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Home , Belene, Blagoevgrad, Karlovo, Pleven, Zlatitsa

TIE-IN OF NPP “” TO THE ELECTRIC GRID Sen.Res.Assoc., El.Eng., Ph.D. Kiril D. TAGAROV “Energy Institute” JSC – , 20, F. Joliot-Curie st., 1113 Sofia, phone: (+3592) 9698 627, FAX: (+3592) 963 40 38, E-mail: [email protected] Sen.Res.Assoc., El.Eng., Ph.D. Christo K. CHRISTOV “Energy Institute” JSC – Sofia Sen.Res.Assoc., El.Eng., Ph.D. Hristo V. VASSILEV “Energy Institute” JSC – Sofia Ph.D. Student Violeta Hr. HRISTOVA Institute of Information Technologies - BAS, 1113 Sofia Keywords: tie-in, stability, reliability, electric grid, alternative options 1. General considerations regarding the tie-in of NPP "Belene" to the power system The earlier studies about the capacity of NPP "Belene" performed in the 1980s considered up to 4 generating units of 1000 MW. It was decided that the electricity output will be on a 400 kV voltage level. The voltage 220 kV was rejected due to the decision not to develop in the future this voltage level. The voltage 750 kV was rejected due to the decline of the growth rate of the electricity demand in and due to the decision that it is not more feasible to import great amounts of electricity. Each 1000 MW turbogenerator must be connected to the 400 kV switchyard by a block-scheme "generator-transformer" through 2 unit transformers of 630 MVA each, operating in parallel. It was considered, that the tie-in of the generating units of NPP "Belene" to the power system will be performed with 4 double-circuit overhead lines at a voltage level 400 kV in the following stages: − 1-st unit: - one double-circuit line to s/s "Tsarevets" c.a. 75 km long - one double-circuit line to a new 400/110 kV s/s "Russe" c.a. 92 km long − 2-nd unit: - no construction of new lines is foreseen − 3-rd unit: - one double-circuit line to a new 400/110 kV s/s "" c.a. 145 km long − 4-th unit: - one double-circuit line to a new 400/110 kV s/s "" c.a. 70 km long. The construction of the above mentioned new 400/110 kV substations near the Russe, Karlovo or Pleven will depend not only on the options for the tie-in of NPP "Belene" but also on the growth of the electric loads in these regions of the country and must be determined by a new Expansion Plan of the high-voltage transmission grid (400 and 220 kV voltage levels) of Bulgaria. The last study on the development of the Bulgarian HV electric grid was "Investigation of the transmission high- voltage network of Bulgaria" conducted 10 years ago (in 1994) in "Energoproekt" PLC as a "PHARE" project of the European Union with the German company "Fichtner" CE. The present situation and the latest forecasts show that the growth rate of the electricity consumption in R. of Bulgaria are much lower than it has been expected in the 1980s. As shown in the new "Least-Cost Plan for the Development of the Electric Energy Sector of the R. of Bulgaria for the period 2004 - 2020" prepared by the National Electric Company NEK - PLC in September 2003, up to the year 2020 the needed capacity of a new nuclear power plant will be not greater than 2 units of 1000 MW and there are also options with one or two 600 (650) MW units. The present situation of the high-voltage transmission network in Bulgaria (400 and 220 kV voltage levels), is shown on Fig. 1. On the same figure with dash-dotted lines are shown the following future developments of the high voltage grid, for which there are decisions to be constructed: − a new 400 kV overhead line from NPP "" to a new s/s 400/110 kV "Bononia"; − the 400 kV overhead line " - " will be finalized; − a new 400 kV interconnection line from s/s "Chervena mogila" (Bulgaria) to s/s "Dubrovo" ().

1 A second 400 kV line from Bulgaria to s/s "Philippi" in is being planned to be constructed, but the 400 kV substation on Bulgarian territory has not yet been chosen. At present, as shown on Fig. 1, the 750 kV interconnection line "Varna" (Bulgaria) - "Isaccea" () is not in service and one of the 750/400 kV autotransformers is being used as a temporary replacement of the 400/220 kV autotransformer on the 400/220/110 kV s/s "Dobrudja", which is at present out of service. In the future it is planned to use the 750 kV line "Varna (Bulgaria) - Isaccea (Romania)" at a voltage level 400 kV. This new situation regarding the forecast of the electricity demand and the development of the power system confirm the results from the studies conducted in the 1980s that the tie-in of NPP "Belene" to the power system must be performed on a 400 kV voltage level. The power plant "Belene" must also have connections to the 110 kV network to ensure back-up power supply of the auxiliaries.

