DC-Bus Voltage Regulation for DC Distribution System with Controllable DC Load Iman Mazhari, Babak Parkhideh University of North Carolina at Charlotte, Electrical and Computer Engineering Department Energy Production and Infrastructure Center Charlotte, USA Email:
[email protected] Abstract— This paper shows a dc-bus voltage regulation for a dc distribution system integrated with a single-phase bidirectional inverter through using a control method to operate with no grid information, and without contributing the solar DCDC converter. This system can intelligently and efficiently operate in islanding mode when load shedding is employed to regulate the DC bus voltage with providing the demand response requirements. Also, this enables controllable loads to ridethrough grid outages, and it can be used as a retrofit solution for existing topologies applicable to any type of controllable DC loads with increasing the efficiency and improving the reliability. The primary simulation results are presented for a system in both grid tied and islanded mode indicates that the proposed system can satisfactorily regulate the common bus voltage, and ride through the blackout. Keywords—DC distribution system; DC controllable load; buck converter; load shedding; bus volatge regulation.
I. INTRODUCTION Considering that most renewable energy sources are either DC or can interface with DC systems much easier than AC systems using power electronic circuits, the existing AC distribution systems can be replaced with a DC distribution systems. Apparently, other types of DC electronics load, such as computers, or high-power application loads, such as adjustable speed motor drives can be tied to the common DC bus. Through this replacement, significant energy saving could be obtained by directly coupling DC power sources with DC loads, thus bypassing DC-AC-DC power conversions and using a DC power distribution system [1-4]. In a dc distribution system compared to an AC system, if DC loads can directly be supplied by a dc voltage source, about 20% of component cost and 8% of power conversion loss can be saved due to removal of AC-DC power conversion unit and the filtering circuit, as shown in Fig.1 and Fig.2 [1]. Due to this benefit and lack of other problems such as frequency instability, reactive power issues, skin effect, and ac losses, DC-internal loads such as all consumer electronics, commercial data centers, efficient DC motors, lighting systems, and electric vehicles continue to grow rapidly [5-6]. In a typical dc distribution system with assuming photovoltaic (PV) as the distributed generation (DG) source as shown in Fig.2, a bi-
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directional inverter must fulfill real power injection (sell power to grid) and rectification (buy power) with power factor correction (PFC) to regulate the dc bus respectively [7]. To avoid any possible technical issue such as power, and energy balance, power quality or protection problem, any distribution system must operate in both grid connected and islanded mode without compromising grid reliability, voltage and frequency balance, or protection schemes, consistent with the minimal standards for all connected devices. One of these requirements needs to be met is the bus voltage regulation within the working range to ensure continuous function of all connected DC loads. In previous studies, different methods for regulating a constant dc-bus voltage were proposed [6, 8-9]. However, a large step load change will cause high dc-bus voltage variation and fluctuation, and the system might run strangely or drop into under or over voltage protection. To suppress these anticipated fluctuation, bulky dc-bus capacitors can be employed, which also increase the size, weight and the cost of a PV inverter system significantly. For example, an on-line regulation mechanism according to the AC inductor current levels to balance power flow and enhance the dynamic performance is presented in [6]. Additionally, for power compensation and islanding protection, the bi-directional inverter can shift its AC current commands according to the specified power factor at AC grid side [6]. This paper shows how the bus voltage can be regulated with varying the load power through an advanced control to avoid utilizing a bulky capacitor. In the proposed control method, any controllable DC load integrated with a DC-DC converter can be directly connected to the PV array. Hence, the PV capacity could be notably smaller than a conventional AC system to produce the same amount of energy from PV to the DC loads. This means considerable reduction in installation costs for the same energy benefit. As the controllable load demands variable power, the DC-DC converter is tuned to supply required current for the load, and correspondingly regulate the bus voltage. In other words, the proposed solution will compensate the power mismatch whenever it is occurred due to a power outage in the islanded mode, when the grid is not available to provide the required voltage.
