1 Design, Control and Application of Modular Multilevel Converters for HVDC Transmission Systems by Kamran Sharifabadi, Lennart Harnefors, Hans-Peter Nee, Staffan Norrga, Remus Teodorescu ISBN-10: 1118851560 Copyright Wiley 2016 Chapter 9 Control and Protection of MMC-HVDC under AC and DC Network Fault Contingencies
Outline (1) Introduction 9.2 Two-Level VSC-HVDC Fault Characteristics under Unbalanced AC Network Contingency 9.2.1 Two-Level VSC-HVDC Fault Characteristics under DC Fault Contingency 9.3 MMC-HVDC Fault Characteristics under Unbalanced AC Network Contingency 9.3.1 Internal AC Bus Fault Conditions at the Secondary Side of the Converter Transformer 9.4 DC Pole-to-Ground Short-Circuit Fault Characteristics of the Half-Bridge MMC-HVDC 9.4.1 DC Pole-to-Pole Short-Circuit Fault Characteristics of the Half-Bridge MMC- HVDC 9.5 MMC-HVDC Component Failures 9.5.1 Submodule Semiconductor Failures 9.5.2 Submodule Capacitor Failure 9.5.3 Phase Reactor Failure 2
Outline (2) 9.5.4 Converter Transformer Failure 9.6 MMC-HVDC Protection Systems 9.6.1 AC-Side Protections 9.6.2 DC-Side Protections 9.6.3 DC-Bus Undervoltage, Overvoltage Protection 9.6.4 DC-Bus Voltage Unbalance Protection 9.6.5 DC-Bus Overcurrent Protection 9.6.6 DC Bus Differential Protection 9.6.7 Valve and Submodule Protection 9.6.8 Transformer Protection 9.6.9 Primary Converter AC Breaker Failure Protection 9.7 Summary
Introduction AC or dc short-circuit fault conditions cannot be avoided in high-voltage direct current (HVDC) schemes, independent of the converter and the transmission link technology (e.g. cables or overhead lines). The probability, frequency, and characteristics of these faults depend on the employed transmission technology, converter technology (e.g. two-level or MMC), and the grounding topology (e.g. direct, resistive, or high-impedance). Generally, the fault conditions for an HVDC scheme can be divided into four main scenarios: AC network faults at point of common coupling (PCC); AC faults inside the converter station; DC faults, e.g. pole-to-pole or pole-to-ground faults; converter internal faults, e.g. submodule faults, modulation and control faults, phase-reactor faults. 4
Two-Level VSC-HVDC Fault Characteristics under Unbalanced AC Network Contingency The two-level VSC- HVDC converters are very vulnerable to ac faults, when the fault location is on the secondary side of the converter transformer. A single-phase to ground fault on the two-level VSC converter terminal bus. Causes the dc-link capacitors discharge through the short-circuit loop rapidly and cause destructive over-currents to the converter semiconductors. The generated negative-sequence component causes second-harmonic dc ripple voltages and currents on the dc bus. The ground potential of the dc-side neutral point will float and the dc-bus voltage will contain high frequency oscillations. charge and discharge of the dc-link capacitors through the short-circuit path loop, the dc-link voltage can reach a peak overvoltage of about two times its nominal voltage. Converter internal ac bus short-circuit and the fault current path loop 5
Two-Level VSC-HVDC Fault Characteristics under DC Fault Contingency Two-level VSC-HVDC converters are very sensitive to dc fault conditions. The dc-side short-circuit faults can cause serious damages to the converter valves. When a dc fault occurs, the dc link capacitors discharge through the fault location and cause a large dc fault current, a rapid decrease in the dc-bus voltage, and a substantial increase of the ac current flow to the converter valves. The VSC cannot control the dc side-fault current even when the insulated-gate bipolar transistors (IGBTs) are blocked, as the antiparallel diodes will act as an uncontrollable rectifier and feed the dc short-circuit with the current from the ac-side network.
2-level VSC DC bus fault condition stages on 2- level VSC Stage 1: Capacitor discharge phase: Immediately after occurrence of fault the converter s large dc capacitors are discharged through the fault location. The equivalent circuit. The magnitude and steepness of the first current peak is determined by the dc link voltage and the cable resistance and impedance. Stage 2: Freewheel diode phase: At this stage, the converter capacitor voltage has decayed to zero and the cable capacitance discharges through the anti-parallel diodes in the converter bridge. Depending on the dc cable design and the cable length, the cable capacitance discharge current can be very large.
2-level VSC DC bus fault condition stages on 2- level VSC Stage 3: AC grid feeding the fault current: Finally, in the third phase, the fault is fed from the ac grid through the anti-parallel diodes acting as an uncontrollable rectifier. At this stage, the fault current peak depends on the short-circuit capacity of the ac grid, the converter filter impedance, the dc-cable impedance, and the fault resistance.
MMC-HVDC Fault Characteristics under Unbalanced AC Network Contingency (1) MMCs are robust against the unbalanced ac network fault conditions. MMC-HVDC converter transformers adapt the star/delta (Y/Δ) configuration with the delta connection on the converter side designed with high zero-sequence impedance. Unbalanced ac grid conditions cause ac-bus-voltage fluctuations and phase shifts, but the converter control strategy are designed handle such conditions without major impact on the dc transmission link. Temporary asymmetrical grid conditions are not seen as harmful to the converter itself. The converter recovers quickly when the ac fault is removed and the converter switches are not exposed to overvoltage or current stresses since the modular converter cell s capacitor is not compromised.
MMC-HVDC Fault Characteristics under Unbalanced AC Network Contingency (2) During unbalanced ac network conditions, the zero-sequence current is almost blocked by the delta connection of the transformer in the converter side, and thus only positive- and negative-sequence currents must be controlled. The distributed location of energy storage capacitors permits the independent control of each phase. Thus, a zero-sequence current control becomes possible in addition to the positive- and negativesequence current control. Unlike the two-level VSC-HVDC, there is no pathway for the zero-sequence components and the discharge of converter capacitors. Therefore, the converter semiconductors will not be compromised under single phase to ground conditions.
DC Pole-to-Pole Short-Circuit Fault Characteristics of the Half-Bridge MMC-HVDC When a dc pole-to-pole fault is detected, the control and protection systems block the converters immediately. When the converter switches are blocked, the anti-parallel diodes act as an uncontrolled rectifier, and a voltage with the opposite polarity of the ac network voltage will be fed into the converter arms as the natural commutation processes of the anti-parallel diodes and the dc shortcircuit current is fed from the ac-side network Short-circuit current path through the MMC during a pole-to-pole short-circuit fault.
MMC-HVDC Component Failures Typical MMC component failures: Submodule Semiconductor Failures Submodule Capacitor Failure Phase Reactor Failure Converter Transformer Failure If a submodule is damaged the high speed bypass switch Is energized, and the submodule will be bypassed and removed from the circuit..
Typical MMC-HVDC protection system