Online Fault Identification Based on an Adaptive Observer for Modular Multilevel Converters Applied to Wind Power Generation Systems
Abstract
:1. Introduction
2. Principle and Fault Mechanism Analysis for MMC
2.1. Operation Principle of MMC
2.2. Fault Mechanism of MMC
- (1)
- Normal operation: In normal operation, as listed in Table 1, when the arm current ip(n),k is positive, if T1 is turned on, T2 is turned off in the ith module which means Sp(n),i,k = 1, the current flows through D1 and C, the capacitor is charged; otherwise, T1 is turned off, T2 is turned on, the current will go through T2 and the capacitor voltage maintains stable; when ip(n),k is negative, T1 is turned off, T2 is turned on, the current flows through D2; oppositely, Sp(n),i,k = 0, the current will go through T1 and C, the capacitor is discharged;
State No. | Current | Status | Gating Signal | Arm Current Goes Through | Capacitor | Capacitor Voltage |
---|---|---|---|---|---|---|
1st | ip(n),k > 0 | T1 on, T2 off | Sp(n),i,k = 1 | D1 and C | Charged | Increased |
2nd | ip(n),k > 0 | T1 off, T2 on | Sp(n),i,k = 0 | T2 | Bypassed | Stable |
3rd | ip(n),k < 0 | T1 on, T2 off | Sp(n),i,k = 1 | T1 and C | Discharged | Decreased |
4th | ip(n),k < 0 | T1 off, T2 on | Sp(n),i,k = 0 | D2 | Bypassed | Stable |
- (2)
- Open-circuit fault in T1 (Figure 3a): as shown in Table 2, the sub-module operates as normal when the arm current ikm > 0, the arm current still goes through D1 and C to charge the capacitor when the gating signal Sp(n),i,k = 1 and the arm current flows through T2 to bypass the capacitor when the gating signal Sp(n),i,k = 0; when ip(n),k is negative, the module is in normal operation when Sp(n),i,k = 0, the arm current flows through D2 and the capacitor voltage is stable; however, when the gating signal Sp(n),i,k = 1, the arm current will be forced to go through D2 instead of T1 and C in the normal condition;
State No. | Current | Status | Gating Signal | Arm Current Goes Through | Capacitor | Capacitor Voltage |
---|---|---|---|---|---|---|
1st | ip(n),k > 0 | T1 on, T2 off | Sp(n),i,k = 1 | D1 and C | Charged | Increased |
2nd | ip(n),k > 0 | T1 off, T2 on | Sp(n),i,k = 0 | T2 | Bypassed | Stable |
3rd | ip(n),k < 0 | T1 on, T2 off | Sp(n),i,k = 1 | D2 | Bypassed | Stable |
4th | ip(n),k < 0 | T1 off, T2 on | Sp(n),i,k = 0 | D2 | Bypassed | Stable |
- (3)
- Open-circuit fault in T2 (Figure 3b): the open-circuit fault is shown in Table 3, the sub-module operates as normal when the arm current ip(n),k > 0 and Sp(n),i,k = 1, if T1 is turned off, T2 is turned on, the arm current is forced to go through D1 and C to charge the capacitor instead of T2 to bypass the capacitor; when the arm current ip(n),k < 0, the module is in normal operation;
State No. | Current | Status | Gating Signal | Arm Current goes Through | Capacitor | Capacitor Voltage |
---|---|---|---|---|---|---|
1st | ip(n),k > 0 | T1 on, T2 off | Sp(n),i,k = 1 | D1 and C | Charged | Increased |
2nd | ip(n),k > 0 | T1 off, T2 on | Sp(n),i,k = 0 | D1 and C | Charged | Increased |
3rd | ip(n),k < 0 | T1 on, T2 off | Sp(n),i,k = 1 | T1 and C | Discharged | Decreased |
4th | ip(n),k < 0 | T1 off, T2 on | Sp(n),i,k = 0 | D2 | Bypassed | Stable |
- (4)
- Short-circuit fault in T1 or T2 (Figure 3c): as shown in Table 4, when the short-circuit fault happens in T1 (T2), the sub-module operates as normal if the corresponding IGBT T1 (T2) is turned on and the complementary IGBT T2 (T1) is turned off; when the complementary IGBT T2 (T1) is turned on, the capacitor discharged through the capacitor discharging loop which is formed by the short-circuited T1 (T2), the complementary IGBT T2 (T1) and the capacitor C. Due to the small time constant of the capacitor discharging loop, the capacitor discharged very quickly which leads to the rapid declines of capacitor voltage and the large short-circuit current in the faulty module. Generally the faulty module is bypassed and the arm current goes through the switch used to do overcurrent protection. However, with MMC topology, the arm current will go through D1 to charge C when the arm current is positive and go through D2 to discharge C with negative arm current. The capacitor voltage of the faulty module changes from zero to a small value compared to the normal capacitor voltages.
