Proceedings of the 2011 International Conference on Power Engineering, Energy and Electrical Drives
Torremolinos (Málaga), Spain. May 2011
Harmonic Rejection Using Multi-Synchronous Reference Frame Technique for CSI-Based Distributed Generation with Grid Voltage Distortion Ahmed Salah Morsy, Mohamed Tawfeek and Adel Lotfy Electrical Engineering Department Alexandria University Alexandria, Egypt
[email protected],
[email protected],
[email protected]
Shehab Ahmed
Ahmed Massoud
Electrical and Computer Engineering Department Texas A&M University at Qatar Doha, Qatar
[email protected]
Electrical and Computer Engineering Department Qatar University Doha, Qatar
[email protected]
Abstract — In the midst of the growing penetration of renewable energy resources and Distributed Generation (DG), power quality is one of the main concerns. Achieving low total harmonic distortion (THD) of exported current using low switching frequency inverters is a challenge, especially under conditions of severe utility voltage distortion. Previous work depended on the wide bandwidth of passive filters accompanying high switching frequency inverters, which enables controlling fast tracking of pure sinusoid reference current. This paper presents a control structure for current source inverter-based distributed generation based on Multi-Synchronous Reference Frame, to reject the effect of utility distortion and attain high quality output current. This solution is applicable for low switching frequency inverters, with limited passive filter bandwidth. Experimental results validate the proposed technique. I.
INTRODUCTION
In the midst of the growing penetration of renewable energy resources and Distributed Generation (DG), power quality is one of the main concerns. Current Source Inverter (CSI) proves to be a natural candidate in Photovoltaic panels and Fuel cells [1, 2]; owing to their voltage boosting capability and smooth DC link current [3]. CSI also has the advantage of inherent protection against short circuit, which is common in distribution level. Unfortunately, due to efficiency constraints, CSI is usually operated at lower switching frequency than that of voltage source inverter (VSI), hence, requiring a CL filter tuned at low cut-off frequency. This creates a low impedance path for grid harmonics which could interfere with the inverter current and increases the total harmonic distortion (THD). This low cut-off frequency puts limitations on control loop bandwidth. Active damping techniques were carried out to weed out resonance problems [4, 5]. For severe voltage distortion, these techniques are insufficient to form a clean DG power source.
978-1-4244-9843-7/11/$26.00 ©2011 IEEE
Also fast switching of VSI connected to the grid through LCL filter tuned at large cut-off frequency was used, hence getting wide current control loop bandwidth [6]. Fast switching VSI with L filter was used to minimize current harmonics and DC Voltage low order harmonic ripples [7]. In this paper, the CSI switching frequency, and controller sampling, is limited to 2.4 KHz, and the CL filter is tuned at 300 Hz. This forms a challenge to eliminate specified harmonics. Since the elimination of one harmonic component increases the others. Consequently an accurate harmonic extraction method was implemented to decompose positive and negative sequences of low order harmonics and convert all these Harmonic components to their dc values in their perspective synchronous reference frames. This extraction method facilitates the use of regular PI controllers in the harmonics cancellation process and helps to overcome the limited bandwidth and switching frequency in controller design. II.
GRID VOLTAGE DISTORTION
In domestic buildings, where DG generally germinates, a large percentage of loads are nonlinear. They can be either single-phase, like printers, computers and electronic equipment, or three-phase, like variable speed drives for centralized HVAC systems. These single-phase loads (and three-phase rectifiers under unbalanced voltage supply) draw unbalanced distorted currents, causing unequal distorted voltage drops on the three phases. Hence, each harmonic will have both positive and negative sequences created on the line voltage. Consequently, for a three-wire DG inverter used for generating power from solar panels or fuel cells to grid, caution should be taken regarding harmonics starting from the third component with positive and negative sequences. Also, the presence of capacitor banks for power factor correction along with inductances of high power rectifiers used in HVAC systems could cause high frequency resonance excited by six-
pulse commutation of HVAC rectifiers. Fig. 1 shows utility voltage at the point of common coupling (PCC) with the DG inverter, while Fig. 2 demonstrates the frequency spectrum of the shown voltage, where resonance can be noticed at about 2.2 KHz and 2.9 KHz. This situation has to be solved on the utility system by either changing capacitor locations or sizes, or by designing a harmonic filter.
