RESULTS of the IIId All-Russian Conference "Technical-Economical Aspects of 20 kV Network Development"
(July 11, 2017, Moscow)
The Conference has been held by the "ELECTRIC POWER. Transmission and Distribution" journal staff with support of JSC "United Energy Company" (JSC UNECO) and participation of the Ministry of Energy of Russia
Participants: 127 persons, including representatives of the Ministry of Energy of Russia, grid companies, including PJSC Rosseti Moscow Region, PJSC Rosseti Center, PJSC Lenenergo, PJSC Rosseti Urals Region, PJSC Rosseti North-West, JSC UNECO, JSC Yugorsk Electric, JSC R&D Centre of FGC UES, engineering companies, operation, design and research companies, higher educational establishments, equipment developers and suppliers, foreign guests (according to the list).
Issues for discussion:
- Is there any reasonable need of 20 kV voltage application in a distribution network of metropolitan cities and power grids of Russia instead of or in addition to the existing 10 kV and 35 kV voltage level? If yes, what are the cases and the criteria?
- What is the economics of construction and operation of 20 kV networks? How do these costs align with those of construction and operation of 10 kV and 35 kV networks? How will distribution network operation efficiency and electricity tariffs improve?
- What is peculiar about 20 kV network operation modes? What neutral operation modes are possible and preferable? What are the risks in solving technical and routine issues, including mandatory use of imported raw materials and process lines for 20 kV XLPE cable production?
- Do they use 20 kV networks in metropolitan cities only or is it possible to view this voltage level as basic in small towns and rural regions as well?
- Are there any technical solutions for integration of "20 kV islands" into a 10 kV network (taking into account different ways of neutral grounding) for provision of mutual redundancy, reliability and cost efficiency of consumer power supply?
- Is there a mechanism of existing 10 kV network transition to 20 kV voltage level? Is there any good example?
- What are (if any) technical peculiarities of 20 kV network application for design of power output diagrams of distributed generation facilities and isolation to the balanced load in conditions of power system failures?
- What is the market situation with regard to proposals of 20 kV equipment and availability (absence) of domestic raw materials for production of 20 kV XLPE cables? Is it important to keep the production lines of MV paper-oil insulation cables in Russia for mastering the production of domestic raw materials for 20 kV XLPE cable manufacture? Is it necessary to improve cooperation of domestic XLPE cable producers and JSC VNIIKP with State Institute of Flint-Organic Compounds (Russian defense contractor supplier) for creation of a new type of insulation for 20 kV cables with respect to import phase out objectives? What is the base cost of equipment for 20 kV networks and their automation means? How does it relate to the import phase out trend?
- What is 20 kV network operation experience in different regions? What are the results of pilot project implementation? What are the key issues to pay attention to in further development and operation of 20 kV networks?
On completion of discussion the participants of the Conference have come to the following conclusions:
- There is a need in legislative and regulatory support of 20 kV network development and operation with provision of feasibility study and consideration of present day requirements to construction of intelligent networks. To provide due elaboration of laws and regulations, it is essential to involve specialists of major power companies with practical experience in electric network operation.
- The main criteria of 20 kV voltage application by reference to foreign experience and pilot project implementation results are as follows:
− Increased load density up to 10 MW/km2
− Need of 3...10 MW power transmission to 25-50 km.
- Variants of 20 kV network construction should be considered with respect to the following advantages:
− Operation experience of 20 kV networks reveals multiple decrease of energy losses regarding similar 6-10 kV network. Such efficiency should be considered when making feasibility study of individual network section development.
− Study of technical-topological solutions in design of outdoor power supply networks shows economic benefit from using 20 kV networks (instead of conventional 6 and 10 kV networks). This benefit comes from reduced number of transformer substations and switchgears and prolongation of cable lines.
− The equipment used in construction of 10 kV overhead lines is mainly designed for 24 kV. It would be reasonable to make use of this fact.
− Design of a 20 kV overhead line is exactly the same as of a 10 kV OHL, no additional requirements to operating personnel are available.
- Possible upgrade of existing utilities and components of electric networks by using 20 kV technologies, based on technical-economical models, risk-oriented approaches and technical condition assessment indices.
