Analysis on IGBT and Diode Failures in Distribution Electronic Power Transformers

Fault characteristics of power electronic components are of great importance for a power electronic device, and are of extraordinary importance for those applied in power system. The topology structures and control method of Distribution Electronic Power Transformer (D-EPT) are introduced, and an exploration on fault types and fault characteristics for the IGBT and diode failures is presented. The analysis and simulation of different fault types for the fault characteristics lead to the D-EPT fault location scheme.


Introduction
Largely improved the intelligent level of power grid, the electronic power transformer (EPT), also as known as smart transformer (ST), normally consists of power electronic converters. Comparing to the traditional transformer, the EPT can control its input and output voltage in magnitude, frequency and phase angle to realize flexible voltage and current regulation, which realizes the assumption made in [1] by providing a customized power supply [2].
The reliability of power electronic components affects the one of converters, hence affects the reliability of power system that are connected with them. A failed power switching component always appears as short circuit or open circuit [3]. The short circuit fault always deteriorate in a very short time [4,5], hence normally protected by hardware protection, and the open circuit situations are also discussed in [6].
The study on switching component failure focus on the converters in motor drive system [9], and EPT structures are discussed in [10], applications in power grid [11]. In [12,13], the fault characteristic analysis of EPT based on MMC in distribution network, and the fault types in the MMC based EPT have greater impact on EPT in distribution network.
In this paper, the topology structures and control schemes of D-EPT are introduced in Section 2. The failure analysis on IGBT and diode of D-EPT is presented in Section 3. The fault location method based on summaries derived from saber simulation is shown in Section 4, and conclusion is given in Section 5.

D-EPT Structures
The D-EPT structure which consists of rectifier section power cell (RSPC), DC-DC converter section power cell (DCPC) and inverter section power cell (ICPC) is shown in Figure 1.

DC-DC converter Section
Inverter Section  Adopted by full-bridge rectifiers, the RSPC of D-EPT transfers the grid AC voltage to DC voltage for DCPC. The DCPC connects the RSPC and the ISPC with a converter, a High Frequency Transformer (HFT) and a diode bridge. The converter will invert RSPC DC-link voltage into AC voltage, which will be transmitted through HFT. The full-bridge inverters that form the ISPC are in parallel with each other, and convert the ISPC DC-link voltage to AC voltage. Figure 2 shows a substation that applies EPT to connect AC transmission system, DC transmission system, and clean energy including hydraulic power, solar power, and wind power.

Control Scheme for D-EPT
The RSPC in Figure 3 includes a current loop to keep the input AC current sinusoidal in phase with the grid voltage for customized power factor, as well as a voltage loop to control the DC-link voltage. The control diagram of RSPC can be given by: DCPC applies an open loop PWM control to generate a high frequency square wave to, and then has it rectified by a diode bridge. As DCPC acts like a proportional amplifier, ISPC DC-link voltage can be given by: By applying carrier phase shifting sinusoidal PWM (CPS-SPWM), the ISPC provides a constant AC voltage output. The RMS value of the output voltage is multiplied by the standard sinusoidal wave to keep the output voltage sinusoidal. The single phase control diagram for ISPC is shown in Figure 4, and its mathematic model can be given by:

Rectifier section power cell IGBT Open Circuit (R1)
Any failed IGBT in the full-bridge will lead to same result because of the CPS-SPWM schemes. Before the R1 fault, the current go through IGBT1 and IGBT3 when Vs>0. Assuming IGBT3 is open circuit, the current will then charge RSPC DC-link through the body diodes BD1 and BD4, which are shown in Figure 7(a). Therefore, the RSPC DC-link voltage will be different from other normal RSPC DC-link voltages.

Rectifier section power cell IGBT and body diode open circuit (R2)
In the normal operation, the voltage drop on IGBT2 equal to RSPC DC-link voltage, and can be described as: When R2 fault happens, the open circuit of IGBT1 equals to an open circuit resistor R in Figure  7(b), hence the voltage drop on IGBT2 equal to RSPC DC-link voltage adding open circuit resistor voltage, which is given by: The IGBT1 current will be reduced due to the open circuit resistor, hence the RSPC voltage will decrease and VIGBT2 will increase largely, which would cause over voltage on IGBT2.