2. Tie-in of NPP "Belene" to the electric power system 2.A. Options for construction of NPP "Belene" with one 600 MW unit or with 2 units In the study "Investigation of the transmission high-voltage network of Bulgaria" conducted in 1994 in "Energoproekt" PLC as a "PHARE" project of the European Union with the German company "Fichtner" CE, NPP "Belene" was considered to be with only one unit of 600 MW. In this case the optimum solution for the tie-in was the following: − double-circuit line 400 kV with length c.a. 75 km to s/s "Tsarevets"; − single-circuit line 400 kV with length c.a. 70 km to a new s/s 400/110 kV "Pleven". The 400 kV connections of NPP "Belene" for the case of 1 unit of 600 MW are shown on Fig. 2.

2 In the case of construction of two 600 (650) MW units or one or two 1000 MW units the options for tie-in must be with four 400 kV circuits.

It is possible to construct a double-circuit 400 kV line "Belene-Pleven" as shown on Fig. 2.a or two single-circuit 400 kV overhead-lines: "Belene-Pleven" and "Belene-Russe-Dobrudja" (or "Belene- Russe-Varna") as shown on Fig. 2.b.

3 For improvement of the transient stability of NPP "Belene" and for the reduction of the active power losses in the high-voltage transmission grid it may be necessary to construct a new 400 kV overhead line from s/s "Tsarevets" to a new s/s "Karlovo" with a length of c.a. 114 km and to break the 400 kV line from s/s "Zlatitsa" to s/s "Plovdiv" in s/s "Karlovo". The technical and economic advantages of a new line "Tsarevets-Karlovo" must be further investigated, taking into account the new conditions of the development of the Bulgarian power system. 2.B. Other options for construction of NPP "Belene" with two nuclear units In the new "Plan for the development of the electric energy sector of the R. of Bulgaria for the period 2004 - 2020" prepared by the National Electric Company NEK - PLC in September 2003, are considered the following cases for finalization of NPP "Belene": • 1-st unit 1000 MW and 2-nd unit (if constructed) - also 1000 МW; • 1-st unit 1000 MW and 2-nd unit (if constructed) - 600 МW; • 1-st unit 600 MW and 2-nd unit (if constructed) - also 600 МW; According the new studies the construction of more than 2 nuclear units until 2020 is not feasible. All electrical and technico-economic studies confirm, that the generating units must be connected to the power system at a voltage level 400 kV, and the power station auxiliaries must have a back-up electricity supply at a voltage level 110 kV. The following double-circuit lines outgoing from NPP "Belene" are possible: • NPP "Belene" - s/s "Tsarevets" 400/110 kV (near the V. Tarnovo) with length c.a. 75 km • NPP "Belene" - s/s "Dobrudja" 400/220/110 kV or s/s "Varna" 750/400 kV (both substations are near the village ) with length c.a. 235 km, one of the circuits being cut-off in a new s/s "Ruse" - 400/110 kV • NPP "Belene" - new s/s "Pleven" 400/110 kV with a length of c.a. 70 km. One of the possible options for the tie-in of NPP "Belene" with one unit of 1000 MW or with two units (either of 600 or 1000 MW) to the 400 kV grid is shown on Fig. 3.

4 In the case of construction of a combined-cycle thermal power plant on natural gas in with either one (two) or more units of 450 (500) MW, the double-circuit 400 kV overhead line from s/s "Russe" to s/s ""Dobrudja" shown on Fig. 3 may be constructed as a single-circuit line as shown on figures 4 and 4.a.

5 It is recommended to construct the new 400 kV lines with bundle steel-aluminum conductors, three conductors per phase with a cross-section of 400 or 500 mm2 , type ACO-400 or ACO-500 (unlike the earlier decisions to construct them with 2 conductors per phase) in order to minimize the noise caused by the corona effect. The tie-in of NPP "Belene" will depend on the growth of the load demand in the different parts of the country and on the necessity of construction of new 400/110 kV substations near the towns Russe, Karlovo and Pleven. In order to make a final decision about the tie-in a new complete study on the development of the high-voltage transmission system (220 and 400 kV voltage levels) in Bulgaria must be conducted. 3. Influence of the stability on the tie-in of NPP "Belene" 3.A. Static stability The necessary number of 400 kV overhead lines has been studied from the point of view of the reserve margin for the aperiodic static stability across the 400 kV lines outgoing from NPP "Belene" according the following formula:

Plim − Prat − Pnonreg K lim = ⋅100% Prat where: Klim is the factor of static stability limit; Plim is the maximum power output from the busbars of NPP "Belene" with which the static stability is preserved; Prat is the rated power of NPP "Belene"; Pnonreg is the power deviation due to unexpected (nonregular) power deviations, usually taken 2% from the rated power. The output of electrical power from a nuclear power station to the electric network is considered to be sufficient if for normal conditions: Klim ≥ 20%. For emergency conditions the outage of two network element (in the case of NPP "Belene" the outage of two circuits from the 400 kV lines) is considered for a nuclear power plant ("N-2" contingency criterion) and for that case the following condition must be satisfied: Klim ≥ 8%.

