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Basic Design Aspects of Ballia-Bhiwadi 2500MW HVDC Power Transmission System

R.K. Chauhan, M. Kuhn, D. Kumar, A. Kölz, P. Riedel

shown in Fig.2. Abstract— The ±500kV, 2500MW Ballia-Bhiwadi HVDC shall transmit energy from Ballia to Bhiwadi stations in India over about 780km. Ballia Converter Station is located in the state of Uttar Pradesh approximately 75km from Ballia District Head Quarter. The Bhiwadi Converter Station is located in the state of Rajasthan approximately 60km from Delhi City. Pole 1 of the ±500kV DC Transmission scheme is planned to be put in operation beginning of June 2009 whereas pole 2 is supposed to follow beginning December 2009. The project is owned and operated by Powergrid Corporation of India Ltd., a Govt. of India Enterprise. The paper deals with the required performance criteria and design studies of the Ballia-Bhiwadi HVDC transmission system. Furthermore it highlights major technical features and main components of the project including state of the art light triggered thyristors, control and protection systems, converter transformers, smoothing reactors, AC/DC filters and DC switches.

Index Terms—HVDC system, Power Transmission, Design Aspects, Performance Requirements Converter transformer, AC/DC filters, Thyristor valves

I. INTRODUCTION ECTION I of this paper gives an overview of the Ballia- S Bhiwadi HVDC power transmission system. Section II 1 Ballia- Bhiwadi and III deal with the design criteria and studies. In section IV 2 Talcher –Kolar (ESI Interconnector) the main equipment and the major technical features are 3 Rihand-Dadri described. Fig. 1. Long Distance HVDC Transmission Systems in India belonging to The ±500kV, 2500MW Ballia-Bhiwadi HVDC project Powergrid Coorporation of India Ltd. shall transmit energy from Ballia to Bhiwadi stations in India over about 780km. Ballia Converter Station is located in the state of Uttar Pradesh approximately 75km from Ballia District Head Quarter. The Bhiwadi Converter Station is located in the state of Rajasthan approximately 60km from Delhi City. Pole 1 of the ±500kV DC Transmission scheme is planned to be put in operation beginning of June 2009 whereas pole 2 is supposed to follow beginning December 2009. The project is owned and operated by Powergrid Corporation of India Ltd., a Govt. of India Enterprise. The Ballia-Bhiwadi system will then be one of three long distance HVDC schemes in operation or construction in India belonging to Powergrid Corporation of India Ltd. (Fig. 1). A single line diagram of the bipolar Ballia-Bhiwadi scheme is

Rajeev Kumar Chauhan is with Powergrid Corporation of India Ltd, Gurgaon (Haryana), India. Matthias Kuhn, Devinder Kumar, Andreas Kölz and Peter Riedel are with Siemens AG, PTD H1, Erlangen, Germany. 2

Ballia Converter Station DC Overhead Line Bhiwadi Converter Station outages, is 97%. 400 kV, 50 Hz Smoothing Reactor Smoothing Reactor 400 kV, 50 Hz AC System AC System In order to ensure the highest level of component and

Thyristor Thyristor system reliability and availability with minimal downtimes, Valves Valves 2 DC Filters: 2 DC Filters: fast fault detection, effective repair and maintenance strategies (DT12/24, (DT12/24, DT12/36) DT12/36) as well as fault-tolerant control systems, redundancy, spare Converter Converter components and quality assurance are required. To provide Transformer Transformer the highest quality of the HVDC control and protection system intensive off-side tests (e.g. functional performance

