3. Integrating active power filters with renewable energy sources

Researches on high-performance power electronic converters combined with renewable energy sources (RENs) capable to extract more energy at a lower cost leads this technology to become technically and economically feasible to meet all the global energy needs. Encompassed by this course of events, there is a novel tendency for replacing the conventional centralized generation systems, with long transmission lines, to the distributed generation (DG) systems. In this novel concept on DG systems, renewable energy sources and storage systems are combined with the existent conventional sources to supply stand-alone or gridconnected loads. Moreover, it provides a better way of using onsite energy resources, minimizing transmission and distribution costs, which is crucial to reduce obstacles for rural or remote areas electrification and to encourage sustainable business development.

In this scenario, which comprehends the real modern power grids, active filters play a key role as an interface for connecting the REN to the power grid. For example, consider the shunt active filter illustrated in Figure 1 with photovoltaic panels and a DC-DC converter connected at the DC-link voltage, as indicated in Figure 17, and the shunt active filter presents an additional feature of controlling the produced energy of the photovoltaic panels to the power grid. Usually, there is a boost converter between the photovoltaic panels and the DC-link voltage (vDC), once the terminal voltage on these panels is much lower than vDC.

For extracting the maximum energy of these panels, the maximum power point tracking (MPPT) algorithm controls the duty cycle of the boost converter. Through the combined operation between the MPPT algorithm and the DC-link voltage controller, it is possible to control the exchange of energy from the PV to the power grid [40]. Consider the output signal of the DC-link voltage controller, labeled in Figure 4 as Ploss, to understand this dynamics. In this scenario, once the produced current by the PV arrays exceeds the active power losses of the converters, Ploss becomes naturally negative. In this power balance, the duty cycle control of the MPPT algorithm increases while the derivative of the PV active power is positive, with the control signal Ploss becomes more negative to keep the DC-link voltage regulated at its rated value. This interactive loop stops when the derivative of the PV active power is equal to zero, which means that the optimal set point (MPPT) was reached. To avoid loss of controllability, it is recommended to include an enable condition to update the duty cycle output of the MPPT

algorithm only when Ploss reaches its steady-state condition. Another alternative is to consider the DC-link voltage controller with a faster dynamics in comparison to the MPPT algorithm. Another possibility is integrating active filters with Doubly-Fed Induction Generator (DFIG) wind turbine as illustrated in Figure 18, with one converter connected to the DFIG (RSC—rotor side converter) and the other one presents shunt connection to the power grid (GSC—grid side converter). It is notorious that RSC controls the flow of energy from DFIG to the DC-link,

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Figure 18. Simplified scheme of back-to-back converters with doubly fed induction generator (DFIG) wind turbine.

In this configuration, the MPPT algorithm is included on RSC control algorithms. Its input is the mechanical speed (ωdfig) with the corresponding produced active power as the output. Moreover, the objective of RSC is to control the reactive power in the stator and the total active power of the DFIG (rotor and stator active powers), controlling the energy flow between the generator and the DC-link voltage. On the other hand, GSC produces controlled currents in

Backing to the MPPT algorithm, a possible algorithm corresponds in incrementing the reference current component related to the rotor active power (i\*r\_dfig) while the derivative of the active power of the generator is positive. In the literature, there are several proposals of MPPT

Nevertheless, there are some bottlenecks for connecting RENs to the power grid. One of them is their intermittent behavior, resulting in voltage- and frequency-deviations, which means an oscillating energy flow. This feature is usual in weak systems, where low inertia dispatchable power source and highly variable RENs are expected. This problem can be mitigated, under

whereas GSC transfers the stored energy on DC-link to the power grid.

counter phase with the grid voltages due to the DC-link voltage controller.

algorithms for wind energy systems as described in [41].

