Capacitor Bank Controller: VarLogic, ABB RVC, BR6000

What it is and where it is used

The power factor controller of an automatic capacitor bank, such as the Schneider VarLogic NR6 or NR12, ABB RVC, Circutor computer Max or Epcos BR6000, measures the installation’s voltage and current and calculates the reactive power. It then switches capacitor steps in or out to keep the power factor near the target. This avoids reactive energy charges and frees capacity in transformers and cables.

They are installed in low-voltage banks in industry, buildings, commercial sites and pumping, and in medium-voltage banks with their own control. For the principle, see capacitor bank, power factor in industry and how to size a capacitor bank.

In medium- and high-voltage banks switched with a circuit breaker, the decision to switch in or out passes to a relay or the bay controller; we explain it in automatic control of capacitor banks, together with the controlled switching that limits the inrush current.

Variants: VarLogic NR6, NR12, RT6, RT12, ABB RVC, Circutor computer Max and Epcos BR6000

Main features

• Reactive power measurement with a current transformer at the service entrance and the bus voltage.
• Step regulation, with reconnection times that respect capacitor discharge.
• Alarms for undercompensation, overcompensation, overvoltage, overcurrent and disconnected current transformer.
• Harmonic distortion measurement on several models, useful to detect resonance risk.
• Four quadrants on some models, for installations with their own generation that export energy.

What data the manufacturer publishes

• ABB RVC (now Hitachi Energy): the instruction manual lists versions with 3, 6, 8, 10 and 12 outputs with user-defined switching sequences. It operates from 100 to 440 V (10% tolerance), at 50 or 60 Hz with automatic frequency adjustment, with a 1 A or 5 A current input and a maximum burden of 8 VA. The target power factor is adjustable from 0.7 inductive to 0.7 capacitive, and the start current threshold (C/k) from 0.01 to 3 A, with automatic C/k measurement. The switching time between steps is programmable from 1 to 999 s. The step relays accept 1.5 A continuous and 5 A peak up to 440 V AC, and the alarm relay 5 A. The manual recommends a class 1.0 CT of at least 10 VA.
• Epcos / TDK BR6000-T (dynamic): the version 5.2 manual lists 6 or 12 transistor outputs, one alarm relay, voltage measurement from 30 to 525 V (phase-neutral or phase-phase), 245 V AC supply and 5 A or 1 A measuring current. The target power factor ranges from 0.1 inductive to 0.1 capacitive. It has 20 preprogrammed control series and an editor for your own series, four quadrants and a second parameter set. It stores the last 8 fault states and its housing is 144 x 144 mm. Switch-on, switch-off and discharge times are selectable between 20 ms and 1 s. The T12/S version adds an RS-485 interface with Modbus or ASCII and comes with the BR7000-Soft software. TDK also lists the BR6000-R6 and BR6000-R12 versions for conventional (contactor) banks; verify with TDK which versions are still current, since some datasheets announce discontinuation.
• Schneider VarLogic: the NR6/NR12 manual covers installation, step programs and manual calculation of the response value, and mentions use at high voltage with a voltage transformer. The RT6, RT8 and RT12 are configured with the number of steps, the program according to the ratio between step ratings, the CT ratio (with a 5 A secondary) and the C/K value.
• Circutor computer Max P&P: the 6-step model has 230 and 400 V versions, measures voltage, current, power factor and THD, and automatically programs the C/K and the phase where the CT is located.

Dynamic or conventional control

A relay-output controller switches contactors: each step operation takes tens or hundreds of milliseconds and must respect the capacitor discharge time. On fast-changing loads, such as welding or cranes, the dynamic controller (BR6000-T or computer Max-f) commands thyristor modules and achieves response times of tens of milliseconds. The BR6000 manual reminds that the programmed discharge time must match the capacitor discharge time. The physical reason is general: reconnecting a step before it has discharged produces an inrush current that damages contactors and capacitors.

Control principles a controller describes

The BR6000 manual defines four switching principles. Sequential is last in, first out, with a long settling time. Ring or circular is first in, first out, and balances wear among equal steps. Intelligent ring is the factory default: it switches several steps at once to reach the target quickly with few operations. Combined is for detuned banks and switches the steps in pairs. When the steps have different ratings, the ratio between them (1:1:2:2, 1:2:4:8, etc.) defines how many combinations the bank has and how fine the control is. The smallest step sets the sensitivity: if the load changes by less than that step, the controller does not act.

