DSP-based Multiple Peak Power Tracking for Expandable Power System1 Wenkai Wu†, N. Pongratananukul†, Weihong. Qiu, K. Rustom, T. Kasparis, and I. Batarseh †
Apecor Corp. Research Pavilion, 12424 Research Parkway, Suite 480 Orlando, FL 32826 Tel: (407) 823-0185
School of Electrical Engineering and Computer Science University of Central Florida Orlando, FL 32816
Fax: (407) 823-6334
Email:
[email protected]
In this paper, a DSP-based multiple peak power tracking for expandable power system is presented. In the system, multiple solar arrays are connected to individual peak power tracker units, which composed of paralleled COTS DC/DC converters. Each of the solar arrays is individually peak power tracked with an improved MPPT algorithm. The outputs of each of the individual tracker units are connected in parallel. In such a power system, new solar arrays may be added to the system in a modular fashion simply by adding additional tracker units and adjusting a control routine to account for the additional units.
Abstract: A DSP-based improved maximum power point tracking (MPPT) approach for multiple solar array application is presented. It incorporates a “shared bus” current sharing method that can regulate many paralleled current mode DC/DC converters. The modular architecture eases the expansion of system power. The current sharing and MPPT performance of the proposed system is validated and evaluated by a 500-W prototype with two solar arrays. Keywords: MPPT; Solar Power System; Current Sharing; Scalable Power System
I. INTRODUCTION II. MPPT CONTROL ALGORITHM
As the need for flexible, scalable space-based power requirements increases, and in an effort to avoid redesign of spacecraft and electric propulsion power systems, a number of concepts have emerged to provide expandable, parallelconnected power converters employing techniques such as maximum power point tracking (MPPT). Such approaches then allow a variety of options with the rest of the power system such as employing standard, modular, power converters that can be connected in parallel. The goal of such structures is to provide a single power system design that can meet a range of power requirements for spacecraft and/or electric propulsion power systems. For such a flexible and scalable power system, a need exists for control of such functionality. Depending on the concept, risks of power system failures exist due to a variety of circumstances. Under any circumstance that causes the output voltage of the power system to lose regulation, MPPT techniques ensure that the power delivered to the load is at the maximum available from the solar arrays. Hence, the control prevents the complete drop out of the system output voltage. Under normal sun insolation and healthy array source conditions, the control will not interfere with the regulation of the system output voltage because the load demand is below the maximum available power of the array source. The expansion capability of the system with such a control provides long-term cost/schedule benefits to the electric propulsion and spacecraft power systems of the next generations. In many cases, Commercial Off-the-Shelf (COTS) power converters can be employed with such control circuitry to meet space needs [1~6]. 1
Curve as shown in Fig. 1 is a typical P-V characteristic of a solar array. Since the curve depend on insolation, temperature conditions, and there is only one single point of operation that will extract maximum power from the array, therefore, MPPT should be implemented to track the changes and extract the maximum power from the solar array. For a solar array source MPPT power system, followed DC/DC converters are usually operated in output voltage regulation mode when system-load demand is less than array peak power. This usually moves the operating point on the IV array characteristic curve to the right side of the array peak power point where the array source behaves similar to a voltage source of low internal impedance. As the load increases, the solar array operating point moves up to the left along the array I-V characteristics until it reaches the maximum power point while the system output voltage remains regulated. Without MPPT control, when the load current is above the level corresponding to the array maximum power, the array I-V operating point will move to the left of the maximum power point, causing the system output voltage to lose regulation. Without a proper controller design, the array voltage can collapse toward zero when load demand is above the maximum power of the array, particularly when supplying a constant-power type of load. When properly applied, a MPPT control can prevent the collapse of the array voltage under excessive load demand. One proper approach is to operate the system in a solar array voltage regulation mode in which the array voltage is clamped to a commanding set point, Vmp, which is
This work is supported by Air Force-SBIR Phase I grant # F29601-02-C-0075
0-7803-7768-0/03/$17.00 (C) 2003 IEEE
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dynamically updated by the MPPT control circuit. The control processes two feedback signals — the array current and the array voltage. Eventually, this continuously updated set point will fluctuate around the voltage corresponding to the array peak power point. By adjusting the operating point of the array to the point Vmp, power output of the array is maximized and the most efficient use of the solar array may be realized.