Figure 17. Shunt active filter as an interface for connecting photovoltaic panels to the power grid.

systems are combined with the existent conventional sources to supply stand-alone or gridconnected loads. Moreover, it provides a better way of using onsite energy resources, minimizing transmission and distribution costs, which is crucial to reduce obstacles for rural or

In this scenario, which comprehends the real modern power grids, active filters play a key role as an interface for connecting the REN to the power grid. For example, consider the shunt active filter illustrated in Figure 1 with photovoltaic panels and a DC-DC converter connected at the DC-link voltage, as indicated in Figure 17, and the shunt active filter presents an additional feature of controlling the produced energy of the photovoltaic panels to the power grid. Usually, there is a boost converter between the photovoltaic panels and the DC-link

For extracting the maximum energy of these panels, the maximum power point tracking (MPPT) algorithm controls the duty cycle of the boost converter. Through the combined operation between the MPPT algorithm and the DC-link voltage controller, it is possible to control the exchange of energy from the PV to the power grid [40]. Consider the output signal of the DC-link voltage controller, labeled in Figure 4 as Ploss, to understand this dynamics. In this scenario, once the produced current by the PV arrays exceeds the active power losses of the converters, Ploss becomes naturally negative. In this power balance, the duty cycle control of the MPPT algorithm increases while the derivative of the PV active power is positive, with the control signal Ploss becomes more negative to keep the DC-link voltage regulated at its rated value. This interactive loop stops when the derivative of the PV active power is equal to zero, which means that the optimal set point (MPPT) was reached. To avoid loss of controllability, it is recommended to include an enable condition to update the duty cycle output of the MPPT

remote areas electrification and to encourage sustainable business development.

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

voltage (vDC), once the terminal voltage on these panels is much lower than vDC.

Figure 17. Shunt active filter as an interface for connecting photovoltaic panels to the power grid.

Figure 18. Simplified scheme of back-to-back converters with doubly fed induction generator (DFIG) wind turbine.

algorithm only when Ploss reaches its steady-state condition. Another alternative is to consider the DC-link voltage controller with a faster dynamics in comparison to the MPPT algorithm.

Another possibility is integrating active filters with Doubly-Fed Induction Generator (DFIG) wind turbine as illustrated in Figure 18, with one converter connected to the DFIG (RSC—rotor side converter) and the other one presents shunt connection to the power grid (GSC—grid side converter). It is notorious that RSC controls the flow of energy from DFIG to the DC-link, whereas GSC transfers the stored energy on DC-link to the power grid.

In this configuration, the MPPT algorithm is included on RSC control algorithms. Its input is the mechanical speed (ωdfig) with the corresponding produced active power as the output. Moreover, the objective of RSC is to control the reactive power in the stator and the total active power of the DFIG (rotor and stator active powers), controlling the energy flow between the generator and the DC-link voltage. On the other hand, GSC produces controlled currents in counter phase with the grid voltages due to the DC-link voltage controller.

Backing to the MPPT algorithm, a possible algorithm corresponds in incrementing the reference current component related to the rotor active power (i\*r\_dfig) while the derivative of the active power of the generator is positive. In the literature, there are several proposals of MPPT algorithms for wind energy systems as described in [41].

Nevertheless, there are some bottlenecks for connecting RENs to the power grid. One of them is their intermittent behavior, resulting in voltage- and frequency-deviations, which means an oscillating energy flow. This feature is usual in weak systems, where low inertia dispatchable power source and highly variable RENs are expected. This problem can be mitigated, under certain limits, through the shunt active filters with an energy storage element, capable to confine the oscillating energy between the active filter and the load [31].

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Other issue is the load power sharing between different power converters in a decentralized microgrid. An alternative to overcome this problem is extending the droop controller concept to the shunt active filters connected in the same power grid. In this case, the active filters modify their output impedance through the virtual impedance method [42]. Basically, once these active filters share the same grid voltage, they are conditioned to produce controlled currents such that their output impedance is modified according to the capabilities of sharing the active- and reactive powers of the load. This issue is one of the most exploited ones by researchers to make the implementation of decentralized microgrids reliable.
