1. Introduction

In this section, we present a discussion involving basic aspects of the active filters for generation and distribution grids. It is important to comment that there are also power electronics compensators for transmission grids presenting different features as, for instance, damping subsynchronous resonance [1], power flow control [2, 3] and improve the stability of a power system [4]. These compensators are known as Flexible AC Transmission System (FACTS), and their study is beyond the scope of this chapter.

Backing to the active power filters, they can be understood as a controlled current sources or controlled voltage sources capable for compensating different power quality problems as, for

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and eproduction in any medium, provided the original work is properly cited.

instance, harmonic and unbalanced components, power factor, voltage sags or swells, damping low-frequency harmonic oscillations, and so on [5, 6]. Moreover, they are used as an interface for renewable energy sources in a new concept of distributed generation or even making the implementation of decentralized microgrids reliable [7–9].

A simplified scheme of the shunt active filter compensating all the harmonic currents drawn by the load is illustrated in Figure 1. An active filter is comprehended by power and control stages. The power stage comprises a voltage source converter (VSI), with a storage energy element (capacitor) at its DC link, inductor filter (Lfp), and small passive filters (Zfp) to provide a low impedance path to the high-frequency components of the produced current by the VSI (iLfp). The control stage presents measurement and instrumentation circuits, microcontrollers, and VSI drivers. As indicated in Figure 1, the reference current produced by the VSI (i \* ) is determined based on the applied control algorithms, which presents the load current (iL), grid voltage (vS) and the DC-link voltage (vDC) as inputs. There is also a pwm controller for keeping iLfp in conformity with the reference current (i \* ). A common point (cp) was considered to indicate that, in a three-phase circuit, the passive filters are connected at this point of the circuit.

It is important to comment that an inductor (LS) is usually applied to represent the grid impedance, which reflects the inductance characteristics of line cables and power transformers. Nevertheless, current researches point out to replace its representation by equivalent impedances that are dynamically modified due to a considerable amount of nonlinear loads, which are dynamically connected and removed from the power grid. This issue becomes more important nowadays due to the proliferation of renewable energy sources with power converter interface [10–13].

An additional storage energy element (SEE) is necessary if sag compensation is required. As depicted Figure 2 with a SEE connected in parallel with the DC-link capacitor, it can be represented as, for instance, ultracapacitors or batteries [14]. There are also other SEEs as superconducting magnetic energy storage (SMES) [15] and flywheel [16]. However, once they are not voltage-source type, it is necessary to use power converters to interface them with the

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Figure 2. Simplified scheme of the series active filter compensating harmonics and voltage sag.

Other issue involving the power stage of the series active filter corresponds to its series connection, which may or may not be done through power transformers. A constraint for implementing active filters without series transformer injection is to avoid short circuits between the phase circuits, which can be done replacing the three-phase VSI by three singlephases VSIs with three independent DC-link voltages as introduced in [17, 18]. Other alternative is the use of high-frequency transformers at the DC-link of the single-phase VSIs, which

Other possible active filter topology consists on the combination of the shunt and series active filters, resulting on the unified power quality conditioner (UPQC). As described in [20], by having these two conditioners connected to the electrical system, simultaneous compensation of the current demanded from the utility and the voltage delivered to the load can be accomplished. As illustrated in Figure 3, the series and shunt active filters compensate at the same time all the harmonic components of the load currents and grid voltages. Its power stage combines all the passive components of the series and shunt active filters as previously exploited. In the same way, its control stage presents all the circuitry, microcontrollers, and control algorithms of both active filters. Particularly, in this configuration, the shunt active filter is responsible to draw a controlled current to keep the DC-link voltage (vDC) regulated. A summary of the UPQC compensation capabilities is shown in Table 1, with the functionalities of the series and shunt active filters well established. Nevertheless, there are proposals in the literature with both active filters compensating the same power quality problem in a complementary way. For instance [21, 22]

DC-link voltage.

are usually applied in isolated DC-DC converters [19].

Figure 2 illustrates a simplified scheme of the series active filter compensating harmonics and voltage sag, with the reference voltage (v\*) being determined through the applied control algorithms, which presents the grid current (iS), grid voltage (vS), and the DC-link voltage (vDC) as inputs. Moreover, there is a pwm controller for producing the VSI filtered voltage (vZsf).

Figure 1. Simplified scheme of the shunt active filter compensating all the harmonic currents drawn by the nonlinear load.

New Trends in Active Power Filter for Modern Power Grids http://dx.doi.org/10.5772/intechopen.72195 67

Figure 2. Simplified scheme of the series active filter compensating harmonics and voltage sag.

instance, harmonic and unbalanced components, power factor, voltage sags or swells, damping low-frequency harmonic oscillations, and so on [5, 6]. Moreover, they are used as an interface for renewable energy sources in a new concept of distributed generation or even

A simplified scheme of the shunt active filter compensating all the harmonic currents drawn by the load is illustrated in Figure 1. An active filter is comprehended by power and control stages. The power stage comprises a voltage source converter (VSI), with a storage energy element (capacitor) at its DC link, inductor filter (Lfp), and small passive filters (Zfp) to provide a low impedance path to the high-frequency components of the produced current by the VSI (iLfp). The control stage presents measurement and instrumentation circuits, microcontrollers, and VSI

based on the applied control algorithms, which presents the load current (iL), grid voltage (vS) and the DC-link voltage (vDC) as inputs. There is also a pwm controller for keeping iLfp in