Begin P&O
measure:V(k), I(k)
∆V (k ) = V ( k ) − V ( k − 1)
P (k ) = V ( k ) × I ( k )
250 Vmp
∆P (k ) = P ( k ) − P (k − 1)
Pow er ripple
Outpur Power (W)
200 a
b
150
∆P > ε 1
Yes
No
100
∆P < −ε 1
Yes
50
∆V > ε 2 No
0 0
10
20
30
40
50
60
∆V > ε 2
70
∆V < −ε 2 Yes
No
Voltage ripple
Yes
Yes
Voltage (V)
∆V < −ε 2 No
Yes
No
Fig. 1. Solar array characteristics
V ref (k + 1)
No
= V ref (k ) + C 1
In distributed solar array system, solar array may have different angles of incidence, and possible problem of having multiple local maxima in the power-voltage curve, as shown in curve b in Fig. 1, due to array damage, unequal cell illumination, shading or dirt. To increase the adaptability of MPPT controller, a hybrid MPPT method combined the advantage of traditional perturb and observe (P&O) and incremental conductance (IncCond) algorithm was implemented in the system. The P&O method is a widely used approach to MPPT. As the name of the P&O method states, this process works by perturbing the system by increasing or decreasing the array operating voltage and observing its impact on the array output power, as indicated in Fig. 1. Figure 2(a) shows a flow chart diagram of the P&O algorithm as it was implemented in the controlling microprocessor. With this algorithm the operating voltage is perturbed with every MPPT cycle. As soon as the MPP is reached, the output voltage of solar array will oscillate around the ideal operating voltage Vmp. This causes a power loss that depends on the step width of a single perturbation. The value for the ideal step width is system dependent and must be determined experimentally to pursue the tradeoff of increased losses under stable or slowly changing conditions. In fact, since the ac component of the output power signal is much smaller than the dc component and will contain a high noise level due to the switching DC/DC converter, an increase in the amplitude of the modulating signal has to be implemented to improve the signal to noise ratio (SNR), however, this will lead to higher oscillations at the MPP and therefore increase power losses even under stable environmental conditions. In
V ref ( k + 1)
V ref ( k + 1)
V ref (k + 1)
= V ref (k ) − C 1
= V ref (k ) − C 1
= V ref (k ) + C 1
(a) P&O MPPT algorithm
Begin IncCond
measure:V(k), I(k)
∆V (k) = V (k) −V (k −1) ∆I (k ) = I (k) − I (k −1)
∆V < ε 1
Yes
No
∆I I + − +ε2 ∆V V
Yes Yes
No
∆I < ε 3
No Yes
∆I > ε 3 Yes No
Vref (k +1)
Vref (k +1)
Vref (k +1)
Vref (k + 1)
= Vref (k ) + C1
= Vref (k) − C1
= Vref (k ) − C1
= Vref (k ) + C1
(b) IncCond MPPT algorithm
Fig. 2 Flow chart of P&O MPPT algorithm and IncCond MPPT algorithm
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MPPT operation could adopt larger modulation amplitude to increase the SNR and system dynamic performance. Furthermore, multiple local maxima problems could be overcome by finding the global maximum through scanning a wide control range P&O MPPT operation. Since the percentage of P&O method is much smaller, the overall system efficiency will be almost the same as IncCond method. Dither signal in P&O MPPT method and proposed hybrid MPPT method helps to increase the system stability, however, it also by-produces one drawback saying output ripple. For multiple channel solar array power system, this issue could be mitigated by inversing the dither signal for the complimentary solar array source, which will be validated in section V.
