The specification and application of large quadrature boosters to restrict post-fault power flows (on behalf on SC A2)

Quadrature Boosters (QBs) have been used in the UK transmission system since 1969 when the first 275 kV unit was installed. In 1997 an enquiry was issued for two 400 kV QBs with a no load phase angle of 11 degrees, a throughput capacity of 2750 MVA and the requirement that they be capable of restricting power flow. This paper describes some of the design issues encountered and the method of modelling the internal voltages and fluxes required to evaluate the performance of the design. Four of these large QBs have now been placed successfully into service together with the existing five 400 kV 2000 MVA and six 275 kV 750 MVA QBs. In the UK transmission system there is a particular need to manage the power flows on certain circuits, following the loss of one or more of the several transmission lines that connect a region of high net generation to one of high net demand. The ability to control the power flow in one or more of the lowest capacity lines linking two parts of the network, allows an increase in the overall secured capacity (the capacity following a fault or faults) of that part of the transmission system. The most effective way of achieving this power-flow control has proved to be by the use of QBs. This ability to provide increased secured capacity whilst avoiding new circuit construction is of particular importance given the need to minimise the environmental impact of the transmission system. Early in the tendering and specification process, it was identified that the QBs would be required to restrict (buck) as well as boost the power flow. Under these circumstances the impedance of the transformers comprising the quadrature booster could increase the phase shift between the input and output terminals well beyond the maximum no-load value. This mode of operation requires very careful modelling and consideration of the voltages and flux levels experienced within the unit to ensure that it has adequate capability. This requirement is referred to but not described in detail in the recent IEC/IEEE guide. Detailed discussions were required with the manufacturers of the units to ensure that the operating conditions were fully understood and taken into account in the design of the QBs. The high throughput rating, transport size and weight restrictions and the bucking operation requirements imposed severe design constraints that had to be overcome by the manufacturers. The requirements for the tap-changers were also a significant challenge requiring additional type testing and a particular winding arrangement. In addition, modelling of the transient conditions that can exist during a through-fault indicated that extremely high voltages can be induced on the terminals of the QB under some, quite likely, circumstances. This highlighted a need for surge arresters with a high capacity connected on both sides of the QB. In order not to make the QBs unnecessarily large, a method of restricting the operating conditions was required. This was achieved using a control scheme that is sensitive to system voltage, frequency and load flow and can restrict the range of tap positions available to the operator accordingly to ensure that design parameters are not exceeded. Testing of these large units was complex, requiring tests on the series and shunt units separately and together. It was not possible to test using current and voltage simultaneously and detailed calculation methods had to be agreed to cover the aspects of performance that could not be adequately tested.