A Comparative Study of FPGA Based Cycloinverter with two Modulation Techniques Vineeta Agarwal
Anshul Agarwal
Electrical Engineering Department MNNIT Allahabad India
[email protected]
Electrical Engineering Department NIT Hamirpur India
[email protected] Abstract—A cycloinverter has been analyzed using two modulation techniques namely delta modulation (DM) and trapezoidal modulation (TM). Field Programmable Gate Array based digital controller has been developed to generate the trigger pulses for cycloinverter using Hardware Description Language. Lowest THD has been found to 2.5% for DM and 5% for TM when modulation index m = 1. There is no specific pattern for the value of THD for different value of frequencies but still DM is better as compared to TM. The circuit has been tested qualitatively by observing various waveforms on DSO. Tests have been carried out to show the effectiveness and flexibility of the proposed method.
I.
INTRODUCTION
capabilities and high current carrying capacities desirable for high-power applications [6]. The output can be synthesized by suitable toggling of the switches subject to the conditions that ensure the switches do not short-circuit the voltage sources and do not open-circuit the current sources [7]. In cycloinverter the IGBTs should be triggered in a proper sequence to synthesize the desired output. The same leg switches should not be triggered at same instant else they will cause short circuit of voltage source. Switching sequence of IGBTs in cycloinverter for an output frequency twice to that of input frequency is shown in Fig. 1(c).
Many digital and transistor logic circuit (such as microprocessor, microcontroller, etc) can develop PWM [12]. The performance of these devices is however limited because these are made with generic hardware, leaving software as the only method to create application-specific functions by the designer [3]. In comparison, FPGAs [4] give designers the freedom to create custom functions, completely adapted to their specific application requirements, by enabling customization of both hardware and software at very low cost [5]. In this paper a digital controller has been designed and implemented on FPGA to generate the trigger pulses for a cycloinverter using hardware description language VHDL. Two modulation techniques namely trapezoidal modulation and delta modulation are proposed to minimize the undesirable harmonic components in order to improve the output of frequency converter. The performance of these modulation techniques are compared in terms of total harmonic distortion factor (THD). II.
(a)
Power Circuit.
(b) Common emitter configuration.
CYCLOINVERTER CIRCUIT
Fig. 1(a) shows the proposed cycloinverter. It requires four bi-directional switches, capable of blocking voltage and conducting current in both directions. In the absence of bidirectional switch module, the common emitter anti-parallel IGBT, with diode pair as shown in Fig. 1(b) is used. The diodes provide reverse blocking capability to the switch module. The IGBTs were used due to its high switching
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(c) Switching sequence of IGBT for the synthesis of output voltage. Fig. 1: Single phase power frequency converter.
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III.
CYCLOINVERTER WITH DELTA MODULATION
The cycloinverter is simulated using SIMULINK software and its facilities with resistive-inductive load taking R = 100 Ω and L = 16.5 mH. Fig. 3 (a) shows the output waveform of cycloinverter along with THD for delta modulation technique when modulation index m = 0.4 and an output frequency of 150 Hz. THD for this case is found to approximately 11.2 %.. When the modulation index is increased to m = 0.8, THD reduces to 8% as shown in Fig. 3 (b) . At unity modulation index, m =1, THD appears to 5% as shown in Fig. 3 (c). When the output frequency is increased to fo = 500 Hz, THD further reduces to approx. 2.8 % shown in Fig. 3 (d). With further increase in the output frequency the THD reduces to 2.5 % and after that it remains almost constant.
V o (Volts)
There is a definite firing sequence to generate the high frequency output fo. Four basic signals are required: two at input frequency and other two at output frequency. For example for 150 Hz (Nr times the input supply frequency that is desired output frequency) the gating pulses are generated using two set of basic signals having frequencies 50 Hz (input supply frequency) and 150 Hz as shown in Fig. 2. Let X1 represents the pulse at a frequency of 50 Hz and X2 of 150 Hz. Compliments of X1 and X2 are shown in Fig. 2(b) as X1’ and X2’. Signal X1 is used to enable the IGBTs for positive input half cycle where as signal X1’ is used to trigger the IGBT for negative half input cycle. Similarly signal X2 is used to enable the IGBTs for positive output where as signal X2’ is used to trigger the IGBT for negative output. Trigger signals required for individual IGBT are shown in Fig 2(c). In order to optimize the harmonics and to improve the output of power frequency converter, gate pulses to different IGBTs are modulated using three modulation techniques: sinusoidal PWM, delta modulation and trapezoidal modulation and their performance are compared in terms of total harmonic distortion in the output.
X1 X2
THD%
(a) X1' X2' (b)
Time (ms)
1a, 4a
(a) Output & THD of cycloinverter with m = 0.4 & fc = 2 kHz
Vo (Volts)
2a, 3a 1b, 4b 10
20
30
40
(c) THD%
2b, 3b
time (ms) Fig. 2 Gate pulses for Different IGBTs
Time (ms) (b) Output & THD of cycloinverter with m = 0.8 & fc = 2 kHz
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Vo (Volts) THD%
Figure 4 Trapezoidal modulation technique
Fig. 5 (a) shows the output waveform of cycloinverter along with THD for output frequency fo = 150 Hz and slope angle, α = 54o. The THD is approximately equal to 12 %. However when fo is increased and say taken as 1 kHz at α = 66o, THD further reduces and coming out to be 11.2 % as shown in Fig. 5(b). Now for output frequency fo= 1.5 kHz and α = 74o, THD is coming out to be approx. 7.5% as shown in Fig. 5 (c) which remains constant there afterwards.
