International Journal of Electrical and Computer Engineering (IJECE) Vol.
No.
June 2016, pp.
ISSN: 2088-8708.
DOI: 10.
11591/ijece.
Direct Instantaneous Power Control for Three-Level Grid-Connected Inverters Yong Yang School of Urban Railway Transportation.
Soochow University.
China
Article Info
ABSTRACT
Article history:
Power electronic grid-connected inverters are widely applied as grid interface in renewable energy sources.
This paper presents direct instantaneous power control of three-phase three-level Neutral Point Clamped (NPC) gridconnected inverters in photovoltaic generation systems.
The system consists of a PV array.
DC/DC converter, three-level NPC inverter.
LC filter and the In order to achieve maximum power point tracking (MPPT), an adaptive perturb and observe MPPT is used.
For balancing the neutral point (NP) voltage, the control scheme through proportional integral (PI) control according to the direction of the NP current based on redundant vector selection is used.
Direct instantaneous power control is developed in a rotating synchronous dq reference frame with space vector modulation with improved operation performance.
In addition, the paper gives a performance study of the positive sequence detector (PSD) plus a synchronous reference frame phase-locked loop (PLL) as the synchronization method.
The performance of the proposed method is investigated by a grid-connected photovoltaic system with a nominal power of 12kW.
The feasibility of the proposed method is verified through experimental results, showing good steady-state and dynamic performance.
Received May 12, 2015 Revised Nov 5, 2015 Accepted Nov 25, 2015 Keyword:
Direct Instantaneous Power
Control
Grid-Connected Inverters MPPT
PSD
Copyright A 2016 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Yong Yang.
School of Urban Railway Transportation.
Soochow University, 2A321.
Yangchenghu Campus.
JiXue Rd.
No.
Xiangcheng Area.
SuZhou.
China.
Email: yangy1981@suda.
INTRODUCTION
Nowadays, the distributed generation based on renewable energies, such as photovoltaic (PV) energy, wind energy and hydropower energy have come to assume an ever-growing .
The renewable energy source will reduce carbon emission.
Photovoltaic generation can be currently regarded as one of the most promising energy sources in the renewable energy sources.
Due to the nonlinear characteristics of PV array, the maximum power point (MPP) tracking (MPPT), which captures the maximum power from the PV array, becomes an essential part of PV power generation systems.
Many MPPT algorithms have been proposed for improving the efficiency of the PV generation systems.
The existing techniques vary in simplicity, accuracy, time response, cost, and other technical aspects.
Among them, perturb and observe (P&O) method is the most common for simplicity, ease of implementation.
However, the method itself is not quite accurate and is prone to failing in quickly tracking the MPP .
The incremental conductance (INC) method will achieve MPPT by comparing the incremental and instantaneous conductance of PV array .
It is much more sophisticated and needs complicated implementation of hardware and software, but it seldom reaches the MPP in practical situations.
Fuzzy logic controllers, genetic algorithms and chaotic algorithms are the most recent advanced MPPT methods, which own their important capability of capturing MPP, even under partially shading for PV systems .
When selecting a MPPT method among many MPPT Journal homepage: http://iaesjournal.
com/online/index.
php/IJECE IJECE
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methods, a DC-DC converter should be connected between the PV array and the load.
The Boost converter has the high efficiency among non-isolated DC-DC converters .
In the last decades, multilevel inverters have attracted great interest in distributed power generation They are many kinds of topologies, such as diode-clamped, clampling-capacitor, isolated H-bridge and so on .
Among these topologies, the three-level neutral point clamped (NPC) voltage source inverter (VSI) has become the most widely used in industry .
It does not require clamping capacitors and isolation transformers and thus its hardware is simple and the total harmonic distortion in its output voltages and currents is smaller than that in the conventional two-level VSI, for which the three level NPC VSI has come to widespread use in distributed power generation systems, especially in PV generation systems.
Yet it raises a widely recognized question how to achieve the neutral-point (NP) voltage balancing.
So far, various strategies have been presented and successful balancing operations have been demonstrated.
One common strategy is to employ two separate dc sources, which are usually supplied by a transformer with two separate windings via diode full-bridge rectifiers.
However, the dc sources are large, expensive and of low efficiency.
Another strategy, as presented in .
, is injecting a current into the NP through an additional converter, which adds to the system cost and control complexity.