addition, in the case of a sudden increase in insolation, the P&O algorithm reacts as if the increase occurred as a result of the previous perturbation of the array operating voltage. The next perturbation, therefore, will be in the same direction as the previous one. Assuming that the system has been oscillating around the MPP, a continuous perturbation in one direction will lead to an operating point far away from the actual MPP. This process continues until the increase in insolation slows down or ends. To avoid the drawbacks of the P&O MPPT method, Incremental Conductance (IncCond) MPPT algorithm was developed [3]. It is based on the fact that the derivative of the output power P with respect to the panel voltage V is equal to zero at the maximum power point. dP d (VI ) dV dI dI = =I +V = I +V =0 dV dV dV dV dV
(1)
One of the advantages of this MPPT algorithm is that it does not oscillate around the MPP. The check of condition (1) and dI = 0 allows it to bypass the perturbation step and therefore maintain a constant operating voltage V once the MPP is found.
Furthermore, conditions
dI I + >0 dV V
III. PARALLELED DC/DC CONVERTERS WITH CURRENT SHARING The application of paralleling modules actually brings benefits in the following aspects: 1) lowering the current stress on each single power semiconductor devices, therefore improves the thermal management and increase the current output capability, 2) achieving so-called N+1 redundant and greatly improves the reliability of the power supply; 3) providing more flexibility for customization, eases the maintenance and repair, and reduce the cycling time. However, connecting converters in parallel also presents several new challenges. The main issue for the parallelconnected converters is how to distribute the current uniformly among the converters. Currently, most of the approaches were mainly intended for uses in large classes of power converters that usually do not exploit current mode control as the innermost basic control loops. When considering a much simpler current sharing control approach, current mode controlled converters become very attractive. Although some approaches adopted current mode control as the innermost control loop, the added complexity makes it unsuitable for expandable power system applications. In this paper, current mode converters are connected in parallel with current sharing bus, as shown in Fig. 4. It can be found after current sharing bus is inserted inside the voltage regulation loop, it becomes the inner loop regulation structure for current sharing. The benefit is that the current sharing loop and current feedback loop can be combined together as one current loop. That makes the total control structure very simple. Also the current sharing response can be much faster because now the current sharing loop is inside and its bandwidth will not be limited by the outside voltage loop. In addition, the automatic master current sharing method for inner loop regulation structure are based on each module's output of voltage compensator rather than the shared current, so there will be no "chattering" or fault tolerance issue as demonstrated in the other structures with current sharing bus being outside of the voltage regulation loop. The current
and
dI > 0 make it possible to determine the relative location of the MPP. This leads to the advantage that an initial adjustment in the wrong direction, as with the “trial and error” P&O method, does not occur. A fast and correct system response to changing operating conditions should be the result – yielding higher system efficiency. A small marginal error could be added to the maximum power condition (1) such that the MPP is assumed to be found if
dI I + < ε2 dV V
. The value of ε 2 was determined with
consideration of the tradeoff between the problem of not operating exactly at the MPP and the possibility of oscillating around it. It will also depend on the chosen perturbation step width C1. Though higher efficiency could be expected, its application is also limited since it cannot overcome local maximum power point issues either.
40ms
12s
40ms
~ ~ Fig. 3 Modified MPPT scheme
The proposed hybrid MPPT method is periodically interrupting normal IncCond MPPT operation by P&O MPPT operation, as shown in Fig. 3. In such an arrangement, a higher efficiency could be expected since the solar arrays primarily operate in IncCond MPPT mode. Periodically P&O
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multi solar array sources share one DSP with different internal A/D and D/A conversion channels. The MPPT controller processes the array output voltage Vin and current signals Iin, thereafter deliver the proper control signal, regulating one or more converter modules through the Shared Bus (SB) as shown in Fig. 5. Shared Bus and parallel pin (PP) pins are interfaced by PNP transistor(s) that provide a distributed current-sink to its respective voltage-error amplifier within each converter. When transistor(s) become active during the MPPT mode of operation, the driving impedance across the PP and return terminals is much lower as compared to that without the transistor, resulting more effective noise attenuation.
