After a severe ice storm destroyed a large section of its transmission network, Hydro-Québec contracted Areva T&D to install the world’s first HVDC based de-icing and power quality system.
In 1998, North America was hit by one of the worst series of ice storms in recorded history. While many parts of the region are accustomed to ice storms, this one was so severe that the weight of the ice literally pulled down hundreds of kilometres of high voltage electrical transmission lines and towers. The Québec region of Canada was the most dramatically affected: more than a million people in the Montreal area were without electricity, some for several weeks.
Hydro-Québec Transàƒâ€°nergie needed to make sure that the consequences of such a severe ice storm in the future would be minimized for the public. However, it was not possible to justify the installation of a de-icing system that would sit idle for the majority of the year. It had to be a two-fold system that could somehow be adapted to their existing systems and transmission lines.
Seven years later, the damaged sections have been rebuilt, but to be in a better position to fight an ice storm of great magnitude, Hydro-Québec had to further invest in security equipment. Their idea was to find a way to effectively and efficiently combine a high voltage de-icing system and a power quality system in one. After considering several options it was concluded that Areva T&D’s proposal was the most appropriate solution for meeting Hydro-Québec’s technical and functional needs.
Finding a solution
Working with Hydro-Québec’s team, Areva T&D’s engineers designed the world’s first HVDC based de-icing and power quality system. The Areva T&D Static Var Compensator (SVC) system is flexible enough to be switched from SVC mode to de-icing mode in less than an hour. When not de-icing, the SVC will function as a reactive power compensator, a classic tool used to give local network voltage support, better utilization of the transmission network, congestion relief, increased system reliability and availability, and improved system stability – especially during and after faults.
The de-icing will be achieved by permitting the flow of high intensity direct current (dc) into the transmission lines, heating the conductors to melt the ice.
The Static Var Compensator (SVC) system being installed on Hydro-Québec’s network can be switched from SVC mode to de-icing mode in less than an hour
The process itself is simple. The lines will be disconnected at the substation level and isolated from the grid via high capacity (7200 A) disconnect switches specially designed by Areva T&D. The SVC will be switched to de-icing mode and the high intensity direct current flowing through the already isolated transmission lines will melt the ice. Once the ice has vanished, the transmission lines are reconnected to the grid. The whole process should take just a few hours. As it is a planned switch, power transmission can be re-routed to other lines to maintain the supply to the end users.
In initial discussions, it was made clear that only rapidly available, proven technologies should be used. The dc de-icing technique, which is appropriate for long lines and offers controllability, provides an economic solution for preventing the build up of ice on the overhead lines. When not in use for de-icing purposes, Transàƒâ€°nergie wanted an SVC power quality system. In this mode, it will improve the voltage regulation while maintaining the dc converter equipment, protections and controls in operation ready for converting into the de-icing mode when needed.
The conductor current needs to melt the accumulated ice without exceeding the thermal limit of the conductor itself. Based on laboratory studies, to de-ice a standard 735 kV line with a bundle of four 1354 MCM conductors per phase, a current of 7200 A per phase for at least 30 minutes is necessary to melt 12 mm of radial ice and free the lines (with a given wind chill factor of -15à‚°C). In addition, all disconnect switches required for the transfer from one mode to the other must have the capability to switch under an ice thickness of 60 mm.
Implementing the solution
In ‘de-icer’ mode, a controlled, high current, dc power source feeds the resistive load of the transmission lines selected for de-icing – just as for an HVDC transmission link, except that only the rectifier station is required. As the converter remains connected to the ac system, it only needs to be line commutated which lends to the use of conventional thyristor based HVDC equipment.
To keep insulation requirements to a minimum, the ac harmonic filters are directly connected to the secondary of the step down transformer. These filters provide 180 MVAr of the 334 MVAr needed to meet the reactive power limits specified for the installation. If the installation were only used for de-icing, the remaining 154 MVAr would have been used as a plain shunt capacitor bank; however, it was more logical to use it as a Thyristor Switched Capacitor (TSC) using classical SVC valves. The TSC represents the only switchable element in the reactive power system during de-icing: the ac filters remain connected and do not require circuit breaker switching.
The step down transformer only needs two windings for the double-six-pulse circuit that was chosen for de-icer operation. However, a 20 kV tertiary winding was provided to achieve an economically effective voltage connection for the TSC valve and (in SVC mode) the HVDC thyristor valves in their configuration as a Thyristor Controlled Reactor (TCR).
The nominal ratings required by Hydro-Québec Transàƒâ€°nergie for the SVC mode are +250/-125MVAr (at a primary reference voltage of 1.05 per unit); but the SVC will normally operate close to float in the range of +75 MVAr. Careful attention was taken to minimize power losses in the ‘float’ range.
More than one million people in the Montreal area were left without power following the 1998 ice storm
The logical SVC design includes a TCR, TSC and ac harmonic filter bank. The TSC requirements were already met and there was more than enough capacitance already designed for the filter needs, so the final item for consideration was the provision of a TCR.
Here again, Areva T&D’s engineers were able to provide a more efficient solution by creating a hybrid arrangement of the HVDC bridge circuits so that not all the thyristors are actively used when in TCR mode. One of the two HVDC bridges is kept blocked and the other is reconfigured in inverse parallel to create the required arrangement for a TCR.
This project requires the use of two types of thyristor valves:
- HVDC valves for the controlled rectifier in de-icer mode and as the TCR in SVC mode
- Classical TSC valves provide the switchable capacitive Vars in both modes.
The suspended thyristor valves use 8.5 kV, 125 mm diameter thyristors. They are equipped with damping and dc grading circuits; a gating/protection unit also protects against forward over voltage, forward dv/dt and forward recovery voltage.
For the Hydro-Québec de-icer project, the HVDC valves are mounted as six Multiple Valve Units of two valves each, with each valve containing two valve modules. There are 24 thyristor levels in series for the de-icer function (one is redundant). While this number seems high for such a low dc voltage (34.8 kV and 250 MW nominal power), it is necessary for operation at full rated current with any control angle up to 90à‚° and also depending on the length of transmission line to be de-iced.
Additional design options
Geomagnetic filters: This system is often subjected to geomagnetic storms, so special consideration was taken during the design process to guarantee that the system will function correctly during these situations. Geomagnetic storms are temporary disturbances in the earth’s magnetosphere resulting from solar flares. When these magnetic fields move in the vicinity of a conductor wire, they can damage the electrical transmission equipment.
Noise Levels: The Lévis substation near Quebec City, which was Hydro Québec’s chosen location for the de-icer is located in a residential area so noise levels must be kept to a minimum. Transàƒâ€°nergie is installing the equipment in the middle of the substation which will maintain the audible noise, even in SVC mode, at 25 dB(A) close to residences within a normal operating range of +75 MVAr. A 40 dB(A) limit is required for the de-icing and testing modes.
Annual Verification: A Verification mode was added to the de-icing and SVC system so that it can be checked at least once a year.
HVDCice, as the system has been appropriately named, is Hydro-Québec’s first step to guarantee a more secure winter transmission system, and fulfilling their promise of delivering electricity to its customers 365 days a year. Areva T&D’s system, which will initially cover 750 km of transmission line circuits, will be operational by autumn 2006.