The grid synchronization techniques can be split into two main categories.
The methods based on zero-crossing detection (ZCD) which does not contain a phase controller and the methods based on phase locked loop (PLL) that involves a phase controller.
Since the ZCD method can be detected only at every half cycle of the grid frequency, so a fast dynamic performance cannot be obtained.
Recently, there has been an increasing interest in PLL techniques for grid-connected inverter systems .
The main task of the PLL algorithm is to provide the phase angle of the grid voltages which is mostly used to synchronize the output currents of the three-phase inverter with the grid voltages at the point of common coupling (PCC).
The dq-PLL method, also known as synchronous reference frame phase locked loop is the classical synchronization algorithm, that is easy to implement, but it is also very sensitive to the utility grid voltage unbalances, which will produces second order harmonics in dq synchronous reference frame .
otating at the angular speed A ) due to the effect of the negative sequence voltage .
otating at the angular speed AA ) of the unbalanced utility grid voltages.
For this reason, a large amount of studies have been carried out in this area in order to find a solution.
The positive sequence detector plus a dq-PLL (PSD dq-PLL) method was proposed in .
, which have analyzed and tested using a model of a grid-connected system and introducing some disturbances to the three-phase utility grid such as voltage unbalances, frequency variations and harmonic distortions.
Various control methods for PWM inverter have also been proposed in recent works.
But most of the works focus on the two-level topology and few dwell on the three-level PWM inverter.
Moreover, owing to the special requirements of the three-level PWM inverter, such as NP voltage balance, the control strategies for the two-level PWM inverter cannot be directly applied.
And the well-known method of voltageoriented control (VOC) that are suitable for the three-level PWM inverter employs an outer dc link voltage control loop and an inner current control loop to guarantee an excellent dynamic performance .
However, the final configuration and performance of the system largely depend on the quality of the applied current control strategy.
In .
, the three-phase three-level NPC inverter using VOC is also applied in photovoltaic generation systems, but NP voltage balance is not considered.
In this paper, direct instantaneous power control with space vector modulation (SVM) is implemented in a rotating synchronous dq reference frame, which possesses the capability of eliminating steady-state error and fast transient response by decoupling control.
The paper uses three-level NPC gridconnected inverters based on Boost converter, which is able to simplify the process of MPPT control and broaden the range of PV array input voltage.
The NP voltage balance is achieved through proportional integral (PI) control according to the direction of the NP current based on redundant vector selection.
The positive sequence detector (PSD) plus a synchronous reference frame phase-locked loop (PLL) as the synchronization method is adopted.
The PV system using a 32 bit digital signal processor (TMS320F2.
is Experimental results obtained on a 12kW prototype show high performance, such as a nearunity power factor .
9%), high MPPT efficiency .
95%), high power conversion efficiency and low current total harmonic distortion (THD) less than 3%.
SYSTEM MODEL AND CONTROL STRATEGY
Figure 1 shows the overall configuration of a transformerless three-phase three-level NPC gridconnected inverter system.
The system is composed of a PV array.
DC/DC Boost converter, three-level NPC grid-connected inverter.
LC filter and the grid.
The Boost converter performs MPPT control.
The Boost converter allows for a wide range of PV voltages.
The output voltage of the PV array is widely varying from 300 V to 900 V.
The dc bus contains two nominally identical capacitors.
The clamped point of the NPC Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
A ISSN: 2088-8708 inverter is connected to the capacitors at the midpoint N which is assumed to be the circuit voltage reference.
The NPC inverter regulates dc link voltage and controls the active and reactive powers.
For the utility grid, the output of the inverter system is defined as 12 kW, 400 V, 50 Hz.
To obtain high system efficiency, a low PWM switching frequency is chosen, which is set to be 10 kHz for both the DC/DC Boost converter and the NPC inverter.
Sa1
I PV
VPV
Sb1 Sc1 U dc Sa 2 Sb 2 Sc 2 Sc3 c Sb 3 Sa3 Sa 4 Sb 4 Sc 4 Figure 1.
Diagram of the three-phase three-level NPC grid-connected inverter system MPPT control A PV array is consisted by numbers of solar cell in series or parallel, and the total power of the PV array is the sum power of all of the individual solar cells.
Many methods have proposed for modeling the PV The PV array produces different levels of power under different solar irradiation and temperature.
