 Open Access
 Total Downloads : 1179
 Authors : Surya Manoj Vuddagiri, K Kiran Kumar
 Paper ID : IJERTV1IS9076
 Volume & Issue : Volume 01, Issue 09 (November 2012)
 Published (First Online): 29112012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Immunity of ACDC Harmonics in VSC HVDC Transmission for MultiLevel Converters
Surya Manoj Vuddagiri Assistant Professor
K Kiran Kumar Assistant Professor
Abstract
They are very attractive techniques for the voltagesourceconverter(VSC) based high voltage dc (HVDC) power transmission systems. The paper discusses optimized modulation patterns which offer controlled harmonic immunity between the ac and dc side. This method based on selective harmonic elimination pulsewidth modulation (SHEPWM) which offer the lowest possible number of switching transitions. This feature also results in the lowest possible level of converter switching losses. The application focuses on the threelevel converter when its dclink voltage contains a mix of lowfrequency harmonic components. Simulation results are presented to conrm the validity of the proposed switching patterns.

Introduction
The continuous growth of electricity demand and ever increasing society awareness of climate change issues directly affect the development of the electricity grid infrastructure. The utility industry faces continuous pressure to transform the way the electricity grid is managed and operated. On one hand, the diversity of supply aims to increase the energy mix and accommodate more and various sustainable energy sources. On the other hand, there is a clear need to improve the efciency, reliability, energy security, and quality of supply. With the breadth of benets that the smart grid can deliver, the improvements in technology capabilities, and the reduction in technology cost, investing in smart grid technologies has become a serious focus for utilities [1].
Advanced technologies, such as exible alternating current transmission system (FACTS) and voltagesource converter (VSC)based high voltage dc (HVDC) power transmission systems, are essential for the restructuring of the power systems into more automated, electronically controlled smart grids. The most important control and modelling methods of VSCbased HVDC systems and the list of existing installations are available in [2].
Fig. 1. Phase of the twolevel VSC for the HVDC power transmission system.
Fig. 2. Threephase threelevel VSC.
The rst generation of utility power converters is based on currentsource converter (CSC) topologies [3], [4]. Today, many projects still use CSCs due to their ultrahigh power capabilities. With the invention of fully controlled power semiconductors, such as insulatedgate bipolar transistors (IGBTs) and integrated gate commutated thyristors (IGCTs), the VSC
topologies are more attractive due to their four quadrant powerow characteristics. A typical conguration of the VSCbased HVDC power transmission system is shown in Fig. 1 as it is shown in [7] and [8].
Multilevel converters [2] can be more efcient but they are less reliable due to the higher number of components and the complexity of their control and construction. Increasing the number of levels above three is a difcult task for the industry. The multilevel converters are beyond the scope of this paper. This paper focuses on the threephase three level VSC topology (Fig. 2) and associated optimized modulation.
In most cases, the voltage of the dc side of the converter is assumed to be constant and the ac network is assumed to be balanced. However, uctuations at various frequencies often occur on the dc side which usually appear as harmonics of the acside operating frequency. The most signicant harmonic introduced to the dcside voltage spectrum by an unbalanced threephase ac network is the 2nd harmonic. Inverters with 2nd harmonic on the dc bus generate the third harmonic on the ac side [10]. The proposed Mtype modulation technique allows 33% reduction in the switching transitions without lowering the order of the predominant harmonic. The geometrical technique of [12] proposes a numerical calculation by modifying the pulsewidth to cancel the harmonics produced by the dcside ripple voltage. It has lower total harmonic distortion (THD) when compared with the conventional triangular sinusoidal PWM in the case where the dclink voltage also uctuates. However, [11] and [12] deal with sinusoidalPWM techniques, which require a relatively high number of transitions per cycle to eliminate the loworder harmonics. Selective harmonic elimination pulsewidth modulation (SHEPWM) is the harmonic control with the lowest possible switching to give tightly controlled voltage spectrum and increase the bandwidth between the fundamental frequency and the rst signicant harmonic.
In the last decade or so, the size and level of power handling capability of the VSCs has increased substantially and has reached new heights for utility applications. As the interaction between the dc and ac systems increases and the power handling capability of these converters increases, it is important to further understand and study the effects of voltage and current harmonics on the converter design and operation and system performance. Any measures to minimize or even eliminate such unnecessary owing of harmonics between the two systems (i.e., the ac and dc) are benecial.
For instance, an approach that determines the harmonic Spectrum of the dcbus currents of VSCs is presented in [14]. For a twolevel threephase
VSC, a general method for calculating the dcbus currents for unbalanced, balanced, linear, and nonlinear loads is described in [15]. However, even if the converters have less dcside ripple voltage with smaller dclink capacitors than conventional methods, the capacitors remain one of the components most prone to failure. Minimizing or eliminating harmonic ows in the dclink capacitors will decrease the dissipated heat and increase overall reliability and efciency.
