HT capacitor banks and detuned reactors: correcting power factor on the 11kV bus

An HT capacitor bank on the 11 kV bus corrects power factor where the utility meters it. That makes it the right tool for plants billed at 11 kV, plants with large HV motors, and plants whose LT correction cannot cover transformer and HV feeder reactive demand. Once drives, DC mills or furnaces are on the same bus, the bank needs a series detuned reactor so it cannot resonate with the supply. New India Electricals Ltd (NIEL) has supplied a 2600 kVAR HT capacitor bank with reactors for a hot strip and cold rolling mill in South East Asia whose plant distribution runs at 11 kV, alongside the 11 kV VCB panels that switch it.

TL;DR

• HT capacitor banks above 1000 V are covered by IS 13925 (Part 1): 2012, the Indian adoption of IEC 60871-1:2005.

• A capacitor on a bus resonates at harmonic order r = square root of (fault level in MVA divided by bank rating in Mvar). Every step switched in moves that resonance lower.

• A detuned reactor in series moves the bank’s own tuning below the lowest harmonic. The common reactor ratings are 5.7, 7 and 14 percent, which tune to about 210, 190 and 135 Hz on a 50 Hz system.

• The reactor raises the voltage across the capacitor, so the capacitor’s rated voltage must be higher than the bus voltage. At 7 percent the rise is about 7.5 percent.

• Capacitor switching needs a restrike-free breaker. Under IEC 62271-100 that means class C2 for banks switched often.

• IS 13925 limits switching inrush to 100 times rated current and requires discharge to 75 V or less within 10 minutes.

• The CEA 0.95 power factor floor applies to distribution systems and bulk consumers, defined as supply at 33 kV or above. An 11 kV plant’s target usually comes from its state tariff order.

When does power factor correction belong on the 11 kV bus rather than the LT side?

Correct on the 11 kV bus when the plant is metered at 11 kV and a large part of its reactive demand sits upstream of the LT boards. That demand includes HV motors, furnace or rectifier transformers, and the magnetising and leakage reactance of the plant’s own distribution transformers. LT capacitors cannot see that demand; an HT bank at the metering point can.

NIEL’s guide to APFC panels sets out the LT case. There, correction at the LT incomer relieves the transformer and the LT distribution. The trade-off at 11 kV is the mirror image:

Large plants often use both: a few-step HT bank for base reactive demand at 11 kV, switched rarely, and LT APFC to trim variable load.

The target figure deserves care. The CEA Technical Standards for Connectivity to the Grid state that “the power factor of the distribution system and bulk consumer shall not be less than 0.95.” The same regulations define a bulk consumer as “a consumer who avails supply at voltage of 33 kV or above.” For an 11 kV consumer, the binding figure usually comes from the state tariff order instead. Aiming above 0.95 keeps a plant clear of both.

Why can an 11 kV capacitor bank resonate, and how do you check?

A capacitor bank and the supply’s short-circuit inductance form a parallel resonant circuit. If that resonance lands on a harmonic that drives, rectifiers or furnaces inject, the harmonic current and voltage are amplified. IS 13925 (Part 1): 2012, Annex D, gives the check: resonance occurs at harmonic order r equal to the square root of S divided by Q.

In that formula, S is the short-circuit power in MVA at the point of connection and Q is the bank rating in Mvar.

First, resonance depends on the supply as much as on the bank. A bus whose fault level changes when a transformer or grid source is switched out moves its resonance with it.

Second, a stepped bank sweeps its resonance across the harmonic spectrum as steps come in. On an illustrative 11 kV bus with a fault level of 100 MVA:

At 4 Mvar, this bank sits exactly on the 5th harmonic, the largest component from six-pulse drives and rectifiers. At 2 Mvar it sits close to the 7th. A plant can commission one step without trouble, add another a year later, and start losing capacitor units with nothing obvious changed.

IS 13925 lists three remedies for excessive harmonic current: move the capacitors elsewhere in the system, add capacitance near the rectifiers, or connect “a reactor in series with the capacitor to lower the resonant frequency of the circuit to a value below that of the disturbing harmonic.” The third is the detuned reactor.

The standard also warns about a less obvious source. At light load, voltage rise from capacitors can saturate transformer cores, producing “harmonics of abnormal magnitude,” one of which “may be amplified by resonance between transformer and capacitor.” That is one reason IS 13925 recommends switching capacitors off at periods of light load.

