# Fault Detection and Location of IGBT Short-Circuit Failure in Modular Multilevel Converters

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Operation Principles, Fault Analysis and Calculation of MMC

#### 2.1. Structure and Control Strategy of MMC

_{0}and an equivalent loss resistance R

_{0}in series. Each SM is composed of IGBT T

_{1}, T

_{2}, anti-paralleled diodes D

_{1}, D

_{2}and a capacitor C

_{0}. The SM is set ON/OFF under the control of a switching function S which is defined as Equation (1)

_{1}and g

_{2}are the gate signals for switches. When S is 1, the SM is “ON” and T

_{1}is conducted and T

_{2}is blocked. When S is 0, the SM is “OFF” and T

_{1}is blocked and T

_{2}is conducted. In normal operation, to maintain the required DC voltage, a Capacitor Voltage Balancing Method (CVBM) is applied in MMC-HVDC system [26,27]. The CVBM, SMs with higher voltages discharging (i

_{arm}< 0) in priority and SMs with lower voltages charging (i

_{arm}> 0) in priority, determines which SMs are ON/OFF, that is the value of S.

#### 2.2. Fault Characteristics Analysis of SM

#### 2.2.1. T_{1} Short-Circuit

_{arm}> 0 and S = 1, T

_{1}short-circuit will have no impact on capacitor charging process. When S is 0 and T

_{2}is conducted, T

_{1}short-circuit will result in the capacitor’s short-circuit via T

_{1}and T

_{2}and the capacitor discharges rapidly. The current path is shown in Figure 2. Similarly, with i

_{arm}< 0 and S = 1, T

_{1}short-circuit will have no influence on capacitor discharging process. Only when S is 0, T

_{1}short-circuit will result in the capacitor short-circuit. The current path is shown in Figure 3.

#### 2.2.2. T_{2} Short-Circuit

_{arm}> 0 and S = 0, T

_{2}short-circuit will have no impact on capacitor charging process. When S is 1, T

_{2}short-circuit will cause the capacitor’s short-circuit via T

_{1}and T

_{2}. As a result, the capacitor switches from normal charging state to fault discharging state. Similarly, with i

_{arm}< 0 and S = 1, T

_{2}short-circuit will result in the capacitor switching from normal discharging state to fault discharging state. The current path is shown in Figure 4 and Figure 5.

#### 2.3. Capacitor Voltage Calculation

_{1}and T

_{2}. The equivalent discharge circuit is illustrated as Figure 6. R

_{1}and R

_{2}are the equivalent on-resistance of T

_{1}and T

_{2}, respectively and R equals R

_{1}+ R

_{2}. u

_{c}is the capacitor voltage and i

_{c}is the capacitor current.

_{c}and its derivation can be deduced as Equations (4) and (5)

## 3. Proposed Fault Detection and Location Method for MMC

_{c}of the fault SM. The criteria of DCLVDM is formulated as Equation (6).

_{α}is reliability coefficient and u

_{ce}is the rated value of u

_{c}under steady state. $\left|{\mathrm{du}}_{\mathrm{act}}\left(\mathrm{t}\right)-{\mathrm{du}}_{\mathrm{cal}}\left(\mathrm{t}\right)\right|\le \mathsf{\epsilon}$ is the differential comparison part. ε is the threshold value accordingly. du

_{act}and du

_{cal}are the actual value and calculation value of change rate of u

_{c}, respectively, and they are formulated as Equations (8) and (9). T

_{s}is the sample period.

_{act}is based on the short-circuit characteristics as Equation (5), hence, the error between du

_{act}and du

_{cal}under fault condition can be much smaller than that under steady state, which can be used to distinguish the fault. Only if Equation (6) is proved to be true and it lasts for a certain period T

_{0}, the IGBT short-circuit fault can be confirmed in a SM. The proposed DCLVDM requires no extra voltage measurement as the capacitor voltage is continuously measured for MMC control purpose.

_{2}(T

_{1}) is proved to be faulty. Hence the key to locate the faulty IGBT is to locate the time spot when the capacitor voltage begins to decrease, that is, the singularity of u

_{c}.

## 4. Case Studies

_{2}occurs in a SM at t = 0.02100 s.

_{1}occurs in a SM at t = 0.03500 s.