2 DC Filters: 2 DC Filters: test) will be performed. (DT12/24, (DT12/24, DT12/36) DT12/36) The performance requirements for dynamic response, reactive power exchange with ac system, overvoltage control, ac voltage distortion, equivalent disturbing current on the dc 3 Filter Banks: 3 Filter Banks: side, radio interference and audible noise have been considered in the system design as per the limits stated in the Owner's Technical specification. Noise filter equipment is provided for the ac switchyards and the dc lines in order to 4 AC Filters: 4 AC Filters: 3 AC Filters: 4 AC Filters: 4 AC Filters: 3 AC Filters: (DT12/24, (DT12/24, meet the specified power line carrier interference limits. (DT12/24, (DT12/24, (DT12/24, (DT12/24, DT12/36, DT12/36, ST12, ST24) DT12/36, DT12/36, ST12, ST24) ST12, ST24) ST12, ST24) 1 C-Shunt ST12, ST24) ST12, ST24) 2 C-Shunts Low loss design was of central importance for technical 1 C-Shunt 1 C-Shunt 1 Shunt Reactor 1 C-Shunt 1 C-Shunt 1 Shunt Reactor 1 Shunt React. and economical optimisations. This resulted in converter Fig. 2. Single Line Diagram of Ballia-Bhiwadi HVDC System station designs with total losses of approximately of 1.3% for both stations at 2500 MW of transmission power. At rated II. DESIGN CRITERIA transmission capacity the main loss sources within the converter station are the converter valves and the converter A. Power Transmission Capacity transformers. The bipolar dc system is rated for a continuous power of 2500 MW (±500 kV, 2500 A) at the dc terminals of the III. DESIGN STUDIES rectifier converter station. The HVDC scheme can be operated in bipolar mode and monopolar mode with ground return or A. Overview of Design Studies metallic return. For maximum ambient dry bulb temperature The design studies for Ballia-Bhiwadi HVDC project can of 50°C the converter stations are designed to transmit be classified in three groups. continuously full rated power without redundant cooling To the first group belong all studies, which results are system in service and for 2 hours an overload of 1.1 p.u. rated required as per the technical specification of the project, like power with redundant cooling in operation. For maximum main circuit parameter study, overvoltage, reactive power, ambient dry bulb temperature of 25°C the converter stations insulation co-ordination, ac/dc filter performance and rating are designed to transmit continuously 1.1 pu of rated power studies, ac breaker, dc switches and interference studies as an without redundant cooling system in service and 1.15 pu of example. These studies have been mainly finalised in July rated power with redundant cooling in service. For half-an- 2007. hour an overload of even up to 1.15pu (bipolar) or 1.2pu The second group of system studies, like the load flow and (monopolar) is possible up to maximum ambient dry bulb stability study, the sub-synchronous resonance and ac temperature. equivalent study, as well as the interaction study for existing The HVDC interconnection scheme is capable of nearby converter stations, affect the stability control continuous operation at any reduced dc voltage level from requirements of the interconnected ac/dc system and have 500 kV down to 350 kV (70%). At 80% dc voltage the been mainly finalized before start of functional and dynamic maximum dc current is 2250 A and at 70% dc voltage the performance tests. maximum dc current is 2145 A without redundant cooling The functional and dynamic performance tests as the third equipment in service. group are studies for control, protection and communication Although the normal power flow direction is from Ballia to which shall commence in mid 2008. Bhiwadi, the HVDC system is designed to transmit power in B. Reactive Power Management the reverse direction. The reactive power compensation elements have been B. Performance Requirements designed to comply with the specified absorption and supply The maximum specified equivalent outage frequency requirements as well as with the specified maximum voltage (EOF= number of one pole outages x 1+ number of other pole change after switching i.e. 3.5% and the maximum size of outages x 1+ number of bipole outages x 2) is 10. subbanks of 150 MVAr. The guarantied energy availability per year of the complete In order to satisfy the maximum reactive power demand of bipole averaged during the three years availability guarantee the converters up to the 2hour-overload and for minimum ac period, considering both forced and scheduled maintenance voltages and frequencies with one subbank out of service, in 3 total 1904 MVAr and 2054 MVAr (at 400kV) are necessary in - deionized water cooling of thyristors, direct water cooled Ballia and Bhiwadi respectively (Fig. 2, Table 1). snubber resistors and valve reactors A special control mode with increased firing/extinction - wire-in-water technology for snubber resistors angles the reactive power consumption of the dc converter can - exclusive use of fire retardant insulating material and wide be increased in order to limit the reactive power flow into the spacing for thermal separation of