What does a detuned reactor do, and which percentage should you choose?

A detuned reactor is an iron-cored or air-cored inductor connected in series with each capacitor step. It tunes the step to a frequency below the lowest significant harmonic on the bus. Above that frequency the step is inductive, so it cannot form a parallel resonance with the supply at the 5th, 7th or higher harmonics.

The reactor is specified by its relative impedance, p, as a percentage of the capacitor reactance at 50 Hz. Schneider Electric’s Electrical Installation Guide gives the common values and their tuning:

The guide notes that 14 percent “is used with high level of 3rd harmonic voltages.” The final choice depends on the measured spectrum. The NIEL article on VFD panels and their effect on supply explains where that spectrum comes from.

A detuned bank is not a tuned harmonic filter. A tuned filter is set close to a harmonic, for example the 5th, so that it absorbs that harmonic current from the bus. A detuned bank is set well below the 5th so that it avoids resonance and absorbs only a modest share of harmonic current. If the aim is to meet a harmonic limit at the point of connection, a filter study is needed; a detuned bank alone may not achieve it.

How does a series reactor change the capacitor rating?

The reactor and capacitor share the same current, but their voltages are opposite in phase. So the voltage across the capacitor rises above the bus voltage by a factor of 1 divided by (1 minus p). IS 13925 (Part 1) states that where inductive elements are inserted in series with the capacitor, “the resultant increase of the voltage at the capacitor terminals above the operating voltage of the network requires a corresponding increase in the rated voltage.”

The arithmetic for an 11 kV bus:

The capacitor’s rated voltage has to cover the maximum operating voltage of the network as well, which IS 13925 requires to be stated by the purchaser. A bank specified with capacitors rated at exactly 11 kV plus a 7 percent reactor is overstressed from the day it is energised.

The effective output of the step also changes. Specify the bank by its reactive output at the bus voltage, with the reactor included, not by the nameplate of the capacitor units. Otherwise the delivered kVAR will not match the calculation that justified the bank.

IS 13925 also flags two reactor details. If iron-cored reactors are used, “attention should be paid to possible saturation and overheating of the core by harmonics.” All loss-producing accessories, reactors included, should be counted in the total bank losses.

What limits does IS 13925 set on an HT capacitor bank?

IS 13925 (Part 1) sets permissible overvoltage, overcurrent, switching inrush and discharge limits for capacitors above 1000 V. These numbers define the envelope the reactor, breaker and protection must keep the capacitors inside.

Three of these drive the bank design directly.

Inrush. IS 13925 warns that transient overcurrents are highest “when a section of a capacitor bank is switched in parallel with other sections which are already energized.” It suggests limiting them with resistance switching “or by the insertion of reactors into the supply circuit to each section.” A detuned reactor on each step does this job at the same time.

Switching frequency. The 2 root 2 overvoltage and 100 times inrush limits assume up to 1000 switchings a year. The standard says that for capacitors switched more often, these values “shall be limited to lower levels” agreed in the contract. An HT bank run like an LT APFC, switching steps many times a day, needs that agreement in writing.

Reclosing. The 10 percent residual voltage limit and the 10 minute discharge time mean an HT step cannot be switched back in immediately after tripping. IS 13925 requires the bank rating plate to state the minimum time between disconnection and reclosure. The control scheme, and any auto-reclose on the feeder, must respect it.

What switchgear and protection does an 11 kV capacitor bank need?

Each step needs a circuit-breaker or switch rated for capacitive switching, overcurrent protection set against the IS 13925 current limit, and unbalance protection to detect failed capacitor units. IS 13925 recommends “circuit-breakers suitable for capacitor switching” such that “restriking during breaking operations, which may result in excessive overvoltages, cannot occur.”

In current IEC terms, that means a capacitive switching class under IEC 62271-100. Schneider Electric’s explanation of class C1 and C2 summarises the standard. Class C1 means “low probability of restrike” and suits banks switched very seldom. Class C2 means “very low probability of restrike” and suits banks operated often, “one per day for instance.” The same note records that IEC 62271-100 defines back-to-back capacitor bank test duties with an inrush current of 20 kA at 4250 Hz. That is why per-step reactors, which cut the inrush, make breaker selection easier.