- The correct load forecast has critical impact on cost efficiency of distribution network development. A more precise and short-term forecast prevents excessive power input. At the same time, the mid-term demand forecast with respect to housing development and process connection requests provides unfair evaluation of future power consumption and estimated figures turn to be overevaluated in more than 10%. This happens because consumers do not use the demanded power in full. It is necessary to elaborate legislative means to improve consumer behavior in respect of planning process connection requests and to provide routine update of demand forecasts.
- To provide the required reliability of power supply when designing 20 kV networks for big cities it is reasonable to use the counter dual-beam diagram based on two distribution substations; in this case the end consumer will have connection with each of four sections, that are actually independent power sources.
- Power grid specialists and designers should take into account the following specific aspects of 20 kV networks:
− Feasibility study of 20 kV cable lines should be carried out with respect to actual conditions of their laying in a metropolitan city and possibility of deep unification of core and shield sections. Based on system positions, it is recommended to use cables with a 630 mm2 section (ground) and a 500 mm2 section (overhead).
− In 20 kV networks with low-resistive neutral grounding the choice of shield sections for phase-wise shielded cables should be made by single phase-to-ground (not two-phase) fault current. The experiment has shown that a single phase-to-ground fault in a 20 kV network in phase-wise shielded cables with closed triangle laying at 860 A and duration up to 1,5 s does not evolve into a multi-phase fault. In other words, the arc power turns to be insuffiсient for complete insulation burning in healthy phases. This is essential from the point of reducing the cable cost and power and energy losses in networks.
− The resistor value for neutral grounding should be selected with respect to permissible touch voltage in a single-phase fault and provision of sensitivity of single phase-to-ground fault protections. Depending on a combination of these requirements, they could use not 12 Ohm but 2-4 times higher resistors.
− The calculated active current value that provides the required sensitivity of non-directional current protections can be less than 1000 A used at present. This decrease results in reduction of thermal load in PT windings and single-core cable shields, as well as of touch voltage and the step.
− Selection of earth fault current in 20 kV overhead lines with low-resistive neutral grounding shall be a compromise. Its recommended value is 200 A (60 Ohm resistor) with the following restrictions observed:
a. Resistance of grounding devices — max. 2 Ohm;
b. Transformer capacity — max. 250 kV·А;
c. If transformer capacity is higher than 250 kV·А, to admit non-selective operation of relay protection and automation in regards to the offset from time-current curves of fuses;
d. Any single phase-to-ground fault is eliminated by non-selective CB tripping within 0,3 s followed by recovery of the single-shot sequence AR circuit with protection acceleration starting from the supply center CB
− 20 kV XLPE cables are self-balanced by reactive power in conditions of current loads close to long-term permissible conductor current that relates to XLPE insulation temperature of 900С. Thus, they hardly generate into the network and do not consume reactive power. This is good for voltage levels in network nodes and power losses. To provide efficient 20 kV network operation, it is recommended to keep high values of the load cos φ, e.g., by installing reactive power compensators at major consumer's.
− Operation of long (50-100 km) 6-35 kV AC cable lines is possible without controlled shunting reactors. However, it is recommended to have the cable core section oversized to improve load mode parameters: reduce active power losses and increase termination voltage levels.
− Application of a 20 kV cable line is efficient for remote (up to 50 km) load supply with high cos φ=1 and oversized core section. For remote
(50-100 km) load supply with cos φ<1, it is preferable to use not a 20 kV, but a 35 kV cable line.
- To complement the agenda of the next Conference on 20 kV network design, construction and operation with the following topics:
− 20 kV network development with respect to that of other voltage level networks, automation technologies and "smart city" systems.
− To cover at length the topic of DC links at 20 kV to provide mutual redundancy and improved reliability.
− To review best domestic and foreign practices in 20 kV network design, construction and operation, complex automation of technological processes and use of advanced technologies.
- The Ministry of Energy of Russia should eliminate inconsistency in valid regulatory (Electric Installation Code, Technical Maintenance Code, etc.) and newly elaborated risk-oriented documents, and in technical condition assessment (including discrepancies in defining intervals of technical maintenance, repair and reconstruction of network facilities).