8.7 Capacitors, Reactors, and Harmonic Filter Banks

Capacitors, Reactors, and Harmonic Filter Banks

Quick Answer: Level II task 2.1h.2 requires the technician to "identify series and parallel components and circuits, determine the proper points at which to connect test equipment, measure capacitance, measure capacitor, reactor, and surge arrester insulation resistances, measure reactor winding resistance, and perform a fuse continuity test," with the knowledge list explicitly including "safety hazards related to working with capacitors." Capacitors store energy, and that single fact drives every safety rule that follows.

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1. Why capacitor banks exist

Inductive loads — motors, transformers, reactors, arc furnaces — draw lagging reactive current that does no useful work but occupies conductor, transformer, and generator capacity, and produces voltage drop. A shunt capacitor supplies leading reactive current locally, cancelling part of that lagging component.

Results: improved power factor, reduced current for the same real power, reduced losses (which fall with the square of current), released system capacity, improved voltage regulation, and reduced or eliminated utility power factor penalties.

2. The discharge hazard

This is the safety point the exam always tests.

A capacitor holds charge after disconnection. NFPA 70 (the NEC) requires capacitors to be provided with a means of discharging stored energy — internal discharge resistors — and specifies the time within which the residual voltage must fall to a safe level: for capacitors rated over 600 V, within five minutes of disconnection; for 600 V and below, within one minute.

But a discharge resistor is a component and components fail. A capacitor with an open discharge resistor holds its charge essentially indefinitely, and it can be at full peak voltage — for a 480 V bank that is roughly 680 V, and for a medium-voltage unit it is lethal without qualification.

The mandatory field sequence:

1. Open the disconnecting means and apply lockout/tagout.
2. Wait the required discharge time — do not shorten it.
3. Measure the terminal voltage with a rated meter to confirm discharge. Waiting is not verification.
4. Short and ground each unit's terminals with a properly rated grounding device, and leave the shorts applied while working. This is the step that protects against a partially failed internal element re-establishing a charge, or against dielectric absorption returning voltage to the terminals after an apparently complete discharge.
5. Only then handle the units.

Dielectric absorption is real and counterintuitive: a capacitor that has been shorted and released can recover a significant terminal voltage over the following minutes as absorbed charge redistributes within the dielectric. This is why the shorting device stays on rather than being touched on and removed.

3. Capacitor construction and fusing

A capacitor bank is built from units (cans), and each unit contains many elements — small individual capacitors — in a series-parallel arrangement inside the case.

The interpretation rule. In a can, elements are arranged in series groups. When an element short-circuits, it removes itself from the series string, and the remaining elements see higher voltage and the unit's capacitance rises. In an internally fused design, the internal fuse then clears that element, and the unit's capacitance falls slightly relative to nameplate. Which direction the measured value moves therefore depends on the fusing design — but any measured deviation of more than a few percent from nameplate indicates internal element failures and is the primary field diagnostic.

Unbalance protection monitors the electrical balance between bank sections. As units fail, the balance shifts. It is set to alarm at a level indicating some failures and to trip before the surviving elements are stressed to cascade failure. Verifying the unbalance relay setting and function is a real commissioning task.

4. Capacitor testing

1. Discharge and ground per Section 2.
2. Visual inspection — Level II names it: corona and thermal discoloration, and connection and mounting condition. Add: case bulging, which is a definite indicator of internal failure and gas generation and condemns the unit; leaks; corrosion; and bushing condition.
3. Capacitance measurement on each unit, compared against nameplate and against the other units in the bank. Comparison across the bank is more sensitive than nameplate comparison because manufacturing tolerance is common to all.
4. Insulation resistance terminals-to-case at the manufacturer's recommended voltage. This checks the case insulation, not the dielectric between plates.
5. Discharge resistor verification — confirm the unit actually discharges within its required time. A failed resistor is a latent lethal hazard that no other test finds.
6. Fuse continuity on externally fused banks — Level II names this explicitly. A blown external fuse means a failed unit, and a bank running with blown fuses is unbalanced and overstressed.

5. Reactors

A reactor is an inductor. Field applications:

• Current-limiting reactors — reduce available fault current so downstream equipment ratings suffice.
• Neutral grounding reactors — limit ground fault current on a resonant or reactance-grounded system.
• Detuning / filter reactors — series with capacitors, discussed below.
• Shunt reactors — absorb capacitive charging current on long lightly loaded lines.
• Smoothing reactors — DC circuits and drives.

Testing:

• Winding resistance with a DLRO or winding resistance ohmmeter, compared phase to phase and against baseline. A rising value indicates a degrading connection or a partially open winding.
• Insulation resistance winding to ground and, on multi-winding units, between windings.
• Inductance / impedance measurement where the manufacturer specifies it.
• Turn-to-turn faults are the difficult case: they may barely change DC resistance while substantially changing inductance, so an impedance or frequency-response measurement is what detects them.
• Visual: on dry-type air-core reactors, check for cracked or degraded encapsulation, contamination between turns, and clearance to surrounding metal. An air-core reactor's magnetic field extends well beyond the coil, and nearby metallic structure or closed metallic loops will heat by induction. Magnetic clearance requirements are a design constraint that field modifications routinely violate.

6. Resonance — why capacitors and harmonics are dangerous together

A shunt capacitor bank and the source inductance form a parallel resonant circuit:

$f_r = \frac{1}{2\pi\sqrt{LC}}$

A useful field approximation expresses the resonant harmonic order as:

$h_r \approx \sqrt{\frac{\text{MVA}_{SC}}{\text{MVAR}_{cap}}}$

where MVA_SC is the short-circuit capacity at the bus and MVAR_cap is the capacitor rating.

Worked example. A bus with 100 MVA short-circuit capacity and a 4 MVAR capacitor bank:

$h_r \approx \sqrt{100/4} = \sqrt{25} = 5$

The bank resonates at the 5th harmonic — which is the dominant harmonic produced by six-pulse rectifiers, meaning virtually every variable frequency drive in the plant. At resonance the parallel circuit presents a very high impedance to harmonic current injection, amplifying the harmonic voltage and driving large circulating currents between the capacitor and the source. The results are capacitor fuse operations, overheated and bulging capacitor cans, transformer overheating, and distorted bus voltage.

This is the classic Level IV failure-analysis scenario: a plant adds power factor correction to reduce a utility penalty, and within months the capacitors are failing repeatedly. The capacitors are not defective; the bank resonates with the source at a harmonic the plant produces in quantity.

The fix: a detuning reactor in series with the capacitor. Adding series inductance lowers the resonant frequency of the branch, and it is deliberately tuned below the lowest significant harmonic — commonly to around the 4.2nd or 4.7th order when the 5th is the concern. Below its tuning point the branch is capacitive and still corrects power factor; at and above it, the branch is inductive and cannot form a parallel resonance with the source at a harmonic that is actually present. A harmonic filter bank is exactly this: a reactor-capacitor series combination tuned to present a low impedance path that sinks a chosen harmonic.

Before installing any capacitor bank in a plant with significant non-linear load, a harmonic study is required. Calculating the resonant order from short-circuit capacity and bank size is the five-minute check that prevents the failure.

Exam trap: A question describes repeated capacitor failures shortly after a power factor correction bank was installed at a plant with many variable frequency drives, and offers "defective capacitor units from the manufacturer" as an option. The mechanism is harmonic resonance between the bank and the source inductance, and the remedy is a detuning reactor, not replacement capacitors.