On the other hand, optimized modulation methods offer many advantages toward tight control of convertergenerated harmonics [19]. A minimization method to nd the complete set of solutions by solving the SHEPWM equations for twolevel inverters is discussed in [20]. In this paper, the dclink voltage is assumed to be constant. In [10], a method is proposed to prevent the dclink ripple voltage from creating loworder harmonics on the ac side of xed and variable frequency inverters. However, only one of the multiple SHEPWM sets [20] of solutions is reported.
An investigation of the harmonic interaction between the ac and dc side for STATCOM is presented in [21] including the socalled dynamic SHEPWM scheme based on precalculated angles for better THD. However, the dynamic SHEPWM scheme is applied only for a threelevel converter and can be applied only for known magnitude and frequency of the ripple. However, only one set of SHEPWM solutions is considered which requires the exact values of magnitude, phase, and frequency of the ripple in order to be implemented.
Control strategies to compensate unbalances are reported in the literature. Mild imbalances caused by unbalanced loads of the ac side are regulated by using separate control loops for the positive and negativesequence components of the voltage as proposed in [23]. Efcient control of unbalanced compensator currents can be achieved by a control algorithm based on the DSTATCOM model [24]. DSTATCOM allows separate control of positive and negativesequence currents and decoupled current control of the dq frame. An advanced strategy based on direct power control under unbalanced grid voltage conditions has been recently presented for a doubly fed induction generator [25]. To take the full advantages of VSCs for HVDC power transmission sstems, an auxiliary controller is added to the main controller which is conventionally implemented in the positivesequence – frame. To compensate for unbalanced acside loads, the auxiliary controller is implemented in the negativesequence – frame.
The objective of this paper is to discuss the effectiveness of optimized modulation based on precalculated SHEPWM in a threelevel three phase VSC to make the ac side immune from the uctuations of the dc link without the use of
passive components. However, since the VSC studied here does not include a closedloop controller, strategies to compensate unbalances are not addressed in this paper.
This paper is organized in the following way. In Section II, a brief analysis of the VSC and the modulation method is provided. Section III contains the characteristics of the method on a VSC with dcside ripple voltage. Section IV provides extensive experimental results to support the theoretical arguments. Conclusions are documented in Section V.

Analysis of the PWM Converter and SHEPWM
The optimized SHEPWM technique is investigated on a threelevel threephase VSC topology with IGBT technology, shown in Fig. 2. A typical periodic threelevel SHEPWM waveform is shown in Fig. 3.
The waveforms of the linetoneutral voltages can be expressed as follows:
Where is the operating frequency of the ac, and Vdc is dclink voltage.
Fig. 3. Typical threelevel PWM switching waveform with ve angles per quarter cycle.
Fig. 4. Solution trajectories. (a) Perunit modulation index over a complete periodic cycle. (b) Five angles in radians.
Thus, the linetoline voltages are given by
— (2)
The SHEPWM method offers numerical solutions which are calculated through the Fourier series expansion of the waveform
— (3)
Where N+1 are the angles that need to be found.
Using five switching angles per quarter wave (N=4) SHEPWM, k=5, 7, 11, 13 to eliminate the 5th, 7th, 11th, and 13th harmonics. During the case of a balanced load, the third and all other harmonics that are multiples of three are cancelled,due to the 120 symmetry of the switching function of the threephase converter. The even harmonics are cancelled due to the halfwave quarterwave symmetry of the angles, being constrained by 0 < 1 < 2 < . . . < N+1 <
/2

Ripple Repositioning Technique
In this section, the technique to reposition the loworder harmonics produced by the dclink ripple voltage of a VSC is described. The switching angles are precalculated for every available modulation index (M) to obtain the trajectories for the SHEPWM, as shown in Fig. 4.
The intersections of the trajectories shown in Fig. 4 with any horizontal straight line, called the modulating signal (Ms) (i.e., an imaginary line of M=0.75 p.u.), give the switching angles of the specific modulation index. Those switching angles are identical to the solution of the conventional SHEPWM method, so when the dc bus voltage is constant, all harmonics before the 17th one are eliminated. However, when the dc bus voltage is fluctuating, other harmonics are introduced. When the dc link has a ripple voltage of constant frequency r and amplitude k times the dcside voltage, the linetoneutral voltage is represented as
V`LN = VLN (1+k sin rt) — (4) Therefore, the modified linetoline voltage of (2) becomes
— (5)
The method is used in the same way as in (5) to derive the other two linetoline voltages of the threephase converter: V`BC and V`CA. As was already mentioned, unbalance on the ac network can cause the 2nd harmonic on the dcside voltage. Hence, r=20, and by substituting n=1 in (5), the lower order harmonics are given by
modulating signal and the trajectories of harmonic elimination solutions.
