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Keywords— Hybrid AC/DC Nanogrid, Renewable energy, Active Power Filter (APF), Harmonics mitigation, Unbalance compensation, Power Factor Correction (PFC).
The power quality of electric power processing and delivery is gaining more attention. Bad quality of the electric power wears out the system equipment rapidly, increasing the cost of maintenance, resulting in system failure or an inconvenient shutdown while leading to strong negative effects on the environment –. Nowadays, the unprecedented expansion usage of the native DC electrical equipment, such as compact fluorescent lamps (CFLs), LED lighting, electric vehicles (EVs), consumer electronics (CEs) and computers, put an extra burden on the existing AC infrastructure . These DC loads require individual rectifiers to be connected to the AC distribution network to achieve two major jobs. One is to convert the energy from AC to DC to be appropriate to supply the dc load, while the second is to work as a power factor corrector.
DC Nanogrids have a centralized rectifier to supply all dc loads, offering a sensible solution for the residential home network , , which replace the different scattered rectifiers. DC distribution system  for electrical energy transmission offers several advantages, such as higher reliability, smaller footprint, lower costs, and higher efficiency, as a result of not only a smaller number of conversion stages, but also the absence of skin effect and reactive power , . Furthermore, most electronic loads, appliances, and variable frequency drives operate with DC voltage. For these reasons, the transition to a DC distribution low voltage network will be the efficient solution for the future home. Therefore, the feasibility of DC systems has been verified for residential systems , commercial systems , shipboard power systems , and industrial systems .
However, the best solution for a contemporary home is to combine a dc network along with the legacy AC network . As a consequence, this paper proposes a hybrid AC/DC Nanogrid for a residential home network in which the AC and DC infrastructures coexist to complement each other, as depicted in Fig. 1. The hybrid Nanogrid comprises of a Photovoltaic (PV) source and DC loads connected to the DC side; legacy AC loads are connected to the current AC infrastructure. A bidirectional AC/DC converter controls the power flow between both sides. Normally, legacy AC loads comply with the electrical regulation standards in distribution networks, such as harmonic levels, but for congested areas, further improvement can be achieved.
The main concern for the utility companies is the huge utilization of nonlinear loads in today’s homes, which degrade the power quality . There are many regulation standards which define the interconnection
requirement of the distributed generation (DG) units, especially in low-voltage distribution systems –. Also, many of regulation standards specify the interconnection regulation for the electric loads, such as home appliances –. Since Nanogrids, such as AC, DC, or hybrid home will be connected to the low- voltage distribution network, the design aspects of the Nanogrid and the interconnection requirement should follow the power quality standard. The injected current harmonics and the total harmonic distortion (THD) is one of the power quality issues that needs to be tackled, especially with increasing usage of nonlinear loads in today’s homes. One of the well-known standards in power quality issues is the IEEE 1547, which defines the harmonics and THD levels.
In order to mitigate harmonic components and compensate for reactive current, negative harmonics and reactive current must be injected into the power network. Many approaches were developed and utilized to mitigate harmonics and provide power factor correction –. One of these approaches is the Active Power Filter (APF), which provides the capability to compensate simultaneously for harmonics and reactive power drawbacks. Moreover, APF can compensate for unbalanced loads and regulate the voltage at the point of common coupling (PCC) with the grid. APF can be connected in shunt or series, and its performance is mainly dependent on the accuracy of the used method to extract undesired current. In three-phase balanced systems, the Synchronous Reference Frame (SRF) method is used –. In three-phase, four wire, unbalanced systems, the instantaneous power theory is usually used –. The disadvantage of this method is that it requires more computation due to measuring the voltage and current for the three phases, which is reflected in the cost of implementation. In this paper, a hybrid AC/DC Nanogrid is proposed for a small-building infrastructure. Unlike the previous research, a control scheme based on modified (SRF) is used to control the bi-directional AC/DC converter linking the AC and DC sides of the Nanogrid. This developed controller allows the bi-directional AC/DC converter to act as APF by forcing the AC grid currents at the PCC to balance three-phase currents with minor harmonics. Furthermore, it maintains power flow controllability between both AC and DC sides of the hybrid Nanogrid. Using the modified SRF for the proposed APF technique, the computational burden has reduced, as it uses only current measurements to support a three-phase unbalanced system.
