Diagnosis Method for GTO Open Switch Fault Applied to Reconfigurable Three-Level 48-Pulse STATCOM

POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE Diagnosis Method for GTO Open Switch Fault Applied to Reconfigurable Three-Level 48-Pulse STATCOM Omar Fethi BENAOUDA1,2, Azzedine BENBIABDELLAH 2, Bilal Djamal Eddine CHERIF 2 1Research Center in Industrial Technologies CRTI, P.O.Box 64, Cheraga 16014, Algiers, Algeria 2Diagnostic Group, LDEE laboratory, Electrical Engineering Faculty, University of Sciences and Technology of Oran MB, BP 1505 El-Mnaouer, Oran 31000, Algeria o.b[email protected], [email protected], [email protected], cherif.do[email protected] DOI: 10.15598/aeee.v17i2.3192 Abstract. In the recent years, several research works are focusing on the use of STATCOM in electrical networks because it is used to regulate the voltage, to improve the dynamic stability of the power system besides allowing better management of the power flow. All these positive tasks have guaranteed an important position of STATCOM within a family of Flexible Alternating Current Transmission System (FACTS). In this paper study, the control and operation of a three levels 48-pulse GTO based STATCOM is implemented with series connected transformers. The system may, unfortunately, be prone to GTO switch faults and therefore may affect reactive power transiting. In this paper, a new diagnostic approach is proposed based on the Single-Sided Amplitude Spectrum (SSAS) method of the three-leg converter currents for detection and localization of open-circuit faults. The integration of the STATCOM reconfigurable fault tolerant to the system is also considered to ensure service continuity. Several results are presented and discussed in this paper to illustrate the performance of the STATCOM faulttolerant diagnostic. Keywords Detection, diagnosis, FACTS, GTO, open switch fault, reconfiguration, SSAS, STATCOM. 1. Introduction The systems of Flexible Alternating Current Transmission (FACTS) have become available worldwide due to the growing demand for energy; these systems generally use power electronics. There are two main trends in the future outlook on the concept of the development of electrical networks; either to improve the quality of the electrical networks or to strengthen these networks [1]; the latter is not an economical solution, the solution is therefore in improving the networks. The use of Flexible AC Transmission Systems (FACTS) has significantly contributed to enhancing both the voltage control and the network stability and hence allowing better management of the power flow [2]. The basic principle of the STATic COMpensator (STATCOM) is in its ability to generate and absorb reactive power between the AC system and the STATCOM [3] and [4]. This is possible thanks to the topology structure of the power electronics converter which often depends on the high-power compensators due to its operating efficiency [5], [6], [7], [8] and [9]. The three-level power converter is the most popular multi-level topology used in the various industrial applications [10]; these converters are particularly used in the FACTS systems which contain GTO thyristor switches since they enable high power management capability. The use of a large number of the structural semiconductor switches is in fact very important in increasing the converter capability according to the required power levels, but is unfortunately reflected negatively by the increase in the converter structure complexity on the one hand and in increasing the possibility of the exposure to faults in the converter on the other hand [11]. In electrical systems that include power converters, several faults may appear in the converter, on its switch control [12] or in the physical semiconductor component itself [13]. Many research concerns are about the behaviour study of the converter; especially after the occurrence of a fault in the semiconductor components or a driver fault [14] and [15]. In the STATCOM sysc 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 114 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE tem, the converter has two operating modes; either as a rectifier or an inverter; in accordance with the source of the voltage change. These devices are very sensitive to a fault in terms of