0.3–0.7 mA/m² of collecting area
Current sizing basis
ESP components · High-voltage power supply
Transformer rectifier sets (TR sets) for ESP fields
A transformer rectifier set converts mains power to the negative DC that energises one ESP field — secondary classes of 45–110 kV, sized at 0.3–0.7 mA per m² of collecting area. Conventional 50 Hz sets deliver that with 30–45 % ripple; high-frequency SMPS units cut ripple below 5 % and run several kV closer to spark-over.
45–110 kV DC (negative)
Secondary voltage classes
30–45 %
Ripple, conventional 50 Hz full-wave
< 5 % at 20–50 kHz switching
Ripple, high-frequency SMPS
01 — Function
One TR set, one field, one point of failure
The TR set is the only component whose fault removes an entire field at once — which is why it is sized, protected and counted per field.
In electrostatic precipitator design, each electrical field gets its own transformer rectifier set: a step-up transformer and high-voltage rectifier, usually in one oil-filled tank on the casing roof, feeding negative DC through a bushing and HV conductor to the discharge frames. Independence per field is deliberate — it is what makes the n−1 arithmetic work. In the four-field bagasse design basis, one TR fault moves outlet dust from 24 to 57 mg/Nm³ @ 6 % O₂ dry; a shared supply would have moved it to hundreds.
Electrically, the ESP field is a corona load: nearly capacitive below onset, then a steep current-voltage curve up to spark-over. The TR set must sit comfortably above both knees — enough voltage class that the gas gap, not the supply, limits operation, and enough current that the discharge electrodes can emit what the dust needs. Total absorbed power stays modest: 177 kW across all four design-basis fields, rapping and heating included.
02 — Sizing
Sizing: current from plate area, voltage from plate spacing
Two numbers fix the rating — both come from the casing, not the catalogue.
How is a transformer rectifier set sized for an ESP field?
Current rating comes from collecting area times a design current density of 0.3–0.7 mA/m²; voltage class comes from gas-passage spacing — around 72 kV for 300 mm passages, 90–110 kV for 400 mm. A design-basis field of 972 m² therefore needs roughly 300–680 mA at a class the gap can use.
The current density is not one number for the whole machine. Inlet fields, working on coarse, heavily space-charged gas, run at the low end; outlet fields, polishing fines, need the upper end. High-resistivity ash near 10¹¹ Ω·cm inverts the logic — current must be capped, not maximised, or the dust layer goes into back-corona, so the outlet-field TR on dry rice-husk silica duty may be specified at 0.2–0.3 mA/m² with the emphasis moved to voltage stability. Voltage class carries margin above expected spark-over: a dusty 300 mm passage sparks around 60–75 kV, so a 72 kV set works at the limit while an undersized 55 kV set clips below it and permanently wastes plate area the Deutsch sizing paid for.
| Duty case | Plate spacing | Voltage class | Current density basis | Notes |
|---|---|---|---|---|
| Biomass inlet field | 300 mm | 72 kV | 0.3–0.4 mA/m² | Space charge from heavy dust suppresses current — voltage headroom matters more |
| Biomass outlet field | 300 mm | 72 kV | 0.5–0.7 mA/m² | Fine dust, low space charge; the emissions-guarding field — first candidate for high-frequency supply |
| Wide-spacing retrofit | 400 mm | 90–110 kV | 0.3–0.6 mA/m² | Fewer, taller passages; existing 72 kV sets cannot exploit the wider gap |
| High-resistivity ash (dry silica, low-S coal) | 300–400 mm | 72–110 kV | 0.2–0.3 mA/m² | Current capped against back-corona; controller must detect and back off |
| Low-resistivity salt cake (recovery boiler) | 400 mm | 90–110 kV | 0.5–0.7 mA/m² | Re-entrainment-limited duty; stable high mean kV plus heavy rapping |
03 — Conventional vs high-frequency
Ripple, and what a high-frequency set buys
Sparks follow peak voltage; collection follows mean voltage. Ripple is the distance between them.
What does a high-frequency TR set actually improve?
Mean kilovolts. A conventional 50 Hz full-wave set carries 30–45 % ripple, so its peaks reach spark-over while its mean — the voltage that collects dust — trails far below. A 20–50 kHz SMPS set holds ripple under 5 %: mean voltage rises several kV at the same spark rate, worth typically 15–30 % more corona power.
