Chandrapur interconnection brings grid benefits

The transmission system of India is divided into five independent regions. Although the nominal frequency is 50 Hz, the regions usually operate at slightly different frequencies, so they are not normally synchronized. Interconnection between the regions offers a number of benefits, particularly where the regions to be linked exhibit different load patterns or a disproportionate mix of hydro and thermal generation. The Western and Southern regions of the Indian transmission network are two such systems.

The Western region has a large amount of thermal generation and a relatively small amount of hydroelectric generation. Considerable inefficiency results from the common practice of backing down the thermal generation during periods of low demand, which may arise as often as every few hours. Conversely, the Southern region has considerable storage-type hydroelectric generation and little thermal. Investment in an interconnector to join the two regions would allow the demand of the Southern region to be met partly by the Western region, conserving the resources stored in the reservoirs of the Southern region. Additionally, during high run-off in the Southern region, such as might occur during the monsoon season, surplus energy could be supplied to the Western region in order to conserve its coal resources.

An ac interconnection already existed via a 400 kV double circuit line from Chandrapur (Western region) to Ramagundam (Southern region). Its utilisation was low, since operational practicalities limited the power transfer to a maximum of 200 MW. Synchronous operation was insecure because the interconnection was relatively weak compared with the size of the two ac systems.

High voltage direct current (HVDC) transmission is an ideal way to link regions since its asynchronous nature allows controlled power transfer to take place under a wide range of ac system operating conditions. In this case the link will enable over 1000 MW to be transferred via the same 400 kV double circuit line. While offering the benefits of resource sharing, it has also restricted the need for additional generation in either region, itself an environmental benefit.

The station is configured as a back-to-back HVDC link and is situated at the Chandrapur end of the existing 180 km, 400 kV double-circuit ac lines between Chandrapur and Ramagundam.

Electrical arrangement

Since a back-to-back convertor station employs no dc transmission line, its parameters are determined solely by the design of the convertor equipment. The direct voltage and current were chosen to be 205 kV dc and 2475 A respectively, yielding a dc power rating slightly greater than 500 MW since the required power transfer is defined at the inverter ac busbars. These values represent an economic compromise between the necessity to minimize the reactive power requirements of the convertor, which requires a low reactance convertor transformer, and the need to incorporate sufficient reactance to limit the maximum valve fault current.

The convertor station is divided into two independent 500 MW poles having the same polarity, phasing and whose layouts mirror each other. To maximize reliability, equipment common to both poles is minimized. Valve halls are located on either side of a service building, which houses the control room and the auxiliary equipment. A 500 MW valve hall contains two 12-pulse convertors, each arranged as three quadrivalves having 54 thyristors per valve.

The convertor transformers are single-phase three-winding units, placed on each side of the valve hall, having horizontal bushings which protrude into it. This arrangement gives immunity from atmospheric over-voltages, and also serves to contain any radio frequency interference generated by the switching action of the convertors because the valve hall is constructed to form a Faraday cage.

High Frequency (HF) containment is further enhanced by connecting the convertors to the ac networks via HF filters. Lower order harmonics are controlled by shunt harmonic filters which also provide reactive power because they exhibit capacitive impedance at low frequencies. To conform with the limits specified for the reactive power exchanged with the ac networks, each side of each 500 MW pole requires a total of 424 Mvar of reactive power. This is configured as three double-damped filters to control characteristic harmonics and one C-type filter to control third harmonic, each of nominally 106 Mvar; an overall total for both sides of 16 filters supplying 1696 Mvar.

The short-circuit level at the Chandrapur Southern busbars may fall to as low as 2300 MVA while maintaining the capability to carry full load power. The Western ac system is stronger, exhibiting a minimum short-circuit level of 3000 MVA. Both ac voltage and frequency are subject to variation on both ac systems; for example power frequency on the Western system is observed to vary from 48 Hz to 51.5 Hz within a single day. The convertor station is required both to operate and to meet the steady state reactive power limits during these voltage and frequency variations.

Both valve halls and the central service building are located in the centre of the 400 kV ac switchyard, the buildings to the right containing the 11 kV and 415 V auxiliary switchgear and the standby diesel generators. The two 400 kV ac lines to Ramagundam enter in the bottom right hand corner of the switchyard and the connection to the Western region exits to the existing 400 kV ac switchyard in the upper right hand corner of the site.

Control arrangements

The operator controls the station by means of a mimic panel, whose general appearance reflects the contents of the single-line diagram.

