Abstract
A multi-terminal high voltage DC (MTDC) grid, is an optimum and cost-effective transmission network to minimize the energy crisis worldwide largely. However, the core demand for a fast DC protection scheme with an extraordinary strict fault clearance time of a few milliseconds, is a key research gap in this network, holding back its development and scalability. To bridge this gap, this paper proposes a novel protection scheme for a meshed MTDC grid. The main goals of the scheme include accurate discrimination of a faulty line, rapid fault detection, fault location estimation, significant fault current reduction, and fully selective isolation of only the faulted line, while continuing normal power flow in the healthy grid zones. Reliability of the scheme for long and extra-long-distance power transmission is increased by aiding the differential protection and Type-D traveling wave (TW)-based algorithms utilizing the distributed optical current sensing technology with the other auxiliary methods and backup plans. These auxiliary methods include independent discrete wavelet transform (DWT), current derivative polarity principles with a minimum sample (short time) window, overcurrent relays, and AC circuit breakers (ACCBs). A faulty segment of a transmission line is accurately discriminated from the healthy ones by measuring a series of multi-point differential currents on it. A faulty line at a particular DC node is accurately discriminated using the differential protection by measuring the current flowing into or out of each line from each side at every node to obtain the algebraic sum. The current sum of a real-time local transient data and a delayed remote data is compared to a preset threshold level. DC fault current is significantly reduced below the breakable levels by coordinating bidirectional hybrid DC circuit breakers (HDCCBs) with the active and passive fault current limiters (FCLs) and the half bridge-VSC-based modular multilevel converters (MMCs). The proposed concepts are successfully verified by the simulation results under a variety of fault scenarios and are found to be accurate.
Highlights
In our modern society, to meet the fast-growing energy demands, tackle economic, technical, environmental concerns of conventional AC networks, deplete costly and vanishing fossil fuels, and prevent the effects of global warming, demand for a bulk integration of renewable energy in power networks is rapidly rising
Most drawbacks and challenges associated with the conventional differential protection and traveling wave (TW) methods, can be eliminated with Type–D TW-based fault location and differential protection methods, utilizing the measurements obtained from the distributed optical sensors on hybrid transmission lines (HTLs) [12]
FAULTY LINE AND FAULTY SEGMENT DETERMINATION RESULTS In Fig. 6, a faulty line L12 300 km long, was accurately discriminated from the healthy ones, for a solid P2P fault at 1 ms between S1 (B1) at 10 km and S2 at 40 km to the fault by measuring the differential current sums on each cable connected to the nodes B1 to B4 in a meshed 4-terminal grid
Summary
To meet the fast-growing energy demands, tackle economic, technical, environmental concerns of conventional AC networks, deplete costly and vanishing fossil fuels, and prevent the effects of global warming, demand for a bulk integration of renewable energy in power networks is rapidly rising. High vulnerability of VSC-based HVDC systems to DC faults, a solid DC line/cable short circuit or pole-pole (P2P) fault, with an extraordinary strict fault clearance time of a few milliseconds, has remained a core technical challenge in both research and practice so far During this fault, even when all the IGBTs are blocked for self-protection, it is impossible to prevent the AC grid from feeding the fault, via the freewheeling diodes which form an uncontrolled bridge rectifier. Most drawbacks and challenges associated with the conventional differential protection and TW methods, can be eliminated with Type–D TW-based fault location and differential protection methods, utilizing the measurements obtained from the distributed optical sensors on hybrid transmission lines (HTLs) [12] These optical schemes accurately discriminate a faulty segment, and require neither high sampling frequency nor accurate GPS time stamping.
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