Low Voltage Direct Current Network Research Articles

Mitigation of overvoltage in LVDC distribution system with constant power load using generic energy storage system

The operation of a high-speed DC circuit breaker can cause overvoltage transients in the Low Voltage Direct Current (LVDC) distribution system, especially when driving a constant power load (CPL). Without adequate damping, the destabilizing effect of the CPL can push the system towards instability. This paper proposes a novel approach to mitigate over-voltage transients caused by DC circuit breaker operation in the LVDC system using a generic energy storage system (GESS). The paper begins by introducing the concept, modeling, coordinated operation and practical application of GESS. GESS employs both power-centric storage, such as supercapacitors, to limit the switching over-voltage within safe limits, and energy-centric storage, such as batteries, to ensure the energy sustainability. A state-space model of a detailed LVDC distribution system with CPL and GESS is subsequently developed. Additionally, to ensure system stability, analytical expressions are derived to determine the required sizing of the power-centric storage under different case scenarios. Simulation analysis using MATLAB's Control System Toolbox validates that GESS can stabilize voltage transients and reduce overshoots within 10 ms. Moreover, the proposed GESS solution is proven to be more reliable and cost-effective than supercapacitor and hybrid energy storage solutions. It is also scalable to larger LVDC networks and adaptable to different load scenarios.

Protection scheme for LVDC distribution network based on stepwise blocking of power electronic transformer

AbstractCascaded H‐bridge power electronic transformers (CHB‐PET) can efficiently connect AC/DC distribution networks, realizing power transmission and electric energy conversion. Once a pole‐to‐pole DC fault occurs on the low‐voltage direct current (LVDC) side of CHB‐PET, the fault current has a fast rise and high amplitude, which easily leads to the rapid blocking of power electronic devices and makes it difficult for DC line protection to obtain fault information. A stepwise blocking scheme of PET is proposed for LVDC network protection. When the fault on the LVDC side of PET is detected, cascaded H‐bridge and low‐voltage H‐bridge of dual‐active‐bridge are blocked first, and then high‐voltage H‐bridge of dual‐active‐bridge is blocked after a 5 ms delay. The stepwise blocking can provide fault information for DC line protection. On this basis, wavelet packet transform is employed to extract different frequency components of fault current. The energy of the specific frequency band corresponding to the switching frequency of DAB and the total energy of all frequency bands contained in the fault current is calculated; then their ratio can be utilized to construct the protection scheme. Finally, the PSCAD/EMTDC simulation results verify the feasibility and effectiveness of proposed protection scheme based on stepwise blocking of CHB‐PET.

Distributed generation in low-voltage DC systems by wind energy in the Baja California Peninsula, Mexico

Mexico's national electric grid comprises ten regional systems, with one isolated in the south of the Baja California Peninsula. In addition, providing electricity access remains a challenge in the country. Therefore, distributed renewable energy solutions are relevant to satisfy the electricity demand and promote grid development. Distributed renewable energy access (DREA) based on a wind energy conversion system integrated into low-voltage direct current (LVDC) networks is presented. The DREA is analysed and simulated under wind speed conditions measured and recorded for one year at Baja California Sur. A representative variability is applied to five type-4 wind turbines based on wind speed turbulence intensity analysis. The generated power is transferred through a bidirectional dual active bridge converter, controlled by a novel small-signal model using a single closed-loop proportional-integral control, owing to its modularity, power density, and ability to transfer power from LVDC to high-voltage direct current. The effectiveness and robustness of the proposed DREA are assessed via a complete mathematical model, corroborated by the simulation results in MATLAB-Simulink®, and validated by experimental results using the real-time simulator Opal-RT Technologies® and laboratory prototyping. The results indicate a DC-link voltage compensation and integration of a power capacity of 150 kW in the LVDC networks with an efficiency of 94%.

Improved voltage‐based protection scheme for an LVDC distribution network interfaced by a solid state smart transformer

The increasing electrification of transport and heat will place increasing demand on low voltage (LV) networks with the potential to overload medium voltage (MV)/LV transformers and LV cables. Deployment of a solid-state transformer (SST) at MV/LV substations and using LV direct current (LVDC) distribution systems offer great potential to address such challenges. However, the SST deployment in addition to the introduction of LVDC will fundamentally change LV fault behaviour and protection requirements due to the limited short-circuit capabilities of such technologies. The SST will deliver limited fault currents, making current-based protection (widely used in LV networks) less reliable. Therefore, this study presents an advanced communication-less protection scheme which can effectively detect and locate DC faults even with reduced fault levels. The developed protection scheme overcomes the selectivity limitations in LVDC voltage-based protection solutions by using a combination of DC voltage magnitude, voltage concavity (sign of d 2 v /d t 2 ) and the sign of the rate of change of current (d i /d t ) regardless of the current magnitudes. The credibility of the developed protection algorithm is tested against different fault scenarios applied on an active LVDC network model built in PSCAD/EMTDC. Noise signals have been included in the simulation to appraise the resilience of the developed scheme.

Protection Scheme of a Last Mile Active LVDC Distribution Network with Reclosing Option

A low voltage direct current (LVDC) distribution network is a promising technology to meet the standards of future energy demands for smart loads. An LVDC distribution network can not only supply efficient, smooth and clean energy, but also makes the integration of renewable energy resources in the distribution system easy. A major obstacle in the implementation of the LVDC distribution network is the protection of the network during abnormal grid conditions, such as transients and faults. This paper analyzes DC fault characteristics considering an LVDC distribution network, highlights the worst case scenario during a fault and protection related issues and proposes the protection schemes for the LVDC network. In the proposed protection scheme, a fault is detected and located through superimposed components. To minimize the effect of the DC fault on the distribution network, distributed fault current limiters are introduced and the final decision to disconnect or reconnect the affected line is made on the basis of the type of fault. In addition, a reclosing scheme for a temporary fault is proposed to avoid high inrush currents and false tripping, which eventually increases the reliability. A fast communication-based backup protection is also suggested, and to reduce dependency, a secondary backup is used in the case of communication delay or failure. The proposed scheme is verified using the modified IEEE 13 node test system, which is implemented in ATPDraw. The results show that the proposed scheme can successfully detect, locate and limit a DC fault in an LVDC distribution network with different fault resistances or locations. Moreover, the network is restored successfully in the case of temporary faults.

Energy Efficiency Analysis and Energy Management of a Superconducting LVDC Network

A novel superconducting low-voltage direct-current (LVDC) power transmission and distribution network consisting of multiple superconducting dc cables and a superconducting magnetic energy storage (SMES) based hybrid energy storage system (HESS) is presented. The superconducting dc cables are favored for both high-efficiency power transmission under normal conditions and self-triggering fault-current-limiting operation under fault conditions. The SMES-based HESS is integrated, with the merits of fast response speed and high-power density from the SMES, and high-energy density and high-economic efficiency from conventional battery energy storage (BES), and thus, is much more effective a sole SMES or BES device for smoothing the line voltage fluctuations caused by some sudden load variations or short-circuit faults. The simulation results obtained from energy efficiency analyses and energy management simulations show that the proposed superconducting LVDC network has significant advantages in high-efficiency, high-quality power transmission and distribution compared to conventional ac networks.