6 These conditions have proven to be fulfilled for the tie-in the options shown on figures 2, 3, 4 and 4.a. 3.B. Transient stability In the power system of Bulgaria 3-phase short circuits in the 400 kV network have not been observed and the probability for their occurrence is considered to be very small. Therefore the most severe case for the transient stability is considered to be the following: − a double-phase-to-ground fault on one of the 400 kV lines outgoing from NPP "Belene" close the 400 kV busbars − a one-phase failure of a 400 kV circuit breaker − switching-off of the remaining short-circuit using the automatic back-up protection which disconnects the failed breaker − these actions lead to the disconnection of two 400 kV lines. In some cases after these actions one of the turbo-generators may remain connected to only one 400 kV line. In the case of a 1000 MW unit this leads to loss of the transient stability of this generator. In order to avoid the loss of the synchronous parallel operation of one 1000 MW unit connected to only one 400 kV line after electrical faults a special automatics of the turbine must be provided for the fast decrease of the turbine power output. The typical characteristic for such fast drop of the turbine load for the Russian made 1000 MW turbines is given on Fig. 5.

Fig. 1.4.1.5. Fast drop of the turbine load of a 1000 MW unit in NPP 1200

1000

800

600 [MW] т P 400 Pт [MW] 200

0 0 1 2 3 4 0.5 1.5 2.5 3.5 t [s]

In the case of a 600 MW turbogenerator which remains connected to only one 400 kV line after faults near NPP "Belene" the transient stability may be preserved without such fast turbine unloading. 4. Choice of the schemes of the400 and 110 kV switchyards of NPP "Belene" 4.A. Scheme of the 400 kV switchyard

The 400 kV switchyard will be constructed as a gas-insulated switchgear (SF6). It will be a double busbar system, sectionalized. To the switchyard will be connected: − the unit transformers (in the case of 1000 MW units there will be two 630 MVA transformers for each turbogenerator, operating in parallel and in the case of 600 (650) MW units there will be one unit transformer of 670 (720) MVA for each turbogenerator) − the 400 kV overhead lines − two 400/110 kV autotransformers After further investigations it may be proved that it may be necessary to connect to the 400 kV switchyard also a 400 kV shunt (compensating) reactor of c.a. 200 MVAr. On Fig. 6 is shown the 400 kV switchyard of NPP "Belene" with a "one-and-a-half breakers" option.

7 On Fig. 7 is shown the 400 kV switchyard of NPP "Belene" with a "4/3 breakers" option.

On Fig. 8 is shown the proposed scheme for the case of 1000 MW units for the 400 kV switchyard of NPP "Belene" with a "two-breakers" scheme for the connection of the generating units and a "one-and-a-half breakers" scheme for the other connections.

For the scheme on Fig. 8 the probability for outage of a generating unit due to electrical faults in the 400 kV network is about 8 times smaller than for the schemes on figures 6 and 7. This is explained by the fact that the generating unit in the scheme on Fig. 8 is not connected directly to transmission lines but only indirectly (through the busbars) and the highest probability of electrical faults is on the overhead lines (1.15 faults per 100 km per year on a 400 kV line which are cleared with successful automatic reclosure and 0.75 permanent faults per 100 km length per year).

8 4.B. Type of the 400/110 kV autotransformers In the Bulgarian power system most of the 400/110 kV power transformers have rated power 250 MVA. There are two main types of such autotransformers: − with short-circuit impedance ek%=18.5% (imported from Germany) and − with short-circuit impedance ek%=12.5% (imported from ). According the practical recommendations of "Atomelektroproekt" - Russia, the equivalent (Thevenin) impedance of the 110 kV part of the power system as "seen" from the 110 kV busbars of NPP "Belene" must be smaller than 5.5 Ω in order to guarantee the conditions for restart of the motors of the auxiliaries of the 1000 MW units after their reconnection from main supply to the back-up supply from the 110 kV grid. This condition is satisfied if there are 2 autotransformers with a short-circuit impedance ek%=12.5%. According to the practice in Bulgaria to connect to the tertiary windings of the autotransformers shunt (compensating) reactors of c.a. 40 MVAr, it is recommended to choose the autotransformers with rated voltage of the tertiary winding of 31.5 kV. 4.C. Connections with the 110 kV network