succeeds as a means to attenuate resonance, but fails against distorted utility. From another standpoint, since the injected active damping current is responsive to harmonics of capacitor voltage and inductor current, this instantaneous dependency lacks the capability of complete cancellation of harmonics, due to the limited switching and sampling frequency. Additionally, the high frequency components in the grid voltage (2.2KHz and 2.9KHz in Fig. 2) and the step changes (noticed on the grid voltage Fig. 1), due to the six commutation pulses of HVAC high power rectifiers connected at the PCC, which excite the CL filter resonance six times per fundamental cycle, complicate the harmonic rejection function of the active damping controller. IV.
Figure 1. Distorted grid voltage
Figure 2. FFT of distorted grid voltage (THD = 3.06%) III.
ACTIVE DAMPING
A typical grid connected CSI has CL filter tuned at low cut off frequency (300 Hz in this work), creating a low impedance path for inverter low order harmonics (parallel resonance) as well as grid harmonics (series resonance). Passive damping techniques discussed in [8] are targeted to raise the filter impedance at its cut off frequency to lower the grid current distortion. They were useful to provide adequate resonance attenuation. However, they all failed to provide filter impedance which is high enough to reject grid current distortion under polluted utility. The active damping concept follows the same target of passive techniques by emulating the presence of passive circuit elements (virtual resistance for damping and virtual negative inductance [4] for better time response and increased stability margins). The virtual damping loop is only placed around filter resonance frequency to prevent interference with fundamental current reference control. Hence, like the passive damping, it
ACTIVE AND REACTIVE POWER CONTROL METHODOLOGY
Normally, power quality is defined by voltage quality served by the utility at different points of distribution network. However, in case of connecting distributed generation DG units to the network, power quality is defined by current quality at the PCC with the grid [9, 10] as in case of grid connected non-linear loads. This requirement is because of the fact that the DG must not actively regulate the voltage at the PCC [9]. In many examples of inverter-based grid connection, where VSI topology is used, the inverter is operated as a voltage source where the inverter voltage applied to the passive filter is controlled. By mimicking the operation of a synchronous generator (through manipulating the phase and magnitude of the voltage) the output active and reactive powers have been successfully controlled. Because in this mode the inverter generates an ideal sinusoidal pulse-width modulated signal at its output, the output current and power quality depend mainly on the grid voltage quality. Only when the grid voltage quality is high, the current and power quality can also be high. If the grid voltage is distorted or if there is voltage imbalance in the grid, the exported output current is distorted or unbalanced. This relation degrades the output power quality, since the generating system adds to the distortion of the grid by presenting a low impedance to existing distortion voltage. Another approach is to operate the inverter (VSI or CSI) as a current source where the inductor current is controlled. This approach improves power quality because the output current quality is influenced less by the grid voltage quality. The main advantage of using a current source instead of a voltage source is that within the control frequency range a higher output impedance is observed from the point of view of the grid voltage. This minimizes the effect of voltage harmonics on the output current and improves the power quality. Current-mode control has been chosen for this work because of its ability to reject existing grid voltage distortion. The control strategy is to control the inverter to generate high quality balanced three-phase currents and to control power at the output of the inverter. If this is achieved, the inverter does not represent a harmonic source connected to the grid, but instead appears as a high quality power generator. This shows an advantage of using CSI over VSI, since the current-mode control is easier with CSI.
The average power control method [6] provides high quality sinusoidal output current and controls the average power flow. The role of the power controller is to generate smooth output current references by filtering out higher harmonic content from the grid voltage spectrum. This slowly changing current reference provided by filtering, along with the harmonic compensator (discussed in later sections) ensure high quality output current. A consequence of average power control is that if the grid voltage is distorted then the instantaneous power fluctuates. The fluctuations are reflected to the dc side as harmonic frequencies that are sourced from the dc-link inductor. Thus increased Volt-Ampere rating of the inverter is required in case of distorted utility.
multiple order harmonics need be compensated, multiple PR controllers can be used for selectively compensating low-order harmonics as in Fig. 3. It can be expressed as in (2): PR Controller S
K , , …
S KR S nω
(2)
Bode Diagram 100
50
0 Magnitude (dB)
There are two methods to control power: instantaneous and average power control. In the instantaneous power control method, the fundamental current component and higher frequency components are controlled to compensate for the grid voltage disturbances in a similar way to the operation of active power filters [11]. A consequence of regulating the instantaneous power is that if the grid voltage is distorted then the current will necessarily be non-sinusoidal in order to keep the power instantaneously constant. If the objective is to provide high power quality (low THD of current), then instantaneous power control should not be used.
