R.K. Chauhan, Power Grid Corporation of India Limited, India & M. Kuhn, Siemens AG, Germany
India’s desire for electricity continues to skyrocket, but capcity increases are not keeping pace. According to the Ministry of Power’s latest figures, in the period 2007-2008 peak demand was 109 GW while peak generation was only 91 GW, representing a deficit of over 16 per cent. However the government has an ambitious plan to add 78 GW of capacity by 2012, not only in generation, but also in the transmission of electricity.
Two of Power Grid Corporation of India’s long distance HVDC schemes that bring welcome extra capacity
One such project is the ±500 kV, 2500 MW Ballia-Bhiwadi HVDC project will transmit energy from Ballia in the state of Uttar Pradesh in the east of the country, to Bhiwadi in the state of Rajasthan. This inter-regional power link covers a distance of 780 km, and once operational later this year will help strengthen the power supply to the growing region around the megacity of New Delhi without the need to build additional power plants.
Pole 1 of the ±500 kV DC transmission scheme is scheduled to go into operation the beginning of June 2009, while pole 2 is anticpated to follow around the beginning December.
The project is owned and operated by state-owned Power Grid Corporation of India, and is one of its three long-distance HVDC schemes in operation or construction in India by Power Grid Corporation.
In the following article we look at the design criteria of the transmission system and highlight some of its main technical features.
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 both bipolar and monopolar mode, with ground return or metallic return. At a maximum ambient dry bulb temperature of 50 °C, the converter stations are designed to continuously transmit full-rated power without a redundant cooling system in service and for two hours an overload of 1.1 p.u. rated power with redundant cooling in operation is possible.
For a maximum ambient dry bulb temperature of 25 °C, the converter stations are designed to continuously transmit 1.1 pu of rated power without a redundant cooling system in service and 1.15 p.u. of rated power with a redundant cooling in service. For 30 minutes an overload of up to 1.15 p.u. (bipolar) or 1.2 p.u. (monopolar) is possible up to maximum ambient dry bulb temperature.
The Talcher station, which is part of the East-South HVDC link between Talcher in the state of Orissa and Bangalore in Karnataka State
The HVDC interconnection scheme is capable of continuous operation at any reduced DC voltage level from 500 kV down to 350 kV. At 80 per cent DC voltage the maximum DC current is 2250 A, while at 70 per cent 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 has been designed to transmit power in the reverse direction, if required.
The guaranteed power availability per year for the complete bipole, averaged over the three years availability guarantee period and taking both forced and scheduled maintenance outages into consideration, is 97 per cent.
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 offside tests (e.g. functional performance test) were performed.
The thyristor valves at the Anshum converter station of the Guizhou-Guangdong I HVDC project in China
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 taken into consideration 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 DC lines in order to meet the specified power line carrier interference limits.
Low loss design was of central importance to ensure technical and economical optimization. This resulted in a converter station design with total losses of approximately of 1.3 per cent for both stations at 2500 MW of transmission power. At the rated transmission capacity the main loss sources within the converter station are the converter valves and the converter transformers.
The design studies conducted for Ballia-Bhiwadi HVDC project can be divided in three groups.
The first group of studies include a main circuit parameter study, overvoltage, reactive power, insulation co-ordination, AC/DC filter performance and rating studies, AC breaker, DC switches and interference studies. The majority of these studies were finalized in July 2007.
The second group of system studies, such as a load flow and stability study, a sub-synchronous resonance and AC equivalent study, as well as the interaction study for existing nearby converter stations, which can affect the stability control requirements of the interconnected AC/DC system, were finalized prior to the functional and dynamic performance tests beginning.
The functional and dynamic performance tests, as the third group, are studies focusing on control, protection and communication. The functional performance test was completed in December last year, and the dynamic performance test is currently underway, and due to be finalized in April.
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 per cent, and the maximum size of subbanks (150 MVAr).
In order to satisfy the maximum reactive power demand of the converters up to the two-hour overload and for minimum AC voltages and frequencies with one sub-bank out of service, 1904 MVAr and 2054 MVAr (at 400 kV) in total are necessary in Ballia and Bhiwadi, respectively.
