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

Transcription

1 1 Basic Design Aspects of Ballia-Bhiwadi 2500MW HVDC Power Transmission System R.K. Chauhan, M. Kuhn, D. Kumar, A. Kölz, P. Riedel 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 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. shown in Fig.2. Index Terms HVDC system, Power Transmission, Design Aspects, Performance Requirements Converter transformer, AC/DC filters, Thyristor valves S I. INTRODUCTION ECTION I of this paper gives an overview of the Ballia- Bhiwadi HVDC power transmission system. Section II and III deal with the design criteria and studies. In section IV the main equipment and the major technical features are described. The ±500kV, 2500MW Ballia-Bhiwadi HVDC project 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 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 1 Ballia- Bhiwadi 2 Talcher Kolar (ESI Interconnector) 3 Rihand-Dadri Fig. 1. Long Distance HVDC Transmission Systems in India belonging to Powergrid Coorporation of India Ltd. 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 2 Ballia Converter Station 400 kv, 50 Hz AC System Converter Transformer 3 Filter Banks: Thyristor Valves Smoothing Reactor 3 AC Filters: 1 Shunt Reactor DC Overhead Line Bhiwadi Converter Station Smoothing Reactor 3 Filter Banks: Thyristor Valves 1 Shunt React. Fig. 2. Single Line Diagram of Ballia-Bhiwadi HVDC System II. DESIGN CRITERIA 400 kv, 50 Hz AC System Converter Transformer 3 AC Filters: 2 C-Shunts 1 Shunt Reactor A. Power Transmission Capacity The bipolar dc system is rated for a continuous power of 2500 MW (±500 kv, 2500 A) at the dc terminals of the rectifier converter station. The HVDC scheme can be operated in bipolar mode and monopolar mode with ground return or metallic return. For maximum ambient dry bulb temperature of 50 C the converter stations are designed to transmit continuously full rated power without redundant cooling system in service and for 2 hours an overload of 1.1 p.u. rated power with redundant cooling in operation. For maximum ambient dry bulb temperature of 25 C the converter stations are designed to transmit continuously 1.1 pu of rated power without redundant cooling system in service and 1.15 pu of rated power with redundant cooling in service. For half-anhour an overload of even up to 1.15pu (bipolar) or 1.2pu (monopolar) is possible up to maximum ambient dry bulb temperature. The HVDC interconnection scheme is capable of continuous operation at any reduced dc voltage level from 500 kv down to 350 kv (70%). At 80% dc voltage the maximum dc current is 2250 A and at 70% dc voltage the maximum dc current is 2145 A without redundant cooling equipment in service. Although the normal power flow direction is from Ballia to Bhiwadi, the HVDC system is designed to transmit power in the reverse direction. B. Performance Requirements The maximum specified equivalent outage frequency (EOF= number of one pole outages x 1+ number of other pole outages x 1+ number of bipole outages x 2) is 10. The guarantied energy availability per year of the complete bipole averaged during the three years availability guarantee period, considering both forced and scheduled maintenance outages, is 97%. In order to ensure the highest level of component and system reliability and availability with minimal downtimes, fast fault detection, effective repair and maintenance strategies as well as fault-tolerant control systems, redundancy, spare components and quality assurance are required. To provide the highest quality of the HVDC control and protection system intensive off-side tests (e.g. functional performance test) will be performed. The performance requirements for dynamic response, reactive power exchange with ac system, overvoltage control, ac voltage distortion, equivalent disturbing current on the dc 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 meet the specified power line carrier interference limits. Low loss design was of central importance for technical and economical optimisations. This resulted in converter station designs with total losses of approximately of 1.3% for both stations at 2500 MW of transmission power. At rated transmission capacity the main loss sources within the converter station are the converter valves and the converter transformers. III. DESIGN STUDIES A. Overview of Design Studies The design studies for Ballia-Bhiwadi HVDC project can be classified in three groups. To the first group belong all studies, which results are required as per the technical specification of the project, like main circuit parameter study, overvoltage, reactive power, insulation co-ordination, ac/dc filter performance and rating studies, ac breaker, dc switches and interference studies as an example. These studies have been mainly finalised in July The second group of system studies, like the load flow and stability study, the sub-synchronous resonance and ac equivalent study, as well as the interaction study for existing nearby converter stations, affect the stability control requirements of the interconnected ac/dc system and have been mainly finalized before start of functional and dynamic performance tests. The functional and dynamic performance tests as the third group are studies for control, protection and communication which shall commence in mid B. Reactive Power Management The reactive power compensation elements have been designed to comply with the specified absorption and supply requirements as well as with the specified maximum voltage change after switching i.e. 3.5% and the maximum size of subbanks of. In order to satisfy the maximum reactive power demand of the converters up to the 2hour-overload and for minimum ac voltages and frequencies with one subbank out of service, in

