage systems
Fig. 9.9 shows a basic configuration of the battery energy storage systems. As the battery unit trades DC power, it is constructed similarly to the PV generation system. It consists of three components; a battery unit, a DC–DC converter, and a utility interactive inverter. By contrast to the PV generation system, the DC–DC converter has to be a bidirectional-type converter. In addition, the battery voltage slightly varies along its state of charge. Moreover, voltage imbalance can be caused among the battery cells in some cases. In this case, the DC–DC converter has to provide a function of balancing control.
Figure 9.9 A basic configuration of the battery energy storage system.
9.4. Power quality problems with related to DERs
For power system analysis, it is important to build a model suitable for analysis. In many cases, the power system is analyzed based on effective values. All the voltages and currents are represented by effective values and the other variables such as active and reactive power will be calculated by them. In such analysis, system behaviors in the range over tens of cycles are considered. In this case, the DER unit can be modeled by current source shown in Fig. 9.10 represented by effective value of its output because the current controller of the DERs works well and the utility interactive inverters can regulate its output voltage at sufficiently quick speed. In this case, various DERs can be modeled by a common style as shown in Fig. 9.10 because engineers need not to consider what the background energy source is but to know how their output are regulated by the DERs controller. This model is effective for study on energy management among the DERs.
Figure 9.10 The simplest current source model of the DER.
On the other hand, some engineers consider more detail phenomena, which vary around the several cycles. For example, transient phenomena after a system disturbance should be considered by more detail model. In such cases, the voltage source model shown in Fig. 9.1 may be useful. By applying a current controller to the voltage source model, the transient phenomena can be analyzed with varying node voltage.
Moreover, some engineers consider more detail phenomena, which vary shorter than several cycles.
For example, harmonics and high-speed transient behaviors should be considered by instantaneous values because other frequency components must be considered in such case. The converters must be configured by a static power converter including switching devices.
There can be some challenges of power quality improvements such as reduction of harmonics or suppression of three-phase imbalance by the utility interactive inverters. Both can be detected as AC components in two-axis theory. Therefore, if the utility interactive inverter has sufficient surplus capacity, the power quality improvement can be realized by the converter control, which regulates the d- and q-axis components as constant values.
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