A special control mode with increased firing/extinction angles enables the reactive power consumption of the DC converter to 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.
During recovery periods, an AC system can experience overvoltage conditions. The impact of overvoltage is minimized by a strategy of restarting the DC system and restoring the power transfer to the pre-disturbance level as soon as possible.
Furthermore, an overvoltage control has been established which prevents the AC bus voltages exceeding the specified limits in order to protect the equipment and at the same time avoids unnecessary filter and shunt capacitor switching. It should also be noted that the overvoltage control strategy will prevent self-excitation of generators in the AC systems as well.
Thyristor Valves and Valve Base Electronic
Ballia-Bhiwadi project will be the first HVDC project in India to use the state-of-the-art technology of direct light-triggered thyristors (LTT) with integrated overvoltage protection eliminating the need for electronic logic at high potential. Keeping the number of components as small as possible without neglecting protection and monitoring, results in high reliability, as well as compact and economical thyristor valves with little maintenance requirements. The very high operating performance of LTTs has already been demonstrated in a number of HVDC schemes, including the Guizhou-Guangdong I and II projects in China.
The valve design is characterized by the following features: a modular design with stacked thyristors and heat sinks, deionized water cooling of the thyristors, direct water cooled snubber resistors and valve reactors, wire-in-water technology for snubber resistors, and the exclusive use of fire retardant insulating material and wide spacing for thermal separation of components. 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, which is the same as used in the Guizhou-Guangdong I and II projects.
One twin tower represents one quadri-valve, comprising the four valves connected to the same AC phase. Each of the four valves in one quadri-valve structure consists of two and a half modular units. Thus, one tower comprises ten modular units. Each valve modular unit in turn includes two valve sections connected in series, with each valve section comprising15 thyristor levels. Thus, the thyristor valve for the Ballia-Bhiwadi project 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.
The towers are suspended from the valve hall roof and all joints between modules, such as suspension insulators, buswork and piping have a flexible design for bearing maximum seismic stresses. Cooling water and fiber optics enter the valve structure from the top, and the aluminum frame of the modules and the large electrode trays at the bottom serve 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, thereby eliminating the need for insulating oil, which can to be a major fire risk. 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 incident.
The valve base electronics (VBE) includes all necessary equipment for thyristor firing and thyristor monitoring, and is a maintenance free system.
The VBE receives signals from the pole control, which are processed and converted into light pulses to turn-on 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.
Control and Protection System
For the Ballia-Bhiwadi HVDC transmission system, Win-TDC, the state-of-the-art technology developed by Siemens in the field of HVDC controls and protections, will be used.
The Win-TDC system is already successfully in operation for the Basslink, a HVDC link that crosses the Bass Strait, connecting the Loy Yang power station in Victoria on the Australian mainland to the island of Tasmania, and the Neptune HVDC transmission systems in the US.
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 bus. This design significantly reduces the complexity of the system, and therefore enhances maintainability and reduces space consumption.
To programme 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.
The converter transformer configuration comprises four single-phase, three-winding transformers for every pole (including one spare). Thus 16 transformers are required in total.
All transformers, including their bushings are arranged outside so that the spare unit is in hot standby mode. In case of an irregularity the station configuration allows the spare unit to be put into service within a few hours.
To improve the low maintenance design the transformers are equipped with vacuum on-load tap changers manufactured by Maschinenfabrik Reinhausen GmbH. The selection of on-load tap changer range is adapted to the requirements of the AC voltage variation range, reduced DC voltage operation and valve capability of operating at high firing angles.
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, the smoothing reactors will limit the transient overcurrents caused by DC side faults or commutation failures, to avoid discontinuous current operation at low DC currents, especially at 70 per cent DC voltage operation with high firing angles, and to reduce both the DC and AC side harmonics. The smoothing reactors are of the air core type.
With its innovative design, once operational later this year, the Ballia-Bhiwadi power link will be able to provide high quality and reliable power. This is great news for the people of Delhi.