Grid Interfacing of Distributed Energy Sources by Three-Level BtB ...

Grid Interfacing of Distributed Energy Sources by Three-Level BtB NPC Converter under Distorted Grid Voltage Marek Jasinski, Krzysztof Rafał, Małgorzata Bobrowska-Rafał, Szymon Piasecki Warsaw University of Technology, Institute of Control and Industrial Electronics, Koszykowa 75, 00-662 Warsaw, Poland [email protected], [email protected], [email protected], [email protected] Abstract- This paper presents complete control algorithm for grid-connected Pulse Width Modulated (PWM) 3-level BtB (back to back) converter intended for industrial drives and renewable energy applications. The objective of control is to limit current harmonics distortion and provide ride through capability during voltage dips. Moreover, Maximum Power Point Tracking (MPPT) and Active Power Feedforward (PF) has been implemented end tested for optimizing energy flow control. Grid Side Converter (GSC) is controlled by Voltage Oriented Control (VOC) supported by synchronization loop with positive sequence extraction, resonant current controllers for higher harmonics compensation, grid voltage feedforward loop for dips compensation and DC voltage filter. At Machine Side Converter (MSC) Direct Torque Control Space Vector Modulated (DTC-SVM) has been used. Algorithm is tested in simulations and experiment from 3kVA up to 15kVA platform using three-level BtB NPC converter with LCL filter. Both controls of GSC and MSC are interacting by Power Feedforward for energy flow optimization between grid and active load.

I.

harmonic distortion, causing current and voltage waveforms deviation from sinusoidal shape. There is wide spectrum of problems caused by harmonic distortion, related to decreased efficiency of power transfer and electrical devices lifetime. Another serious power quality problem are voltage dips, swells, flickers, frequency variation, etc. Most commonly used devices from adjustable speed drives to microprocessors are not able to operate properly during voltage dips, causing manufacturing process interruption, which can be very costly to industrial customers and households [1]. Another aspect of power quality concerns power generation by renewable energy sources, which usually utilize power electronics to adapt generated power parameters to those required by grid [2]. Strict requirements for power quality are imposed by grid codes. These regulations give limits of harmonic distortion of injected current, define required low voltage ride through capability, disconnection conditions and others. Based on above mentioned issues it can be concluded, that power quality should be always fulfill in grid-connected power electronic applications. On the one hand, converters should not introduce distortions, on the other, they should be resistant to distortions existing in the grid. Solution, to these problems is implementation of advanced converter control algorithm. It is important to use as much energy from alternative source as possible, and it is also important to control an energy flow between one AC system to DC-link and to second AC system. Accurate and fast energy flow

INTRODUCTION

Electric power systems are continuously developed and modified. Significant contribution is brought by power electronic devices connected to electrical grid. Numerous applications ranging from low power rectifiers up to HVDC systems give new possibilities to electrical power conversion, transmission and consumption. At the same time power quality requirements have became more and more important. Conventional rectifiers, based on diodes or thyristors, introduce reactive power and significant

Fig. 1. Three-Level Neutral Point Clamped BtB converter as wind turbine interface with LCL filter and local load for islanding operation.

978-1-4577-1914-1/11/$26.00 ©2011 IEEE

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in this paper is three-level BtB Neutral Point Clamped (NPC) topology shown in Fig. 1. It is connected to utility grid using LCL type filter, providing reduction of choke inductance, therefore filter size and cost are minimized. In studied case values of the filter were chosen using method described in [3]. In multilevel converters significant distortion of voltage and current waveforms appears in unbalanced DC-link voltage conditions. Capacitor voltage balancing has to be included in modulation algorithm. Proposed solution includes use of additional control loop in Space Vector Modulation (SVM) method. It bases on fact, that two of redundant voltage vectors have opposite influence on capacitor voltage balance [4]. Proportion between vectors switching times is controlled by PI controller. Contrary to other model-based or predictive solutions proposed in literature, this solution is easy to implement and requires low computational effort. Another hardware-related phenomena, that distort waveforms are dead-time effect and narrow-pulse elimination. Influence of dead-time depends on phase current sign, causing deviation on every zero crossing. Due to coupling between phases in three-wire system, current distortion appears in every phase, every 60 electrical degrees. Compensation of this effect is done by adding values proportional to dead-time to duty cycles calculated by SVM, with a sign depending on current [5]. Control accuracy of the GSC is important for the grid and islanded grid in case of islanding operation. Because the quality of this control determines the shape of the grid current, grid and stability of DC-link voltage [13].