3.1 Simulation Result
The MATLAB/SIMULINK software is used to demonstrate the dclink ripplevoltage repositioning technique. Key results are presented in Figs. 57. Fig. 5 shows the simulation results of the method for the case that the dcbus voltage has no ripple. The modulating signal is equal to the modulation index since the dclink voltage is constant. Hence, the results are identical to the ones taken by using conventional SHEPWM with a fixed modulation index (i.e, Ms=M=0.75).
(a)
— (6)
The negativesequence fundamental component and the positive sequence 3rd harmonic are created on the ac side since it is proven in (6). For a constant dcbus voltage, the modulating signal is a straight line of magnitude equal to the modulation index. For the fluctuating dcbus voltage, the modulating signal is divided by, (1+k sin r t) which is the sum of the average perunit value of the dc link and the ripple voltage in order to satisfy the repositioning technique. So when the magnitude of the dclink voltage is instantaneously increased by a certain amount, the modulating signals amplitude is reduced by using the switching angles of a lower modulation index. Therefore, by using the higher modulation index at the instants that the voltage is reduced and lower modulation index at the instants that the voltage is increased, the amount of ripple is reversed.
According to Fourier transform properties, multiplication in one domain corresponds to convolution in the other domain. So even if one frequency is removed from the modulated signal, it is expected to appear as sidebands of the switching frequency. SW is the switching function of the conventional SHEPWM and the new switching function is represented by
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 5. Simulation results for SHEPWM (a) DClink voltage. (b) and (c) Solution trajectories to eliminate harmonics and intersection points with the modulating signal (M = 0.75). (d) Linetoneutral voltage. (e) Line toline voltage. (f) and (g) Positive and negative sequence linetoline voltage spectra, respectively.
Fig. 6 shows what happens when 10% of
Therefore, the relevant linetoneutral voltage is given by
The new switching function has the property of nullifying the loworder harmonics of the ac side, produced by the ripple of the dcside voltage. This new switching function is generated from the respective intersections of the modified
2nd harmonic is added to the dclink voltage. The switching angles are unchanged but the amplitude of the output voltage is fluctuating. The modulating signal is forced to be constant to give the same results with the conventional SHEPWM. The value of the fundamental component is increased by 5% and a value of the 3rd harmonic is equal to 5% of the fundamental that appears in the spectrum of Fig. 6(f).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 6. Simulation results for SHEPWM (a) DClink voltage. (b) and (c) Solution trajectories to eliminate harmonics and intersection points with the modulating signal (M = 0.75). (d) Linetoneutral voltage. (e) Line toline voltage. (f) and (g) Positive and negative sequence linetoline voltage spectra, respectively.
By applying the dclink ripplevoltage repositioning (Fig. 7), it is observed that the switching angles have slightly shifted. As shown in Fig. 7(f), the value of the fundamental component is equal to the one of Fig. 5(f). The 3rd harmonic no longer exists.
The modulating signal can be represented by the equation
Where Vdc mean is the average value of the dclink voltage and Vdc real is the online dclink voltage with 2nd harmonic ripple voltage on the dc bus, both per unit
Conclusion
An optimized SHEPWM technique, which offers immunity between the ac and dc side in a threelevel threephase VSC, is discussed in this paper. The technique is highly signicant in HVDCs due to the elimination of every loworder harmonic of the ac side produced by the dclink ripple voltage.
The dclink ripple repositioning technique regulates the magnitude of the funamental component and eliminates the low order harmonics of the ac side even when the dc bus voltage uctuates.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 7. Simulation results for 10% ripple of the 2nd harmonic at the dc bus by using the repositioning technique. (a) DClink voltage with 10% ripple. (b) and
(c) Modied modulating function and its intersection with the solution trajectories. (d) Linetoneutral voltage.
(e) Linetoline voltage. (f) and (g) Positive and negativesequence linetoline voltage spectra,
respectively.
This is an online method which can be applied for eliminating any loworder harmonic frequency regardless of amplitude or phase shift of the ripple. There are some limitations related to the maximum modulation index available for SHEPWM angles.
The repositioning technique also causes a reection with respect to the midpoint between the fundamental component and the rst signicant harmonic. On the other hand, it eliminates all low order acside harmonics for every dcbus ripple voltage of frequency below the midpoint harmonic.
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