It has been assumed that the voltage level of the controllable load is lower than the common bus voltage such that a step down (buck) converter is required between the bus and the controllable load. II. CONTROL STRATEGY AND OVERLOAD CONDITIONS
η= 98% ⨯ 97% ⨯ 98.5%⨯ 97%⨯95%= 85% Fig. 1. General Scheme for a conventional AC microgrid with estimated
η= 98% ⨯ 95% = 93%
To prove the proposed solution, a system consists of a grid-tied PV inverter which has been integrated with a DC/DC converter is the main requirement. Fig.3 illustrates the implemented setup including the proposed control method for the grid tied PV inverter integrated with a controllable DC load which is paired with the common dc bus through a DCDC converter. For the load connected buck converter, two current control methods including average current control for the load, and PV current control for the maximum power point tracking (MPPT) control are employed and utilized alternatively. For grid connected inverter, the outer loop controller maintains the maximum power point (MPP) voltage for the DC link voltage, and the inner loop controller regulate the AC current to preserve the unity power factor. This bidirectional inverter is integrated to preserve the MPP in the grid connected mode, and compensate the probable power mismatch between the PV source and DC controllable load. More details about robustness analysis, filter design of the given inverter for high current applications, and distributed control for cascaded topology have been discussed in [10-13].
Fig. 2. General Scheme for a conventional DC microgrid with estimated conversion efficiencies
Fig.3. Implemented setup including a bidirectional inverter, and PV integrated with the controllable DC load
Pload
Pload
× Vequi _ low
P
P
pv
power is fixed, and if overgeneration happens, the extra source power from the PV supplies the grid, and inversely if overloading happens, the remaining power is provided by the grid. In this paper, only overload condition is discussed as shown in Fig.4 considering that system is automatically stable in the overgeneration mode when the bus voltage is increased due to the power mismatch, and will intersect the PV curve at the equilibrium point. Contrary, if islanding happens at overloading mode, bus voltage drops to meet the new intersection on the PV curve which is called Vequi _ low . The reason is that the load power needs to be lowered to meet the diminished source power ( Ppv ) in lower bus voltages due to
pv
the DC link capacitor discharging caused by the power mismatch ( Ppv < Pload ). The moment that load shedding is
Fig.4. Operation modes of the source (PV) and controllable DC load integration before and after islanding
In grid connected mode, it has been assumed the system operates at MPP through a bi-directional inverter as shown with a constant power line which is shown with green color at Fig.4. With assuming that the load demanded power is greater than maximum available PV power, the load nominal power is considered aligned with a virtual PV curve. In islanded mode, as the controllable load demands variable power, the DC-DC converter is tuned to supply required current for the load, and correspondingly regulate the bus voltage. In other words, load power is adjusted to meet the new equilibrium point ( Vequi _ low ) to meet the power balance on the actual PV curve as shown in Fig.4. To detect islanding, a novel method with providing significant advantages compared to other methods including lack of non-detection zone can be utilized as tested and verified in [14]. In Fig.4 power curves for both PV, and DC controllable load respect to the bus voltage in overloading mode when load power is greater than source power before the moment that islanding happens are shown. In contrast, there is another mode called overgeneration in which load power is less than the source power as long as the grid is connected. In other words, the extra power generated by the PV is delivered to the grid in overgeneration mode. This will cause the bus voltage to increase, and consequently the DC link capacitor is charged. In this mode, the load power will intersect the PV curve, so the system will operate automatically in a stable region and will meet the power balance autonomously. This feature helps us raise the load power without reaching the maximum allowable bus voltage to meet the power balance at a voltage below the maximum allowable bus voltage to increase the over voltage protection (OVP) margin of the system During the system startup, it can be assumed that the system operates at the grid connected mode, and the MPP is sustained by the bi-directional inverter. In other words, load