This paper is arranged as follows: in Section II, some of the most common active power filtering techniques adopted in the literature are discussed. In Section III, a description of the system under study and the suggested control technique is demonstrated. In Section IV, simulation results are presented and inquired to validate the developed controller. In section V, experimental results are presented to evidence the validation of the controller’s execution experimentally. Finally, in Section VI, some of the conclusions that can be derived from this paper are listed.
II. Hybrid DC/AC NANOGRID Description AND ACTIVE FILTERING TECHNIQUES
In this section, a brief overview of the various techniques adopted for inverter control and active power filtering will be presented to clarify the contribution of the proposed technique.
A. AC/DC Converter Control Techniques
The first stage in APF control requires the generation of a reference current which represents harmonic and reactive current contents. The reference current can be calculated in the time or frequency domain. In the time domain, most compensation techniques are based on the synchronous reference frame and instantaneous power theory and are accomplished through the instantaneous derivation of the compensating signals from the distorted ones. In the frequency domain, Fourier analysis of the distorted signal is used to extract the harmonics, a process which requires significant computation power that can lead to a slow response.
A.1. Instantaneous Power Theory
The instantaneous power theory method remains one of the most popular APF control schemes. It transfers the three phase voltages and currents from the abc coordinates to the αβ0 coordinates via Clark’s transformation. As a result, the active and the reactive instantaneous powers can be calculated. Generally, each of the active and the reactive powers are composed of both continuous and alternating terms. The continuous term corresponds to the fundamentals of the current and the voltage. The alternating term represents the power related to the sum of the harmonic components of the current and the voltage. A low-pass ﬁlter or a high-pass ﬁlter is required to separate the continuous and alternating terms of the active and the reactive instantaneous powers. A block diagram depicting APF control using instantaneous power theory is shown in Fig. 2. This type of control can be applied to both balanced and unbalanced systems in a three-phase, four-wire system as well as systems containing voltage harmonics. However, as a result of measuring voltages, line currents, and an online transformation to αβ0 coordinates, its effective implementation requires a complex hardware infrastructure.
A.2. Synchronous Reference Frame
The SRF compensation method uses the Parks transformation to represent the distorted signal in the d-q plane, as given in (1):
In which, the fundamental component will be represented by a DC value in the d-q plane. Moreover, the harmonic component will be represented by an AC component with a frequency of 120 Hz and/or other multiples of 60 Hz. The component of the current in the d-axis will represent the active power, while the component of the current in the q-axis will represent the reactive power. Hence, bi-directional control of the flow of active and reactive power can be done separately. Moreover, the harmonic compensation current can be extracted from the d-axis current using a high pass filter. One of the limitations of this method is that when applying Parks transform, an AC component appears in the d-q plane with a frequency of 120 Hz in 60 Hz networks due to unbalancing. This AC component is equal to the AC component produced by the third harmonic, which leads to the injection of 3rd harmonic to the grid .
B. Active Power Filter configuration
APFs can be configured in a number of different ways to meet a variety of load compensation requirements. An APF can be classified according to the load type and supply into the following 3 categories: three-phase, three-wire APF, three-phase-four-wire APF, and single-phase APF. A single or three phase APF can operate as a current or voltage source converter which can be either connected in series, as a shunt, or as a unified power quality conditioner. In the case of a three-phase, four-wire system with unbalanced load, an APF can be designed in two different ways. In the first way, three independent single phase filters with isolation transformers are designed for independent phase control. In the second way, a four-wire filter is designed to compensate the neutral current and balance the load. Two configurations exist for three-phase, four-wire active filters. In the first configuration, known as the four-arm converter type, the fourth converter-arm is used to compensate the neutral current. In the second configuration, known as a capacitor mid-point type, the entire neutral current flows through the DC bus capacitors. A three-phase, three-arm voltage converter is then utilized as a link converter between both DC and AC sides where the same converter is used to control the power flow while acting as an APF. The APF proposed in this paper utilizes a link converter operating in current control mode to inject the reactive and harmonic currents as required by the nonlinear load.