reliability and therefore the detection and localization of faults become a necessity and are the most profitable. Thus, in order to ensure planned maintenance or improve reliability, diagnostic and fault detection methods must be applied. In general, the faults can be classified as short-circuit faults and open-circuit faults [16] and [17]. Some diagnostic methods have been developed for converters in the last decade by many researchers like Mendes, Abramik, Peuget and all [18] and [19]. In the case of open-circuit faults, most of these fault diagnostic methods for inverters have been reviewed by Bin Lu [20] and [21], while a few researchers take care to study the diagnosis of the three-phase rectifier [22]. At this stage, it should be noted that it is somehow difficult to apply diagnostic techniques on the STATCOM system because the converter may work either as a rectifier or an inverter and it is possible that the switching from one mode to another may occur at any time. There is a number of researchers that have gone beyond the diagnosis and detection tasks by introducing the concept of fault tolerance in the design of converter topologies. To maintain the continuity of service of the converter with an acceptable performance after the fault occurrence, either reconfiguration based on redundancy or control techniques are applied to the converter [23]. The three-level converter is considered to be one of the most famous of tolerant topologies described in the literature [24]. There are many techniques to reconfigure the converter; either by adding an extra (or redundant) leg or by changing or readapting the control of the healthy legs. This tolerance is achieved by adding some devices to the fundamental topology. This paper proposes a diagnostic technique based mainly on the method of calculating Single-Sided Amplitude Spectrum (SSAS) for the GTO open-circuit fault in the three-level converter operating under the two modes; rectifier or inverter. First, a comparison of the GTO switch opening fault is indicated in the STATCOM. Subsequently, the methods for diagnosing SSAS faults are applied and then proposed to the STATCOM converters. Finally, A multi-level fault tolerant converter is incorporated in the system and discussions about the effects of an opencircuit fault on the entire system ±100 MVAR 48-pulse GTO STATCOM is presented. 2. System Description Figure 1 depicts the 48-pulse GTO STATCOM configuration which is composed of four converters (Conv1, Conv2, Conv3 and Conv4) where each one of them has its own switching frequency. These converters are connected in series with four transformers (Trans1, Trans2, Trans3 and Trans4). The phase shifts of the four transformers are −15◦,−7.5◦,+7.5◦and +15◦, respectively. The voltages obtained on the 500 kV sides of the transformers are connected in phase after the series connection of the primary windings, where the STATCOM has behaviour according to the relation voltage generated by the component of voltage source converter with the system voltage. Fig . 1 : Three - level 48 - Pulse STATCOM system configuration ( ± 100 MVA) . S 4 11 S 3 11 S 2 11 S 1 11 Conv2 V b V a - V dc2 + V dc1 N Conv4 Conv3 Conv1 Trans4 Trans3 Trans2 Trans1 N N N N V a V S 1 11 S 2 11 S 3 11 S 4 11 V V dc 2 Vc Fig. 1: Three-level 48-Pulse STATCOM system configuration (±100 MVA). c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 115 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE If the STATCOM is operating in a steady state, this means that there is parity between the Voltage-Sourced Converter (VSC) and the system voltage, but if they are not equal, the STATCOM will intervene either to absorb or generate reactive power. In order to know the dynamic response of the STATCOM with respect to changes in the system voltage, one of the three voltage sources used can be modified. Figure 1 shows the three-level 48-Pulse STATCOM system configuration. The phase shift expression of each 12-pulse converter is defined as follows: The 12-pulse compound voltage generated by each of the four converters are given as: The first 12-Pulse Converter (Conv1): Vab12(t)1= 2 [Vab1sin (ωt + 30◦) + Vab11 sin (11ωt + 195◦) +Vab13 sin (13ωt + 255◦) + Vab23 sin (23ωt + 600◦) +Vab25 sin (25ωt + 120◦) + . . . ]. (1) The second 12-Pulse Converter (Conv2): Vab12(t)2= 2 [Vab1sin (ωt + 30◦) + Vab11 sin (11ωt + 15◦) +Vab13 sin (13ωt + 75◦) + Vab23 sin (23ωt + 60◦) +Vab25 sin (25ωt + 120◦) + . . . ]. (2) The third 12-Pulse Converter (Conv3): Vab12(t)3= 2 [Vab1sin (ωt + 30◦) + Vab11 sin (11ωt + 285◦) +Vab13 sin (13ωt + 345◦) + Vab23 sin (23ωt + 240◦) +Vab25 sin (25ωt + 300◦) + . . . ]. (3) The fourth 12-Pulse Converter (Conv4): Vab12(t)4= 2 [Vab1sin (ωt + 30◦) + Vab11 sin (11ωt + 105◦) +Vab13 sin (13ωt + 165◦) + Vab23 sin (23ωt + 240◦) +Vab25 sin (25ωt + 300◦) + . . . ]. (4) The output voltage of the 48-pulse neutral line from the STATCOM model is expressed by: Van48(t) = ∞ X n=1 Vabnsin (nωt + 18.75◦n+ 18.75◦i) ∀n= 48r±1, r = 0,1,2,.... (5) Fig.2 : System simulation with a 48 - pulse VSC based  100 MVAR STATCOM in a 3 - bus power system . 300MW Programmable [48] B 2 B B 5 00KV R L Q L1_200Km 500KV 9000MVA L 3_180Km L2_75Km 200MW 6500MVA 500KV STATCOM pulse Vabc_B1 Iabc_B1 Vdc Voltage Source 1 3 Fig. 2: System simulation with a 48-pulse VSC based ±100 MVAR STATCOM in a 3-bus power system. 2.1. Simulation Results and Discussions In the following section, some simulation results of the healthy STATCOM using the MATLAB/ SIMULINK environment are obtained and discussed. The simulation results depicted in Fig. 3 show the dynamic response of a healthy STATCOM for the source voltage change. The programmable voltage source is changed to determine the reaction of the STATCOM. It is important to note that before the instant 0.05 s, the source voltage equals 1pu, while the secondary voltage Vsa of the bus B1 equals 1pu and the primary current Ia Prim takes the value 0pu. This situation indicates in fact, the non-intervention of the STATCOM; the DC voltage equals 1pu and the reactive power is stable to its value of zero. After that instant, the source voltage decreases to the value of 0.979 pu hence allowing the intervention of the STATCOM to generate 70 MVAR of the reactive power with a dynamic response time equals to 47 ms. At this point, the DC voltage has increased to 1.07 pu, the primary current IaPrim is advanced by supplying the voltage of bus B1. At the instant 0.15 s, the source voltage is increased to 1.025 pu of its nominal value, which makes the STATCOM absorbs 40 MVAR of reactive power and the value of DC voltage decreases to 0.97 pu. At the instant 0.25 s, the source voltage is equal to 1pu which implies the non-intervention of the STATCOM and therefore proving that the reactive power is equal to 0MVAR. c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 116 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Va Sec [pu], Va [pu], Ia Prim [pu] -2 -1 0 1 2Va Sec Va Ia Prim Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 -60 -40 -20 0 20 40 60 80 Time [s] Q [MVAR] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Vmeas [pu], Vref [pu] 0.97 0.98 0.99 1 1.01 1.02 Vmeas Vref Time [s] Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Vdc [V] ×104 1.8 1.9 2 2.1 2.2 Fig. 3: Healthy STATCOM dynamic response waveforms related to the system voltage change. 3. STATCOM Under Open Switch Fault 3.1. GTO Open-Circuit Fault of the Conv1 In order to study the effect of the fault on the behaviour of the STATCOM, an open-circuit fault at the switch S1 12 in the conv1 is created at the instant t= 0.12 s; see Fig. 4. As for the rest of the converters, they are supposed to remain all healthy. In reference to Fig. 5, it can be observed that the open-circuit fault does not affect directly the voltage Va but it affects significantly the current Ia Prim as a result of the distortions (harmonics). It can be noticed Fig.4 : Conv1 with open switch fault. i V dc1 V dc2 i S 1 11 S 1 12 S 1 13 S 1 14 S 1 21 S 1 22 S 1 23 S 1 24 S 1 31 S 1 32 S 1 33 S 1 34 i a i b i c Fig. 4: Conv1 with open switch fault. Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Vasec [pu], Va [pu], Ia prim [pu] -2 -1 0 1 2Vasec Va Iaprim Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Q [MVAR] -50 -25 0 25 50 75 100 With Fault Without Fault Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Vmeas [pu], Vref [pu] 0.96 0.98 1 1.02 Vmeas Vref Time [s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Vdc [V] ×104 1.6 1.8 2 2.2 2.4 2.6 Fig. 5: Faulty STATCOM dynamic response waveforms related to the system voltage change, (Case of a Conv1 GTO open-circuit fault at the instant of 0.12 s). that after the fault occurrence, the reactive power flow increases in the inductive mode but decreases when the STATCOM is operating in the capacitive mode. It can c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 117 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE also be seen that many perturbations appear on the DC voltage. Before proceeding with the removal of these fault effects, one must rely on the diagnostic method that enables detecting the faults and their localization. The current signals are used in this paper study as they help well in the diagnosis and the detection of the faults. 4. Method of Detection and Localization of Open-Circuit Faults In this section, the use of an adopted diagnostic method called the Single-Sided Amplitude Spectrum (SSAS) method is proposed to detect and localize open-circuit fault applied to STATCOM converters. The novelty of this work lies in the fact that the proposed SSAS method previously used in signal processing applications is applied for the first time in the field of diagnosis of converters and more particularly in the STATCOM converters diagnosis applications. The important advantage of the SSAS method compared to Park vectors method, for example, is that it has the ability to diagnose each phase on its own hence facilitating the detection and localization process, especially in the case of faults in the two operations (capacitive or inductive). The results obtained using the SSAS method depict well its merits and effectiveness. 4.1. Fault Detection of the Faulty Leg by the SSAS Spectral analysis is a technique widely used in signal processing; it consists of transposing the signal from the time-space to the frequency-space [25] and [26]. The spectral representation is obtained by the Fourier transform which was first introduced by Joseph Fourier. The Fourier transform S(f)of a signal s(t)is given by the following equation: Ia,b,c(f) = ∞ Z −∞ ia,b,c(t)e−j2πftdt. (6) The positive aspect of this technique is that it is based on the study of the harmonic analysis of each phase current and differs from the Park vectors method which depends on the transformation of the threephase currents to the two d–qcurrents in the d–qaxis frame [27]. 4.2. Results Discussion In this case, the SSAS method is applied to the threelevel converter conv1 currents under the healthy condition. This diagnostic method consists of calculating the amplitude spectrum at the fundamental frequency. Figure 6 shows the spectral analysis in the case of a healthy state of the three legs currents (iai,ibi,ici) for the inverter and (iar,ibr,icr) for the rectifier, the amplitude of the fundamental spectrum is equal 106 % in both operating modes. In Fig. 7, it is noticed that during the occurrence of an open-circuit fault introduced in the conv1 S1 12 switch when operating in the capacitive mode, the spectral amplitude of the current of the first faulty leg Iai(f)is equal to 16 %. For the remaining healthy legs, the value of Iai(f)is greater than or equal to 120 %. In the inductive operating mode, the value of Iar(f)of the first faulty leg is equal to 50 %. For the other healthy legs, Iar(f)is greater than or equal to 120 %. Frequency [Hz] 0 50 100 150 200 |Iai(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of iai(t) (a) Frequency [Hz] 0 50 100 150 200 |Ibi(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ibi(t) (b) Frequency [Hz] 0 50 100 150 200 |Ici(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ici(t) (c) Frequency [Hz] 0 50 100 150 200 |Iar(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of iar(t) (d) Frequency [Hz] 0 50 100 150 200 |Ibr(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ibr(t) (e) Frequency [Hz] 0 50 100 150 200 |Icr(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of icr(t) (f) Fig. 6: Simulation results of spectral analysis of three currents of healthy conv1, under two operating modes: ((a), (b), (c)): capacitive and ((d), (e), (f)): inductive. c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 118 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE Frequency [Hz] 0 50 100 150 200 |Iai(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iai(t) (a) Frequency [Hz] 0 50 100 150 200 |Ibi(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ibi(t) (b) Frequency [Hz] 0 50 100 150 200 |Ici(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ici(t) (c) Frequency [Hz] 0 50 100 150 200 |Iar(f)| 0 20 40 60 Single-Sided Amplitude Spectrum of iar(t) (d) Frequency [Hz] 0 50 100 150 200 |Ibr(f)| 0 50 100 150 Single-Sided Amplitude Spectrum of ibr(t) (e) Frequency [Hz] 0 50 100 150 200 |Icr(f)| 0 50 100 150 200 Single-Sided Amplitude Spectrum of icr(t) (f) Fig. 7: Simulation results of spectral analysis of three currents with an open-circuit fault introduced in conv1 GTO S112 switch under two operating modes: ((a), (b), (c)): capacitive and ((d), (e), (f)): inductive. 4.3. Fault Localization of the Faulty Open-Circuit Switch After discovering the faulty leg by applying the SSAS method, in order for the diagnosis to be completed, the faulty switch should be located, for that, this method is deepened in reading the signal of spectral analysis of phase currents. 4.4. Simulation Results Each time an open-circuit fault is created at one switch in the first leg, then the value of |IaS1 xy(f)|is calculated in the two operating modes as illustrated in Fig. 8. Through the spectra depicted in Fig. 8, it can be concluded that the fundamental amplitude of the spectrum |IaS1 xy(f)|of the phase current helps to localize the open-circuit faults in any switch because this value is reduced in the case of a fault occurrence, where each faulty switch has a certain value of |IaS1 xy(f)|. The following table shows the values of |IaS1 xy(f)|under the two operating modes. Frequency [Hz] 0 50 100 150 200 |IaS1 11(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iai(t) (a) Frequency [Hz] 0 50 100 150 200 |IaS1 12(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iai(t) (b) Frequency [Hz] 0 50 100 150 200 |IaS1 13(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iai(t) (c) Frequency [Hz] 0 50 100 150 200 |IaS1 14(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iai(t) (d) Frequency [Hz] 0 50 100 150 200 |IaS1 11(f)| 0 20 40 60 Single-Sided Amplitude Spectrum of iar(t) (e) Frequency [Hz] 0 50 100 150 200 |IaS1 12(f)| 0 10 20 30 40 Single-Sided Amplitude Spectrum of iar(t) (f) Frequency [Hz] 0 50 100 150 200 |IaS1 13(f)| 0 20 40 60 Single-Sided Amplitude Spectrum of iar(t) (g) Frequency [Hz] 0 50 100 150 200 |IaS1 14(f)| 0 20 40 60 Single-Sided Amplitude Spectrum of iar(t) (h) Fig. 8: Simulation results of spectral analysis of phase current ia with an open-circuit fault introduced in switches (S111, S112, S113, S114 ) of conv1 GTO switch, under two operating modes: capacitive ((a), (b), (c), (d)) and inductive ((e), (f), (g), (h)). Tab. 1: Amplitude of the fundamental spectrum. |IaS1 xy(f)| Operation S111 S112 S113 S114 Capacitive 30 % 16 % 13 % 27 % Inductive 55 % 38 % 43 % 50 % Table 1 summarizes the characteristics of the various faults of the GTO switch in the conv1. Since Tab. 2 below is large, the values of |IaS1 xy(f)|are replaced by the value of AF a. It is important to note that the lower the value of |IaS1 xy(f)|, the fault effect is increased and is more sensitive when the faulty STATCOM is under the capacitive operating mode compared to the inductive operating mode. c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 119 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE Tab. 2: Characteristics of the different open-circuit GTO switch faults in conv1. States Leg1 Leg2 Leg3 Capacitive Operation No fault AF a =|Iai|AF b =|Ibi |AF c =|Ici| S111 open |Iai|in < AF a <|Iai|AF b >|Ibi|AF c >|Ici | S112 open AF a <|Iai|ex AF b >|Ibi|AF c >|Ici | S113 open AF a <|Iai|ex AF b >|Ibi|AF c >|Ici | S114 open |Iai|in < AF a <|Iai|AF b >|Ibi|AF c >|Ici | S121 open AF a >|Iai| |Ibi|in < AF b <|Ibi|AF c >|Ici | S122 open AF a >|Iai|AF b <|Ibi|ex AF c >|Ici | S123 open AF a >|Iai|AF b <|Ibi|ex AF c >|Ici | S124 open AF a >|Iai| |Ibi|in < AF b <|Ibi|AF c >|Ici | S131 open AF a >|Iai|AF b >|Ibi| |Ici|in < AF c <|Ici | S132 open AF a >|Iai|AF b >|Ibi|AF c <|Ici |ex S133 open AF a >|Iai|AF b >|Ibi| |Ici|in < AF c <|Ici | S134 open AF a >|Iai|AF b >|Ibi|AF c <|Ici |ex Inductive Operation No fault AF a =|Iar |AF b =|Ibr |AF c =|Icr | S111 open |Iar |in < AF a <|Iar |AF b >|Ibr |AF c >|Icr | S112 open AF a <|Iar |ex AF b >|Ibr |AF c >|Icr | S113 open AF a <|Iar |ex AF b >|Ibr |AF c >|Icr | S114 open |Iar |in < AF a <|Iar |AF b >|Ibr |AF c >|Icr | S121 open AF a >|Iar | |Ibr |in < AF b <|Ibr |AF c >|Ici| S122 open AF a >|Iar |AF b <|Ibr |ex AF c >|Icr | S123 open AF a >|Iar |AF b <|Ibr |ex AF c >|Icr | S124 open AF a >|Iar | |Ibr |in < AF b <|Ibr |AF c >|Icr | S131 open AF a >|Iar |AF b >|Ibr | |Icr |in < AF c <|Icr | S132 open AF a >|Iar |AF b >|Ibr |AF c <|Icr |ex S133 open AF a >|Iar |AF b >|Ibr |AF c <|Icr |ex S134 open AF a >|Iar |AF b >|Ibr | |Icr |in < AF c <|Icr | The lower the value |IaS1 xy(f)|, the effect is increased when the faulty STATCOM is under the capacitive operation and is sensitive compared to the inductive operation. The application of the single-sided fundamental amplitude spectrum method presents an advantage since it has the ability to detect and localize the faulty switch of each leg separately. For example, if each of the switches (S1 13, S1 21, S1 34) are faulty, then three different amplitude spectra are shown where each phase has a certain value, but in the case of the Park vectors method, any faulty switch cannot be determined. There are six values for the SSAS in the case of the healthy state of conv1. Note that all the phases, in this case, have the same amplitude spectrum value |Iai|=|Ibr|=|Ici|. By cons, in the case of an opencircuit fault for example, if the value of |Iai|is reduced, the values of |Ibr|and |Ici|in the remaining two phases are increased which means that the fault occurred in the first leg and therefore here the faulty leg is directly localized. In the second, the SSAS value is to be exactly calculated because it has four values, two large and two small, where the large values are external. The flowchart shows the different steps of the switches and the other two are the internal switches as they are defined as |Iai|ex and |Iai|in, here the faulty switch is localized. Fundamental amplitude spectrum current value by the SSAS method is shown in Fig. 9. 5. Faulty Converter Reconfiguration 5.1. Reconfigured Faulty Converter Topology According to Fig. 10, the structure of a three-level converter is considered and is changed by adding active switches to the converter clamping diodes where it is considered as a new application for the STATCOM. The importance of this change is to allow the restoration of the level of voltage loss in the case of open-circuit faults in addition to the appearance of bidirectional current paths [28]. The integration of the two NC relay types: (S0 U1, S1 U1, . . .) in each leg is to isolate the open-circuit fault. The importance of the relay (SNP ) is to isolate the Neutral Point (NP) during a failure mode. In order to protect the freewheeling diode fault problem, the SNP relay is linked at the neutral point to control the connection depending on the type of fault. For example, if there is a fault in the switch S1 11 in the first leg of conv1, the SNP relay opens for c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 120 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE Fig.9: Flowchar t of the amplitude spectrum current value by SSAS method . NO YES If: [A F (a,b,c) ≠ │I (a,b,c) i/r │] Localization of open extern switch Detection of faulty leg Localization of open extern switch If : │I (a,b,c) │ in  A F(a,b,c)  │I (a,b,c) i/r │ If : A F(a,b,c)  │I (a,b,c) i/r │ ex Calculation of SSAS Value of I a , I b , I c Fig. 9: Flowchart of the amplitude spectrum current value by SSAS method. neutral point disconnection in order to allow the exploitation of alternative configurations. After completing the SNP opening, the converter will operate in an alternative mode to meet the required voltage in the faulty leg. Opening through SNP switching, the converter will operate in an alternative mode to meet the required voltage in the faulty leg. Fig.10: Reconfiguration of three - level NPC converter. S 1 11 S 1 12 S 1 12R S 0 U1 S 1 U1 S 1 13 S NP S 1 13R U c0 U dc U c1 D 0 u1 D 1 u1 S 1 21 S 1 22 S 1 23 S 1 24 S 1 34 S 1 33 S 0 U2 S 0 U3 S 1 U2 S 1 U3 S 1 32R S 1 32 S 1 23R S 1 33R S 1 31 D 0 u2 D 1 u2 D 1 u3 i c0 i c1 C 0 C 1 NP D 0 u3 S 1 22R S NP ’ S 1 14 Fig. 10: Reconfiguration of three-level NPC converter. 