sharing bus actually carries a common reference for the current loops in all modules based on the current reference each module provides through its voltage compensator. The current sharing accuracy is then decided by the current control accuracy of each module. As shown in Fig. 4, innermost current mode regulation method combined with automatic master current sharing bus becomes very attractive. First of all, shared bus with the fault tolerance makes the power system a real scalable system just through adding/removing the power modules and without conflict with the running modules. Secondly, such arrangement also has the benefit to eliminate the non-uniform output characteristics caused by different reference voltage among the converters. Besides, the shared bus can also be implemented to track the maximum power point, and thereafter to further simplify the system and make it possible to carry out MPPT control with the standalone Commercial Off the Shelf (COTS) power modules. Therefore, such a current sharing techniques is suitable for solar-based expandable parallel-connected power system. Module #1
Vin
Filter channel 1
~ ~
~ ~
DC-DC Vout+ pp Converter with CMC 11 Vout-
C
~ ~ ~ ~
DC-DC Converter with CMC 1N Vout-
I1in AD0
v3in + -
+ v 1in AD1
~ ~
SB1
Vout+ pp
Vo
~ ~ ~ ~ ~ ~
Load
DA0
+ -
DA2
DSP MPPT Controller
+
-
+
-
Hi +
Verr1 Hv+
Vref1
AD2
L O A D
I2in
Shared_Bus Vref2
Hi +-
DA1
AD3
+ v 2in -
+
Hv Verr2 -
~ ~
~ ~
~ ~ ~ ~ DC-DC Vout+ Converter pp with CMC 2N Vout-
Module #2
v4in
DC-DC Vout+ Converter pp with CMC 21 Vout-
Filter channel 2
+ -
DA3
+
~ ~
~ ~
SB2
Fig. 5 Basic configuration of paralleled converters with multiple peak power tracking control
Fig. 4 Paralleled converters with current sharing bus
All the solar array output voltage and current are fed into the DSP controller for tracking their respective array peak power. Each channel either automatically adjusts its operating condition to be near or at the maximum power point of its solar array source(s) or regulates its system output voltage when the net load demand is below the peak power. At the same time, nearly uniform current sharing among DC/DC converter modules connected in parallel is also achieved by the concept of shared-buses. In this distributed MPPT approach, each current mode DC/DC converter module has an additional control port for the purpose of MPPT. It is possible that, while many converter modules are operating in their output voltage regulation mode, the remaining modules may operate in MPPT mode individually, depending on the characteristics of their respective solar array sources.
IV. MULTIPLE SOLAR ARRAY POWER SYSTEM WITH MAXIMUM POWER POINT TRACKING The power system with the multiple solar array voltage regulation and MPPT control is shown in Fig. 5, the configuration is an output-paralleled DC/DC converter system with distributed solar sources, each of which is connected to the input of respective DC/DC converter. A simple DSP-based controller dedicated to all the solar array sources is implemented to control their respective DC/DC converter modules, therefore track the multiple peak power points of the distributed solar arrays. The benefits of DSPbased MPPT controller can be numerated as follows: 1) Improved Signal to Noise Ratio (SNR); 2) Inter-modal system stability since operating points are very repeatable and free from life and environmental drift; 3) Easier for control laws optimization; 4) Minimum system complexity since
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V. EXPERIMENTAL PROTOTYPE OF A POWER SYSTEM WITH TWO SOLAR ARRAYS
Total output current
According to the system block diagram shown in Fig. 5, a two solar array power system with MPPT, as shown in Fig. 6, is developed to validate the concept. In the built system, each solar array coupled by two current mode DC/DC converters, two solar array simulators (E4350B from Agilent) were used to represent two solar arrays and four converters (HDM-45 from Rantec) are COTS products with the parallel pin (PP). TMS320C24x evaluation board from Texas Instruments was used to track the maximum power point of two solar array simulators.
