Smoothing reactors are the devices that can help you resolve the issues caused by harmonic and transient over currents. They smooth the direct current and also serve as a buffer between the converters and the DC line. They are connected in series with the converters before the DC filter. Their sizing is determined based on various requirements, including the reduction of ripple in the current.
They are used for various purposes in different applications. Some of them are given below:
• They help reduce the frequency of commutation failures caused by dips in the AC voltage at the converter bus in inverters.
• By minimizing the rate of rise of direct current in the bridge, they also help in preventing the resulting commutation failures in inverters when the direct voltage of another bridge (connected in series) collapses.
• In the case of light loads, smoothing reactors can smooth the ripple in the direct current so that it doesn’t become discontinuous.
• Besides smoothing ripples, they decrease harmonic voltages and currents in the DC line.
• Smoothing reactors also limit the crest current caused in the rectifier that occurred due to the short circuit on the direct current line.
• They also help in limiting the current flowing in the valves during the converter bypass pair operation due to the discharge of shunt capacitances of the DC line.
It is important to note that the last three functions are not relevant in back to back HVDC systems. The inductors of value from 0.27H to 1.5H are generally used for systems with DC transmission lines and the value ranges from 12mH to 200mH for back to back HVDC power transmission systems.
The sizing of the smoothing reactor is also determined from the point of view of minimizing the effect of low order harmonic resonances in AC and DC systems. It is critical to avoid and prevent series resonance of the DC system at the cardinal frequency and also at the second harmonic. The value of inductance must remain constant with variations in direct current. Therefore, it requires an air-core based construction. It is feasible only if the reactor does not get saturated by the fault current.
However, saturation reactors cannot replace smoothing reactors as smoothing reactors don’t get affected by saturation reduce commutation failures.
The location of the smoothing reactor can be chosen at either a high voltage terminal or ground terminal. But, in the second case, it is vital to install a small reactor of the order of 5 to 10mH on the line side so that the converter station can be protected from the consequences of lightning strokes to the line. The main advantage of installing the reactor at the ground terminal is that it allows the converter ground faults to be cleared by converter control. Plus, the insulation of the reactor is also reduced.
A smoothing reactor is a device connected in series with a direct high voltage line as part of either converter station or back-to-back interconnection between different networks of transmission. Its main functions are to reduce harmonic or ripple current, limit the inrush current during fault conditions, limit the rate of rise of DC phase current and improve the dynamic stability of the power transmission system.