The mechanism is purely geometric. Spark-over is decided by the waveform's crest; precipitation is driven by its average. Flatten the waveform and the average moves up toward the same crest. High-frequency sets add three further advantages: spark energy is lower and recovery faster, because the controller can interrupt within a switching cycle rather than a mains half-cycle; three-phase input loading is balanced, where a conventional single-phase set loads one phase of the plant supply; and the unit is smaller and lighter for the same kVA, which simplifies roof work at retrofit. The candid limits: SMPS electronics demand cleaner ambient conditions and more specialised service than a 50 Hz tank, and on a heavily space-charge-limited inlet field the mean-kV gain buys less than it does on the outlet field — which is why staged upgrades put the first high-frequency set on the field nearest the stack.
Oil versus dry is a siting decision. Oil-filled sets remain the roof standard: mature insulation at 110 kV, thermal mass, long life, with oil sampling, temperature monitoring and containment as the running obligations. Dry epoxy and SMPS designs remove the oil — welcome indoors, near sensitive areas, or under strict environmental rules — and in exchange need dust-protected, ventilated enclosures and closer attention to internal temperatures.
04 — Protection and retrofit
Protection settings, and matching TR to field at upgrade
A TR set protects itself, the field, and the stack figure — in that order of speed.
The protection stack works on three timescales. Milliseconds: arc detection blocks the supply before a spark hardens into a sustained arc that pits collecting plates and burns discharge emitters; secondary over-current trips on dead shorts — a fallen wire, a tracking bushing. Seconds: spark-rate control, executed by the field's ESP controller, quenches each spark, sets voltage back a few percent and re-ramps, holding a deliberate spark rate near the optimum instead of zero. Minutes to hours: oil temperature, level and Buchholz-type pressure devices on oil-filled tanks, plus insulator heating supervision on the insulators and conductors that carry the HV into the casing — most "TR faults" called in from site are, on inspection, tracking support insulators upstream of a healthy tank.
At upgrade, the matching rule is unforgiving: every change inside the casing re-opens the TR sizing. Added collecting area raises current demand at 0.3–0.7 mA/m²; a set already near nameplate clips, and the new plates run at reduced field strength — the upgrade under-delivers precisely where it was justified. Conversion to 400 mm spacing raises the required voltage class beyond what a 72 kV set can use. Our ESP upgrade assessments therefore treat TR sets and fields as one system: measured V-I curves per field first, then plate work, then TR and controller selection against the new geometry — with n−1 emissions restated for the rebuilt machine. CONFIRM: list of existing TR set makes and controller protocols supported for retrofit integration.
FAQ
Engineering questions, answered
How do you size a transformer rectifier set for an ESP field?
From collecting area, not gas flow: multiply the field's plate area by a design current density of 0.3–0.7 mA/m², then pick the next standard secondary rating. A field with 972 m² of plates needs roughly 300–680 mA. Voltage class follows plate spacing: about 72 kV for 300 mm passages, 100 kV and above for 400 mm.
What secondary voltage does an ESP transformer rectifier set supply?
Standard classes run 45–110 kV negative DC. The needed class follows gas-passage geometry: spark-over in a dusty 300 mm passage sits near 60–75 kV, so a 72 kV class set lets the controller work at the spark limit; 400 mm spacing raises the ceiling toward 90–110 kV. A TR rated below the gap's spark-over wastes collecting area permanently.
What is the difference between conventional and high-frequency TR sets?
A conventional set rectifies 50/60 Hz mains, leaving 30–45 % ripple, so peak voltage — which sets sparking — sits far above the mean voltage that collects dust. A high-frequency SMPS switches at 20–50 kHz, cutting ripple below 5 %: mean kV rises several kilovolts at the same spark rate, and corona power into the field typically gains 15–30 %.
Are oil-filled or dry TR sets better?
Oil-filled remains standard for roof-mounted ESP duty: proven insulation at 110 kV, good cooling, decades of service life, at the cost of oil condition monitoring and fire and containment provisions. Dry epoxy-insulated and SMPS units avoid oil entirely — attractive indoors and where environmental rules bite — but run hotter internally and need stricter ambient and dust protection.
What protections does an ESP TR set need?
Spark-rate control that quenches each spark and resets voltage before energy dumps into an arc; arc detection that blocks the supply within milliseconds; secondary over-current and under-voltage trips for dead shorts such as a fallen discharge electrode; and oil temperature, level and pressure monitoring on oil-filled units. Protection settings belong to the field, so they are re-verified at every retrofit.
Can one TR set feed a larger field after an ESP upgrade?
Only if both current and voltage margins survive the change. Added collecting area at 0.3–0.7 mA/m² raises current demand — a set already at nameplate mA will clip and the new area collects at reduced field strength. Wider spacing raises required voltage class. Match each TR to its enlarged field, or the upgrade under-delivers where it was meant to gain.
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