The switchyard 110 kV of NPP "Belene" must be of the type "gas insulated switchgear" (SF6) with a double busbar system, sectionalized. The 110 kV lines are determined by the following requirements: − fulfillment of the need for power supply of the region; − guarantee for the successful reconnection of the motors for the auxiliaries by their reconnection from the main to the back-up supply taking into consideration the "N-2" contingency criterion. The requirements of the region are out of the scope of the present report. In order to comply with the requirements of "Atomelectroproekt" - Russia to achieve an equivalent (Thevenin) impedance of the 110 kV network after outage of "N-2" elements (both autotransformers) to be below 5.5 Ω in the case of 1000 MW units the number of the 110 kV overhead lines outgoing from the power plant must be at least five: − to cut-off in the 110 kV switchyard the existing line from s/s "Belene" to TPP ""; − to construct a new line from NPP "Belene" to TPP "Svishtov"; − to construct a new double-circuit line from NPP "Belene" to Pleven. To the 110 kV switchyard will be also connected the 110/6/6 kV, 63/31.5/31.5 MVA transformers for the back-up supply of the auxiliaries. In the case of 1000 MW units each unit will have two such transformers. It is recommended to leave space for three reserve bays in the 110 kV switchyard. The single line diagram of the 110 kV switchyard is shown on Fig. 9.

9 5. Recommendations for the connections of NPP "Belene" to the electric power grid • 400 kV lines Irrespective of the capacity of the generating units of NPP "Belene" (600 or 1000 MW) and of their number (one or two) it is recommended to construct two double-circuit 400 kV overhead lines. It is recommended to construct one of the double-circuit lines to the existing 400/110 kV substation "Tsarevets". For the second double-circuit line can be recommended one of the following two options, which must be chosen in a new Expansion Plan of the Bulgarian high-voltage electric grid: − to a new 400/110 kV s/s "Pleven"; − to the existing s/s 400/220/110 kV "Dobrudja" (or to the existing s/s 750/400 kV "Varna") with a break of one of the circuits in a new 400/110 kV s/s "Russe". The decision about the choice of one of these two options may be given only in a new HV-grid Expansion Plan. If the first nuclear unit is of 600 MW it is possible to attach initially the conductors of only one 3-phase circuit on the towers for the above mentioned double-circuit lines. It is recommended additionally to construct a 400 kV line from s/s "Tsarevets" to a new 400/110 kV s/s "Karlovo", probably together with the unit. The answer about the necessity of this line may be given only after an extensive study on the development of the Bulgarian HV grid. • 400 kV switchyard It is recommended to construct the 400 kV switchyard as a GIS (SF6) with two busbars, sectionalized, according a "combined" scheme as follows: − "two breakers" scheme for the generating unit transformers; − "one and a half" breakers scheme for all other connections. • 110 kV lines It is recommended to have 5 110 kV lines: − one line to s/s "Belene"; − two lines to TPP "Svishtov"; − two lines to the existing 220/110 kV s/s "Pleven" (or to a new 400/110 kV s/s "Pleven"). The final decision about the 110 kV lines outgoing from NPP "Belene" can be given after a complete study on the development of the electrical loads in the Belene-Nikopol-Svishtov region. • 110 kV switchyard It is recommended to construct the 110 kV switchyard as a GIS (SF6) with two busbars, sectionalized, with a "one breaker" scheme for all connections, with five 110 kV overhead lines and three reserve bays. • 400/110 kV autotransformers It is recommended to install two 400/110/31.5 kV, 250/250/80(100) MVA autotransformers with a short-circuit impedance ek% ≤12.5%. • Interconnection lines with the neighboring countries The existing interconnection lines of Bulgaria in operation (double 400 kV line "Kozloduy (Bulgaria) - Tsintsareni (Romania)", 220 kV line "Kozloduy (Bulgaria) - Ishalnitsa (Romania)", 400 kV line "Dobrudja (Bulgaria) - Vulkaneshti (Moldova)", 400 kV line "Sofia (Bulgaria) - Nish (Serbia)", 400 kV line " (Bulgaria) - (Greece)" and two 400 kV lines from TPP "Maritsa East 3" to Turkey, together with the 750 kV line "Varna (Bulgaria) - Isaccea (Romania)" which is now disconnected and is expected to operate on 400 kV and the planned new 400 kV lines (to Macedonia and Greece) are sufficient to keep the power system stability after outage of a 1000 MW nuclear unit.

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