The mimic panel is integrated into a local area network (LAN) linking it to a pair of workstations for event recording. Five bay controllers, one serving each 400 kV bus diameter, are connected to the LAN to accomplish control and monitoring of the switchyard. The convertor control equipment is serviced by separate special-purpose serial and parallel links. The LAN itself and its interface computers, including the serial links to convertor control, are duplicated.

Control of the convertors is divided into functional blocks called master control, reactive power control, pole control, valve control and valve base electronics.

Master control embodies the ac system-related functions, such as frequency control, power modulation and power demand override. Master control also coordinates the power orders for the two poles.

Pole control contains not only the fast closed-loop features which impose the static characteristics on the convertors, but also phase control, tapchanger control, start-up and shutdown sequencing, sub-synchronous damping control and also the specialized convertor protection functions which are duplicated.

The static characteristics imposed on the Chandrapur back-to-back scheme have been tailored to this application. Each section of the static characteristic is produced by one element of the phase locked oscillator per 12 pulse convertor. Normal operation takes place at point X. However, abnormal ac network voltage conditions may cause either characteristic to sweep along the other.

Valve control responds to signals from phase control by producing the pulse sequence which permits the thyristor gate units to trigger the thyristors. The pulse sequence carries thyristor junction temperature data which adjusts the sensitivity of the electronic protection located at each thyristor level. This temperature is calculated by a thermal analogue, which is a digital implementation of a mathematical model of the thermal characteristics of a worst-case thyristor and its cooling arrangement.

Valve base electronics (VBE) is an optical interface for sending firing information to the gating unit of each thyristor and receiving status data in return (Databack). It provides indications of failed (short-circuit) thyristors and non-functioning gate units. There is also an optical monitoring system for valve coolant leak detection.

Reactive power control acts to deliver the var exchange and harmonic performance required from the HVDC link. While coarse control of reactive power is achieved by switching ac filters, to conform with the tight reactive power limits (±50 Mvar on the Western system) requires the convertors to absorb excess Mvar generated by the filters.

Thus, even when meeting the harmonic performance criteria means energising more filters than the minimum number which provides adequate reactive power, the convertor station still does not export excessive Mvar to the ac network. This is achieved by operating the convertors at an artificially reduced direct voltage and increased direct current, thus maintaining ordered power but increasing the reactive power absorbed by the convertors. This mode of operation is called reactive power mode.

Temporary convertor action is also used during filter switching to limit the voltage step imposed on the ac system. The reactive power controller meets the reactive power limits by switching filters and varying the reactive absorption of the convertors. Reactive power control combines these features with strategies for controlling the convertors and ac filters to limit ac system voltages under temporary and fault conditions.

The convertor control system utilises the GEC Alsthom Series IV logic controller. Different applications, such as reactive power control and the pole control command system (PCCS) communicate via optical links.

Additionally, all Series IV control applications report their status to the master processor monitor (MPM) via optical links. MPM is a computer-based diagnostic tool which monitors the software systems to which it is connected.

It also incorporates monitoring and recording of the status of every thyristor level in every valve, using the information provided by Databack via VBE. MPM also has secure facilities for exercising the hardware and software of the control applications in order to perform off-line testing and for parameter adjustment.

The HVDC control system design was validated at GEC Alsthom's power electronics technology base at Stafford, UK, using both analogue and digital simulators. An analogue simulator was used to establish the control parameters for the fast low level features which confer stable operation and good fault recovery on the link. A real time digital simulator (RTDS) was used to carry out a detailed examination of both fast and slow control/power system interactions, because it has the capacity to model more comprehensively the ac systems to which the convertor station is connected.

A model of the adjacent Chandrapur-Padghe 2 x 750 MW HVDC transmission project was implemented at the same time on the RTDS in order to search out possible control interactions between the two projects. An extensive series of tests was performed in order to ensure that neither HVDC project had an adverse effect on the other during steady state or transient conditions.

Reliable power

The lack of synchronism between the Western and Southern regions of the Indian power system limits the power transfer capability and security of an existing ac interconnection, allowing only limited power exchange to take place when conditions permitted. The Chandrapur 2 x 500 MW HVDC back-to-back link provides an asynchronous link between the two systems to enable power to be exchanged at will, in a reliable and controllable manner.

The design of the Chandrapur HVDC back-to-back link incorporates a number of novel features which are specially tailored to this location. The use of the convertors to provide fine control of the reactive power exported to the ac system allows the HVDC link to continue to transfer power under all the combinations of operating circumstances which fall within the boundaries of the specified conditions.

As a result of this flexibility, the dc link is able to exchange at least 1000 MW of power under a wide range of ac system operating conditions.