20kV Voltage Adaptation Problems in Urban Electrical Networks Olegs Borscevskis, Gerards Gavrilovs Riga Technical University [email protected], [email protected] Abstract-Nowadays due to electric power consumption’s subsystems or stages of hierarchical structure are external increasing in big towns, as in foreign countries, as in Latvia, supply system of the city with voltage 330kV and higher, the exsist necessity in electric system elements, maximum electric load’s and power consumption’s appreciate on initial projection internal supply system with voltages 110(220)-10(20)-0.4 kV stages. In this paper the current distribution network of Riga and aggregate of urban consumers. Connections between city is analysed and the development of a new network concept is subsystems are carried out through high-voltage transformer described. substations (TS) and networks at the corresponding levels of the UPSS hierarchy [1]. Old equipment and grown network structures make the I. INTRODUCTION maintenance and operation of the network difficult and expensive, because major part of the current medium voltage Historically the major part of the current Latvian electrical (MV) electrical network was built using old technology, oil- network has been built in 60 th and 70 th of XX century using filled cables and old equipment with long-life not more than old Soviet Union technology. The main part of the electrical 25 years [3]. equipment, cables and overhead lines (OHL) in the Latvian From 20 to electrical network is reaching the end of its lifetime in the From 15 to coming years. Due to the historical developments in the 25 years, 7% From 10 to 20 years, 7% network structure, reliability is low compared to European 15 years, 4% standards [1]. In Fig.1 the structure of voltage levels of >25 years, Latvian power supply system is shown. 60% From 5 to 10 years, 7% Up to 5 years, 15% Fig. 2. MV OHL and cable lines division by age Situation with distribution 20-10/0.4-0.23kV transformers not better then with cables, mostly of them are oil filled non hermetic transformers without any temperature protection and no possibility to connect temperature protection. MV distribution transformers division by age is illustrated in Fig.3. From 20 to From 15 to 25 years, 9% >25 years, 20 years, 32% 16% Fig. 1. The structure of voltage levels of Latvian power supply system From 10 to 15 years, 1% The urban power supply system (UPSS) of state can be presented as a hierarchical structure of the voltage levels From 5 to 10 Up to 5 (Fig.1). Highest level of hierarchy is the power system of years, 15% years, 15% state or the region, which includes the UPSS. The UPSS Fig. 3. MV distribution transformers division by age 68 The same situation is with MV switchgears. More then half II. 20 KV VOLTAGE USING EXPIERENCE IN LATVIA AND of them are old type switchgears, with old generation `s OTHERS COUNTRIES circuit-breakers. Others TS and feeders–distribution points (FP) are completed with modern type air insulated or SF6 gas At present, the world's already dozens of countries and filled switchgears. regions will 20kV voltage level as a medium voltage distribution network of the standard voltage, and have listed From 15 to From 20 to the International Electrotechnical Commission standards for 20 years, 9% 25 years, 9% people with a very mature Technology and experience. 1948 using 20kV voltage parts of the United States, France From 10 to 15 years, 3% >25 years, and Germany since the 20th century, 60, began uniform 62% application of 20kV distribution network, other European From 5 to 10 countries such as Italy, Austria, Bulgaria, Poland, Hungary, years, 3% etc. (accounting for 80% of Europe) also were powered by 20 ~ 25kV as the piezoelectric. In Asia, including Singapore, Korea, Taiwan, China and other countries and regions have Up to 5 adopted 20kV medium voltage power distribution [4]. Very years, 14% widely 20kV voltage use in Finland. They also adopted 20kV voltage from rural to urban networks. In Fig. 6 the 110/20kV Fig. 4. MV switchgears division by age TS scheme in Helsinki is shown. In 2010 total electricity consumption of Latvian power supply system amounted to 7573 GW/h, the maximal peak load 1385 MW. Two-thirds of this load consumes the largest Latvian city and the largest consumer of the Latvian republic – Riga. In 2010 the peak load of Riga was 524MW. It is about 38% of common load of Latvian power supply system [6]. The current situation of Riga`s urban distribution network is not