Getting the FACTS right3 June 2003
An innovative solution with critical advantages over conventional static var compensators has been chosen by a US utility to improve recovery from voltage fluctuations on its network. John Loughran*
Northeast Utilities Service Company, based in Connecticut, USA, recently awarded a contract to the transmission and distribution sector of Alstom to supply a FACTS device called Statcom to improve voltage recovery on its network. The decision followed a lengthy investigation by NU into the after-effects of a voltage drop in the Norwalk-Stamford area of the grid. After the fault cleared, it took 8-10 seconds for the system to recover to 90 per cent of its former level. This prompted NU to undertake studies on possible solutions that would avoid the reccurrence of such an event.
Flexible AC transmission system, or FACTS, is the generic name for a group of devices (see panel, right) gaining in popularity in the power transmission industry. Why? because they provide a flexible means of strengthening weak areas of the grid. And they meet a present need. Utilities confronted with network problems meet difficulties in carrying out major improvements to transmission networks. New transmission lines take time to build, and in many countries such projects are vigorously opposed by local residents. And then there is the cost. With utilities operating in deregulated markets under fierce pressure to keep maintenance costs as low as possible, constructing new lines is an expensive option.
It is no longer feasible for utilities simply to add generation capacity or reinforce transmission lines to improve the grid. It is likely that transmission operators, with no control over generation capacity and less freedom to manoeuvre on transmission infrastructure, will turn to more technologically advanced solutions to grid problems. For this reason, Statcom, along with other FACTS devices, is generating interest from utilities keen to find affordable solutions to network problems.
NU considered three options to provide voltage support to the NorwalkStamford area:
• installation of fixed shunt capacitor banks;
• upgrade of the area's 115 kV transmission system to 230 kV; and
• application of FACTS technology.
The use of fixed shunt capacitors was the lowest cost solution but considered the least satisfactory from an operational viewpoint. The connection of shunt capacitors causes a rise in system voltage appropriate to countering the effect of predictable or staged increases in load, when banks of capacitors can be switched in sympathy with the load increase. However fixed shunt capacitors are less satisfactory in response to dynamic situations, or unexpected disturbances, as such incidents are inherently unpredictable. The capacitors cannot be connected permanently because of the unacceptable overvoltages that would result, in the absence of the disturbances, from the large capacitance required. This solution was not therefore considered to be viable.
Upgrading the local transmission system from 115 kV to 230 kV was also soon recognised as non-viable as the system had already been upgraded from 66 kV to 115 kV, leaving insufficient insulation clearances for subsequent extension to 230 kV. Consequently, there would be a need for complete overhead line rebuilds, for which the cost could not be justified on economic grounds nor the system disruption accepted during the considerable down-time required for replacement.
The option selected was the implementation of a dynamic solution based on FACTS technology employing power electronic converters. Such systems provide high-speed dynamic solutions to adverse network conditions of the type encountered by NU, both during the contingency itself and in the critical immediate post-event period, to ensure that the network is returned to normal, stable, operating conditions. It was determined that a dynamic source of reactive power injection of the order of 150 MVAr was required to stabilise the NorwalkStamford system during disturbances. This could be provided either by a conventional static var compensator (SVC), based on thyristor controlled reactors (TCR) and thyristor switched capacitors (TSC), or by a newer FACTS technology, the static synchronous compensator, aka Statcom. This latter option was chosen because of its technical and economic merits. The principal advantage of Statcom over the classical SVC is its ability to generate full capacitive current output at extremely low voltage, which the SVC cannot. Other merits of Statcom that helped clinch the decision included compactness, good harmonic performance with little or no filtering, a superior dynamic response, a better transient rating, lower noise generation and lower associated magnetic fields
To reduce the cost by minimising the required rating of the more expensive dynamic Statcom element it was decided to install a ±150 MVAr unit in conjunction with three 50 MVAr fixed capacitor banks.