Figure 1 displays the P-V curve of a PV array.
As shown in Figure 2, there is one operating point where the PV array generates maximum power.
Figure 2.
Characteristic of a PV array P&O method is the most widely used in commercial PV converters, where the method is considered as Autrial and errorAy.
The main advantage of the MPPT algorithm is its simple control and structure, but it has some drawbacks such as oscillations in steady-state around MPP and failing to track the MPP under sudden irradiation change .
For P&O algorithm, a larger perturbation size will lead to faster dynamics for capturing power from the PV array but will causes oscillations in PV current and PV voltage, a small perturbation size will reduces in PV current and PV voltage but will produce a slower dynamics of extracting power from the PV array .
As a compromise, a variable step P&O method is employed to extract maximum power from the PV array and to deliver it to the inverter.
The reference voltage for the PV arrays is calculated as follows .
Vref,k 1 A Vref,k A M AEPk AEVk where k and k A 1 are the sampling instants.
M is the step size and AEPk / AEVk is the instantaneous power slope at the PV array output.
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The main job is to choose and design a highly efficient converter when the variable step P&O method is used, which is considered as the main part of the MPPT.
The Boost converter has low switching losses and high efficiency among nonisolated DC-DC converters.
Thus, the Boost converter is employed in designing the MPPT.
Control of Boost converter is based on two control loops and two serial-connected PI in outer loop is set by an controllers, which is shown in Figure 3.
The reference PV array voltage VPV is compared with the measured PV array adaptive P&O MPPT method.
The reference PV array voltage VPV voltage VPV and the error is sent to proportional integral (PI) controller.
The output signal from the PV array is the reference current of the PV array.
Then the output of current control loop is used by simple voltage I PV modulation to generate switching pulse of Boost converter.
I PV
VPV
I PV
VPV
VPV
VPV
I PV
I PV
Figure 3.
Control scheme of MPPT based on Boost converter Grid synchronization PLL will detect the phase angle and the magnitude of the three-phase utility grid voltages with good steady-state and dynamic response.
There are many studies which show different structures and algorithms for PLL methods.
The synchronous reference frame PLL method, which has used in many renewable generation systems, is displayed in Figure 4.
The structure is consisted by Clarke transformation and Park transformation as phase detection (PD), the PI regulator as the loop filter, and the integrator as the voltagecontrolled oscillator (VCO).
As shown in the Figure 4, the input variables of the PLL are the three-phase utility grid voltages, and the output variables of the PLL are the phase angle of the three-phase utility grid
ed A v
A' 1
Figure 4.
Block diagram of the dq-PLL method The measured signals of the three-phase utility grid voltages are contaminated with harmonics, voltage unbalances, and frequency variations.
A solution for these problems caused by the unbalanced threephase utility grid voltages is to add a positive sequence detector (PSD) block, which is based on the symmetrical component method.
Applying the theorem, the unbalanced three-phase utility grid voltages can be decomposed into its positive, negative and zero sequences.
The instantaneous positive sequence components ( eaA , ebA , ecA ) of an unbalanced utility grid voltages ( ea , eb , ec ) is given by:
Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
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EeaA E
E AE 1E 2
Eeb E A 3 E A EecA E
E E
A 2 E Eea E
EE E
A E Eeb E 1 EE EE ec EE EE A A e j 2A /3 A A1/ 2 A 3.
A jA / 2 ) / 2
E 2
j 4A /3 A A1/ 2 A 3.
A jA / 2 ) / 2 EE A A e Using .
, the instantaneous positive sequence components ( eaA , ebA , ecA ) can be obtained as:
E A 1
S90 .
b A ec ) E ea A 3 ea A 6 .
b A ec ) A E
E A 1
S90 .
c A ea ) A A.
aA A ecA ) E eb A eb A .
c A ea ) A E
E A 1
S90 .
a A eb ) Eec A ec A .
a A eb ) A E where S90 is a 90 degree phase shift operator, which can be realized by a simple first-order filter and its following transfer function can be obtained as:
S90 .
= 1 A ( s / A0 ) 1 A ( s / A0 ) .
where A0 is the angular frequency of the utility grid voltages.
1/ 3
1/ 2
S90 ( s ) S90 ( s ) S90 ( s ) eaA 1 / .
1 / .
1/ 2
1/ 3
Figure 5.