II. System and Controller Description
A. System and proposed control description
The system under study is a hybrid AC/DC Nanogrid to represent future homes integrated to a power system, as depicted in Fig. 3. It consists of an AC zone, a DC Zone, and bidirectional AC/DC converter. The AC zone represents the legacy AC infrastructure with the AC loads connected to it. This AC low-voltage network is connected to the utility low-voltage network at the PCC. The DC zone, with a 380 V DC voltage as the main DC bus, is used to integrate the alternative DC sources and load to the system. The 5 kW PV is connected to the main DC bus through DC/DC boost converter. The DC-powered appliances are connected to the main bus through multiple stages of DC/DC buck converters based on their voltage level. A 10 kW hysteresis current-controlled bi-directional DC/AC converter connects both zones together to maintain bidirectional power flow and act as an APF to the AC loads connected directly to the utility grid. Therefore, it is used to create a DC zone network in the low voltage home infrastructure while working as an APF to improve the power quality of the system at PCC. The proposed control technique will consist of two parts. One is the bi-directional control for the AC/DC converter which maintains the power balance into the proposed hybrid DC/AC Nanogrid. The other one is responsible for the APF functionality.
A.1. Power Flow Control
In order to maintain the power balance through the Nanogrid, the amount of power controlled by the linking converter needs to be calculated, as given by equation (2), which represents the difference of power between the PV power and the local DC load power.
Based on the instantaneous power theory, the active power is given in equation (3).
Since the synchronous reference frame d-axis is aligned with the three-phase voltage angle, Vq will be equal to zero. Then, the Id reference is calculated from equation (4).
A.2. APF Control
The filtering functionality serves three purposes: compensating the unbalance due to the existence of different appliances supplied with single and three-phase loads, mitigating harmonics due to non-linear loads, and correcting the power factor at the PCC. In the proposed system, the reference current signal is obtained from the measured load current through the use of a modified synchronous reference frame based method. The modified synchronous reference frame method is capable of dealing with a balanced and unbalanced system with a three or four wire connection. A block diagram depicting the reference current generator is shown in Fig. 4. This type of controller is based on park’s transformation. In which the three-phase current are converted to direct and quadrature current ild, and ilq respectively. This calculated current ild is analyzed. This analysis gives a DC component represents the fundamental active component current, while the AC component represents the harmonic components. Unlike the synchronous reference frame method, which uses the high pass filter to isolate the AC component from ild. The proposed modified synchronous reference frame method uses a low-pass filter with a cut-off frequency of 75 Hz. The DC component of ild is obtained by passing ild through the low pass filter. The output of the low pass filter will represent the magnitude of the fundamental active current component existing in the load. A dc voltage regulator is used to calculate the active current component required to regulate the DC bus voltage. The reference current from the DC voltage regulator, in addition to the reference current calculated from a power balance controller represented in (4), is added to the calculated ild. A three-phase sinusoidal current reference is calculated by using inverse Park’s transform, where ilq and il0 will be set to zero since we need to obtain the active fundamental component only. Consequently, the calculated sinusoidal component is subtracted from the load current. The obtained component is a reference current that represents all harmonics and reactive components existing in the load current plus a fundamental component necessary for power balancing. By this method, we can manage to overcome the problem of an unbalanced AC current component where the calculated three-phase sinusoidal reference is being subtracted from the load current to obtain the final current injected to the AC side. The developed controller detects power-quality problems resulting from the loads and automatically generates the needed compensating currents. Hence, it prevents power- quality issues from propagating to the network of the grid as the harmonics or distorted current are not seen beyond the PCC. Also, it maintains the load generation balance inside the Nanogrid.
III. SIMULATION RESULTS AND DISCUSSION
In order to investigate the effectivity, efficiency, and reliability of the proposed control technique, the proposed APF model was built in a MATLAB/Simulink environment. The system running time is time scaled to 24 sec to assimilate a 24-hour period. During the running time, the controller will be examined with severe loading conditions to verify the capability of the control model with such a load pattern.