Output current in one converter
Fig. 7 Current sharing among the paralleled DC/DC converters (2A/div) Array Voltage
OVR mode MPPT mode
DSP EVM Array Current IDC
Solar Array Output DC/DC converter
Fig. 8 Dynamic response of array voltage and current (Upper trace: 10V/div, Lower trace: 2A/div) Output DC Bus
I – V trajectory
Fig. 6 500W two solar array power system with DSP-
based MPPT control
P – V trajectory
Figures 7–9 give the experimental results of power system with single solar array and two parallel-connected DC/DC converters. Fig. 7 indicates the active current sharing method can achieve near uniform current among the paralleled DC/DC converters. The innermost current mode regulation plus automatic master current sharing solution could be implemented to expand the power in distributed power system. Fig. 8 illustrated the dynamic response of the array voltage and array current, revealing transitions between the maximum power point tracking and output voltage regulation modes of operation. From the figures, 240W of the array peak power is transferred to the power system during the MPPT mode and 120 W of the array power is transferred during the OVR mode in which the system output voltage is regulated at 28V. During the MPPT mode, the output voltage loses its regulation and settles below 28 for this particular load condition without backup battery. Fig. 9 shows the trajectories of array Power-Voltage and Current-Voltage under MPPT operation. The P-V trajectory reveals the array output moving around its peak value resulted from added dither signal, which indicates the MPPT control achieved expected performance.
Fig. 9 Trajectory of array P-V and I-V
Figures 10–11 present the experimental results of power system with two solar arrays. Fig. 10 indicates the Vmp waveform for two solar arrays, it can be found two periodical dither signals are superimposed upon the operating points of two solar arrays with the same amplitude and 180° phase shift. As afore-mentioned, the periodical dither signal with higher amplitude was implemented in the system to overcome solar array local maxima. Phase-shifted dither signal in different solar arrays, therefore, can deliver improved output performance since the output ripple caused by the dither signal could be cancelled by the counterpart unit. In the scalable power system with distributed solar arrays, it is possible that, while many converter modules are operating in their output voltage regulation mode, the remaining modules may operate in MPPT mode individually, depending on the characteristics of their respective solar array
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sources. To evaluate such a performance of developed power system, two solar arrays were deliberately set with different characteristics. Fig. 11 illustrated both solar array’s PowerVoltage trajectories under different load. Fig. 11(a) shows the light load case, it can be found, in the solar array with less power rating, its output moving around its peak value resulted from added dither signal, while in the other solar array, two converter modules are operating in their output voltage regulation mode. With the load increases, the operating point of another solar array also climbs up to its maximum power point, while the other one stays in its peak power point, as shown in Fig. 11(b).
maximum power points tracking. Advantages of the proposed system can be summarized as follows: 1) DSP-based controller is cost efficient for tracking multiple solar array peak power points, and flexible for system power expansion and control algorithm optimization. Improved control scheme increases MPPT efficiency and its adaptability to assort solar array characteristics. 2) Use of external parallel pin to meet an added-on control purpose such as maximum power tracking without internal circuit modifications of the paralleled COTS DC/DC converters. VI. CONCLUSION An expandable power system with robust multiple power point tracking capabilities is presented in this paper. The system incorporates a DSP controller to track multiple peak power points of a plurality of solar arrays. Paralleled current mode DC/DC converters, coupled between a solar array and the load acts as a peak power track module for each solar array. The performance of the proposed system is validated and evaluated by a built 500W prototype. ACKNOWLEDGMENT The authors would like to thank Dr. Siri Kasemsan for his technical insight, without him this paper could not have been accomplished.
Fig. 10 Operating point for two solar arrays 300 250
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Power (W)
200
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Voltage (V)
(a) Light load
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(b) Heavy load Fig. 11 Two solar array’s P-V trajectory under different load
7. Batarseh, I.; Siri, K.; Lee, H. “Investigation of the output droop characteristics of parallel-connected DC-DC converters” Power Electronics Specialists Conference, PESC '94 Record., 25th Annual IEEE, pp. 1342 -1351 vol.2. 1994
The experimental results validated the feasibility of constructing scalable power system with parallel-connected DC/DC converters and distributed solar arrays with multiple
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