Rectifier section power cell IGBT short circuit (R4)
As shown in Figure 7(c), the RSPC DC-link capacitors will be short circuit by conducting IGBT1 and IGBT2 simultaneously. As a result, the RSPC as well as ISPC DC-link voltage will be reduced and IGBT1 as well as IGBT2 will be over current.

DC-DC converter section power cell IGBT open circuit (D1)
As shown in Figure 8(a), the DC-DC converter of DCPC acts like a two-switch forward converter when IGBT5 is open circuit, and the body diodes of BD5 and BD8 will prevent the magnetic flux of the HFT core build-up gradually. As shown in Figure 8(b), fault D2 makes the body diodes BD5 and BD8 impossible to prevent the saturation of transformer core, as the current to reset the magnetic flux is blocked.

DC-DC
3.6. DC-DC Converter section power cell IGBT short circuit (D4) As shown in Figure 8(c), the RSPC DC-link capacitors will be short circuit by conducting IGBT5 and IGBT7 together. As a result, the RSPC as well as ISPC DC-link voltage will be reduced and IGBT5 as well as IGBT7 will be over current.

DC-DC Converter section power cell diode open circuit (D5)
As shown in Figure 9(a), the open circuit on RD1 will block the current flow from rectifier diodes RD1 and RD4. The energy of the current will be saved in the HFT and then releases by RD2 and RD3, which might cause over current on IGBTs of DC-DC converter, RD2 and RD3.

DC-DC Converter section power cell diode short circuit (D6)
As shown in Figure 9(b), the HFT will be shorted as RD1 and RD3 conducts together. As a result, RSPC as well as ISPC DC-link voltage will be reduced and RD1 as well as RD3 will be over current.

Inverter section power cell IGBT open circuit (I1)
As shown in Figure 9(c), the failure on IGBT9 forces inverter to work in the negative half cycle, making output current only flow in the negative half cycle.

Inverter section power cell IGBT short circuit (I4)
As shown in Figure 9(d), the faulty currents will connect the positive and negative poles of ISPC DClink capacitors on one hand, and connect positive and negative poles of output filter on the other hand. As a result, the ISPC DC-link voltage and ISPC output voltage will be reduced, and IGBT9, IGBT10 and IGBT11 will be over current.

Simulation Results Switching Component Fault Analysis of D-EPT
The Saber simulation is performed based on the proposed D-EPT in Figure 1, which transforms the 10kV AC voltage (5774V in phase) to 400V AC voltage (230V in phase The failure IGBTs and diodes are in accordance with fault types illustrated in Figure 5, and the abbreviations "error1", "error2", "error3", "error4" as well as "norm" in the summary based on saber simulation in Table 1 refer to errors including under voltage, over voltage, over current, currents unbalance as well as normal, respectively.  It_1  norm error3  norm  norm error3  Io_1  error4 error4   It_2  norm  norm  norm  norm  norm  Io_2  error4 error4   It_3  norm  norm  norm  norm  norm  Io_3  error4 error4   It_4  norm  norm  norm  norm  norm  Io_4  error4 error4 By monitoring the parameters mentioned in Table 1, almost all IGBT and diode fault can be identified and located by applying a fault location scheme in Figure 10. By monitoring input current, one can make a distinction between open circuit fault and short circuit fault, as the latter one always accompanies with over current and requires hardware protection circuit embedded in IGBT gate driver. The input power factor calculation program on input current can identify R4, D4, D6 and I4 fault. By monitoring DC-link voltages such as RSPC and ISPC DC-link voltages, one can check the voltage abnormality including over voltage, under voltage and voltage unbalance, which can tell I4 fault and R1 fault in Figure 10.
As faults like C1 and E1 will distort the HFT currents, the monitor on HFT currents on needed. Moreover, different algorithms are needed for the measurement on HFT currents, including RMS value for current unbalance, maximum value on amplitude for over current and average value for saturated core in HFT.
In order to identified G1 fault whose average value of output is not zero, both RMS value and average value calculation on ISPC output voltage and current are required.

Conclusions
Based on different types of IGBT and diode failures in Distribution Electronic Power Transformer (D-EPT), the theoretical analysis as well as the simulation results is presented. By monitoring abnormalities in parameters including input current, DC-link voltages, HFT currents, and ISPC voltages which are summarized in Table 1, the fault location scheme that is based on unique feature to each fault can be applied to identify the failure IGBT or diode.