There are two main ways of arranging GTO converters to minimise residual harmonics and render the generated alternating voltage acceptable. These methods entail introducing complexity into either the coupling transformers or the power electronic converters (Figure 3).
Conventional multi-pulse rectifiers employ transformers with phase-shifted secondary windings to develop, for example, 12-pulse, 24-pulse and sometimes 48-pulse systems which effectively suppress low order harmonic currents under balanced system conditions. A corresponding phase-shifting transformer arrangement can be adopted to combine the outputs of a multiplicity of simple, small, voltage-sourced converters into an acceptable waveshape when the supply system is balanced, though unbalanced and fault conditions result in poor shaping. This is the approach employed by a number of Statcom suppliers.
An alternative method of achieving pulse multiplication and minimising harmonics adopted uniquely by Alstom is to develop the near-sinusoidal waveshape directly in the power electronic converters. A multi-level converter approach has been adopted, using multi-link 'chain circuits'. Alstom has successfully employed this approach in a +75 MVAr Statcom in service with National Grid in the UK. The Statcom converter currently being manufactured for the Glenbrook substation likewise employs Alstom's chain-circuit topology (Figure 4). In this configuration a converter valve comprises a chain composed of a number of individually switched, series-connected, single-phase bridges referred to as links. Each link of a chain is identical and consists of a single-phase bridge arrangement of four GTOs and freewheel diodes, together with its own DC capacitor as a voltage source; in normal operation the voltages of all the capacitors are kept almost equal. Each link is able to generate three voltage levels and to give a positive, negative or zero voltage contribution to the overall generated voltage. Three single-phase chain circuit converters, connected together in delta, are used to form a complete Statcom. The mechanical analogy of a chain with three links is illustrated in Figure 5a, 5b, together with the corresponding output voltage (Figure 5c). This shows a four level voltage waveform (three levels plus zero) for a chain circuit comprising three links, As more links are added, the number of voltage steps increases, giving a closer and closer approximation to a sine wave. The physical arrangement of a single link is illustrated in Figure 6.
With such a converter arrangement, the need for complex magnetic combining transformers to achieve an acceptable waveform is avoided. This is the key differentiating factor between the approaches of Alstom and its competitors. Moreover, to create the output voltage, each GTO is only required to turn-on once and turn-off once per power frequency cycle, which minimises switching losses and gives a very efficient converter design compared with other designs that use PWM high-frequency switching techniques. As well as low switching losses, the harmonic generation for the converter is negligible. Thus there is no requirement for any harmonic cancellation within the step-up transformer windings nor for parallel connected harmonic filters.
The Statcom can be housed in a shipping container, as in the case of a unit being employed by the National Grid in the UK, and can thus be moved in its entirety from one location to another should the need arise. This gives the customer a truly portable solution for improving networks. Typically it would takes three months to transfer and set up the containerised unit between locations.
The Statcom at NU's Glenbrook substation is split into two halves, each rated at ±75 MVAr and in normal operation the two halves are connected to separate buses in the substation via separate step-up transformers. To allow for operation at full output with one transformer out of service, each transformer is rated at 150 MVA, so that both Statcoms can be connected to the secondary of one transformer. A bus-tie circuit breaker connects the two converters together. In the event of failure of a transformer or feeder, it is possible to reconfigure the circuit quickly and return the compensator to service at full rating. In addition, either converter can be disconnected in the event of a fault, and the Statcom can be operated at 50 per cent rating. In order to be within standard current and voltage ratings for the converter equipment, the voltage on the secondary of the step-up transformers was chosen to be 14.6 kV rms. The rated current of the converters is 1712 A rms. Because of the high pulse number obtained within the converter, as described above, harmonic filters are not required as part of the installation.