Time (ms) Time (ms)
THD%
THD%
Vo (Volts)
Vo (Volts)
(c) Output & THD of cycloinverter with m = 1 & fo = 150 Hz
Time (ms) (d) Output & THD of cycloinverter with m =1 & fo = 1 kHz
Time (ms)
Figure 3. Simulation Results with Delta Modulation (a)
Fig. 4 shows the trapezoidal modulation technique. This is based on the classical uni-polar PWM switching. In this method a modulating trapezoidal signal Vtrapz ( t ) with an amplitude ( Ar ) and the frequency (fs) is compared with a carrier triangular signal Vtri ( t ) which is a train waveform with a frequency fc and amplitude Ac. The output frequency of the converter is decided with the frequency of the modulating wave. The intersection between modulation signals and carrier defines the switching instant of the PWM pulses. Trapezoidal itself comprises of two linear segments, namely the slope line and the horizontal line. The waveform of the trapezoidal is depend on the location of it slope angle, α. Different locations of α will result in different shape and harmonic contents.
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Vo (Volts)
CYCLOINVERTER WITH TRAPEZOIDAL MODULATION
THD%
IV.
Output & THD of frequency converter at α = 54o & fo =150 Hz
Time (ms) (b) Output & THD of frequency converter at α = 66o & fo = 1 kHz
Vo (Volts)
VI.
EXPERIMENTAL RESULTS
THD%
Experimental results are obtained qualitatively by observing the waveforms on digital storage oscilloscope (DSO) at salient points of the control circuit. A laboratory prototype (Fig. 7) includes an input interfacing circuit consisting of zero crossing detector (ZCD), power circuit comprising of eight IGBTs (BUP 314D) with common emitter configuration, driver circuit and isolation circuit using opto- couplers (4N35), resistive-inductive load and a controller (FPGA kit). The 220 Vrms (50 Hz) single-phase supply is used with a low pass LC filter with 2 mH inductor and 10 µF capacitor. The inductive load used consists of L = 16.5 mH and R = 100 Ω.
Time (ms) (c) Output & THD of frequency converter α = 74o & fo =1.5 kHz. Figure 5. Simulation Results with Trapezoidal Modulation
V.
CYCLOINVERTER REALIZATION OF FPGA
The principle of above two modulation techniques is implemented on FPGA using Xilinx programming. The block diagram of cycloinverter realization on FPGA is depicted in Fig. 6. Reference wave (RW) generator produces the required reference wave whereas carrier wave (CW) generator process carrier wave of desired switching frequency. The magnitude of these waves is determined by using look-up table technique [8-9]. The carrier wave and reference waveform will depend on the selected modulation technique for trigger pulse generation. A comparator in on line process finds the intersection points of CW and RW [10]. A synchronizing logic synchronizes the trigger pulse with the input supply in accordance with the zero crossing detector circuit (ZCD) output. The pulses produced by the switching pulse generator and by the intersection of RW and CW are multiplied by an AND gate. The multiplied output will be the required modulated trigger pulse.
Figure. 7. Experimental setup
ZCD is used to synchronize the controller pulses with the input supply. Fig. 8 shows the input-output voltage & current waveforms along with FFT analysis of DM cycloinverter at output frequency, fo = 1 kHz. After computing FFT analysis in MATLAB, THD is acquired as 4.8 %. Fig. 9 shows the output voltage-current for trapezoidal modulation technique along with the FFT of converter at output frequency of 1 kHz where THD retrieved is 10.4 %. THD has been significantly reduced in DM cycloinverter as compared to TM cycloinverter. The inverter has been tested in the output frequency range of 1 kHz to 100 kHz but it can work for other frequencies also by taking high bit up/down counter.
Figure 6. Block diagram for FPGA implementation
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VII. CONCLUSIONS
(a)
Input voltage, (upper trace: 100 V/div, 2ms/div) output current (middle trace: 2 A/div, 2ms/div) output voltage (lower trace: 50 V/div, 2ms/div)
A cycloinverter is developed through Xilinx field programmable gate array (FPGA) which generates the trigger pulses for various IGBTs used in the power circuit of the converter to produce an output frequency that is an integer multiple of the input supply frequency. Two modulation techniques are implemented on FPGA SPARTAN-3E kit, which relieves the controller from the time consuming computational task of PWM signal generation using Hardware Description Language VHDL in Xilinx 9.2i Web Pack software. The converter has been tested from 1 kHz to 100 kHz and operation of the circuit has been found to be satisfactory. The cycloinverter output improves with delta modulation scheme where the total harmonic distortion is found only 2.5 %, whereas in case of trapezoidal modulation, THD illustrated as 7.5 %. It is therefore recommended that delta modulation is the optimum choice for cycloinverter. REFERENCES [1]
(b) FFT (5 dB/div, 2ms/div) with DM at fo = 1 kHz
[2]
Figure 8 Experimental Results with DM at fo = 1 kHz
[3]
[4]
[5]
[6] (a) Input voltage, (upper trace: 100 V/div, 2ms/div) output current (middle trace: 2 A/div, 2ms/div) output voltage (lower trace: 50 V/div, 2ms/div)
[7] [8] [9]
[10]
(b)
FFT (5 dB/div, 2ms/div) with DM
Figure 9 Experimental Results with TM at fo = 1 kHz
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