It is important to comment that an inductor (LS) is usually applied to represent the grid impedance, which reflects the inductance characteristics of line cables and power transformers. Nevertheless, current researches point out to replace its representation by equivalent impedances that are dynamically modified due to a considerable amount of nonlinear loads, which are dynamically connected and removed from the power grid. This issue becomes more important nowadays due to the proliferation of renewable energy sources with power con-

Figure 2 illustrates a simplified scheme of the series active filter compensating harmonics and voltage sag, with the reference voltage (v\*) being determined through the applied control algorithms, which presents the grid current (iS), grid voltage (vS), and the DC-link voltage (vDC) as inputs. Moreover, there is a pwm controller for producing the VSI filtered

Figure 1. Simplified scheme of the shunt active filter compensating all the harmonic currents drawn by the nonlinear

\*

). A common point (cp) was considered to indicate that,

) is determined

making the implementation of decentralized microgrids reliable [7–9].

66 Power System Harmonics - Analysis, Effects and Mitigation Solutions for Power Quality Improvement

drivers. As indicated in Figure 1, the reference current produced by the VSI (i

\*

in a three-phase circuit, the passive filters are connected at this point of the circuit.

conformity with the reference current (i

verter interface [10–13].

voltage (vZsf).

load.

An additional storage energy element (SEE) is necessary if sag compensation is required. As depicted Figure 2 with a SEE connected in parallel with the DC-link capacitor, it can be represented as, for instance, ultracapacitors or batteries [14]. There are also other SEEs as superconducting magnetic energy storage (SMES) [15] and flywheel [16]. However, once they are not voltage-source type, it is necessary to use power converters to interface them with the DC-link voltage.

Other issue involving the power stage of the series active filter corresponds to its series connection, which may or may not be done through power transformers. A constraint for implementing active filters without series transformer injection is to avoid short circuits between the phase circuits, which can be done replacing the three-phase VSI by three singlephases VSIs with three independent DC-link voltages as introduced in [17, 18]. Other alternative is the use of high-frequency transformers at the DC-link of the single-phase VSIs, which are usually applied in isolated DC-DC converters [19].

Other possible active filter topology consists on the combination of the shunt and series active filters, resulting on the unified power quality conditioner (UPQC). As described in [20], by having these two conditioners connected to the electrical system, simultaneous compensation of the current demanded from the utility and the voltage delivered to the load can be accomplished. As illustrated in Figure 3, the series and shunt active filters compensate at the same time all the harmonic components of the load currents and grid voltages. Its power stage combines all the passive components of the series and shunt active filters as previously exploited. In the same way, its control stage presents all the circuitry, microcontrollers, and control algorithms of both active filters. Particularly, in this configuration, the shunt active filter is responsible to draw a controlled current to keep the DC-link voltage (vDC) regulated. A summary of the UPQC compensation capabilities is shown in Table 1, with the functionalities of the series and shunt active filters well established. Nevertheless, there are proposals in the literature with both active filters compensating the same power quality problem in a complementary way. For instance [21, 22]

Algorithms for determining the reference current are related to which features we expect that the active filter be able to compensate. It is important to comment that control algorithms for shunt active filters have been proposed in the literature for more than 30 years. Among all these proposals, those derived from the instantaneous power theory [23–25], dq reference frame [26–28], conservative power theory [7], and the active and non-active currents [29–31]

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The instantaneous power theory, or p-q theory, was emerged at the beginning of the 1980s, with the main purpose to provide new power definitions in time domain for three-phase threewire circuits and, in sequence for three-phase four-wire circuits. Based on the αβ0 system coordinates, the p-q theory has the advantage of instantaneously separating the homopolar (zero-sequence) from the nonhomopolar (positive- and negative-sequence) components [31]. This issue allowed new proposals on control algorithms to three-phase four-wire active filters. An enhanced version of the p-q theory, known as the p-q-r theory, was conceived based on a different coordinate translation, where voltages and currents are translated from αβ0 to p-q-r system coordinates [32, 33]. Another approach is the use of Park transformation with a synchronizing circuit (d-q coordinate system) to conceive control algorithms based on the dq reference frame. A comparison involving all of these algorithms for active power filters was

A different methodology from the aforementioned corresponds to the active and non-active currents, which does not present any kind of coordinate translation. It derives from Fryze active current concept and presents a very simple formulation as introduced in [34]. Essentially, this algorithm determines the minimum (active) current component that transports the same energy of a generic three-phase load current. Due to its simplicity, we choose the control algorithms based on the active and non-active currents as basis to exploit the performance of the active filters, considering a power grid with unbalanced voltages and nonlinear loads.

Figure 4 presents a control algorithm for constant instantaneous active power concept, whereas Figure 5 for sinusoidal grid current concept, with the grid voltages (vSa, vSb, vSc) replaced by the control signals plla, pllb, pllc. These signals are unitary sinusoidal waveforms synchronized with the fundamental positive-sequence component of the grid voltages

Figure 4. Control algorithm based on the Fryze active currents for constant instantaneous active power concept.

are widely applied.

introduced in [33].

Figure 3. Simplified scheme of the unified power quality conditioner (UPQC).


#### Table 1. UPQC functionalities.

present a sag compensation proposal through the combined operation of the series and shunt active filters for the maximum utilization of both active filters.

In next section, we exploit their control algorithms.