management in BtB converter gives such features as: better stabilization and utilization of the DC-link voltage and moreover improve the quality of the power at grid side (reduce current overshoots during transients).

dSPACE DS1005

Static switch

u abc

PLL

Load

i abc LCL

*

udc

PI

id

*

iq

*

udc iPF

PF

NPC GSC

VOC SVM

udc * MPPT Ω

PI

Te Ψs

Ω

*

*

NPC MSC

DTC SVM

i uvw IM

Ω

III. Active Load

vwind

Te

*

Fig. 2. Three-Level BtB NPC Converter with Energy Flow Control. PLL – Phase Locked Loop, PF – Power Feedforward, MPPT – Maximum Power Point Tracking, IM – Induction Machine, DC – DC Machine, GSC – Grid Side Converter, MSC – Machine Side Converter, VOC-SVM – Voltage Oriented Control, DTC-SVM – Direct Torque Control Space Vector Modulated, NPC- Neutral Point Clamped

II.

CONTROL ALGORITHM

DC

GRID INTERFACING BTB CONVERTER

Recently Voltage Source Converters (VSC) based on IGBT switches have gained popularity, mainly due to advantages as: flexible, bi-directional power flow control, sinusoidal-like current and reduction of passive elements (L, C). Application of multilevel converters brings further advantages: operation with higher voltage and power levels, reduced switching losses, better electromagnetic compatibility due to reduced dv/dt, reduced AC filter size and others. Converter analyzed

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A. Voltage Oriented Control Presented control structure bases on Voltage Oriented Control (VOC) scheme [6], where current is controlled in Synchronous Rotating Frame (SRF). PI controller gains are tuned using symmetrical optimum criterion. For precise control of grid current, measurement is placed on the grid side of LCL filter. When operating with distorted and unbalanced voltage, current drawn from the grid can reach high values of THD factor. Limited bandwidth of PI controllers do not allow to compensate for higher harmonics and negative sequence appearing in grid voltage, therefore additional control loops have to be used [7]. Control GSC’s algorithm is presented in Fig. 3, additional control loops are marked with dashed lines and discussed in following subchapters. Because full control schemes for BtB converters is quite complicated the most important blocks are shown in Fig. 2. Voltage dips and harmonics cause distortions in phase angle, causing improper transformations between stationary and rotating frames. A lot of Phase Locked Loop (PLL) algorithms have been proposed to deal with this problem [8], offering filtration of higher harmonics and negative sequence.

value of protection systems. Basic method to improve converter operations under distorted grid conditions is application of voltage feedforward to PI current controllers. Although it assures great improvement it does not solve problem completely. Also problems of phase angle distortion and power oscillations remain. For voltage sensor-less operation, grid voltage estimation algorithm can be introduced, however it usually has limited dynamics and current overshoot can appear when voltage dip appears. Moreover, during grid voltage interruption the control system should move to islanding mode and return to grid connected mode when the grid voltage appears. Therefore, measured voltage feedforward is strongly recommended. During voltage unbalance negative sequence causes 2nd harmonic oscillations in power, which is seen as DC voltage oscillations [12]. This causes active current reference distortion (Fig. 4), magnifying oscillations and resulting in higher current THD. Resulting current distortion depends on implemented DC voltage filter and controller gains. Simplest solution would be either to use low controller gains or low frequency low-pass filter. However, this would drastically reduce control dynamics causing high overshoots in transients. Solution for this problem is a specific DC voltage filter design. Therefore, third order block combining notch and low-pass filter was used. It was developed using Matlab Filter Design and Analysis Tool and implemented as direct form IIR filter structure. The filter was designed such as 100Hz component is strongly attenuated, making control system immune to DC voltage oscillations without reducing control dynamics. 2nd harmonic variation in DC-link voltage does not affect AC waveforms, because value of DC voltage is compensated by proper calculation of modulation index in SVM algorithm. Moreover, unbalanced voltage conditions do not have significant influence on capacitor voltages balancing.

