Increasing power transfer capacity within a network is a problem easily fixed with the application of series compensation. PEi looks at one project in China, where strong industrial growth is placing a strain on the grid.

Lutz Kirschner, Quan Bailu, Volker Lehmann, Siemens Power Transmission and Distribution, Germany

The growing power demand of industries in the eastern provinces of China is putting a strain on the regional grid and an increase in power transfer capability is required. One of the most economical solutions to the problem is the installation of fixed series compensation, which would not only enable the desired power increase but also stabilize the interconnected networks by reducing the connecting line impedance.

In the summer of 2005, Siemens received an order for the Fengjie series compensation system located in the Wanzhou district 500 kV AC network. Siemens is responsible for design, delivery, erection and commissioning, and the start of commercial operation of two capacitors located in two parallel lines was scheduled for June 2006.

The two parallel lines in which the capacitors have been installed are part of the Chuanyu Grid to Central China Mains project. This consists of two circuits of 500 kV lines, which start in the west from Wanxian 500 kV substation and extend to the east to Longquan 500 kV HVDC converter station. Each circuit has a line length of 360 km. Fengjie 500 kV substation is located about 140 km away from Wanxian substation along the two circuits, with the site at Fengjie County, Wanzhou District. Due to the demanding power increase, the implantation of transmission reinforcements in Wanzhou area is foreseen. The AC network at Wanxian is coupled through this corridor to the Three Gorges HVDC converter station in Longquan. The series capacitor system will have a capacity of 2 x 610 MVAr, a compensation degree of 35 per cent.


Figure 1. Location of the Fengjie fixed series capacitor in China
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The most economical design has been found for a gap protected metal oxide varistor (MOV) scheme with an MOV total energy capability of 35 MJ per phase. Future AC system configurations have been investigated during the final design to determine the necessary energy requirements of the MOV. The calculations are based on a PSCAD/EMTDC model of the AC system corridor with both different actual and future AC system configuration stages including the detailed representation of the capacitors and their protection features.

Single line and main data

The AC fault calculation is the basis for the type of protection for the series capacitor bank. The single line diagram comprising the main components is shown in Figure 2. The capacitive reactance is 35.3 ohms for each of the two parallel series capacitors. The nominal continuous current both capacitor banks are designed for is 2400 A rms with temporary overload currents up to 3600 A rms for 10 minutes. Under normal conditions the AC line current flows through the capacitor resulting in a steady state voltage drop across capacitor and varistor. In case of AC line faults the short circuit currents increase the voltage drop across the capacitor until it is clipped by the MOV. During voltage clipping the MOV dissipates energy. To protect the MOV against excessive energy dissipation, a fast operating spark gap is connected in parallel to the capacitor performing an immediate bypass after gap triggering. To protect the gap against excessive arcing stresses, the bypass switch closes about 50 msec after gap firing. Depending on the severity of the AC line fault the capacitor units can be protected by different bypassing strategies.

The damping circuit is connected in series with the spark gap and bypass switch. Sufficient damping must be provided to limit the high frequency discharge current of the capacitor. To safely insert and bypass the installation, motorized disconnects and earthing switches are provided.

Component ratings

For a cost and performance optimized design, various studies with different system conditions have been carried out. To determine the component ratings under worst- case conditions, not only steady state voltage and current stresses need to be examined but also the transient stresses during severe AC system faults have to be investigated in detail.

The series capacitor consists of one segment with a differential protection detecting capacitor unit unbalances. It is built up of 480 capacitor units per phase. Six units are connected in series to cover the 85 kV rms total nominal voltage. Four racks constitute such a configuration. Fuseless capacitor unit technology offers the lowest losses during operation and therefore contributes to the high efficiency of series capacitor installations. Due to insulation requirements, double bushings are used per unit.


Figure 2. Fixed series capacitors Fengjie single line diagram
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The MOV housings are of polymer type. They are equipped with a pressure relief membrane and a flange with a gas diverter nozzle. In case of an MOV failure and internal short circuit, the pressure relief system releases the pressure inside the housing. Due to the overload conditions after an AC system fault, the rated voltage of the MOV has been chosen to be 143 kV rms. To determine the number of MOV housings which have to be installed on the platform, AC system fault calculations have been carried out. Different AC system configurations as well as different fault types and point on wave fault inceptions need to be considered for the design, to establish the proper operation of the capacitor system also for future stages of the growing AC network.