Block diagram of the PSD block According to .
, the PSD block can be achieved, which is shown in Figure 5.
The PSD dq-PLL synchronization algorithm will be achieved by adding the PSD block to the classical dq-PLL structure shown in Figure 4, which can achieve a reliable detection of the positive sequence voltage of the phase and frequency of the unbalanced three-phase utility grid voltages.
The overall structure of the PSD dq-PLL is displayed in Figure 6.
edA A v
A' 1
Figure 6.
Block diagram of the PSD dq-PLL method IJECE Vol.
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PWM modulation The three-level NPC generates 27 vectors as shown in Figure 7 .
: 3 zero voltage vectors (ZVV.
,o,.
, 12 small voltage vectors (SVV.
(ONN,POO.
OON.
PPO.
NON.
OPO.
OPP.
NOO.
OOP.
NNO.
POP.
ONO), 6 medium voltage vectors (MVV.
(PON.
OPN.
NPO.
NOP.
ONP), and 6 large voltage vectors (LVV.
(PNN.
PPN.
NPN.
NPP.
NNP.
PNP).
LVVs divide the plane into six sectors, where each sector covers the space corresponding to 60 degree.
For the sake of brevity, mathematical formulations are presented only for the first sector.
Figure 7 .
shows the space vector representation of the first sector and consists of four triangles numbered from 1 to 4.
Based on the space vector modulation (SVM) theory, the expected voltage vector of an NPC converter is synthesized by three adjacent voltage vectors.
In the NPC converter.
ZVVs or LVVs do not affect the neutral point (NP) balance because they connect the phase currents not to the NP, but to the positive or negative dc rail.
MVVs connect one of the phase currents to the NP thus making the NP voltage dependent partly on the loading conditions, which become the main factor resulting in NP voltage unbalance.
SVVs come in pairs and each pair exports a pair of voltages of the same value but in opposite directions.
Hence.
SVVs can be further divided into positive SVVs and negative SVVs.
Consequently, in order to maintain balanced voltages in the dc-link capacitors, the present voltage imbalance and the direction of the instantaneous output should be known.
If the NP current iN is positive/negative, it will discharge/charge the lower capacitor.
Take Figure 1 for example, if the output current ia is positive.
ONN will discharge the lower capacitor ( iN A i a ), and POO will charge the lower capacitor ( iN A ib A ic A Aia A 0 ).
A positive SVV and a negative SVV form a pair, and they exert exactly opposite effects on the NP voltage.
Consequently, control of the NP voltage is achieved through selecting the switching pattern of the vectors.
Modulation for the NPC converter utilizes symmetrical placement vectors, of which the first effective vector is a negative SVV.
Switching patterns for different voltage vectors in the first sector are presented in Table 1.
Supposing the total onduration time of SVVs is TSVV , and the onduration time of a negative SVV is Tsn A mTSVV ( 0 C m C 1 ), that of the corresponding positive SVV Tsp can hence be determined via the following expression:
Tsp A .
A .
TSVV .
The NP voltage can be balanced through a linear PI controller according to the direction of the NP current, the relationship between switching states of SVVs and output phase currents which is shown in Table 2.
Control scheme for balancing NP voltage is displayed in Figure 8 and the symbol AEud is defined as the difference between VP and Vn .
U ref .
Figure 7.
Space voltage vector for three-level NPC inverter Table 1.
Vector sequence in the first sector
Region
Vector sequence ONN-OON-o-POO-POO-o-OON-ONN
ONN-PNN-PON-POO-POO-PON-PNN-ONN
ONN-OON-PON-POO-POO-PON-OON-ONN
OON-PON-PPN-PPO-PPO-PPN-PON-OON
Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
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Relationship between switching states of SVV and output phase currents Negative SVV
ONN
NP current
Positive SVV
POO
NP current
iN A ia OON
iN A Aic
PPO
iN A ic
iN A Aib
iN A Aia
NON
iN A ib
OPO
NOO
iN A Aia
OPP
iN A ia NNO
iN A ic
OOP
iN A Aic
ONO
iN A Aib
POP
iN A ib AEu d AEud .
Figure 8.