For Glenbrook, each three-phase converter is delta connected and has 15 links in series in each phase. To satisfy the worst-case operating point for the converter, a minimum of 14 links must be continuously available. One additional link is provided in each phase as redundancy. The air-cored buffer reactors, which connect each phase of the converter to the LV busbars, are chosen to ensure that the control system can provide fast and stable control of the converter over the full operating range. The reactors also limit the maximum voltage and current stresses on the converter valve in the event of an insulation fault on the LV busbars. The physical arrangement of the power electronic equipment for each link has been carefully engineered to deliver a compact modular unit that is still adequately accessible for maintenance. Figure 7 shows the power electronic link assemblies being mounted in an enclosure. The link DC capacitors are mounted beneath the power electronic assemblies (interconnected via the aluminium busbars, which are visible at the bottom of the photograph across the width of the enclosure).
As the compensator's primary function is to help the system to recover from faults that would otherwise depress the network voltage for a significant length of time, it is designed to operate at full output at voltages down to 40 per cent for 30 seconds. In normal operation, the operating point of the Statcom is adjusted automatically by the controls, by switching the mechanically switched capacitors (MSCs) at Glenbrook and three other substations in the locality. This maintains the dynamic margin of the unit. In addition, the voltage at the three remote substations is monitored by the Statcom controls and the MSCs are switched at these substations to maintain the 115 kV bus voltage within limits. The availability of the compensator is designed to be 98 per cent with respect to forced and scheduled outages. The availability with respect to forced outages is increased by the use of two transformers, each feeding one 75 MVAr Statcom with the transformer LV buses separated, but connected by a circuit breaker. This configuration enables the full output of the compensator to be quickly and automatically restored following the loss of a substation 115 kV bus or a transformer.
An artist's impression of Glenbrook Statcom, with two step-up transformers, is shown on page 21. The two Statcom converters are mounted in a building as six rows of chain links side by side, as illustrated in Figure 8. The connections to each phase are made via through-wall bushings on the end wall of the building. Each converter is connected to its respective LV busbars through pairs of air-cored buffer reactors. To save space, the coil pairs are stacked. The disconnect switches on either side of the transformer and the Statcom disconnects are motor operated and are mounted outdoors. The 115 kV circuit breakers are located in the substation and feed the Statcom via cables terminated adjacent to the transformers. The converter building also contains the control and protection panels, battery and chargers, AC and DC distribution boards, the fire protection and heating, ventilation and air-conditioning equipment. The cooling plant pump station is housed in a specially modified shipping container close to the side of the building, with the air-blast radiators ground-mounted next to the container. The cooling system has a single hydraulic circuit filled with a mixture of pure de-ionised water and monoethylene glycol coolant.
Operation and maintenance
By appropriate control of the Statcom's output voltage (Vc) to be higher or lower than the bus voltage on the secondary of the step-up transformer, the compensator will draw a capacitive or inductive current from the system. As the Statcom comprises single-phase converters connected in delta, a separate and individual current order can be given to each phase to match prevailing conditions during system disturbances and to reduce the negative phase sequence, or unbalanced voltage on the AC system. The power supply for the electronics on the links is derived either from the DC capacitor voltage or from a separate inverter that feeds each link via an auxiliary power interposing transformer mounted on each link. This circuitry is utilised to charge the DC capacitors and for equalising the voltages of the individual links.
With this type of equipment, very little maintenance is required. The outdoor and conventional indoor plant will be maintained in accordance with NU's normal practice. The power electronics and cooling plant require inspection annually to ensure that there are no coolant leaks or high resistance electrical joints. The cleanliness of the power electronics is also checked. Faulty components will be replaced as and when they fail and sufficient redundancy is built in to allow the Statcom to continue running until a suitable outage can be arranged to replace the failed item. Pump seals and de-ionisers will be changed on a regular basis. Air filters in the ventilation plant must be replaced regularly to prevent pollution from settling on the valves and to maintain adequate air-flow. The controls, including fibre-optic cables, require annual inspection to ensure that no faults are developing.
The Glenbrook project is critical to the satisfactory operation of the southwest Connecticut system, especially under contingency conditions. Being able to achieve implementation in a very short lead time, in this case 12 months, was a factor in the selection of Alstom's solution. The compensator is due to be in service by the end of December 2003.