There are two different types of faults which have to be considered for the design of the number of MOV housings:

External faults – External faults are those occurring outside the series compensated line terminated by breakers. The MOV must be designed to withstand external faults without damages, as the series capacitor will not be bypassed during external faults.

The fault is applied and normally cleared by a line protection breaker within 100 msec. Considering an unsuccessful line clearing this duration increases by another 100 msec until final tripping of the line. The analysed external fault locations taking into account different fault types like single and multi phase faults show that this type of fault leads to the highest MOV current stress without any allowed bypassing action.

Internal faults – Internal faults are those occurring within the series compensated line terminated by breakers. If an internal fault is detected, the series capacitor is allowed to be bypassed by the triggered spark gap. This type of fault leads to the maximum overvoltage stress of MOV and capacitor. As soon as the instantaneous value of the MOV current exceeds 8 kA the protection issues a signal to trigger the gap firing within 1 msec. The voltage immediately before spark gap firing is limited to a protective level of 275 kV = 2.3 per unit by the MOV.

The highest energy consumption for the MOV has been found to be 30 MJ per phase considering a single phase AC fault with unsuccessful line reclosing. It appears if the fault location is near the end of a line. Fifteen per cent spare energy has been added for the final energy rating of the MOV.

Spark gap

The spark gap has to protect the MOV against overload in case of internal faults. The protection fires the gap within 1 msec as soon as a threshold value of 8 kA is exceeded. It is absolutely necessary that the gap bypasses the MOV as soon as possible because of the high rate of rise of energy dissipation in the MOV during internal faults.

The gap consists of two housings installed on top of each other. The main trigger circuit is located in the lower housing while the upper one is passive. It fires successively after the lower gap has been triggered. The thermal fault current carrying capability is 40 kA rms. To avoid excessive arcing the spark gap is always triggered in conjunction with a closing impulse to the bypass switch. Therefore the arcing stresses only occur for about 50 msec on the gap electrodes.


Figure 3. Layout drawing of fixed series capacitor installation in Fengjie County
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The damping circuit is designed to discharge the capacitor in case of an internal fault when the voltage across the capacitor reaches the protective level. It is located in series to the spark gap and bypass switch and is therefore stressed by line current only in case of bypass. To limit the peak current value of the discharging capacitor, a reactor of 600 µH has been chosen for both series capacitors. The appropriate damping is achieved by a resistor located in parallel to the damping reactor. To avoid a steady state current drain in the resistor, a small spark gap is also included in series with the resistor. The impulse energy rating of the resistor is sufficient for a duty cycle of two consecutive discharges.

Bypass switch

The 550 kV SF6 bypass switch is designed as three single-phase units, one for each phase a closing time of 50 ms. It consists of porcelain insulator columns with a hydraulic opening mechanism. The switch is used for insertion and disconnection of the bank. In case of internal faults, it closes so the spark gap current commutates to the switch releasing the gap from further current stress. Due to this requirement the switch is designed for high frequency capacitor discharges as well as power frequency currents and dynamic short circuit currents up to a thermal limit of 50 kA rms.

The series capacitor components are installed on a platform that is connected to the high potential of the AC transmission line. This design leads to the lowest creepage distances and flashover voltages across the components. The platform itself is built up on reinforced structures of porcelain insulators to provide an appropriate line to earth creepage distance with a height of approximately 6 m. The bypass switch is not installed on the platform but located beneath the platform on the ground. Due to the optimized design the platform size is only 15 m x 8 m and has a total weight of about 40 tons. Figure 3 shows the total installation for one phase.

References:

Anderson, Farmer, “Series Compensation of Power Systems”, ISBN 1-888747-01-3, 1996

Kirschner, Baoshu, Gong, and Breuer, “Design Aspects of the Chinese 500kV Thyristor Controlled Series Compensation Scheme TCSC Tian Guang”, DRPT Conference April 2004, Hong Kong

Bohn, Kirschner, and Kuhn, “TPSC: Thyristor Protected Series Compensator: Design and Control Concepts”, CIGRE-XERLAC Conference May 2003, Iguazu, Argentina