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Minimized Switching Loss Bidirectional AC/DC Converter With PI Voltage Regulation for Grid Connected Micro Grid Tied Systems
1Radha P, 2Anita Ravi
1,2Department of EEE, DBIT, Bangalore, India
Abstract-The simplified pulse width modulation (PWM) strategy for the bidirectional ac/dc three phase converter in a micro grid system is proposed. A high efficiency isolated bidirectional ac to dc converter is proposed to control the bidirectional power flows and to improve its power conversion efficiency. The number of switching’s of the proposed simplified PWM strategy is one fourth that of the conventional unipolar PWM (UPWM) and bipolar PWM (BPWM). Based on the novel simplified PWM strategy, a proportional integral (PI) control scheme and SVPWM technique is developed to achieve better rectifier mode and inverter mode performance compared with the conventional dual loop control scheme. The proposed simplified PWM strategy with the proposed proportional integral control scheme has lower total harmonic distortion than the bipolar PWM and higher efficiency than both unipolar and bipolar PWMs. Furthermore, the proposed simplified PWM operated in the inverter mode also has larger available fundamental output voltage VAB than both the unipolar and bipolar PWMs. The simulation results verify the validity of the proposed PWM strategy and control scheme.
Index Terms—bidirectional AC/DC converter, simplified pulse width modulation (PWM), total harmonic distortion (THD).
I. INTRODUCTION
The three phase ac/dc PWM converter is widely used in many applications such as adjustable-speed drives, switch-mode power supplies and uninterrupted power supplies. The three-phase ac/dc PWM converters [1] [2]
are usually employed as the utility interface in a grid- tied renewable resource system, to utilize the distributed energy resources (DERs) shown in figure 1,which is efficiently and retain power system stability the bidirectional AC/DC converter plays an important role in the renewable energy system [3]. When DERs have enough power the energy from the DC Bus can be easily transferred into the AC grid through the bidirectional AC/DC converter.
In contrast, when the DER power does not have enough energy to provide electricity to the load in the DC Bus, the bidirectional AC/DC converters can simultaneously and quickly change the power flow direction from AC grid to DC grid and give enough power to the DC load and energy storage system [4]. There are many requirements for ac/dc PWM converters as utility interface in a grid-tied system; for instance, providing
power factor correction functions, low distortion line currents high quality dc output voltage and bi-directional power flow capability Moreover, PWM converters are also suitable for modular system design and system reconfiguration.
In the existing PWM control strategies of a three-phase Ac/dc converter, the converter switches are operated at higher frequency than the ac line frequency so that the switching harmonics can be easily removed by the filter [1], [3], [7]. The ac line current waveform can be more sinusoidal at the expense of switching losses. The PWM strategies are;
1 unipolar pulse width modulation 2 bipolar pulse width modulation 3 hysteresis pulse width modulation
Until now several PWM strategies have been utilized in a three-phase ac/dc converter such as bipolar PWM (BPWM), unipolar PWM (UPWM) [12]–[14].UPWM results in a smaller ripple in the dc side current and significantly lower ac side harmonic content [14]
compared to the BPWM. The UPWM effectively doubles the switching frequency in the ac voltage waveform harmonic spectrum allowing the switching harmonics to be easily removed by the passive filter.
II. THREE PHASE INVERTER TOPOLOGY
Three phase voltage-fed PWM inverters are recently showing growing popularity for multi-megawatt industrial drive applications. The main reasons for this popularity are easy sharing of large voltage between the series devices and the improvement of the harmonic quality at the output as compared to a two level inverter.
In the lower end of power, GTO devices are being replaced by IGBTs because of their rapid evolution in voltage and current ratings and higher switching frequency. The Space Vector Pulse Width Modulation of a three level inverter provides the additional advantage of superior harmonic quality and larger under-modulation range that extends the modulation factor to 90.7% from the traditional value of 78.5%.
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Figure1. Distribution energy system
Figure2. Three-phase VSI
Fig 3: Waveforms of Harmonics and its Effects
Harmonics and its effects
Harmonics are mainly due to the introduction of non linear loads into the system which draws non sinusoidal voltage and current across network. Some examples of loads which produce harmonics are VAR compensators, inverter, and DC converter. The presence of harmonics results in poor power factor, failure of operation, and poor efficiency, shortens equipment life. And even it leads to overheating of lines, transformers and generators to excessive iron losses. Due to these problems, energy delivered to the end user is an object of concern and power engineer faces the challenge of solving the problem harmonics.
Filter
To avoid effect of harmonics on the operation of sensitive equipments, it is necessary to have harmonic contents within limit and it is achieved by installing filter at load end. Filters are specially designed to remove unwanted frequency component which is present in the signal. Different types of filters are passive, active and hybrid filters. Passive filter are simple in designing and low cost.
Pulse width modulation
The basic concept of PWM technique is comparing carrier with reference wave generates the pulse accordingly.
Modulation index controls the harmonic content of output voltage waveform.