In Fig. 5(a), the DC voltage of the main DC bus is shown. It can be seen that the voltage is stable during the whole operation at 380 V. The ripple in the voltage fluctuates between 370 V to 390 V at the instant of load transition, revealing that the ripple in the DC voltage does not exceed 5%, which complies with the standards. Fig. 5(b) shows the linearized PV output power. Fig. 5(c) shows the linearized DC load power. Fig. 5(d) shows the power for the bi-directional AC/DC converter. The power flow in the hybrid Nanogrid can be clearly seen in Fig. 5(d). It can be noticed that the power in the intervals between 0 to 7 sec and 17 to
24 sec is positive, which indicates that the converter is working in rectifier mode. In these intervals, since the PV power is not sufficient to feed the local DC load, the converter extracts power from the AC zone to feed the deficit in the DC zone. On the other hand, in the interval between (7- 17 sec), the power is negative, which means the converter is working in the inverter mode. It can be noticed that the PV power is more than the local DC power in that interval, so the inverter extracts power from the DC zone to support a local load in the AC zone. In response, the converter control verified its capability to control the power flow between both sides of the hybrid Nanogrid. The three-phase AC load currents and source currents are shown in Fig. 5(e) and Fig. 5(j), respectively. It can be seen from Fig. 5(e) that the load current is an unbalanced, three-phase current which is one of the problems that the controller will tackle. Fig. 5(f) and Fig. 5(g) give a close view of Fig. 5(e). It is noticed that the current waveforms suffer from noticeable distortion due to the existence of a nonlinear load which injects harmonics to the system. Fig. 5(f) and Fig. 5(g) show the load current at two different intervals to show the performance of the system in both rectifier/inverter modes. In addition, it clarifies the ability of the controller to mitigate harmonics compensating for unbalances under both modes of operation. It can be seen in Fig. 5(f) and Fig. 5(g) that the three phases are balanced and the current waveforms are uniform.
|16.5 Sec||18.5 Sec||16.5 Sec||18.5 Sec|
|THD||6.42 %||5.42 %||4.23 %||2.73 %|
|Fundamental||100 %||100 %||100 %||100 %|
|3rd||0 %||0 %||0.97 %||0.78 %|
|5th||4.14 %||3.49 %||0.91 %||0.61 %|
|7th||2.95 %||2.49 %||0.88 %||0.46 %|
|9th||0 %||0 %||0.6 %||0.03 %|
|11th||1.88 %||1.59 %||0.89 %||0.49 %|
This means that the inverter controller succeeds to isolate the load problems (imbalance and distortion) from the source side. The harmonic compensation is another objective of the proposed control technique. Although the legacy ac loads comply with the electrical regulation standards in the distribution network, such as harmonic levels, further improvement can be achieved in the congested areas. Harmonic analysis for this case is provided in Table I, where the total harmonic distortion (THD) is reduced from 5.42% in the load current to be 2.73% in the source current at instant 18.5 sec in the rectifier mode. Also, the total harmonic distortion (THD) is reduced from 6.42% in the load current to be 4.23% in the source current at 16.5 sec in the inverter mode. This indicates the controller’s effectiveness in harmonic reduction, which has a favorable impact on the system efficiency while preventing a wide harmonic propagation to the source side.
Fig. 6(a) shows the voltage and current waveforms for one of the load phases (phase A). It is seen that the current and voltage are shifted from each other due to the low load power factor. Fig. 6(b) shows that the source voltage and current waveforms are in phase, which reflects the PF improvement to unity. This is done by setting the quadrature component of the reference current to zero.