components ac systems. Available shunt reactors at respective converter - 5 inch LTTs station may also be used to limit reactive power exchanges The same valve design is adopted for the rectifier and with the grid under certain AC / DC system conditions. inverter station. The thyristor valves of Ballia-Bhiwadi project are arranged in three twin towers for one pole same as for C. Overvoltage (OV) Control Tian-Guang and Gui-Guang I and II projects (Fig. 3). One The overvoltage condition of the AC system may faced twin tower represents one quadrivalve comprising the four during recovery periods. The impact of overvoltages are valves connected to the same ac phase. Each of the four valves minimised by the strategy of restarting the dc system and in one quadrivalve structure consists of two and a half restoring the power transfer to the predisturbance level as modular units. Thus one tower comprises 10 modular units. soon as possible. Furthermore an overvoltage control has been Each valve modular unit in turn includes two valve sections established which prevents the ac bus voltages to exceed the connected in series and each valve section comprises 15 specified limits in order to protect the equipment and at the thyristor levels. Therefore a thyristor valve for Ballia-Bhiwadi same time avoids unnecessary filter and shunt capacitor project with two and a half modular units comprises five valve switching. It is to note that the OV control strategy will sections with 75 thyristors connected in series. A valve section prevent self-excitation of generators in the ac systems as well. also includes the thyristor heat sinks, a clamping structure, the It comprises: snubber circuits, thyristor voltage monitoring boards, valve 1. Re-Start Strategy of the converters incoordination with reactors and a steep front grading capacitor. The snubber AC system recoveries. circuits consist of the series connection of one single capacitor 2. Normal Voltage Limit Control (Voltage Dependent and one resistor with wire-in-water technology for the most Interlocking strategy for control and protective actions efficient cooling possible. and Sequential Switching) 3. Fast Overvoltages Limitation Control (Voltage Dependent Filter/Shunt switching) 4. Control to prevent self-excitation of generators D. Insulation Co-ordination In the insulation co-ordination of Ballia-Bhiwadi project the basic insulation levels of the equipment, the arrangement and ratings of the arresters and the requirements of air clearances and creepage distances have been defined for indoor as well as outdoor equipments. With respect to the creepage distances the design is based on the assumption of heavily polluted environmental conditions at both converter stations. Fig. 3. Existing thyristor valves at the Anshun station of the Guizhou- Guangdong I transmission project The towers are suspended from the valve hall roof and all IV. MAIN EQUIPMENT AND MAJOR TECHNICAL FEATURES joints between modules like suspension insulators, buswork, A. Thyristor Valves and Valve Base Electronic (VBE) and piping are flexibly designed for bearing maximum seismic stresses. Cooling water and fiber optics are entering the valve Ballia-Bhiwadi project will be the first HVDC project in structure from the top. The aluminum frame of the modules India using the modern state of art technology of direct light- and the large electrode trays at the bottom act as a corona triggered thyristors (LTT) with integrated overvoltage shield. The thyristors can be replaced without opening any protection eliminating the need for electronic logic at high water connections. All non-metallic materials used were potential [1]. Keeping the number of components as small as selected in order to minimize the risks of destructive fires. possible without neglecting protection and monitoring aspects Capacitors are filled with insulating gases, thus the insulating results in high reliability, as well as compact and economical oil was eliminated which used to be a major risk of fire. thyristor valves with little maintenance requirements. The Plastic materials for tubing and insulation have flame excellent operating performance of LTTs has been already retardant, self-extinguishing characteristics. These measures demonstrated in the HVDC schemes of Pacific Intertie, Moyle combined with good aeration of all components and a fast fire Interconnector, Gui-Guang I and II, Basslink and Neptune detection system make it extremely difficult to envisage a project. credible fault scenario resulting in a serious fire. The valve design is characterised by the following features: The valve base electronics (VBE) includes all equipment - modular design with stacked thyristors and heat sinks 4 necessary for thyristor firing and thyristor monitoring. The C. Converter Transformer VBE receives signals from the pole control which are The converter transformer configuration comprises four processed and converted into light pulses for the turn-on of (including one spare) single-phase three-winding transformers the thyristors. The light pulses for one valve section are for