6. STATCOM Control by Integrating Multi-Level Fault Tolerant Converters This section tackles the STATCOM control through the application of multi-level fault tolerant converters. The reconfigurable fault tolerant is necessary to maintain and ensure service continuity of the STATCOM system. The general detailed structure of the reconfigurable multi-level fault tolerant converters with their diagnostic blocks is shown in Fig. 11. Each converter in the general structure is connected with three blocks; Fig.11: General diagram of four faul t - tolerant converters with their detection blocks and locations. i a1 [12] [12] Decision block SSAS SSA S SSAS Detection of the faulty leg i b 1 i c 1 Location of the Faulty switch Decision block SSAS SSAS SSAS Detection of the faulty leg Decision block SSAS SSAS SSAS Detection of the faulty leg Decision block SSAS SSAS SSAS Detection of the faulty leg Location of the Faulty switch Location of the Faulty switch Location of the Faulty switch i a 2 i b2 i c2 i a 3 i b3 i c3 i a 4 i b4 i c4 Conv 1 Conv2 [12] Conv 3 Conv 4 Fig. 11: General diagram of four fault-tolerant converters with their detection blocks and locations. c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 121 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 17 |NUMBER: 2 |2019 |JUNE the first is to detect the faulty leg, the second is to localize the faulty switch, and the last block is for the re-configuration action which takes place once the fault is localized. There are basically 12 currents which are all subject to spectral analysis simultaneously. 7. Simulation Results and Discussion Figure 12 illustrates the fault tolerant converter performance in the STATCOM under the influence of the fault occurring in the two instants 0.5s and 0.7s and Time [s] 0 0.2 0.4 0.6 0.8 1 Va Sec [pu], Va [pu], Ia Prim [pu] -2 -1 0 1 2Va Sec Va Ia Prim Time [s] 0 0.2 0.4 0.6 0.8 1 Q [MVAR] -50 -25 0 25 50 75 100 Time [s] 0 0.2 0.4 0.6 0.8 1 Vmeas [pu], Vref [pu] 0.96 0.98 1 1.02 Vmeas Vref Time [s] 0 0.2 0.4 0.6 0.8 1 Vdc [V] ×104 1.8 2 2.2 2.4 Fig. 12: Timeline of the healthy operation, the fault duration and the post-fault operation of an open-circuit fault introduced at GTO switch S112 of conv1. its impact on the absorption or generation of the reactive power. The results obtained in Fig. 12, show that after the fault occurring at the instant 0.5s, it results in a slight decrease in the reactive power Qfollowed by a slight increase at the instant 0.7s. After the re-configuration, the reactive power returned to its initial value. It should be noted that a slight distortion is visible at the level of the primary current IaPrim at the instants 0.5s and 0.7s; then the current signal immediately returns to its initial sinusoidal form. Ripples also occurred in the DC voltage between 0.97 pu and 1.16 pu at 0.5s and between 0.94 pu and 1.04 pu at 0.7s; then immediately returns to its initial DC form. From all these presented results, it can be deduced that the integration of the threelevel converter fault-tolerant has brought profitable solutions to the STATCOM system in terms of avoiding harmonics caused by current distortions as well as the amount of loss or excess of the reactive power. In this last part of this section, as depicted by Fig. 13, two very important tasks are added to represent the isolation of the faulty switch and the reconfiguration of the control strategy, which is to be added to the flowchart of Fig. 9, in order to ensure the process continuity of the entire STATCOM fault-tolerant system. Fig.13: Flowchart of the isolation and reconfiguration tasks for the faulty conv1. Location of open (exter n /intern) switch according to the A F(a,b,c) Isola tion of switch faulty Reconfiguration of the control strategy Fig. 13: Flowchart of the isolation and reconfiguration tasks for the faulty conv1. Figure 14 shows the different instants before and after the fault occurrence. The time taken by the detection, localization and reconfiguration operations is 0.005 s, which is very short and sufficient enough to ensure the continuity of service of the STATCOM system. 8. The Fault Effect of the First Leg Switches on the Value of SSAS In this case, an open-circuit fault is introduced in the first leg switches each one separately, in order to understand the effect of the voltage source changes on the value of SSAS in the faulty state under capacitive and c 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 122