IV. Hardware Implementation & Experimental Results
To investigate the feasibility of the proposed control technique, a hardware setup has been established as depicted in Fig. 7. The notional hybrid AC-DC Nanogrid, shown in Fig. 7, has been implemented in our hybrid AC-DC power system test-bed (Energy System Research Laboratory, Florida International University) . It consists of AC and DC zones connected through an AC-DC converter. The DC zone is represented by a DC bus, where its voltage was set to 380 V, a DC load emulator to represent load appliances connected to the DC side, and a PV emulator to represent a roof-top PV source. The PV emulator, represented by XR SERIES DC power supply offered by the MAGNA-POWER ELECTRONICS, is programmed to emulate the PV I-V characteristics . It is interfaced with the DC bus through DC-DC boost converter to extract maximum power from the PV system. The PV emulator model and the converter controllers are built within the MATLAB/SIMULINK environment and executed with the DSPACE 1104 real-time interface. The DC load emulator consists of a combination of eight resistors with different values (60 Ω, 50 Ω, 40 Ω, 20 Ω, 20 Ω and 1 Ω) in a certain arrangement . However, changing the values of the equivalent resistors can yield different combinations and load patterns. Eight controlled switches are used to alter the system’s topology. The main concept of operation is based on sending control signals to the switches to change their states (on/off), and by changing their states, the equivalent circuit is changing. The control commands are initiated from a control program developed in the LabVIEW environment. The control commands are sent through the PCI 6025E card to a circuit. This circuit is based on TEXAS INSTRUMENT inverting buffer module sn7406n. This module contains six inverters with open collector output. The RMS values for the voltage in the AC zone is set to 208 V. The AC bus is connected to the utility grid at PCC. Also, different load models were designed to represent the AC load pattern . One of the passive loads built has a switching capacity of 10 levels parallel of resistive loads from 300-W to 3-kW power in steps of 300-W at a nominal voltage that can be switched to emulate various load patterns. The parameters of the main components of the hybrid AC-DC Nanogrid are given in Table II. The experimental results are shown in Fig. 8; they show the capability of the proposed algorithm to compensate for unbalance and nonlinearity of the load current in different modes of operation. This figure is divided into four regions separated by the red-dashed line. These four regions represent three different modes of operation. In mode one, (interval between 0-28 sec), the converter is working as an APF for the AC load but without any exchange in the power between AC and DC zones. In mode two, (interval between 28-88 sec and 188-278 sec), the converter works as an APF and rectifier in which the power is transferred from the AC side to the DC side. In mode three, (interval between 88-188 sec), the converter works as an APF and inverter in which the power is transferred from the DC side to the AC side. In Fig. 8(a), the DC voltage of the main DC bus is shown. It can be seen that the voltage is stable during the whole operation at 380 V. The ripple in the voltage fluctuates between 370 V to 390 V at the instant of load transition. This small fluctuation in the DC bus voltage is compatible with the standards, as it does not exceed 5%. Fig. 8(b) shows the power for the bi-directional AC/DC converter. Also, the bidirectional power flow in the hybrid Nanogrid can be clearly seen in Fig. 8(b). It can be noticed that the power in the intervals between 30 to 100 sec and 188 to 278 sec is positive which indicates that the converter is working in rectifier mode. Since the PV power is not sufficient to feed the local DC load in these intervals, the converter extracts power from the AC zone to feed the deficit in the DC zone. On the other hand, in the interval between (100- 188 sec), the power is negative, which means the converter is working in the inverter mode because the PV power exceeds the local DC power in this interval. Therefore, the inverter extracts power from the DC zone to support the local loads in the AC zone.
|Boost Converter||power rating||2500 W|
|switching frequency||5 kHz|
|LBC, RLBC||6 mH, 0.21Ω|
|Bidirectional AC/DC Converter||power rating||1800 W|
|switching frequency||10.89 kHz|
|AC Filter||L AF, RLAF||12 mH, 0.31 Ω|
Therefore, the inverter extracts power from the DC zone to support the local loads in the AC zone. In response, the converter control verified its capability to control the bidirectional power flow between both sides of the hybrid Nanogrid. The three-phase AC load currents and source currents are shown in Fig. 8(c) and Fig. 8(j), respectively. Fig. 8(d), Fig. 8(e) and Fig. 8(f) give a close view of Fig. 8(c) at modes one, two, and three, respectively. It is noticed that the current waveforms suffer from noticeable distortion due to the existence of nonlinear loads, which inject harmonics into the system. Fig. 8(g), Fig. 8(h) and Fig. 8(i) give a close view of Fig. 8(j) at modes one, two, and three, respectively.
In addition, these figures clarify the ability of the controller to mitigate the harmonics and compensate for unbalances under all modes of operation. As is seen in Fig. 8(g), Fig. 8(h) and Fig. 8(i), the three-phase currents at the source side are balanced and uniform. This means that the inverter controller succeeds to isolate the load problems (imbalance and distortion) from the source side.
