Dr. Asok Mukherjee, Siemens Power Transmission and Distribution, Germany
India’s East-South interconnector project is a key link in the country’s plan to have a fully interconnected national grid by 2012. Completed ahead of schedule in 2003, this 2000 MW project is the largest power transmission project in India.
Linking the Indian states of Karnataka and Orissa over a distance of some 1450 km, the East-South interconnector project is a state-of-the-art high voltage direct current (HVDC) transmission system. Completed last year, the link transfers a bulk power of up to 2000 MW from the Talcher power generation centre in the eastern part of India to Kolar near Bangalore, the hub of a rapidly developing industrial and high-tech area in the south.
The East-South interconnector project is the largest power transmission project in India, and was completed in record time and ahead of schedule by Siemens Power Transmission and Distribution Group (PTD) of Germany.
The East-South project is the eighth in a series of HVDC links in India, interconnecting its five asynchronous regional power grids, thus making a flexible power transfer between them based on supply and demand possible. Taking advantage of the local resources, the HVDC links have contributed successfully to the creation of a nation-wide grid, which when completed in 2012 will have 30 000 MW of inter-regional transmission capacity. This major project is being planned and overseen by Powergrid, the largest power transmission utility of India.
The HVDC alternative
The network planners of India have in this regard made use of the excellence of HVDC technology as the only alternative, when conventional alternating current transmission system proves neither technically nor economically feasible for interconnection of asynchronous (dissimilar) grids, and/or for power transmission over large distances between power generation and load centres.
An HVDC system operates on the principle of conversion of alternating current (AC) into direct current (DC) and vice versa by using converter valves. The East-South project is a long distance HVDC transmission system, with the rectifier (AC into DC) and inverter (DC into AC) stations very distant from each other. The rectifier station is located at Talcher in Orissa and the inverter station in Kolar near Bangalore.
At the heart of an HVDC system are its converter valves for AC-DC conversion. HVDC converter valves are made of powerful thyristors with high current ratings and steadily increasing blocking voltages. Figure 2 shows the completely assembled converters, arranged as three quadruple valves, for one pole of the East-South system. Three valve modules make one valve, and four such valves are needed for a quadruple valve. The 12 modules in total needed to make a quadruple valve are best arranged as a twin tower as can be seen in Figure 2. The number of thyristors per module (including redundancy) is 28 for Talcher, the rectifier station, and 26 for Kolar, the inverter station. The total number of thyristors for the complete bipole is thus 3888.
Figure 2. Completely assembled converters, arranged as three quadruple valves, for one pole of the Indian East-South HVDC transmission system
Figure 3. The three converter transformers of one pole of the Talcher station during erection. They are of single phase, three winding design, rated at 397 MVA. Six such transformers plus one spare belong to each station
A thyristor level consists of one 102 mm (4″), 8 kV flatpack thyristor, an individual snubber circuit comprising a snubber resistor and a capacitor, and the thyristor electronic card. The snubber circuit protects the thyristor against transient overvoltages and currents during turn-off and turn-on respectively, as well as limits the rate of rise of current (di/dt) at turn-on and of voltage (dv/dt) at turn-off. The thyristor electronics has multiple functions and comprises an opto-electronic system for conversion of light signal – coming from the valve base electronics via fiber optic – to an electrical signal for firing the thyristor. It has also an electronic logic system to determine not only that all requirements for triggering the thyristors are met, but also for monitoring the thyristor levels and specially protecting them, in case an individual thyristor does not receive a gate pulse due to an electronic failure.
As shown in Figure 2, the converter twin towers are suspended from a special ceiling construction of the valve hall, and all connecting components between the modules such as suspension insulators, buswork and pipings are of flexible design to ensure maximum seismic stress withstand capability.
Skill in design
Between the converter valves and the AC grids, on both sides of the 1400 km-long ±500 kV DC transmission line, are the converter transformers, another key component of an HVDC system. In the rectifier station at Talcher, they transform the eastern AC grid voltage of 400 kV down to a value, as is optimal for the converter valves, based on design calculations. In Kolar, where DC is converted back to AC, the converter transformers do the reverse, i.e. step up the voltage from the valve side to the level of the southern AC grid (also 400 kV), thus completing the interconnection. Converter transformers experience combined AC and DC stresses in the winding insulations, high harmonic content in the current and need special competence and skill in design, construction and testing, compared to conventional AC power transformers.