every pole. Therefore 16 transformers are required in total. generated by three laser diodes (one of them being redundant) All transformers including theirs bushings are arranged and transmitted via separate fibre optic cables to a Multimode outside so that the spare unit is in hot stand bye mode. In case Star Coupler (MSC) situated in the valve modular unit. There of an irregularity the station configuration allows putting the the light firing pulses are distributed to the individual thyristor spare unit in service within a few hours. To improve the low levels via separate fibres. The VBE also converts the optical maintenance design the transformers are equipped with signals received from the thyristor monitoring board to vacuum on load tap changers from Maschinenfabrik electrical signals. The VBE is a maintenance free system. Reinhausen. The selection of on load tap changer range is B. Control and Protection System adapted to the requirements about ac voltage variation range, reduced dc voltage operation and valve capability of operating For the Ballia-Bhiwadi HVDC transmission system Win- at high firing angles [2]. The transformer leakage impedances TDC, the actual state-of-the-art technology from Siemens in were determined by taking several factors into consideration the field of HVDC controls and protections will be used. Win- like permissible short-circuit current of the thyristor used, TDC is based on the SIMATIC WinCC Human Machine optimized ratio between rating and construction cost etc. The Interface (HMI) and the SIMATIC TDC (Technology and converter transformers have the following main data (same for Drive Control) control system which leads to the name Win- Ballia and Bhiwadi as specified): TDC. The system is already successfully in operation for the Rated power [MVA] 498 Basslink [3] and Neptune HVDC transmission systems. Rated Voltages [kV]: Based on well established and widely used industry process - line winding 400/√3 controllers and due to the hot standby redundancy configuration, Win-TDC realizes a high degree of reliability - valve wye winding 211.1/√3 and performance while guaranteeing a long product lifecycle - valve delta winding 211.1 and professional support. Since the mid 1980's Siemens has Leakage Reactance 17 % applied the powerful SIMADYN D and SIMATIC control Tap Changer Range -6.6% to +19.8% system technology to realize the HVDC control and protection Step Size 0.825% system for many HVDC systems worldwide [4]. Combining Insulation Levels [kV]: this experience with recent technology developments in the - line side LIWL 1300 field of industrial controls, Win-TDC provides the following - valve wye side LIWL 1550 improvements compared to its predecessor: - valve delta winding LIWL 1050 • High integration and processing power leads to a D. Smoothing Reactors reduction of processor boards and components and thus to A 250 mH smoothing reactor(s) per pole is provided to a significant saving of space and improving reliability. avoid resonances at low order harmonics taking the different • Fast communication links allow an independent, central dc circuit configurations including dc filter outage into and redundant measuring system, resulting in a highly account. Further tasks of the smoothing reactors are to limit reliable design the transient overcurrents caused by dc side faults or • The use of Microsoft Windows® based systems for all commutation failures, to avoid discontinuous current operator control, monitoring and engineering purposes operation at low dc currents especially at 70 % dc voltage enhances acceptance of users and reduces training efforts. operation with high firing angles and to reduce the dc side as One major innovation of Win-TDC is the pole related well as ac side harmonics. The smoothing reactors are of air central measuring system connected to Pole Controls and DC core type and have following main data: Protections. It provides the interface to the Siemens hybrid Inductance 250 mH optical DC measuring system as well as to the AC values Rated Voltage 512 kV dc required by the HVDC control and protection system. The AC Rated Current 2500 A dc and DC system quantities are transmitted to the various Insulation Level 1425 kV LIWL to ground control and protection processors via a high-speed optical 850 kV LIWL terminal-terminal Time Division Multiplexing (TDM) bus. This design significantly reduces the complexity of the system thus E. DC Breakers enhancing maintainability and reducing space consumption. The metallic return transfer breaker (MRTB) and ground For programming the HVDC control and protection return transfer breaker (GRTB) located in the dc yard of the systems a powerful standard function block library is used. It Ballia converter station are designed to allow transfer from allows graphical programming and enables a high integration ground return operation to metallic return operation and vice of control and protection functions while maintaining versa up to dc currents for operation with up to 1.2 pu redundancy [4]. overload without interruption of transmission. The MRTB and 5

