Harmonic influence analysis of unified power flow controller based on modular multilevel converter

J. Mod. Power Syst. Clean Energy (2016) 4(1):10–18 DOI 10.1007/s40565-015-0175-2 Harmonic influence analysis of unified power flow controller based on modular multilevel converter Yubo YUAN1, Peng LI1, Xiangping KONG1, Jiankun LIU1, Qun LI1, Ye WANG1 Abstract The unified power flow controller (UPFC) based on modular multilevel converter (MMC) is the most creative flexible ac transmission system (FACTS) device. In theory, the output voltage of the series MMC in MMCUPFC can be regulated from 0 to the rated value. However, there would be relatively large harmonics in the output voltage if the voltage modulation ratio is small. In order to analyze the influence of MMC-UPFC on the harmonics of the power grid, the theoretical calculation method and spectra of the output voltage harmonics of MMC are presented. Subsequently, the calculation formulas of the harmonics in the power grid with UPFC are proposed. Based on it, the influence of UPFC on the grid voltage harmonics is evaluated, when MMC-UPFC is operated with different submodular numbers and voltage modular ratios. Eventually, the proposed analysis method is validated using digital CrossCheck date: 13 December 2015 Received: 16 November 2015 / Accepted: 17 December 2015 / Published online: 16 January 2016  The Author(s) 2016. This article is published with open access at Springerlink.com & Xiangping KONG Yubo YUAN Peng LI Jiankun LIU Qun LI Ye WANG 1 State Grid Jiangsu Electric Power Company Research Institute, Nanjing 211103, Jiangsu, China 123 simulation. The study results would provide guideline for the design and operation of MMC-UPFC project. Keywords Unified power flow controller (UPFC), Modular multilevel converter (MMC), Harmonic features, Voltage modulation ratio, Submodular number 1 Introduction In order to improve the performance of the existing power systems, the flexible ac transmission system (FACTS) devices have been widely applied in the power systems [1]. Among the FACTS device family, the unified power flow controller (UPFC) is the most creative FACTS device, which can simultaneously regulate multi electrical parameters to improve the power transfer capability and stability level of the power grid [2]. Since the modular multilevel converter (MMC) has the advantages of modular structure, inherent redundancy and ease of expandability, it has successfully made its way into industrial high voltage and larger power applications [3–5]. The carrier phase shifted sinusoidal pulse width modulation (CPS-SPWM) or the nearest level modulation (NLM) is usually adopted by the MMC to reduce the switching frequency, voltage and current harmonics [4–6]. The basic principle of NLM is to calculate the number of submodules (SMs) which should be put into operation based on the voltage modulation ratio, and then generate trigger pulses according to the sort of the SMs’ capacitor voltages. If the voltage levels reach to a large number (such as tens or hundreds), the NLM stands out among other modulation methods, since it has the following advantages: ease of implementation, low switch frequency, and small output voltage harmonics [7, 8]. Harmonic influence analysis of unified power flow controller based on modular multilevel converter For the voltage source converter based high voltage direct current (VSC-HVDC) application, the single phase unit of the MMC consists of hundreds of SMs, and the output voltage of MMC is usually around the rated value under normal operation condition. In this context, the harmonics in the output voltage are very small [4, 8].The dc side harmonic in MMC is studied in [9, 10]. The analytical approaches to calculate the harmonics for MMC with different modulation methods are presented in [7, 11]. A modular harmonic domain modeling technique for UPFC is developed in [12]. However, being different from the operation condition of the VSC-HVDC, the output voltage of the series MMC in MMC-UPFC can be regulated from 0 and the rated value [13, 14]. The waveform of the output voltage would be distorted if the voltage is not so large, and the harmonics would be rather remarkable. Hence, the research on the harmonic characteristics of MMC-UPFC is of great significance to the safe and stable operation of the power grid with UPFC. To this end, the influence of the MMC-UPFC on the harmonics of the power grid is analyzed in this paper. The structure and operating principle of MMC-UPFC is put forward. The theoretical calculation method and spectra of the output voltage harmonics of the MMC based on NLM are proposed. Then the harmonic study approach of the power grid with MMC-UPFC is presented. Finally, the simulation study which validates the proposed harmonic analysis approach is demonstrated. The harmonic suppression measures are discussed. 11 SM MMC id P SMa1 SMb1 SMc1 SMa2 SMb2 SMc2 upa SMaN SMbN SMcN iPa ipb ipc a Udc c O una N b iNa iNb iNc SMa1 SMb1 SMc1 SMa2 SMb2 SMc2 SMaN SMbN SMcN Phase unit D1 T1 + VC Vc1 T2 D2 isa Rs Ls Vsa isb Rs Ls Vsb isc Rs Ls Vsc Arm Fig. 1 The topology of MMC Fig. 2 System structure of UPFC 2 MMC-UPFC model The topology of the MMC is depicted in Fig. 1 [15]. It can be found that the MMC consists of three parallelconnected phase units, each with upper and lower arms. The single arm has a modular structure with N seriesconnected submodules (SMs) and a series inductance. Each SM contains two insulated-gate bipolar transistor (IGBT)/diode switches and a capacitor [4, 5]. The MMC-UPFC consists of shunt MMC and series MMC which are connected back-to-back with the common dc bus [13], as shown in Fig. 2. The shunt MMC interfaces to the power grid through the shunt transformer, and can absorb or generate controllable reactive power for AC bus voltage regulation. Meanwhile, it can exchange the active power with the power grid to maintain the dc bus voltage constant. In addition, the series MMC interfaces to the power grid through the series transformer. The power flow of the transmission line can be regulated by adjusting the magnitude and phase angle of the voltage injected by series MMC [1, 2]. 3 Harmonic spectra of MMC based on NLM 3.1 Basic principle of NLM At present, the NLM is the most commonly used modulation technology for MMC, whose basic principle can be elaborated as follows [7]: According to the operating principle of MMC, the number of SMs which are put into operation at every switch cycle should always be N (N is number of the SMs in the single arm). uref is the modulation voltage, VC is the capacitor voltage of single SM. With the increase of the instantaneous value of uref, the number of SMs which are put into operation in the lower arm should increase, and the number of SMs which are put into operation in the upper arm should decrease at the same time. Consequently, the output voltage would increase. At every switch cycle, the numbers of SMs which are put into operation in the upper and lower arms can be de (...truncated)