#### 4.1. Fault Characteristics

#### 4.2. Fault Detection

_{α}, ε and T

_{0}are set as K

_{α}= 0.8, ε = 0.7, T

_{0}= 1 ms.

_{act}and du

_{cal}in steady state is much larger than that of faulty state, which is very useful in fault detection.

_{α}, ε, T

_{0}remain the same as mentioned above. The simulation performance is similar to Figure 9. Because of space constraints, the simulation performance is not displayed. The proposed DCLVDM combined with the proper value of related parameters can detect the short-circuit fault accurately. For reliability coefficient K

_{α}, a larger value will improve the sensitivity of the low-voltage part to voltage decline, which can shorten the detection time. However, too large a value will reduce the reliability because potential disturbance or noise, etc. which may cause minor voltage decline, can be mistaken for short-circuit fault. For the threshold, ε, the value of ε should be set between the minimum of the differential comparison under steady state and the maximum under faulty state combined with a margin. In this paper, the range of ε is about 0.35–1.48 according to the simulations. Therefore, setting ε as 0.7 is sufficient and reasonable to detect the fault. As for T

_{0}, a larger value will improve the reliability of the fault detection process but lengthen the detection time. Because the IGBTs are of the same model in the same MMC converter and each SM has the same operation characteristics, the proposed DCLVDM and the parameter value setting are universally effective, which has been validated by numerous simulations. Actually, there is no a specification standard for the parameters setting with SM short-circuit, the parameters setting should take into consideration the sensitivity, speed, and reliability of the detection process comprehensively.

#### 4.3. Fault Location

_{2}is proved to be faulty. Similarly, T

_{1}can be proved to be faulty according to Figure 12b at t = 0.03500 s. The proposed faulty IGBT location method based on CWT can accurately locate the singularity of the voltage capacitor caused by the fault, thus locating the faulty IGBT efficiently and precisely.

#### 4.4. Comparison Analysis

## 5. Conclusions

## Author Contributions

## Acknowledgments

## Conflicts of Interest

## Appendix A

Quantity | Value |
---|---|

DC nominal voltage (U_{dc}) | ±20 kV |

Number of SMs per arm (N) | 20 |

Power transmission (P) | 20 MW |

Arm inductor (L_{0}) | 15 mH |

SM capacitance (C_{0}) | 6 mF |

AC line voltage (Vac) | 10 kV |

Loss resistance (R_{0}) | 0.1 Ω |

SM capacitor voltage | 2 kV |

equivalent on-resistance (R_{1}, R_{2}) | 0.01 Ω |

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**Figure 11.**Fault location result with 40-db white Gaussian noise for Case 2: (

**a**) Contaminated capacitor voltage; (

**b**) Wavelet Transform Coefficients under various scales.

Reference | Requires | Complexity | Detection Time |
---|---|---|---|

[21] | output voltage and gate signal | simple calculation | about 1 cycle |

[22,23] | phase voltage | complex model and additional sensors | numbers of sampling intervals |

[24,25] | gate voltage or current | additional sensors and circuits | tens of microseconds |

Fault IGBT | SM State | S | i_{arm} | Capacitor State |
---|---|---|---|---|

T_{1} | ON | 1 | >0 | Normal |

<0 | Normal | |||

OFF | 0 | >0 | Short-circuit | |

<0 | Short-circuit | |||

T_{2} | ON | 1 | >0 | Short-circuit |

<0 | Short-circuit | |||

OFF | 0 | >0 | Normal | |

<0 | Normal |

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**MDPI and ACS Style**

Jiang, B.; Gong, Y.; Li, Y. Fault Detection and Location of IGBT Short-Circuit Failure in Modular Multilevel Converters. *Energies* **2018**, *11*, 1492.
https://doi.org/10.3390/en11061492

**AMA Style**

Jiang B, Gong Y, Li Y. Fault Detection and Location of IGBT Short-Circuit Failure in Modular Multilevel Converters. *Energies*. 2018; 11(6):1492.
https://doi.org/10.3390/en11061492

**Chicago/Turabian Style**

Jiang, Bin, Yanfeng Gong, and Yan Li. 2018. "Fault Detection and Location of IGBT Short-Circuit Failure in Modular Multilevel Converters" *Energies* 11, no. 6: 1492.
https://doi.org/10.3390/en11061492