State No. | Current | Status | Gating Signal | Arm Current Go through | Capacitor | Capacitor Voltage |
---|---|---|---|---|---|---|
1st | ip(n),k > 0 | T1 on, T2 off | Sp(n),i,k = 1 | D1 and C | Bypassed | Increased |
2nd | ip(n),k > 0 | T1 off, T2 on | Sp(n),i,k = 0 | D1 and C | Bypassed | Increased |
3rd | ip(n),k < 0 | T1 on, T2 off | Sp(n),i,k = 1 | D2 and C | Bypassed | Decreased |
4th | ip(n),k < 0 | T1 off, T2 on | Sp(n),i,k = 0 | D2 and C | Bypassed | Decreased |
3. Proposed Fault Identification Method for MMC
3.1. Fault Detection
Features | Phase A | Phase B | Phase C | ||||||
---|---|---|---|---|---|---|---|---|---|
Open-Circuit Fault | Short-Circuit Fault | Open-Circuit Fault | Short-Circuit Fault | Open-Circuit Fault | Short-Circuit Fault | ||||
T1 | T2 | T1 | T2 | T1 | T2 | ||||
e1 | >kOT1 | <−kOT2 | >kS | <−kOT1 | >kOT2 | >kS | 0 | 0 | <−kS |
e2 | >kOT1 | <−kOT2 | >kS | 0 | 0 | <−kS | <−kOT1 | >kOT2 | >kS |
e3 | 0 | 0 | <−kS | >kOT1 | <−kOT2 | >kS | <−kOT1 | >kOT2 | >kS |
3.2. Fault Localization
3.2.1. Mode 1: Normal Operation
3.2.2. Mode 2: Open-Circuit Fault in T1
3.2.3. Mode 3: Open-Circuit Fault in T2
3.2.4. Mode 4: Short-Circuit Fault in T1 or T2
Fault Type | Specification | Modification Strategy |
---|---|---|
Mode 1 | Normal condition | Sp(n),i,k = 1 |
Mode 2 | Open-circuit fault in T1 | S'p(n),i,k = (ip(n),k > 0 && Sp(n),i,k = 1) |
Mode 3 | Open-circuit fault in T2 | S''p(n),i,k = not (ip(n),k < 0 && Sp(n),i,k = 0) |
Mode 4 | Short-circuit fault in T1 or T2 | S'''p(n),i,k = 0 |
4. Simulation Results
Parameter | Value | Parameter | Value |
---|---|---|---|
Rated active power P | 20 MW | Arm resistor R | 0.05 Ω |
Rated DC-link voltage Vdc | 60 kV | Number of sub-module N | 12 |
Rated AC grid voltage Vac | 30 kV | Switching frequency fs | 1.2 kHz |
Sub-module Capacitor C | 0.8 mF | Fundamental frequency f | 50 Hz |
Arm inductor L | 5 mH | Modulation index M | 0.9 |
4.1. Normal Operation
4.2. Open-Circuit Fault in T1
4.3. Open-Circuit Fault in T2
4.4. Short-Circuit Fault in T1 or T2
5. Conclusions
Author Contributions
Conflicts of Interest
References
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Liu, H.; Ma, K.; Loh, P.C.; Blaabjerg, F. Online Fault Identification Based on an Adaptive Observer for Modular Multilevel Converters Applied to Wind Power Generation Systems. Energies 2015, 8, 7140-7160. https://doi.org/10.3390/en8077140
Liu H, Ma K, Loh PC, Blaabjerg F. Online Fault Identification Based on an Adaptive Observer for Modular Multilevel Converters Applied to Wind Power Generation Systems. Energies. 2015; 8(7):7140-7160. https://doi.org/10.3390/en8077140
Chicago/Turabian StyleLiu, Hui, Ke Ma, Poh Chiang Loh, and Frede Blaabjerg. 2015. "Online Fault Identification Based on an Adaptive Observer for Modular Multilevel Converters Applied to Wind Power Generation Systems" Energies 8, no. 7: 7140-7160. https://doi.org/10.3390/en8077140