The C/K parameter and its relation to the CT

The C/K response threshold is the reactive current, measured on the CT secondary, that justifies switching in the smallest step. It is obtained from that step’s rating, the network voltage and the CT ratio: C/K = step Q / (√3 x line voltage x CT ratio). If C/K is too low, the controller hunts, switching without stopping; if it is too high, it leaves reactive power uncompensated. Several controllers calculate it automatically at start-up, but that calculation assumes the CT is well located and the load varies enough during the measurement.

CT location and other installation errors

The current transformer is installed at the service entrance, upstream of the load and the bank, so that it measures the total current of the installation. If it is installed downstream of the bank, the controller does not see what the bank contributes and may switch steps in and out erratically. The BR6000 manual also points out the causes of several symptoms. «Measures wrong» is due to a short-circuited or badly connected CT. A badly compensated line is due to reversed polarity. The alarm relay activates after one minute for under- or overcompensation. Steps disconnect because of excessive cabinet temperature: the controller measures temperature and disconnects in stages from the programmed threshold, between 50 and 85 °C. On networks with very unbalanced loads, a controller that measures a single phase can differ from the three-phase power factor.

Harmonics and resonance: why capacitors fail

A capacitor and the network inductance form a resonant circuit; the parallel resonance frequency falls at harmonic order h = √(Ssc / Qc), with Ssc the short-circuit power at the connection point and Qc the bank rating. If that order is close to the 5th or 7th, and there are drives or rectifiers, the harmonic current is amplified and overloads the capacitors. The standard solution is the detuned bank: a reactor in series with each step, with tuning frequency f = f1 / √p, where p is the detuning factor. For example, 7% on a 50 Hz network gives 189 Hz, below the 5th harmonic. A prior study defines whether it is needed and with what factor (IEEE 519 harmonic study, resonance in capacitor banks). The controller should measure harmonics and alarm on overcurrent when the bank nears its limits.

Installations with their own generation

If the plant exports energy, reactive power can change direction; a four-quadrant controller (BR6000, computer Max and others) keeps the bank from worsening the power factor. For a solar plant, see solar panels and power factor. Reactive power rules depend on the country and the utility, and so do the penalties for a low power factor; check the tariff that applies to the site.

Communications and integration

Several models have Modbus RTU over RS-485 to bring the power factor, connected steps and alarms to the energy management system or SCADA.

Frequently asked technical questions

• VarLogic NR6 and NR12 controller manuals: Schneider publishes guides and configuration FAQs for each model.
• The bank does not connect steps: usually the current transformer is badly located (after the bank or on another phase), reversed polarity or a wrongly calculated C/K.
• Low power factor with the full bank: the bank is too small for the current load, or steps are out because of fuses or contactors.
• Capacitors that fail often: there may be harmonics or resonance; the bank may need detuning reactors.
• Reactive power penalty: depends on the country and the utility tariff; compare the penalty with the cost of the bank.

How we help

We do not sell banks or controllers. At Tiza Group we measure the installation, do the compensation and harmonic study, define the bank and its controller, and verify the result, in substations and in industry, in Colombia, or elsewhere on request; the studies are also done remotely, worldwide. See power quality studies within engineering design and studies. Write to us through contact.

Related guides

• Capacitor bank
• How to size a capacitor bank
• Power factor in industry
• Active vs passive harmonic filter
• IEEE 519 harmonic study
• AccuSine and PQF active filter, SVC and STATCOM
• Fluke 435-II, 1760, Hioki PQ3198 and PQ3100 network analyzers
• Automatic control of capacitor banks at MV and HV
• Controlled switching (point-on-wave): what it is and where it is used

References

• IEC 60831-1, Shunt power capacitors of the self-healing type for AC systems having a rated voltage up to and including 1000 V.
• IEC 61921, Power capacitors – Low-voltage power factor correction banks.
• Schneider Electric, VarLogic datasheet (Modbus Organization) and VarLogic RT configuration FAQ.
• ABB, RVC controller instruction manual (2GCS201086A0050) and announcement of the new RVC controller generation.
• TDK Electronics, BR6000-T user manual (version 5.2, March 2011).
• Circutor, computer Max P&P, computer Max-f and computer SMART III pages; EEPower, launch of the Epcos BR6000 series.

Frequently asked questions