started, is related to the size of the capacitor such that with choosing a bigger DC link capacitor, the time interval to hold the load power at the nominal point, and correspondingly the bus voltage variation to operate in constant power load (CPL) mode is increased. Also, with considering direct coupling of PV and common dc bus, if the irradiance drop is raised, the power imbalance which is directly related to the current difference between the nominal load and actual PV curve is amplified, and the bus voltage drop will be faster. III. SIMULATION RESULTS The system depicted in Table. I is simulated in both grid tied and islanded mode to prove the proposed paired control method. Table. I illustrates the parameters selected for simulation, and experiment for the PV, bi-directional inverter, and the controllable load. It has been assumed that at the startup, system is running in the grid-tied mode with the Pload −in = 160w which is the demanded power by the buck converter, and bus voltage ( V pv = Vdc ) is regulated at Vmpp =32.9V after almost 0.2 sec as shown in Fig.5. When
irradiance happens at Time = 1.2 sec, because without contributing the grid power, load power is greater than the maximum available PV power, the DC link capacitor needs to be discharged, and bus voltage has to be decreased. TABLE I. DESIGN PARAMETERS OF THE IMPLEMENTED SETUP PV Parameters
Value
Voltage at Maximum Power ( Vmppt )
32.9V
Current at Maximum Power ( I mppt )
Value
Load Resistance
2.8 Ω
Switching Frequency
40Khz
8V
4.6 A
Open Circuit Voltage ( Voc )
Load and Inverter Parameters Load Trip off Voltage
40V
Short Circuit Current
( f sw ) DC Link Capacitor
( I sc )
5A
( C DC )
Maximum Power (MPP)
151 W
Grid Voltage ( Vg )
10 mF
25 V
Transient
MPP Region
Islanding Region
MPP is maintained at Time = 0.2 sec
DC Bus Regulated Voltage after Islanding with employing the load shedding
Fig.5. DC bus voltage ( V pv ), and PV current ( I pv ) before and after islanding in overloading mode Fig.8. Grid voltage ( Vg ), and current ( I g ) before and after retaining MPP in
Pload − in
Ppv < Pmppt
overloading mode. There is 180-degree phase shift between grid voltage, and grid current
observed in Fig. 6, and due to the lower PV power before retaining the MPP, the generated grid power is slightly higher at the startup as depicted in Fig.6.
Ppv = Pmpp
Pg = 0
Fig.6. Ppv ,
Pg , Pload −in before and after islanding in overloading mode when Ppv + Pg = Pload − in
*
When MPP is maintained, the bus voltage is regulated around 32.9 which is the MPPT voltage, and when islanding happens at 1.2sec, the bus voltage drops because of DC link capacitor (Cdc ) discharging to compensate the power mismatch for the load. With employing the proposed control method operating at load shedding region, the Pload − in is lowered to 145w to match the PV power at the new regulated ∗ is reduced V pv = Vdc = 30v .To lower the load power, I dc corresponding to the lowered bus voltage to avoid significant drop of Vdc , and meet the new demanded power. Fig.7 also shows the PV curve and equilibrium points before and after islanding with employing the load shedding.
*
DC Bus Regulated Voltage after Islanding with employing the load shedding DC Bus Regulated Voltage before Islanding Vmpp = 32.9v
Pmpp = 150w
As expected, and shown in Fig. 6, the generated grid power becomes zero after islanding occurrence. Fig. 8 also shows that when PV power reaches the MPP voltage, grid current, and grid voltage have 180-degree phase shift representing that the generated power by the grid is negative to compensate the extra power (10w) demanded by the load to meet the power balance. IV. EXPERIMENTAL RESULTS
Fig.7. P vs. V characteristics of the chosen PV panel
For this reason, the PV voltage is decreased and simultaneously the PV current is also increased, moving slightly to the short circuit current. As the Pmpp = 150 w < Pload −in = 160 w for the selected PV panel, the power difference (10w) is injected from the grid, as can be
For the hardware setup, a multi-purposed generic converter (MPGC) which can be operated in both DC/DC and DC/AC modes is utilized as shown in Figs 9. For the controllable DC load, a minimum threshold voltage source connected in series with a constant resistor is considered. This threshold voltage indicates the minimum voltage needs to be provided by the buck converter for the load to avoid system shut off. This gives us an equivalent model of a DC controllable load such as energy storage system (ESS), or LED lighting. For the implemented setup, Vth = 7.5V , and Rload = 2.8Ω .