Control scheme for balancing NP voltage .
when iN A 0 .
when iN A 0 NPC inverter control In the stationary AA reference frame for a balanced three-phase system, the inverter output currents in Figure 1 can be expressed as follows:
E diA EE L dt A ua A eA A RiA E L diA A u A e A Ri EE dt where uA and uA are the A and A components of NPC inverter output voltages respectively.
eA and eA are the A and A components of grid voltages, respectively.
iA and iA are the A and A components of inverter output currents, respectively.
L is the filter inductance.
R is the total resistance of the NPC inverter.
Transformation .
from stationary AA reference frame to rotating dq coordinates, the synchronous dq reference frame grid currents can be obtained as E did EE L dt A ud A ed A Rid A A iq E L diq A u A e A Ri A A i EE dt where ud and uq are d-axis and q-axis output voltages of the NPC inverter respectively.
ed and eq are d-axis and q-axis grid voltages, respectively.
id and iq are d-axis and q-axis grid currents, respectively.
A is grid voltage angular frequency.
A id and Aiq are induced voltages due to the transformation of filter inductance from AA frame to dq .
Applying a sampling period Ts , the equation .
can be discretized as follows:
Ts R EEid .
A .
A .
A L )id .
) A L .
) A ed .
)) A ATs iq .
) E Ei ( k A .
A .
A Ts R )i .
) A Ts .
) A e .
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The instantaneous power can be computed in stationary AA coordinates as follows:
E P A eA iA A eA iA E EQ A eA iA A eA iA .
After this transformation, the three-phase active power and reactive power can be obtained in dq rotating frame as follows:
EE P A ed id A eq iq EEQ A eq id A ed iq The active and reactive powers in the rotating reference frame at .
sampling instant can be given as EE P .
A .
A ed ( k A .
id ( k A .
A eq .
A .
iq ( k A .
E EEQ ( k A .
A eq .
A .
A .
A ed ( k A .
A .
For a small enough sampling time, it can be obtained as ed .
A .
A ed .
) E eq ( k A .
A eq .
) .
By substituting .
, we obtain:
Ts R E P .
A .
A [.
A L )id ( k ) A L .
d ( k ) A ed .
)) A ATsiq .
)]ed ( k ) E E A[.
A Ts R )i ( k ) A Ts .
) A e .
)) A AT i .
)]e ( k ) E EQ .
A .
A [.
A Ts R )i ( k ) A Ts .
( k ) A e .
)) A AT i .
)]e .
) E
E A[.
A Ts R )iq ( k ) A Ts .
q ( k ) A eq ( k )) A ATsid ( k )]ed .
) E
By using PSD dq-PLL synchronization algorithm, the q-axis grid voltage eq will be zero, and the above equation .
for the instantaneous powers can be simplified as:
Ts R EE P .
A .
A [.
A L ) P .
) A L .
)ed .
) A ed .
)) A ATs ed .
)iq ( k ) E EQ .
A .
A .
A Ts R )Q ( k ) A Ts u .
)e .
) A AT e .
)i .
) EE
In order to make the NPC inverter output active and reactive powers at the .
sampling instant equal to the given active and reactive powers at the .
sampling instant, it can be obtained as:
EE P .
A .
A Pref .
) E EEQ.
A .
A Qref .
) .
Substituting .
and rearranging the results, ud .
) and uq .
) be calculated as:
Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
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Eud .
) A e [ Ts ( Pref A P .
)) A RP .
)] A ed A e A Q( k ) E Eu ( k ) A 1 [ L (Q .
) A Q ) A RQ ( k ))] A L A P .
) EE q ed Ts The equation .
contains the d-axis and q-axis components of the NPC inverter output voltages in the rotating reference frame.
After transformation from rotating dq coordinates to static AA reference frame, the gating signals will then produced according to above mentioned PWM technique for three-level NPC The direct instantaneous power control with space vector modulation control algorithm shown in Figure 9 contains the following blocks: three-level three-phase SVM, the three-phase active power and reactive power calculation, one outer dc-link voltage control loop, grid synchronization (PSD dq-PLL) and required NPC inverter voltage calculation.
The dc-link voltage difference .
of the given dc-link voltage and the measured dc-link voltage U dc is delivered to the linear PI controller, which will achieve U dc stabilizing the dc-link voltage of the three-level three-phase inverter.
The reactive power reference can be set depending on the needs of the power system and the three-phase inverter can send or absorb reactive power, while the reference value of the active power is calculated based on measured dc-link voltage of the inverter and the output value of the outer dc-link voltage control.