-50
-100
-150
-200
2
10
10
3
Frequency (Hz)
Practically, to avoid measuring far or un-accessible points, only the capacitor voltages and output currents are measured and hence the grid voltage is calculated from the capacitor voltage and estimated steady state voltage drop on the inductor as in (1). v
v
0
v
v
ωL
ωL 0
.
i i
(1)
The reference output current is calculated after filtering the estimated grid voltage, and then added to a feed-forward value of capacitor current predicted from its measured voltage. A compensator, of limited bandwidth, is added to reach zero steady state error without interfering with other control loops [4]. V.
PROPORTIONAL RESONANT CONTROLLER
High-order harmonics in output current caused by switching of the inverter are mainly eliminated by passive CL filter, while low-order harmonics caused by the inverter itself or due to grid voltage distortion should be suppressed by appropriate control. Harmonics that can be compensated by control are relevant to the control bandwidth. The higher the order of harmonics is required to be compensated, the higher the control bandwidth of the system is needed. Bandwidth of system is associated with the cut-off frequency of CL filter, inverter switching frequency, delay of the system and so on. The Proportional Resonant PR Controller is used to track sinusoidal waveform similar to the use of Proportional Integral Controller in tracking DC values. PR Controller provides infinite gain at the specified frequency to be tracked; by placing complex poles at this frequency. Therefore steady state error at this specified frequency can be eliminated. When
Figure 3. Resonant controller at 3rd, 5th and 7th harmonics
In [12], proportional resonant controller was used for the cancelation of 3rd, 5th and 7th harmonics. However, PR controller is very sensitive to measurement precision, quantization error and discretization effect. This discretization effect is demonstrated clearly when dealing with low switching and sampling frequency. These problems can be overcome by using PI controllers in multi-synchronous reference frames as shown in the next section. VI.
HARMONIC EXTRACTION AND CANCELLATION
In order to eliminate low order harmonics from the grid current, they should be first extracted from the current waveform. Multi-Synchronous Reference Frame (MSRF) approach for harmonic extraction used in [7] is shown in Fig. 4. To extract a specified sequence (+ or -) of a given Harmonic, it is transformed to its perspective synchronous reference DQ frame, where it is represented by two DC values. A low-pass filter is then used to block other frequency components and a notch filter is required to eliminate the fundamental component. Amelioration of MSRF approach is introduced. This is done by subtracting other estimated current harmonic components from the loop of the component to be extracted, instead of excessive dependence on low pass filters and notch filters to exclude other frequency components as indicated in Fig. 5. This extraction method facilitates the use of regular PI controllers to take their roll in harmonic rejection and helps to overcome the limited bandwidth and switching frequency in controller design.
Figure 4. Harmonic extraction and elimination using MSRF
Figure 6. Experimental setup
Figure 5. Harmonic extraction and elimination using Ameliorated MSRF
Resonance is shown in Fig. 7(a). After, active damping employing virtual negative inductance was applied Fig. 7(b) 5th & 7th Harmonic components were attenuated, but insufficiently. Harmonic Rejection using MSRFs showed more selectivity and better attenuation than Proportional Resonant Controller when considering rejection of 3rd Harmonic only (Fig. 7(c, d)). (Fig. 7(e, f)) tipped the balance towards using MSRFs for harmonic extraction and rejection against PR controllers, by more than 10db harmonic rejection. VIII.
VII.