Basing on authors analysis, PLL based on Second Order Generalized Integrators (SOGI) [9] was chosen. Apart from higher harmonic attenuation, it enables estimation of positive sequence of grid voltage in stationary frame by utilizing simple coupling network (PSE on Fig. 2). This method provides fast synchronization independent of grid voltage conditions. B. Higher Harmonics Compensation Application of grid voltage feedforward consolidates compensation for higher harmonics, but does not eliminate them completely. Delay of digital control system causes outof-phase compensation. Effect magnifies with increase of rated power, because filter size and switching frequency decrease. This imposes, use of additional control loops to deal with most serious harmonic components. Among many proposed methods resonant controllers are particularly notable [10]. However, idealized resonant circuit is very sensitive to frequency variations. In presented scheme controller design is based on bandpass filters. Integral character is not preserved, but robustness against frequency deviations is achieved. Implementation of resonant structures in SRF allows to compensate a pair of harmonics with one controller [11]. In proposed control algorithm two controllers have been implemented: one tuned at 6th harmonic and second at 12th harmonic appearing in SRF. Thus compensation of 5th, 7th, 11th and 13th harmonic of grid voltage is possible. C. Voltage Dips and Unbalance Compensation Normally, to preserve constant power under voltage dip, current in faulted phase would rise proportionally to depth of the dip. This is dangerous situation, which can lead to thermal damage of semiconductor components or disconnection of device by protection controls. Proposed method for compensation for power deficit under voltage dip is to increase current equally in all phases, which should not cause overheating in faulted phase, and maintain it below critical

Fig. 3. Improved Voltage Oriented Control scheme. PLL – Phase Locked Loop, PSE – Positive Sequence Estimator, PI-RES – Proportional-Integral Resonant Controllers, LPF – DC-Link Low-Pass Filter 32

a)

IV.

SIMULATION RESULTS

Presented converter system and control algorithm has been implemented in Matlab/Simulink environment. Parameters of simulated system are given in Tab.1. Control has been implemented using C code allowing easy implementation on DSP DS1005 experimental platform. Fig. 4. shows DC voltage filter behavior during voltage dip. 2nd harmonic oscillation is eliminated from active current reference, reducing current negative component, while DC voltage remains almost the same as without filter. Presented results in Fig. 5. show transients in respect to voltage dip of converter in rectifier mode, operating at 80% of rated power. Grid voltage is polluted by 5th, 7th, 11th and 13th harmonic with 5%, 4%, 2%, and 1% of magnitude, respectively. A single phase voltage dip down to 0% magnitude appears between 0.12s and 0.18s (Fig.5a). To prove, that every improvement in control is necessary, single blocks have been excluded from the algorithm: • Use of simple arctan function instead of proposed PLL results in very high current distortion (Fig.5b), • Disconnection of voltage feedforward causes high current unbalance during voltage dip (Fig.5c), • Using unfiltered DC voltage for regulation results in current unbalance during voltage dip (Fig.5d).

b)

c)

d)

a)

e)

b)

Fig. 5. Simulation results – grid current and voltage.

• Fig. 4. Simulation results – DC voltage filter.

Complete control algorithm assures balanced undistorted currents as shown in Fig.5e. During voltage dip current rises proportionally to voltage drop to preserve constant power and maintain DC voltage. Small oscillations on the beginning and end of voltage dip are result of resonance in LCL filter.

V.

Experimental verification has been made on laboratory setup shown in Fig.6. Parameters of BtB laboratory model are the same as in simulation. LSC was operating as a rectifier loaded with 60% of nominal power. Distorted grid voltage was simulated using California Instruments 5001iX programmable source. Measurement equipment included sunchronously triggered scopes (Tektronix 3034B and 5034B) and power quality analyzer (Fluke 434). Harmonic compensation was tested under voltage polluted by most characteristic harmonics (Fig.7a). To verify effectivness of resonant controllers voltage feedforward was disabled during experiment. As expected, basic VOC scheme is not able to compensate distortions. Current THDI ≅ 17% is

TABLE I DATA OF SIMULATED AND EXPERIMENTAL SYSTEM Rated Power Switching Frequency Grid voltage Filter Parameters Lc / Cf / Lg DC Voltage DC Capacitance Cdc1=Cdc2