C dc
Controllable DC Load PV Source
MPGC in DC-DC mode
Fig.9. Implemented setup of PV source integrated with DC controllable load by using a MPGC
For the results provided here, it is assumed that the system is tested in the islanded mode, so we can ignore the grid connected operation. In other words, PV is assumed to be directly connected to the common bus, and the grid is islanded. Grid connected operation is very similar to a singlephase grid-tied PV inverter as explained in [14, 15]. Initially, MPPT controller starts running to provide maximum power out of the PV source. As shown in Fig.10, at the start up, PV and load currents start increasing to reach the MPP point, and simultaneously PV voltage which is same as the bus voltage is decreasing to reach V pv = Vmppt = 32.9v .
Regulated load current after employing load
Fig.12. Experimental result with employing the load shedding after islanding. To avoid saturation of the duty cycle, it is assumed that load current reference is reduced to 5.5A with applying load shedding. Blue is bus voltage, green is PV current, pink is the load current
MPP Voltage
Fig.10. Experimental result to maintain the MPP V pv = Vmppt = 32.9v at the startup. Blue is bus voltage, green is PV current, pink is the load current
Fig.11. Experimental result without employing the load shedding after islanding. Blue is bus voltage, green is PV current, pink is the load current
Based on the timing scale shown in this figure, it takes almost ten seconds for the system to regulate the MPP voltage for delivering maximum available PV power which is about 150W. Two different scenarios are tested and presented that in both, irradiance is suddenly dropped 20% representing the impact of abrupt shading. Fig.11 demonstrates when no load shedding is applied. As it can be seen, the system shuts down due to losing the control, and it cannot reach any stable point. The reason behind that is when irradiance drops, the bus voltage drops in response of DC link capacitor discharging. As demonstrated in Fig.11, the PV voltage keeps decreasing constantly, and at the same time, the duty cycle of the buck converter keep increasing to compensate the reduced amount of the load current. As demonstrated, by increasing the duty cycle, and correspondingly reducing the bus voltage, the generated power by PV is reduced more, and the power mismatch in intensified. So, the system moves to unstable region, and it is shut down when the duty cycle reaches the maximum possible limit (duty cycle =100%). Fig.11 indicates that for the selected PV panel in which duty cycle gets saturated at V pv = Vdc = 13.3v . After this point, both PV, and load current become zero, and bus voltage goes to the open circuit condition.
Fig 12. shows the experimental results of a PV source system integrated with DC controllable load after applying a load shedding control when the irradiance drops. It has been assumed that the duty cycle of the buck converter has been limited to some threshold values to avoid system shut down. When the irradiance drops, bus voltage is decreased and duty cycle is increased continuously such that it will hit the limit. At this point, which is indicated by V pv = V dc = 26.7v in Fig.12, the reference load current is decreased to the new * = 5.5 A ). It should be noted that the new chosen value value ( I dc for the reference current load should be selected such that it will be below the maximum current that the system can provide after irradiance drop. Also, with referring to Fig.3, when load current reference drops, the error of comparator for the current controller loop becomes negative, and duty cycle is decreased. That’s why the bus voltage is regulated at a value higher than MPP voltage. Fig.12 clearly shows after employing the load shedding, the load current is regulated at the new value ( I dc = 5.43 A ) which is pretty much close to the set * = 5.5 A ), and bus voltage is regulated at reference value ( I dc V pv = Vdc = 34.9v which is higher than the MPP voltage.
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V. CONCLUSION This paper presents a control method paired with controllable DC load to regulate the bus voltage in DC distribution systems. Compared to other common AC and DC distribution systems, not only does it brings advantages such as lower power losses, lower maintenance, less investments, and more compatibility with more efficient DC loads, it also provides a reliable, efficient system to properly regulate the bus voltage in both grid-tied, and islanded mode through a controllable DC load. In this paper, detailed description of the proposed control method is explained and its performance and efficacy are evaluated through simulation and hardware experiments for both grid-tied and islanded modes with considering irradiance drop in islanded mode that creates power imbalance. Provided results also prove that employing load shedding is required to avoid system failure, and meet the power balance. REFERENCES [1]
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