III. OPERATION PRINCIPLE OF THE PROPOSED PWM STRATEGY WITH
LINEAR CONTROL TECHNIQUE
A bidirectional single-phase ac/dc converter is usually utilized as the interface between DERs and the ac grid system to deliver power flows bidirectionally and maintains good ac cur- rent shaping and dc voltage regulation, as shown in Fig. 2. Good current shaping can avoid harmonic pollution in an ac grid sys- tem, and good dc voltage regulation can provide a high quality dc load. To achieve bidirectional power flows in a renewable energy system, a PWM strategy may be applied for the single-phase full-bridge converter to accomplish current shaping at the ac side and voltage regulation at the dc side.
Generally, BPWM and UPWM strategies are often utilized in a single-phase ac/dc converter. In this paper, a novel simplified PWM strategy with linear control technique is pro- posed. In this control scheme, the compensation current or voltage signal is compared with its estimated rated reference signal through the compensated error amplifier to produce the control signal. The resulting control signal is then compared with a saw tooth signal through a pulse width
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modulation (PWM) controller to generate the appropriate gating signals for the switching transistors.
The frequency of the repetitive saw tooth signal establishes the switching frequency. This frequency is kept constant in linear control technique.
As shown in Figure, the gating signal is set high when the control signal has a higher numerical value than the saw tooth signal and vice versa. With analogue PWM circuit, the response is fast and its implementation i s simple. Nevertheless, d u e to inherent problem of analogue circuitry, the linear control technique has an unsatisfactory harmonic compensation performance.
This is mainly due to the limitation of the achievable bandwidth of the compensated error A.PI controllers for DC-bus voltage control .
IV. PROPORTIONAL INTEGRAL CONTROLLER
PI controllers for DC-bus voltage control .A PI controller used to control the DC-bus voltage is shown in Figure. Its transfer function can be represented as Kp +Ki/S. where Kp is the proportional constant that determines the dynamic response of the DC-bus voltage control, and Ki is the integration constant that determines it's settling time. It can be noted that if Kp and Ki are large, the DC-bus voltage regulation is dominant, and the steady-state DC-bus voltage error is low. On the hand, if Kp and Ki are small, the real power unbalance gives little effect to the transient performance.
In general it can be said that P controller cannot stabilize higher order processes. For the 1st order processes, meaning the processes with one energy storage, a large increase in gain can be tolerated. Proportional controller can stabilize only 1st order unstable process. Changing controller gain K can change closed loop dynamics. A large controller gain will result in control system with:
a) Smaller steady state error, i.e. better reference following
b) Faster dynamics, i.e. broader signal frequency band of the closed loop system and larger sensitivity with respect to measuring noise.
c) Smaller amplitude and phase margin When P controller is used, large gain is needed to improve steady state error.
Stable systems do not have problems when large gain is used. Such systems are systems with one energy storage (1st order capacitive systems).
If constant steady state error can be accepted with such processes, than P controller can be used. Small steady state errors can be accepted if sensor will give measured value with error or if importance of measured value is not too great anyway. PI controller will eliminate forced oscillations and steady state error resulting in operation of on-off controller and P controller respectively.
However, introducing integral mode has a negative effect on speed of the response and overall stability of
the system. Thus, PI controller will not increase the speed of response.It can be expected since PI controller does not have means to predict what will happen with the error in near future. This problem can be solved by introducing derivative mode which has ability to predict what will happen with the error in near future and thus to decrease a reaction time of the controller.
PI controllers are very often used in industry, especially when speed of the response is not an issue. A control without D mode is used when:
a) Fast response of the system is not required
b) Large disturbances and noise are present during operation of the process
c) There is only one energy storage in process(capacitive or inductive)
d) There are large transport delays in the system
P-I controller is mainly used to eliminate the steady state error resulting fron P controller. However, in terms of the speed of the response and overall stability of the system, it has a negative impact. This controller is mostly used in areas where speed of the system is not an issue.
V. SVPWM TECHNIQUE
SVPWM different approach to SPWM is based on the space vector representation of voltages in the d, q plane.
The d, q components are found by Park transform, where the total power, as well as the impedance, remains unchanged. M is the best computational PWM technique for a three phase voltage source inverter because of it provides less THD & better PF.
Principle;
When upper transistor is switched ON; corresponding lower transistor is switched OFF. The ON and OFF state of the upper switches (S1, S3, S5) evaluates the output voltages. Space vector modulation (SVM) is an algorithm for the control of pulse width modulation (PWM). It is used for the creation of alternating current (AC) waveforms; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multiple class-D amplifiers. There are various variations of SVM that result in different quality and computational requirements. One active area of development is in the reduction of total harmonic distortion (THD) created by the rapid switching inherent to these algorithms.
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Figure4. Block diagram of PI controller
Space Vector Modulation (SVM) was originally developed as vector approach to Pulse Width Modulation (PWM) for three phase inverters. It is a more sophisticated technique for generating sine wave that provides a higher voltage to the motor with lower total harmonic distortion. The main aim of any modulation technique is to obtain variable output having a maximum fundamental component with minimum harmonics. Space Vector PWM (SVPWM) method is an advanced; computation intensive PWM method and possibly the best techniques for variable frequency drive application.
Fig. below shows that the inverter switching state for the period T1 for vector V1 and for vector V2, resulting switching patterns of each phase of inverter are shown in Fig. pulse pattern of space vector PWM.