EXPERIMENTAL RESULTS

15kVA 5kHz 3x400V 2.8mH / 6uF / 2.2mH 600V 2200uF

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a)

high, drastically exceeding acceptable limits (Fig.7b). Application of resonant controllers is an effective way of compensation, THDI is ≅ 1,6% (Fig.7c). LSCs behavior under grid voltage dips is presented in Fig.8. Two cases are considered: a) single phase drop down to 0%, b) two phase drop down to 40%. In first case some voltage still remains in faulted phase, because in three-wire connected system artificial neutral point of measurement circuit shifts with respect to grid’s neutral point. During voltage dips current in all phases rises and remains almost balanced. DC voltage oscillations due to unbalance are visiable, also relatively small transients on the beginning and on the end of the dip can be observed. Final performance under grid voltage dips depends also on the temporary operating conditions. In presented case rectifier is operating at 60% load, and such current growth is acceptable. If voltage dip occurs while working close to rated power, current limitation can cause temporary DC voltage drop. During sudden voltage drop/rise on the beginning and on the end of voltage dip current spikes can be observed. Due to one sample delay, control algorithm is not able to compensate for this effect. This is however the most critical case with very high dv/dt, which is not possible in real power grid.

b)

Fig. 8. Experimental results – grid voltage dips . a) single phase drop down to 0%; b) two phase drop down to 40%. From the top: grid voltage 200V/d, DC-link voltage 50V/d; grid current, 10A/d.

Finally, in Fig. 9 results for generating operation mode are presented. In Fig.9a steady state are presented, while in Fig 9b transient in respect to wind step change is presented. Further, in Fig. 9c and 9d wind turbine speed control method are shown: constant speed and and incremental Maximum Power Point Tracking (MPPT) operation respectively. a)

b)

c)

d)

Fig. 6. Experimental setup. a)

b)

c)

Fig. 9. Experimental results: a) steady state with 7m/s of wind speed. From the top: machine current (10A/d), grid current (5A/d.), grid voltage (200V/d), DC-link voltage (50V/d, offset 400V). b) transient in respect to load torque (0 to 50% MLN) with Power Feedforward. From the top: machine current(10A/div), grid current (5A/d), electromagnetic torque (10Nm/d), DC-link voltage (20V/d offset 400V). Transients in respect to wind speed change 4-7 m/s. c) constant speed d) incremental MPPT. From the top: 1. Wind speed (5m/s/d), 2. Turbine mechanical torque (10Nm/d), 3. Rotating turbine speed (100rad/s/d), 4. Generator Power (1kW/d)

Fig. 7. Experimental results – harmonics compensation - HC. a) grid voltages 200V/div THDU≅7.3%; b) without HC grid current, 5A/div THDI≅17% and c) with HC grid current, 5A/div THDI≅1,6%. 34

VI.

CONCLUSION

[4]

The algorithm to control grid-connected three-level NPC converter was presented. It has been proved, that proper control of grid interfacing PWM converter under distorted grid voltage is a complex problem and requires coordination of many control loops. Following improvements were introduced to control algorithm to ensure stable operation and sinusoidal currents: • deadtime compensation, • DC-link voltages balancing with active Power Feedforward from MSC to GSC, • DC voltage filter eliminating negative sequence, • positive sequence extraction for proper synchronization with distorted grid, • grid voltage feedforward, • harmonics compensation by resonant controllers, • cooperation with incremental MPPT of wind turbine. Presented methods are robust and require low computational effort, thus are possible for industrial applications. Thanks, to the filter in DC voltage measurement which has very good dynamic the overal dynamic and accuracy of DC voltage control is obtained. Simulation and experimental results prove effectiveness of proposed algorithm. System presents good dynamics in transient states. In steady state resonant controllers assure very low THD factor. Implementation of presented control algorithm results in two major benefits: • BtB converter is resistant to voltage distortion appearing in power system. • Additional distortion injected to the grid by BtB converter is significantly reduced. • The power from wind turbine can be utilized more efficiently.

[5]

[6] [7]

[8]

[9]

[10]

[11]

[12] [13]

ACKNOWLEDGMENT This work has been supported by Polish Ministry of Science and Higher Education grant no N510 331637 and by Ventures programme of Foundation for Polish Science, cofinanced from European Union, Regional Development Fund and by European Union in the framework of European Social Fund through the Warsaw University of Technology Development Program. REFERENCES [1] [2]

[3]

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