TRANSMISSION & DISTRIBUTION
Upgrade for a grid hot line1 March 2007
The transmission capacity of the ‘East-South’ long distance HVDC interconnector in India is to be upgraded from 2000 to 2500 MW by optimal utilisation of its overload margin using a novel line management solution.
The East-South long-distance HVDC transmission link between Orissa and Bangalore was completed in 2003 by main contractor Siemens PTD. It was designed for a power rating of 2000 MW at a transmission voltage of ±500 kV DC. In 2006 Powergrid Corporation of India awarded Siemens the contract to increase the transmission capacity of the link by 500 MW, a feat that was to be achieved solely by making use of the system’s maximum overload capacity of 2500 MW. The specified project completion time was 15 months.
Since 2003 the East-South Interconnector II (Figure 1) has linked the power generation centre of Talcher in Orissa in eastern India with the rapidly developing industrial region around Bangalore 1450 km away in the south. HVDC technology is the only reasonable way to transfer bulk power between the two regional asynchronous grids in the states of Orissa and Karnataka. It also exhibits relatively low power loss even over this great distance.
An HVDC connector consists essentially of two substations for the AC-DC-AC conversion and a DC transmission line joining them. At any given time one of the converter stations operates as the rectifier (ie the sending station) and the other as the inverter (at the receiving end). The main components of each converter are the thyristor valve groups, the converter transformers and smoothing reactors, and DC filter banks and reactive power elements. The AC yards at each end of course provide the connection to the AC grids and to the reactive power elements such as shunt reactors, AC filters and shunt capacitors. PLC filters attenuate the electrical interference, to ensure that the behaviour of the system is as environmentally friendly as possible.
To increase the capacity of the link, Siemens engineers found a way to utilise more effectively the overload capacity of the system without having to install additional components – a way to increase the DC transmission current by modelling the thermal characteristics of the power converter transformers and the smoothing reactors to ensure that the planned service life of the system at rated load is still attainable despite the increase in power. The ageing curve of the transformers and the reactors is calculated on the basis of this modelling. It is necessary also to monitor the fluid conditions in oil cooled components and compute the maximum possible load taking into account the oil temperature, the ambient temperature and the availability of cooling units.
Combined, these two techniques allow power transmission of between 2000 and 2500 MW on this line at the present transmission voltage of ± 500 kV DC for a substantially prolonged period.
The conventional method of increasing the power rating is to increase the transmission voltage or the line current, both of which require extensive and costly modifications of the system.
The adopted alternative solution is not usually found in HVDC systems. With the aid of software packages known as Relative Ageing Indication (RAI) and Load Factor Limitation (LFL) and additional hardware measures, it is possible to utilise the overload capacity of the system more effectively without installing additional thyristors connected in series or in parallel to increase the transmission voltage or the current respectively.
The new concept is based on the supervision of the relative thermal ageing rate of components like transformers or reactors which is mainly influenced by their steady-state hot-spot temperature. The steady-state hot-spot temperature depends on various factors such as the ambient temperature, the load level and the type of cooling. The relative ageing or relative loss of life follows from the summation of time intervals at which the equipment is operated with a certain relative ageing rate.
Each component of the East South interconnector has a certain overload capability depending on design, thermal time constant and heat dissipation conditions. An acceleration of ageing may occur if the design hot-spot temperature is exceeded. The objective therefore is by improving the system to prevent a deterioration of system elements due to higher load and to avoid a relative thermal ageing that would reduce the design lifetime.
Generally the weakest element characterises the overload capability of the whole system. Based on individual overload capability of the components, Siemens concluded that the converter transformers (Figure 2), smoothing reactors, AC PLC line traps (at one station) and the LV DC bushings were the most limiting system elements in extending overload.
The most economic solution was to replace only the PLC line traps and LV DC bushings but continue using the existing converter transformers and smoothing reactors together with an upgraded transformer cooling system and an air handling unit to cool each smoothing reactor. This additional cooling dissipates the extra heat generated during operation at high load. Meanwhile the on-line RAI&LFL system models the thermal behaviour of the converter transformers and smoothing reactors, indicates the relative ageing experienced and prevents the design values of hot-spot and oil temperatures being exceeded.
RAI and LFL systems
The RAI on-line system is incorporated within the human machine interface, ie the control room., while the LFL system is implemented within the pole control. An off-line software based load scheduling tool (LST) assists the operator in preparing the long term load scheduling. This tool calculates the loss of expected life and the maximum hot-spot temperatures for the entire future scheduled period. It indicates whether the load scheduling conforms to the limits of loss of life of the equipment. The relative ageing that the equipment has already experienced is considered. This scheduled load profile can be exported from the off-line tool and can be used as the daily schedule file for the HMI power scheduler.
The standard HMI power scheduler is used to execute the daily load profile. The operator may enable the power scheduler in automatic power mode, where the power order follows the set of reference values as exported from the off-line load scheduling tool.
Irrespective of whether the power scheduler is enabled or not, the RAI system indicates an on-line value for the relative ageing of the converter transformers and smoothing reactors that they have experienced (historical values) and the relative ageing at the end of the pre-set load curve (future values). Should a hot-spot temperature limit be exceeded, a warning will be displayed to the operator. He can take the appropriate measures such as re-scheduling of the pre-set load curve.
The on-line pole control LFL system serves as a protection function for the converter transformers by the real-time supervision of the hot-spot temperatures based on the actual measurements. It calculates a maximum permissible load factor depending on recorded ambient and top-oil temperatures, actual load, operational status of cooling devices and permissible internal hard limits for hot-spot temperatures. If the hot-spot thresholds are exceeded, the LFL system limits the actual power to a permissible load factor, overriding relative ageing considerations.
Cooling of smoothing reactors
The forced air cooling of smoothing reactors is a novel way to enhance the duration and magnitude of overload in existing units (Figure 3). In comparison to procurement of new reactors, substantial savings in cost can be achieved. The cooling of each pair of smoothing reactors is performed by an air handling unit as shown during the testing at the coil manufacturer’s factory (Figure 4). The design target is to achieve a lowering of the hot-spot temperature by at least 10K with sufficient air flow even at an ambient temperature of 50°C.
Together, these measures ensure that the planned service life of the system at rated load is still attainable despite the increase in power.
In addition a retrofit of the AC yards at both converter stations by new AC filter banks will be carried out in order to satisfy the increased reactive power consumption. The open and closed-loop control concept and the protection concept will be adapted to the new operating mode of the HVDC transmission system as part of the upgrade, and the system’s own auxiliary power supply is to be augmented.
In addition to the RAI&LFL system, Siemens is also retrofitting a number of components to increase the performance of the system. These include new cooling units for the power converter transformers at both stations designed to dissipate the higher heat in operation, and additional alternating current filter banks to cover the increased reactive power. Further, new reactors for PLC filters, circuit breakers for the new AC filter banks, arresters and current transformers will also be installed. The open and closed-loop control concept and the protection concept will be adapted to the new operating mode of the HVDC transmission system as part of the upgrade, and the system’s own auxiliary power supply is to be augmented.
Figure 2. Valve hall building with converter transformers to be retro-fitted with a new cooling system to cope with the greater power load Figure 3. Smoothing reactor coils Figure 4. Test assembly for reactor unit cooling by air handlers Figure 1. India’s regional networks with major HVDC links