GRTB are of the proven design as used in ESI Interconnector V. REFERENCES and Gui-Guang projects. The passive design which comprises [1] J. Holweg, H.P. Lips, Q.B. Tu, M. Uder, Peng Baoshu, Zhang Yeguang, a dc high speed switch (DCHSS), i.e. a single phase unit of an “Modern HVDC Thyristor Valves for China's Electric Power System," ac breaker appropriately modified for the dc application, and in Proc. 2002 IEE-PES/CSEE International Conference on Power Systems Conf. additional equipment like reactor, capacitor, energy absorber [2] K. Eckholz, P. Heinzig, “HVDC-Transformers - A Technical for the required current transfer. Fig. 4 shows the principal Challenge," in Proc. 2002 IEE-PES/CSEE International Conference on arrangement for the MRTB and GRTB. Power Systems Conf. [3] Dr. M. Davies, A. Kölz, M. Kuhn, D. Monkhouse, J. Strauss – "Latest Control and Protection Innovations Applied to the Basslink HVDC Lp Cp Rp Interconnector", in Proc. 2006 IEE ACDC Conf. [4] Georg Wild, “Win-TDC The New Powerful HVDC Control and DCHSS Protection System”, CIGRE-Colloquium on Role of HVDC, FACTS and Arrester Emerging Technologies in Evolving Power Systems, 17-24 September 2005, Bangalore, India Fig. 4. Principal Arrangement of MRTB and GRTB F. AC Filters The following performance requirements are specified - individual harmonic distortion Dn ≤ 1.0 % - total harmonic distortion D ≤ 4.0 % - total effective distortion Deff ≤ 3.0 % - telephone influence factor TIF ≤ 40 - generator harmonic current content Ig ≤ 1.0 % - arithmetic sum of 5th and 7th harmonic currents in any generator Ig5/7 ≤ 0.6 % The performance limits shall be met during whole load range with any subbank out of service. The required compensation equipment in order to meet the performance is listed in Table 1. The assembly of the filter types are shown in Fig. 5. Type A B C D E L Ballia 3 2 3 3 3 1 Bhiwadi 3 2 3 3 4 2 Table. 1. AC Filters and Shunt Reactors for Ballia-Bhiwadi

L A B C D E C-Shunt L-Shunt DT12/24 DT12/36 ST 12 ST24 72.6MVAr 120MVA 97MVAr 150 MVAr 150 MVAr 150 MVAr

C1 C1 C1 C1 C1 Arr Arr L1 R1 Fac1 L1 R1 Fac1

L2 C2 L2 C2

Fig. 5. AC Filter types for Ballia and Bhiwadi G. DC Filters Two double-tuned dc filters as shown in Fig. 6 are installed in every converter pole in order to reduce harmonic currents flowing in the dc lines. HVDC-Bus

C1

Arr L1 Fdc1

Arr Fdc2 L2 C2

Neutral Bus Fig. 6. DC Filters for Ballia-Bhiwadi (DT12/24 and DT12/36)