In order to validate the controller capability to improve the power factor, Fig. 9 is shown. Fig. 9(a) shows the phase a’s voltage versus phase a’s load current. Fig. 9(h) shows the phase a voltage versus phase a supply current. Fig. 9(b), Fig. 9(c) and Fig. 9(d) give a close view of Fig. 9(a) at modes one, two, and three, respectively. It is noticed that the current waveform is out of phase and lagging the voltage waveform, which means lag PF. Fig. 9(e), Fig. 9(f) and Fig. 9(g) give a close view of Fig. 9(h) at modes one, two, and three, respectively. It can be clearly seen that the current waveform is in phase with the voltage waveform, which means unity PF. Thus, the proposed control algorithm id succeed to correct the power factor for the system.
In this paper, a control technique for hybrid AC/DC Nanogrid is proposed to offer a solution for the near- future smart building. The controller is based on a modified synchronous reference frame algorithm. This control technique enables the linking converter to act as APF in order to force the AC supply current at the PCC to be a balanced, three-phase current regardless of the loading conditions.Moreover, the controller provides harmonic mitigation and power factor correction at the PCC. The controller is verified in MATLAB/SIMULINK for simulation purposes. The developed controller is then tested experimentally. Different loading conditions are simulated to verify the effectiveness of the system. Simulation results show that the controller deals successfully and automatically with various severe power quality issues isolating these problems and preventing them from being seen by the supply. The experimental results also showed the effectiveness of the proposed controller in mitigating the power quality issues and ensuring load generation balance inside the Nanogrid.
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List of Figure
Fig. 1 The main configuration for the hybrid AC–DC Nanogrid.
Fig. 2. Block diagram of instantaneous power theory APF control.
Fig. 3. Block diagram for the hybrid Nanogrid under study
Fig. 4. A block diagram of the reference current generator.
Fig. 5. simulation result: (a) Main DC bus Voltage (V), (b) PV system output power (Watt), (c) DC load power (Watt), (d) Power of the bi-directional DC/AC converter, (e) Three phase AC load current, (f) Zoom in for three phase AC load current between (16-16.1) sec, (g) Zoom in for three phase AC load current between (18.5-18.6) sec, (h) Zoom in for three phase AC supply current between (16-16.1) sec, (i) Zoom in for three phase AC supply current between (18.5-18.6) sec, (j) Three phase AC supply current.
Fig. 6. A block diagram of the reference current generator.
Fig. 7. Expermental setup block diagram .
Fig. 8 . Experimental results show the unbalance compensation and harmonics mitigation: (a) Main DC bus Voltage (V). (b) Power of the bi-directional DC/AC converter. (Watt). (c) Three phase AC load current (d) Zoom in for three phase AC load current between (25.03-25.1) sec. (e) Zoom in for three phase AC load current between (150.03-150.1) sec. (f) Zoom in for three phase AC load current between (200.03-200.1) sec. (g) Zoom in for three phase AC supply current between (25.03-25.1) sec. (h) Zoom in for three phase AC supply current between (150.03-150.1) sec. (i) Zoom in for three phase AC supply current between (200.03-200.1) sec. (j) Three phase AC supply current.
Fig. 9 . Experimental results show the power factor correction: (a) Phase a’s Voltage (V) and phase a load current, (b) Zoom in for Phase a Voltage (V) and phase a load current between (25.03-25.1) sec, (c) Zoom in for Phase a Voltage (V) and phase a load current between (150.03-150.1) sec, (d) Zoom in for Phase a Voltage (V) and phase a load current between (200.03-200.1) sec, (e) Zoom in for Phase a Voltage (V) and phase a supply current between (25.03-25.1) sec, (f) Zoom in for Phase a Voltage (V) and phase a supply current between (150.03-150.1) sec, (g) Zoom in for Phase a Voltage (V) and phase a supply current between (200.03-200.1) sec, (h) Phase a Voltage (V) and phase a supply current
List of Tables
Table I HARMONIC ANALYSIS FOR five cycle of PHASE C
Table II Hybrid AC-DC Nanogrid System Parameters