Figure 3 shows the three converter transformers of one pole of the Talcher station during erection. They are of single phase, three winding design, rated at 397 MVA. Six such transformers plus one spare belong to each station. Because of the identical design of the transformers, the spare one from one station can be used in the other on need. The valve side secondary bushings of the transformers – four in number – protrude into the valve hall, inside which the star-delta crossings and connections to the suspended valves are made.
Figure 4. The outdoor AC switchyard, where the AC harmonic filters are installed and connected to the 400 kV AC bus
Another key component is the smoothing reactor which limits the DC fault current and also suppresses the DC and AC harmonics to a permissibly low level. In the design phase, calculations considering different DC circuit configurations were carried out for an adequate dimensioning of the smoothing reactor, which is installed outside the valve hall and connected to the 500 kV DC valve hall bushing. Per station and per pole a 250 mH reactor was chosen. The smoothing reactor is of dry air-core type and comprises two coils of 125 mH each. The rated voltage is 500 kV DC, rated current 2000 A.
The harmonics mentioned above, which the smoothing reactor is supposed to limit, are a necessary evil of the current conversion process in the converter valves. The converters are sources of harmonics, which if allowed to infiltrate unhindered into the AC or DC systems, would distort the system voltage. The DC harmonics can be kept within specified levels by an adequately designed smoothing reactor in combination with DC harmonic filters. For absorption of the AC harmonics, AC harmonic filters are needed. They are tuned to the specific frequencies of the harmonics aimed for elimination. The AC harmonic filters are installed in the outdoor AC switchyard and connected to the 400 kV AC bus. The DC filters, located behind the smoothing reactor, are connected to the outgoing or incoming 500 kV DC line at the rectifier or the inverter station respectively.
A fully redundant and digitised powerful control and protection system has been implemented in the East-South system, that guarantees not only an optimally controlled energy transmission and adequate protection of all station components, but is also easy to handle and user-friendly, including an operator interface based on the most recent computer technology.
The control system maintains the power transmitted at the desired level, coordinating the switching of reactive power elements as per reactive power demand over the specified range of operation, optimising the power ramp-up or ramp-down rate with predefined values, and performs numerous other control and monitoring functions during dynamic power changes, based on predetermined AC system parameters. The protection system ensures selectively a safe disconnection and isolation of the faulty equipment, avoids unnecessary shut downs of the system, and prevents as far as possible damage to HVDC components caused by faults or overstresses. Down from the process level in the control and protection system hierarchy – where the important system parameters are measured, e.g. DC current, DC voltage – the data are passed on to the control level via a redundant optical field bus. This level comprises the AC and DC station control system and the pole control system for thyristor valves via valve base electronics.
Also, for fast exchange of control and protection data between the two converter stations at Talcher and Kolar, the telecontrol is connected directly to the related pole control. The information exchange between the operator control level at the top of the hierarchy and the control level below takes place by means of a redundant local area network (LAN). This operator control level is responsible mainly for the fully redundant AC and DC station initiation and system monitoring. Also, interface to remote control facilities from load dispatch centres and telecom interface to corresponding converter stations for exchange of monitoring data are integrated into this level. The eastern or the southern region load dispatch centre of the Indian grid can thus take over the control of the HVDC transmission system as needed.
Figure 5 shows the control room at Talcher converter station with visual display units and mimic boards. The complete hardware and software of the East-South control and protection system were subjected to intensive off-site testing in Germany. All control and protection functions as also the redundant systems could thus be thoroughly checked prior to shipment. This has reduced the on-site commissioning time contributing substantially to the completion of the project before schedule.
The East-South HVDC interconnector incorporates the second longest HVDC transmission line in the world. In operation since February 2003, a bulk amount of power is being transferred to the south via this link. Besides, by making use of the inherent dynamic power change possibilities through HVDC control when AC system disturbances or faults occur, this link will contribute to the stability of the existing regional AC grids. With the high reliability and availability of this HVDC transmission system, low losses and the long lifetime – all based on a high grade of redundancy and modern techniques developed out of decade long operational experiences – this link will serve as a vital element of the Indian national grid.
On February 14, 2003, the HVDC East-South interconnector project was dedicated to the nation by the Prime Minister of India, Mr. A. B. Vajpayee, in an inaugural ceremony at Kolar, in the state of Karnataka.