The required NPC inverter voltage in synchronous rotating frame in each sampling period can be directly calculated according to the given active and reactive powers, the measured active and reactive powers, the measured grid voltages, the measured grid currents, filter inductance and the total resistance, through simple mathematical calculations.
After coordinate transformation ( dq / AA ) from synchronous rotating frame dq into stationary coordinates AA using grid synchronization (PSD dq-PLL), the transformation signals are given to SVM block input.
And in order to balance NP voltage through the redistribution of the positive and negative small vectors usage, a PI controller has been included.
The output SVM signals determinate current states of the power switches.
There is a possibility to control both active power and reactive power independently.
P A ed id A eq iq Q A eq id A ed iq Q .
) U dc iA iA Qref P( k ) Q .
) Pref P( k ) U dc ea eb uA uA U dc AEud Figure 9.
Direct instantaneous power control scheme for three-level NPC inverters EXSPERIMENTAL RESULTS In order to verify the performance of the proposed control strategy, an experimental test bench has been developed as shown in Figure 10.
Some experiments have been carried out on a laboratory setup of a 12-kW three-phase three-level NPC inverter which consists of a microprocessor based on a control circuit and a power circuit.
For the control circuit, the control strategy is implemented in a software adopting a 32bit fixed-point DSP TMS320F2808, the PWM pulses are generated through the internal pulse generator of the DSP and extended by the complex programmable logic device (CPLD) EPM7256, and voltage and current signals are measured via a 12-bit resolution of internal analog-to-digital (A/D) converter integrated in the DSP TMS320F2808.
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Table 3 presents the experimental parameters.
To facilitate the operational evaluation of the system performances, a PV array emulator is of particular importance in order to avoid any significant impact on the MPPT and direct power control of the three-phase three-level NPC inverter.
In the experiment, the Topcon Quadro 32K programmable DC power supply is utilized as the PV array emulator, which functions as a realtime emulator of the PV array output characteristics.
The PV characteristic curves and operating points can be graphically monitored through communication between the setup and a computer.
In the experiment, there are two PV array curves.
The first PV array curve is set at the PV array open-circuit voltage Voc1 A 650V , the PV short-circuit current I sc1 A 27A and the PV array MPPT voltage VMP1 A 520V while the second PV array curve is set at the PV array open-circuit voltage Voc2 A 650V , the PV short-circuit current I sc2 A 13.
5A and the PV array MPPT voltage VMP2 A 520V .
Figure 10.
Photograph of experimental test bench Table 3.
Experimental parameters Symbol Description Rated output power Value Symbol Description Nominal grid phase voltage .
Value Switching frequency Nominal grid frequency PV input voltage 300V-900V
Grid filter inductance Boost inductance Grid filter capacitor
Boost capacitor
Total resistance 1A
Dc-link bus voltage 800 F
MPPT step size
VPV
PSD dq-PLL method operation In this section, the performance of the designed PSD dq-PLL method is evaluated through In the first study, the steady-state performance is investigated.
The cosine value of the grid angle cos A and the sine value of the grid angle sin A as well as the phase a grid voltage ea under the nominal grid are shown in Figure 11 .
Figure 11 .
displays the grid angle cos A and the grid angle sin A as well as the phase a grid voltage ea under the distorted grid.
In the second study, the dynamic performance is investigated.
Figure 12 .
illustrates the experimental results of the phase a grid voltage ea , the grid angle cos A and grid voltage frequency f g when the grid voltage undergoes a frequency step change from 50 Hz to 56 Hz.
Figure12 .
shows the phase a grid voltage ea , the grid angle cos A and grid voltage frequency f g when the grid voltage changes a frequency step from 56 Hz to 50 Hz.
From the Figure 11, it can be concluded that the designed PSD dq-PLL method has excellent steady-state performance even under Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
A ISSN: 2088-8708 the distorted grid.
As shown in the Figure 12, it depicts that the dynamic response of PSD dq-PLL method is very fast.
The output grid frequency gets to steady state less than 20ms even the grid frequency changes 6 Hz, which is little occurrence in practical situation.
Steady-state operation of NPC Inverters In order to achieve unity power factor, the q-axis reference reactive is set to zero.
Figure 13 .
show the steady-state experimental waveforms of phase a grid voltage ea , phase a output current ia , phase b output current ib .