3 3 total 1904 MVAr and 2054 MVAr (at 400kV) are necessary in Ballia and Bhiwadi respectively (Fig. 2, Table 1). A special control mode with increased firing/extinction angles the reactive power consumption of the dc converter can be increased in order to limit the reactive power flow into the ac systems. Available shunt reactors at respective converter station may also be used to limit reactive power exchanges with the grid under certain AC / DC system conditions. C. Overvoltage (OV) Control The overvoltage condition of the AC system may faced during recovery periods. The impact of overvoltages are minimised by the strategy of restarting the dc system and restoring the power transfer to the predisturbance level as soon as possible. Furthermore an overvoltage control has been established which prevents the ac bus voltages to exceed the specified limits in order to protect the equipment and at the same time avoids unnecessary filter and shunt capacitor switching. It is to note that the OV control strategy will prevent self-excitation of generators in the ac systems as well. It comprises: 1. Re-Start Strategy of the converters incoordination with AC system recoveries. 2. Normal Voltage Limit Control (Voltage Dependent Interlocking strategy for control and protective actions 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. IV. MAIN EQUIPMENT AND MAJOR TECHNICAL FEATURES A. Thyristor Valves and Valve Base Electronic (VBE) Ballia-Bhiwadi project will be the first HVDC project in India using the modern state of art technology of direct lighttriggered thyristors (LTT) with integrated overvoltage protection eliminating the need for electronic logic at high potential [1]. Keeping the number of components as small as possible without neglecting protection and monitoring aspects results in high reliability, as well as compact and economical thyristor valves with little maintenance requirements. The excellent operating performance of LTTs has been already demonstrated in the HVDC schemes of Pacific Intertie, Moyle Interconnector, Gui-Guang I and II, Basslink and Neptune project. The valve design is characterised by the following features: - modular design with stacked thyristors and heat sinks - deionized water cooling of thyristors, direct water cooled snubber resistors and valve reactors - wire-in-water technology for snubber resistors - exclusive use of fire retardant insulating material and wide spacing for thermal separation of components - 5 inch LTTs The same valve design is adopted for the rectifier and inverter station. The thyristor valves of Ballia-Bhiwadi project are arranged in three twin towers for one pole same as for Tian-Guang and Gui-Guang I and II projects (Fig. 3). One twin tower represents one quadrivalve comprising the four valves connected to the same ac phase. Each of the four valves in one quadrivalve structure consists of two and a half modular units. Thus one tower comprises 10 modular units. Each valve modular unit in turn includes two valve sections connected in series and each valve section comprises 15 thyristor levels. Therefore a thyristor valve for Ballia-Bhiwadi project with two and a half modular units comprises five valve sections with 75 thyristors connected in series. A valve section also includes the thyristor heat sinks, a clamping structure, the snubber circuits, thyristor voltage monitoring boards, valve reactors and a steep front grading capacitor. The snubber circuits consist of the series connection of one single capacitor and one resistor with wire-in-water technology for the most efficient cooling possible. 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 joints between modules like suspension insulators, buswork, and piping are flexibly designed for bearing maximum seismic stresses. Cooling water and fiber optics are entering the valve structure from the top. The aluminum frame of the modules and the large electrode trays at the bottom act as a corona shield. The thyristors can be replaced without opening any water connections. All non-metallic materials used were selected in order to minimize the risks of destructive fires. Capacitors are filled with insulating gases, thus the insulating oil was eliminated which used to be a major risk of fire. Plastic materials for tubing and insulation have flame retardant, self-extinguishing characteristics. These measures combined with good aeration of all components and a fast fire detection system make it extremely difficult to envisage a credible fault scenario resulting in a serious fire. The valve base electronics (VBE) includes all equipment