so critical due to economical global crisis from the point of view of new loads connection to the grid like few years ago. But it is still problem to connect powerful consumers (more then 1 MW) to the concrete 110/20-10kV TS, because some of TS are overloaded, some underloaded and sometimes in underloaded TS no free feeders on MV side. New connections to grid are eliminated by unavailable loads of power 110/10-10kV transformers too-mostly of them are overloaded. In Fig.5 the dynamic of replaced high voltage (HV) 110/20-10kV transformers is shown. Replaced HV transformers 2010 2008 Fig. 6. The standard 110/20kV power supply scheme of Helsinki 2006 Time period (Years) 2004 Historically in Latvian urban networks was using 6 and 10kV MV voltages. Only in the end of XX century was 2002 happened great reconstruction, when old 6kV MV voltage 2000 was replaced to 10kV MV voltage. 20kV voltage probably 0 5 10 15 20 25 use in rural networks and in suburban areas, where 10kV MV Quantity (pieces) electrical networks (mainly cable lines) border with 20kV MV electrical networks (mainly OHL). In this reason the main Latvian power supply company use 110/20-10kV TS Fig. 5. The number of replaced HV transformers where put in HV transformers with three coils. It is mean that HV transformer transform 110kV voltage to 20kV and also from 110kV to 10kV. 69 by the load radius of the size and power constraints, per square kilometer need to build a ~ M-1 2 110kV TS. Parts of the power supply capacity of the existing lack of distribution network in urgent K-2-110 K-1-110 need of expansion of power distribution T-1 T-2 T-3 substation, the power load faster growth areas, the ARS increase in load led to expansion of distribution T-2-NZ T-3-NZ T-1-NZ substations is a must; TNr.2 TNr.3 TNr.1 63,0 25,0 63,0 TNr.15 TNr.16 by the supply voltage and wire cross-section NTNr.11 NTNr.12 NTNr.13 constraints, 10kV distribution line very large number. Therefore in equipment, infras area, line corridor, network structure, operation T-13A T-11 T-12 T-13B management, and reduced energy loss is ARS ARS M-11 KS-1-10 KS-2-10 inevitable in such aspects as there were many ARS problems; Gen F-979 distribution losses are too high. 2.5 MW T-21 T-22 T-23 ARI In addition, the continuous expansion of urban and rural M-21 KS-1-20 KS-2-20 power grid and density of the rapidly increasing, and ARI gradually exposed the power distribution network level, substation level, the layout structure, etc. not meet with the irrational. The existing power distribution system from the Fig. 7. The standard scheme of Latvian suburban 110/20-10kV TS power supply capability, power distance and line losses, etc. are difficult to meet the load demand for development [2]. At present, Latvia commonly used medium voltage Usually the big industrial consumers like factories or some distribution network 10kV power supply voltage level. This is production object or new live districts are lying in the border a low power load in our country against the backdrop of of city. Therefore load density in suburban areas is smaller evolving. Many years of operation proved that in a particular then in the centre of city, but larger then in rural areas (5- stage of development is suitable for Latvian national 7MVA/km 2). conditions, to promote the development of power industry, safety plays a bigger role in electricity supply. Economically 0.45 2x63kVA 0.4 developed regions and urban-suburban industrial area near 2x100kVA ) ) the load density increased rapidly, more and more clearly 2 0.35 2x160kVA 0.3 2x250kVA revealed our existing distribution network for medium-sized 2x400kVA 0.25 cities have been incompatible with the economic 2x630kVA 0.2 development and urban modernization needs. At present, 0.15 some of our big cities like Riga, the load density has reached 0.1 2 (km area service TS 20 ~ 25 MVA/km even higher. Therefore in these areas with 0.05 large load density is economically gainfully to use 20kV 0 3 5 8 10 13 15 17 20 23 25 27 30 33 36 voltage network. Load density (MVA/km 2) In Fig. 8 the dependence of service area from load density for 110/20kV is described. Fig. 9. The 20/0.4 kV TS service area dependence from load density ) ) 2 2x63kVA 1000.00 3 0.5 36 5 2x100kVA 100.00 0.4 2x160kVA 33 8 0.3 2x250kVA 10.00 2x400kVA 0.2 2x630kVA 1.00 30 0.1 10 TS service area (km area serviceTS 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 0 Load density (MVA/km 2) 27 13 25 15 2x16MVA 2x25MVA 2x32MVA 2x40MVA 2x63MVA 2x80MVA 23 17 Fig.