Eight Switching state topologies of VSI:
Figure 5. Determination of Switching State Topologies of VSI
The Pulse Width modulation technique permits to obtain three phase system voltages, which can be applied to the controlled output. Space Vector Modulation (SVM) principle differs from other PWM processes in the fact that all three drive signals for the inverter will be created simultaneously. The implementation of SVM process in digital systems necessitates less operation time and also less program memory.
The SVM algorithm is based on the principle of the space vector u*, which describes all three output voltages ua, ub and uc :
u* = 2/3 . ( ua + a . ub + a2 . uc ) ………(2) where a = -1/2 + j . v3/2
We can distinguish six sectors limited by eight discrete vectors u0…u7
TABLE 1. Switching states
VI. CONVENTIONAL DUAL LOOP CONTROL SCHEME
Fig 6: Dual Loop Control Scheme
The conventional dual loop control scheme applied to the proposed simplified PWM cannot produce good performance in a three-phase bidirectional AC/DC converter. In this section, based on the proposed simplified PWM strategy, a PI control scheme and SVPWM technique is also developed to provide better line current shaping and better output voltage regulation compared with the conventional dual loop control scheme. In the conventional dual loop control scheme applied to the three-phase bidirectional AC/DC converter, the inner current loop and outer voltage loop
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are utilized as shown in figure. Where Vdc* is the DC voltage command, Vdc is the actual DC voltage; iL* is the AC current command, iL is the actual AC current.
The voltage controller calculates the voltage error and generates the current amplitude command iL multiplied by the unit sinusoidal waveform, obtained from the phase lock loop to generate the current command iL*. In general, a proportional-integral (PI) controller is adopted as the voltage controller and current controller to achieve power factor correction at the AC side and voltage regulation at the DC side.
For a convenient explanation the converter operated in the rectifier mode is discussed first. The rectifier mode switching combination is listed in Table 1. One can choose operation Status A and Status E during the condition Vs > 0, Status C and Status E during the condition Vs < 0. It should be noted that the selection of Status A or B for increasing inductor current, and status C or D for decreasing inductor current are all allowable in the proposed simplified PWM strategy. To derive the state-space averaged equation for the proposed simplified PWM strategy the duty ratio Don is defined as Don = ton / T, where ton is the time duration when the switch is turned ON, ie. Son=1, and T is the time period of triangular waveform. The duty ratio Doff is defined as Doff =1- Don, which is the duty ratio when the switch is turned off. While the ac grid voltage source is operating in the positive half-cycle Vs > 0, the switching duty ratio of status A is defined as Don and that of status E is defined as Doff. The corresponding circuit equations of status A and status E were obtained in equations (1) and (2), respectively. By introducing the state-space averaged technique and volt-second balance theory the state-space averaged equation is derived as follows:
Vs-(1-D)Vdc=0.
When the converter is operated in the steady state, the DC voltage is equal to the desired command Vdc=Vdc*, equation (12) also can be expressed in the following form.
Don = [1 – Vs / Vdc ]
Consider that the converter is operated in the inverter mode One can choose Status F and Status H for increasing and decreasing the inductor current, respectively, during condition Vs > 0, and Status I and Status K for decreasing and increasing the inductor current, respectively, during the condition Vs < 0. Note that selecting Status F or Status G for increasing the inductor current and Status I or Status J for decreasing the inductor current is all allowable in the proposed simplified PWM strategy. While the converter is operated in the inverter mode, the control signal v’cont can be obtained using a similar manner in the rectifier mode. After calculation the control signal v’cont operated in the inverter mode is the same as that in the rectifier mode. Because the control signal v continuous proportional to Don, one can regard the calculated signal v’contin as the duty ratio feed-forward control signal
vff to add into the dual loop feedback control signal vfb.
The feed-forward control signal vff can enhance the control ability to provide fast output voltage response as well as improve current shaping. The switching signals TA +, TA-, TB +, TB–,Tc+,Tc- are generated. The feed forward system is applicable for all BPWM
VII. CONCLUSION
This paper presented a novel simplified PWM strategy using PI control and SVPWM technique scheme in the bidirectional three-phase ac/dc converter. The proposed simplified PWM strategy only requires changing one active switch status in the switching period instead of changing four active switch statuses as required in the UPWM and BPWM strategies. The efficiency of an ac/dc converter operated in the proposed simplified PWM strategy is higher than that in the UPWM and BPWM strategies. Based on the proposed SVPWM technique control scheme, both ac current shaping and dc voltage regulation are achieved in both the rectifier and inverter operating modes. In addition, the proposed simplified PWM operated in the inverter mode has larger available fundamental output voltage VAB than both BPWM and UPWM, and the THD of the proposed simplified PWM is less than that of BPWM, and close to that of UPWM. Finally, the prototype system was constructed and tested, using FPGA Spartan-3E XC3S250E as the controller. Both the simulation and experimental results verify the validity of the proposed PWM strategy and control scheme.
ACKNOWLEDGEMENT
The author would like to thank Prof. H. C. Chen of the NCTU in Taiwan for his kind assistance.
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