Grid capacity and efficiency enhancement by operating medium voltage AC cables as DC links with modular multilevel converters

Abstract

a b s t r a c tIt is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5-20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50-60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.

Boundaries of Operation for Refurbished Parallel AC–DC Reconfigurable Links in Distribution Grids

“…The concept of converting the existing ac links for refurbished dc operation to maximize the power transfer capacity has recently gained significant attention [4]- [7]. An illustration of capacity deficit benchmark ac distribution network is shown in Fig.…”

Recent contributions and future prospects of the modular multilevel converters: A comprehensive review

“…In Shekhar et al, grid capacity and efficiency improvement for MMC‐based HVDC transmission system is presented by considering voltage regulation, capacitive leakage currents, skin and magnetic proximity effect, and load power factor. Different AC and DC circuit topologies are investigated by changing cable lengths and conductor cross‐sectional areas.…”

Reconfigurable DC Links for Restructuring Existing Medium Voltage AC Distribution Grids

“…In case of overhead lines, modification to dc transmission can have considerable increase in power transmission capacity [27] Similarly, with use of dc links, the power transfer capacity of existing underground cable infrastructure can be significantly enhanced [4] as shown in Figure 2. A detailed quantification of capacity enhancement under different scenarios is offered in [28].…”