4 4 necessary for thyristor firing and thyristor monitoring. The VBE receives signals from the pole control which are processed and converted into light pulses for the turn-on of the thyristors. The light pulses for one valve section are generated by three laser diodes (one of them being redundant) and transmitted via separate fibre optic cables to a Multimode Star Coupler (MSC) situated in the valve modular unit. There the light firing pulses are distributed to the individual thyristor levels via separate fibres. The VBE also converts the optical signals received from the thyristor monitoring board to electrical signals. The VBE is a maintenance free system. B. Control and Protection System For the Ballia-Bhiwadi HVDC transmission system Win- TDC, the actual state-of-the-art technology from Siemens in the field of HVDC controls and protections will be used. Win- TDC is based on the SIMATIC WinCC Human Machine Interface (HMI) and the SIMATIC TDC (Technology and Drive Control) control system which leads to the name Win- TDC. The system is already successfully in operation for the Basslink [3] and Neptune HVDC transmission systems. Based on well established and widely used industry process controllers and due to the hot standby redundancy configuration, Win-TDC realizes a high degree of reliability and performance while guaranteeing a long product lifecycle and professional support. Since the mid 1980's Siemens has applied the powerful SIMADYN D and SIMATIC control system technology to realize the HVDC control and protection system for many HVDC systems worldwide [4]. Combining this experience with recent technology developments in the field of industrial controls, Win-TDC provides the following improvements compared to its predecessor: High integration and processing power leads to a reduction of processor boards and components and thus to a significant saving of space and improving reliability. Fast communication links allow an independent, central and redundant measuring system, resulting in a highly reliable design The use of Microsoft Windows based systems for all operator control, monitoring and engineering purposes enhances acceptance of users and reduces training efforts. One major innovation of Win-TDC is the pole related central measuring system connected to Pole Controls and DC Protections. It provides the interface to the Siemens hybrid optical DC measuring system as well as to the AC values required by the HVDC control and protection system. The AC and DC system quantities are transmitted to the various control and protection processors via a high-speed optical Time Division Multiplexing (TDM) bus. This design significantly reduces the complexity of the system thus enhancing maintainability and reducing space consumption. For programming the HVDC control and protection systems a powerful standard function block library is used. It allows graphical programming and enables a high integration of control and protection functions while maintaining redundancy [4]. C. Converter Transformer The converter transformer configuration comprises four (including one spare) single-phase three-winding transformers for every pole. Therefore 16 transformers are required in total. All transformers including theirs bushings are arranged outside so that the spare unit is in hot stand bye mode. In case of an irregularity the station configuration allows putting the spare unit in service within a few hours. To improve the low maintenance design the transformers are equipped with vacuum on load tap changers from Maschinenfabrik Reinhausen. The selection of on load tap changer range is adapted to the requirements about ac voltage variation range, reduced dc voltage operation and valve capability of operating at high firing angles [2]. The transformer leakage impedances were determined by taking several factors into consideration like permissible short-circuit current of the thyristor used, optimized ratio between rating and construction cost etc. The converter transformers have the following main data (same for Ballia and Bhiwadi as specified): Rated power [MVA] 498 Rated Voltages [kv]: - line winding 400/ 3 - valve wye winding 211.1/ 3 - valve delta winding Leakage Reactance 17 % Tap Changer Range -6.6% to +19.8% Step Size 0.825% Insulation Levels [kv]: - line side LIWL valve wye side LIWL valve delta winding LIWL 1050 D. Smoothing Reactors A 250 mh smoothing reactor(s) per pole is provided to avoid resonances at low order harmonics taking the different dc circuit configurations including dc filter outage into account. Further tasks of the smoothing reactors are to limit the transient overcurrents caused by dc side faults or commutation failures, to avoid discontinuous current operation at low dc currents especially at 70 % dc voltage operation with high firing angles and to reduce the dc side as well as ac side harmonics. The smoothing reactors are of air core type and have following main data: Inductance 250 mh Rated Voltage 512 kv dc Rated Current 2500 A dc Insulation Level 1425 kv LIWL to ground 850 kv LIWL terminal-terminal E. DC Breakers The metallic return transfer breaker (MRTB) and ground return transfer breaker (GRTB) located in the dc yard of the Ballia converter station are designed to allow transfer from ground return operation to metallic return operation and vice versa up to dc currents for operation with up to 1.2 pu overload without interruption of transmission. The MRTB and

5 5 GRTB are of the proven design as used in ESI Interconnector and Gui-Guang projects. The passive design which comprises a dc high speed switch (DCHSS), i.e. a single phase unit of an ac breaker appropriately modified for the dc application, and additional equipment like reactor, capacitor, energy absorber for the required current transfer. Fig. 4 shows the principal arrangement for the MRTB and GRTB. Lp Cp DCHSS ester Rp Fig. 4. Principal angement of MRTB and GRTB V. REFERENCES [1] J. Holweg, H.P. Lips, Q.B. Tu, M. Uder, Peng Baoshu, Zhang Yeguang, Modern HVDC Thyristor Valves for China's Electric Power System," in Proc IEE-PES/CSEE International Conference on Power Systems Conf. [2] K. Eckholz, P. Heinzig, HVDC-Transformers - A Technical Challenge," in Proc IEE-PES/CSEE International Conference on 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 Interconnector", in Proc IEE ACDC Conf. [4] Georg Wild, Win-TDC The New Powerful HVDC Control and Protection System, CIGRE-Colloquium on Role of HVDC, FACTS and Emerging Technologies in Evolving Power Systems, September 2005, Bangalore, India 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 Bhiwadi Table. 1. AC Filters and Shunt Reactors for Ballia-Bhiwadi A DT12/24 120MVA B DT12/36 97MVAr C ST 12 D ST24 E C-Shunt L L-Shunt 72.6MVAr Fac1 L1 R1 Fac1 L1 R1 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 Fdc1 L1 Fdc2 Neutral Bus Fig. 6. DC Filters for Ballia-Bhiwadi (DT12/24 and