1. Research Background 2. Fault Characteristics of Internal AC Bus faults 3. Fault Characteristics of DC faults 4. Requirements for protection 5. Conclusions
Fig. 1. Topology of modular mul2level converter.
Advantages of Modular Multilevel Converter (MMC) Modular topology Low switching frequency and low losses Realize multilevel output voltage Flexible controlled
The typical terminal bus faults include : single- phase to ground fault phase to phase short circuit, etc. Regarded as permanent faults Fig. 2. Single line diagram of MMC terminal bus fault.
2.1 Fault Mechanism The significant differences about the fault characteris2cs of the internal AC bus faults between the VSC and MMC converter sta2ons, mainly exist in whether there is a path for the zero- sequence components or discharge. Take the single- phase to ground fault for example. b Fig. 3. Discharging path under the single- phase to ground terminal bus fault: (a) Based on the VSC, and (b) MMC converter.
2.1 Fault Mechanism Unlike the VSC converter, there is no pathway for the zero- sequence components and the discharge of capacitors in such isolated neutral system, a metallic single- phase to ground fault can result in the following characteris?cs: the fault current components allocated to the AC bus would not exist according to the current distribu2on with an infinite zero- sequence resistance, the currents of AC bus maintain what the pre- fault converter sta2on delivers. The voltage of the fault phase would sag to zero as well as the non- fault phase clamp to the value of line- voltage which is symmetric.
2.1 Fault Mechanism Although the upper and lower DC line voltages become unbalanced due to the admiped second harmonic components produced by the nega2ve sequence, since the symmetry of the bus line- voltage and the consistency of MMC s the phase unit, the DC- link voltage U dc will stay stable depending on the alterna2ng voltages compensa2on of each polar line. In addi2on, taking account of the performances as the above men2oned, the effect under the fault can be decreased dras2cally in terms of the power transmission.
2.2 Simula2on and Results The parameters: U d = ±200kV The number of each bridge arm=20 the arm inductance = 20.80 mh the capacitor value of submodule = 1.76mF the transformer ra?o = 220/245 kv the DC transmission line using lumped parameter model =0.07+j0.11Ω. reac?ve power is set to 0 Mvar, ac?ve power is 400MW, AC voltage is 0.9 p.u. The carrier phase shiyed pulse width modula?on scheme (CPS- SPWM) is u?lized [21] and the capacitor voltages among different SMs are balanced [22].
2.2 Simula2on and Results (a) (b) (c) (d) Fig. 4. Simula2on waveforms of A- phase to ground fault: (a) Terminal bus phase- current, and (b) voltage, (c) DC- link and bipolar voltage. (d) Ac2ve and reac2ve power transmission.
The typical DC faults include : posi2ve or nega2ve line to ground fault line break fault line- to- line fault Take the DC line to ground fault for example The fault characteris2cs are analogous to the conven2onal VSC 5
3.1 Fault Mechanism Figure. 6(a) shows the posi2ve line to ground fault in the conven2onal VSC- HVDC When systems, the in posi2ve which the line DC- link to ground capacitors fault discharge occurs, through only the the poten2al pathways Assuming the electrical iden2fica2ons that are consistent with Fig. constructed reference point by the changes grounding theore2cally loop from the (from neutral of to the DC in capacitors Fig. 6(b)). to The the 1, by the varia2ons of Up and Un analyzed above, the AC voltages of ground important fault, conclusion making it much is that easier the to DC- link cause a voltage large discharge (U dc ) and current the flow. power Different converter from terminal the VSC- HVDC, bus also transmission s2ll hold the the same DC make side corresponding level of MMC of what converter changes. the pre- fault can be considered Take A- system to phase produces ground for through instance: under an two u accidental clamped a0 =U p - u a1 resistances. DC line to with ground large fault. value to provide the poten2al reference point and clamp the DC voltage. (a) Fig. 6. Comparison about the posi2ve line to ground fault: (a) Based on the VSC, and (b) MMC converter. (b)
3.2 Simula2on and Results Only the MMC terminal phase- voltages get down followed by the muta2on of DC voltage, which contain direct current (a) (b) components with high- frequency fluctua2ons, increasing the root mean square (RMS) voltage. (c) (d) Fig. 7. Simula2on waveforms of posi2ve line to ground fault: (a) DC- link and bipolar voltage. (b) bipolar current and short circuit current. (c) phase- voltages of terminal bus. (d) ac2ve and reac2ve power.
When the two ground faults happen, the system could transform power as usual, and at the same 2me the current of the DC lines would not exceed the limita2on. So (1)Considering the appropriate standard for the insula2on of HVDC system, the insula2on level of the corresponding devices must to be designed reasonably, such as DC arrester, AC and DC bus. (2) the set value of the protec2on devices of the DC and the AC system should be regulated correspondingly to match the control strategies.
MMC- HVDC system possesses a stronger fault through capability compared with the conven2onal VSC- HVDC systems. This paper mainly focuses on analysis of the fault mechanisms under the single- phase to ground terminal bus fault and DC line to ground fault in MMC based HVDC system. In both cases, the electrical quan22es of important parts remain normal, and just need to consider the problems of insula2on damage brought by the clamping voltage of AC bus or DC line. Therefore, with proper insula2on of the power line, the reliability will be improved by adjus2ng the protec2on values of AC and DC system.