Understanding Current Limiting Reactors (CLRs)
Purpose of the CLR
- Provide wave shape smoothing and absorb transients caused by phase fired SCRs.
- Provide current limiting during transient overload (sparking) conditions.
The typical T/R set power supply uses phase fired Silicon Controlled Rectifiers (SCRs) to regulate current flow to the T/R set during each half cycle of the power line. The precipitator load is capacitive under normal operating conditions and low resistance or shorted load during a spark. The precipitator equivalent load is reflected to the T/R set primary side. Without inductance of the CLR, there would be a significant primary current surge (high di/dt) each half cycle of the line at SCR turn on, as the T/R primary circuit voltage makes a step change to the line voltage.
The CLR provides an inductive load regardless of what is happening on the T/R set secondary, providing sinusoidal current waveforms on the primary and secondary sides of the T/R set.
During sparking, the secondary of the T/R set is effectively shorted. The T/R set typically has an impedance of 6%, meaning that during a spark condition, the primary current could rise to 1/0.06 or more than 16 times the T/R set rated primary current. The SCRs and other primary circuit elements would be severely stressed and would fail from the repeated surges. The CLR has a typical impedance of 30-60%, which limits surge currents to 1.66: 3.33 times the rated primary current. This is within the repetitive surge current ratings of well-designed SCR assemblies, ensuring long service life.
CLRs should be designed to complement T/R set ratings to provide full primary current at a target conduction angle of 120-140 degrees. It has been documented that, for most precipitators, as conduction angles decrease below about 100 degrees, some losses in collection efficiency can be expected, while conduction angles above 140-150 degrees have negligible benefit, and in some cases can cause operational problems.
Spark over voltage in a precipitator limits the peak voltage that can be applied to the bus section, while average voltage determines the collection efficiency. In a normally operating precipitator, any mechanism that can raise the average voltage without increasing the peak voltage will improve collection. A T/R set energized with full conduction sinusoidal current produces the maximum ratio of average secondary voltage to peak secondary voltage. By targeting 120 degrees, an operating margin allows increasing power if needed, while only slightly reducing the average to peak secondary voltage and current ratios.
We attempt to select CLR and T/R sizes and taps to create a system that reaches primary and secondary current limit at 120-140 degrees conduction angle. Unfortunately, many of the installed high voltage systems have components that are improperly sized. As long as the T/R set is not undersized, the problem can be fixed by proper selection of CLR parameters.
The most common errors are CLRs that have insufficient inductance and/or T/R sets that have a substantially higher milliamp rating than needed. If the T/R set current rating is oversized (a common problem in inlet fields) or the CLR has insufficient inductance, the T/R will run at low conduction angles, reducing collection efficiency. In either case, increase the CLR inductance until a 120-140 degrees conduction is achieved at the highest bus section operating current. If full conduction angle is achieved without sparking or reaching current limit, try a lower inductance CLR tap.
If the T/R set has multiple primary taps, selecting a different tap may improve performance. If full conduction angle is achieved without sparking, try a lower voltage/higher current T/R set primary tap. If sparking typically occurs at conduction angles below 120 degrees, select a higher voltage/lower current primary tap. If primary current limit is reached substantially before secondary current limit, increase the CLR inductance and/or select a different T/R set primary tap.
CLR inductance (mHy) = [10 * (CLR target percent impedance) * (Line Voltage)] / [377 * (T R primary current rating)]. If the T R set impedance is significant, deduct it from the CLR target impedance.
Typically, CLRs are sized to give 50% impedance and are rated for the T/R nameplate primary current rating. Installing a multi-tap CLR with taps providing 40%, 50% and 60% impedance provides flexibility in tuning the system and is recommended.
As an example, for 50% impedance with a 480 Volt line supply and a 146 Amp T R set, the required inductance would be:
L (mHy) = (10 * 50 * 480) / (377 * 146) = 4.3 mHy.
If the T/R set is oversized for the electrical bus section, sizing the CLR inductance to the current rating of a properly sized T/R set can improve performance. If this is done, use a multi-tap CLR. Select the lowest inductance tap to give 40% impedance for the T/R nameplate current rating and select the mid or high impedance tap to match 50% impedance for the ideal size T/R set. The CLR current rating should be sized to the highest anticipated operating current, and the AVC primary current limit must be set no higher than the lower of CLR or T/R current rating.