EXPERIMENTAL RESULTS
An experimental rig Fig. 6 has been built, with a 10KVA CSI connected to a 50Hz distorted grid at 415V via an isolation transformer. Filter parameters are C=86µF, L=3.5mH (fcut off = 290 Hz). TMS320C28335 floating point DSP is sampling voltage and current measurements at 2.4 KHz and generating Space Vector PWM signals to the inverter gating circuit.
a)
Resonance effect 5th harmonic (~ -9db)
CONCLUSION
DG interface with distorted utility is accompanied with challenges to attain acceptable power quality. Active damping is proven to be insufficient means of THD fixing of DG output current under severe utility voltage distortion. Two Techniques were introduced to reject low order harmonics. Experimental results substantiated the validity of both techniques and preference of MSRF method.
b)
Active damping without cancellation 5th harmonic attenuated (~ -16db)
c)
e)
PR controller at 3rd harmonic in stationary reference frame ±3rd harmonic (~ -39db)
PR controllers (at 3rd harmonic in stationary frame & 6th harmonic in fundamental synchronous reference frame) ±3rd , -5th & +7th harmonics cancelled (~ -31db)
d)
PI Controllers at ±3rd harmonic synchronous reference frames ±3rd harmonic (~ -50db)
f)
PI Controllers at multi-harmonic synchronous reference frames ±3rd , ±5th & ±7th harmonics cancelled (< -41db)
Figure 7. Experimental results of grid current under distorted grid with different controllers IX. [1]
[2]
[3]
[4]
[5]
REFERENCES
Klumpner, C.; , "A New Single-Stage Current Source Inverter for Photovoltaic and Fuel Cell Applications using Reverse Blocking IGBTs," Power Electronics Specialists Conference, 2007. PESC 2007. IEEE , vol., no., pp.1683-1689, 17-21 June 2007 Photong, C.; Klumpner, C.; Wheeler, P.; , "A current source inverter with series connected AC capacitors for photovoltaic application with grid fault ride through capability," Industrial Electronics. IECON '09. 35th Annual Conference of IEEE , vol., no., pp.390-396, 3-5 Nov. 2009. Mohr, M.; Fuchs, F.W., "Comparison of three phase current source inverters and voltage source inverters linked with DC to DC boost converters for fuel cell generation systems," Power Electronics and Applications, 2005 European Conference on , vol., no., pp.10 pp.-P.10 Morsy, A.S.; Ahmed, S.; Enjeti, P.; Massoud, A.; , "An active damping technique for a current source inverter employing a virtual negative inductance," Applied Power Electronics Conference and Exposition (APEC), 2010 Twenty-Fifth Annual IEEE , vol., no., pp.63-67, 21-25 Feb. 2010. Wiseman, J.C.; Bin Wu; , "Active damping control of a high-power PWM current-source rectifier for line-current THD reduction," Industrial Electronics, IEEE Transactions on , vol.52, no.3, pp. 758764, June 2005.
[6]
Prodanovic, M.; Green, T.C.; , "Control and filter design of three-phase inverters for high power quality grid connection," Power Electronics, IEEE Transactions on , vol.18, no.1, pp. 373- 380, Jan 2003. [7] Hwang, J.-W.G.; Winkelnkemper, M.; Lehn, P.W.; , "Control of ACDC-AC Converters with Minimized DC Link Capacitance under Grid Distortion," Industrial Electronics, 2006 IEEE International Symposium on , vol.2, no., pp.1217-1222, 9-13 July 2006. [8] Ahmed, K.H.; Finney, S.J.; Williams, B.W., "Passive Filter Design for Three-Phase Inverter Interfacing in Distributed Generation," Compatibility in Power Electronics, 2007. CPE '07 , vol., no., pp.1-9, May 29 2007-June 1 2007 [9] "IEEE Standard for Interconnecting Distributed Resources With Electric Power Systems," IEEE Std 1547-2003 , vol., no., pp.0_1-16, 2003. [10] "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems," IEEE Std 519-1992 , vol., no., pp.0_1, 1993. doi: 10.1109/IEEESTD.1993.114370 [11] Akagi, H.; Watanabe, E.; Aredes, M.; , "," Instantaneous Power Theory and Applications to Power Conditioning , vol., no., pp.400. [12] Chunxi Liu; Chi Sun; Wenhua Hu; , "Proportional-resonant controller of high power 400Hz inverter in stationary frame," Electrical Machines and Systems, 2008. ICEMS 2008. International Conference on , vol., no., pp.1772-1777, 17-20 Oct. 2008.