PV array input voltage VPV .
PV array input current iPV and NP fluctuation voltage AEud under different PV array curve conditions after the NPC grid-connected inverter has reached the MPP.
Figure 13 .
show waveforms obtained under the first PV array curve and Figure 13 .
display waveforms obtained under the second PV array curve.
It can be seen from Figure 13, for the first PV array curve, the voltage is about 520 V and the current is around 24 A, and for the second PV array curve, the voltage is about 520 V and the current around 12 A.
This figure demonstrates that the proposed MPPT control strategy is capable of reaching the MPP and of producing a nearly perfect sinusoidal current with few steady-state error and low THD less than 5% , which is recommended in Ie Std 929-2000.
Moreover.
Figure 13 reveals that output currents of the NPC inverter are very well synchronized with grid voltages and the power factor is larger than 0.
99 as indicated by the digital power meter (WT1.
Additionally, from Figure 13 .
and Figure 13.
, it can be found that the NP fluctuation voltage is less than 8 V, which indicates that the NP voltage balance control scheme is fairly effective.
Figure 13 .
shows system efficiency measured by the WT1600 digital meter.
It is clear that the proposed 3-level NPC inverter can achieve 98.
2 % maximum efficiency and 97.
5% European efficiency.
Figure 13 .
displays MPPT efficiency measured by the SAS Control software of the Topcon Quadro 32K programmable DC power supply, it can achieve 99.
95 % MPPT
Dynamic performance of NPC Inverters Figure 14 .
shows the dynamic experimental waveforms of phase a grid voltage ea , phase a output current ia , phase b output current ib , phase c output current ic .
PV array input current I PV , dc-link bus voltage U dc and NP fluctuation voltage AEud under different conditions.
Figure 14 .
displays the dynamic performance when the given PV array curve is abruptly changed from the first PV array curve to the second PV array curve, and Figure 14 .
shows the similar transient response when changing from the second PV array curve to the first PV array curve.
From this Figure, it can be concluded that the three-phase NPC inverter exhibits excellent dynamic response.
Following a stepwise changing of the input PV array, the output currents of the NPC inverter can achieve steady state only needing a half cycle of the grid voltage.
Figure 14 .
and Figure 14 .
show that the dc link voltage variation is small when the PV array power is changed abruptly, and it only needs about 3 grid voltage cycles to achieve steady state for the NPC inverter.
From Figure 14 .
, it can be observed that the balance control method for the NP voltage adopted in the NPC inverter is effective, and the NP voltage is balanced not only in steady state but also in dynamic .
Figure 11.
Steady-state experimental results using PSD dq-PLL method .
under the nominal grid condition .
under the distorted grid condition IJECE Vol.
No.
June 2016 : 1260 Ae 1273
IJECE
A 1271
ISSN: 2088-8708
Figure 12.
Dynamic-state experimental results using PSD dq-PLL method .
grid voltage frequency step change from 50Hz to 56Hz .
grid voltage frequency step change from 56Hz to 50Hz .
Figure 13.
Steady-state experimental waveforms of NPC inverters Direct Instantaneous Power Control for Three Level Grid Connected Inverters (Yong Yan.
A ISSN: 2088-8708 .
Figure 14.
Dynamic experimental waveforms of three-level NPC
CONCLUSION
The paper adopts a new application of three-level NPC inverter with a Boost converter as the conversion system for photovoltaic generation systems.
Compared with two-level inverter.
NPC inverter is capable of providing higher voltage which is desired to meet the requirement for higher efficiency.
Due to the fast-changing irradiation, the simple variable step P&O method is employed to extract maximum power from the PV array.
The direct instantaneous power control of three-level NPC inverter is based on required converter voltage in each sampling period being directly calculated with SVM and the NP voltage is balanced via PI control.
Such a developed algorithm is featured in the following characteristics: .
constant switching frequency of power transistors.
decoupling control for active and reactive power.
a simple MPPT control with high-accuracy tracking.
a simple control with good NP voltage balance.
All algorithms and controllers are implemented on the TMS320F2808 microcontroller.
The proposed control strategy is verified by experimental results under various steady-state and dynamic Experimental results obtained on a 12-kW prototype show high performance of the proposed ACKNOWLEDGEMENTS This work was supported by the Research Fund for the National Young Science Foundation of China under Grant 51407124.
REFERENCES