NIEL’s Power+ V indoor VCB panels cover 3.3 kV to 11 kV, 630 A to 3150 A and short-time ratings from 18.4 kA to 40 kA. The page lists ABB Vmax breakers up to 31.5 kA and VD4 at 40 kA, designed to IEC 62271-100. For a capacitor feeder, confirm the breaker’s capacitive switching class and rated capacitor bank breaking current for the specific rating ordered. NIEL’s guide to specifying MV panels covers the rest of the lineup.

On protection, IS 13925 recommends overcurrent relays “adjusted to trip the circuit-breakers when the current exceeds the permissible limit,” and adds that “fuses generally do not provide suitable overcurrent protection.” Unbalance protection, which IS 13925 expects to appear on the bank connection diagram, detects failed capacitor units early. Overvoltage protection, or at minimum an interlock that trips the bank at light load, handles the voltage rise that IS 13925 warns about.

How should an HT capacitor bank be commissioned on site?

Commission an HT bank as a system: capacitors, reactors, switchgear, protection and control together. Verify the bank’s actual resonance behaviour before leaving it in automatic service. The measurements that matter are capacitance per phase, reactor inductance, protection settings against IS 13925 limits, and a harmonic measurement with the bank in and out.

The practical sequence:

• Measure capacitance of each unit and phase. IS 13925 limits the ratio of maximum to minimum capacitance measured between any two line terminals to 1.08.

• Confirm reactor inductance and the resulting tuning frequency against the design value.

• Check discharge devices and the reclose time delay in the control scheme.

• Test unbalance, overcurrent and overvoltage protection by secondary injection.

• Energise one step at a time and record bus voltage, step current and harmonic spectrum at each stage.

• Compare the measured voltage total harmonic distortion with and without the bank in service.

If voltage distortion rises as steps are added, the bank is amplifying a harmonic.

The CEA Safety Regulations, 2023 also require site tests “as per relevant standards” before any new apparatus above 650 V is commissioned.

What did an 11 kV capacitor bank look like on a rolling mill project?

On a hot strip and cold rolling mill project in South East Asia, NIEL supplied the electrical balance of plant as one scope. The 11 kV side included an 11 kV VCB panel lineup with one incomer and three outgoing feeders, 25/31.5 MVA 66/11 kV power transformers, and a 2600 kVAR HT capacitor bank with reactors. The mill load included 750 kW DC motors, thirty-one further DC motors and four 690 V AC motors at 1000 kW and 1100 kW.

DC mill drives are converter loads, so they draw harmonic current and consume reactive power that swings with rolling load. A capacitor bank on that bus without reactors is exposed to exactly the resonance sweep in the table above. The project scope put the bank, its reactors, the switchgear and the transformers under one supplier, and NIEL supplied and commissioned the equipment. NIEL’s guide to substations for industrial plants explains why that single scope matters at the interfaces.

What should an HT capacitor bank specification include?

• System voltage, maximum operating voltage and frequency

• Fault level at the point of connection, minimum and maximum, from the power system study

• Measured harmonic spectrum at the 11 kV bus, with voltage THD and individual orders

• Required reactive output in kVAR at bus voltage, number of steps and step sizes

• Detuning ratio, or a requirement for the supplier to propose it from the spectrum

• Capacitor rated voltage, accounting for the reactor voltage rise

• Connection: double star with unbalance protection, or as agreed

• Expected switching operations per year, if above 1000

• Breaker capacitive switching class under IEC 62271-100, and rated capacitor bank breaking current

• Discharge time to 75 V and minimum reclose interval

• Protection: overcurrent, unbalance, overvoltage and light-load tripping

• Target power factor and the tariff clause it serves

• Indoor or outdoor installation, ambient temperature and enclosure

Correcting power factor on your 11 kV bus?

NIEL supplied a 2600 kVAR HT capacitor bank with reactors for a hot strip and cold rolling mill in South East Asia, together with the 11 kV VCB panels and power transformers on the same bus. Send us your single line diagram, fault levels and a harmonic measurement, and our engineers will review the resonance check and switching arrangement before a kVAR figure is fixed.

Find the equipment around your capacitor bank:

• Power+ V indoor VCB panels: 3.3 kV to 11 kV, 630 A to 3150 A, for the capacitor feeder.

• Power transformers: the source impedance that sets the resonance point.

Call +91 80 42434343, email support@newindiaelectricals.com, or contact our team with your bus details.

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