Sizing servo voltage stabilizers for a multi-site rollout

An organisation rolling out voltage stabilizers across branches, offices or service points usually wants one answer for every location. A single rating is easy to buy and easy to stock, but sites differ in load, supply condition and growth. The useful standard in a multi-site programme is the sizing method and the specification, not one number applied everywhere.

This article sets out the inputs that decide a servo voltage stabilizer rating, how margin is added, and how sites can be grouped into a small number of rating bands that keep procurement and service manageable.

Start with the load at each site

The rating of a stabilizer is the apparent power it must deliver, in kilovolt-amperes (kVA). It follows from the load connected downstream, not from the size of the building or the incoming supply.

• Connected load: the sum of nameplate ratings of everything the stabilizer will feed.
• Operating load: what actually runs together at the busiest time of day. This is normally lower than the connected load and is the better basis for sizing.
• Power factor: loads are often listed in kilowatts (kW). Dividing kW by the power factor gives kVA. Where the power factor is not known, 0.8 is a common working assumption that is replaced by measured values when they exist.

Motor starting and inrush

Compressors, lift motors, pumps and air-conditioning plant draw several times their running current for a short period when they start. A direct-on-line motor start is typically five to seven times full-load current. The stabilizer and its protection must carry this without tripping or a large output dip, so the largest motor start at each site belongs on the survey sheet alongside the steady load.

Record the incoming voltage, not the nominal

A site described as having a 415 V supply may see far less at the end of a long feeder during peak hours, and more at night. Sizing needs the observed minimum and maximum, ideally recorded over at least a week that includes peak and off-peak periods. A short spot reading during a site visit can miss the conditions that matter.

The stabilizer's input window has to cover the recorded range with some allowance. For three-phase servo voltage stabilizers a typical wide input window is 340–480 V AC, with a nominal output of 400 or 415 V AC. Narrower windows, such as 360–450 V AC, suit steadier supplies. A site whose recorded minimum falls below the window needs a different configuration, not simply a larger rating.

Why low input voltage matters for sizing

At the bottom of the input window the stabilizer still delivers its rated output, so it draws more current from the supply. As a rough guide, the input current rises in the ratio of output voltage to input voltage. At 340 V in and 415 V out, that ratio is about 1.22, so the incoming current is around 22% higher than at nominal input. Incoming cables and upstream protection are selected with this in mind.

Add margin deliberately

A margin covers measurement uncertainty and modest load growth. A typical allowance is 20–25% above the peak operating load, applied once. Margins added at every stage (by the site survey, the consultant and the buyer) compound into equipment that is much larger than the duty and runs lightly loaded for its life.

Group sites into rating bands

Once each site has an indicative requirement, sites can be grouped into three or four rating bands. Each band shares one specification: enclosure, protection, bypass arrangement, indication and documentation. Within the programme, a site moves to the next band only when its survey data justifies it.

Bands reduce the variety of spares, keep installation and maintenance procedures common, and make it easier to compare what was supplied across locations. They also give procurement a stable basis for comparing complete scope rather than equipment price alone.

Choose the phase arrangement

Most rollout sites take a three-phase supply but feed a mixture of single-phase loads: lighting, computers, small air-conditioners. Phase loads are then unequal and phase voltages can drift apart. A stabilizer with independent control on each phase corrects each phase to its own setting. Balanced control, where all phases are corrected together, suits loads that are themselves balanced, such as three-phase motors. The phase arrangement belongs in the band specification.

Conditions that change the rating

• Ambient temperature and ventilation: ratings are stated at a defined ambient temperature. A plant room that runs hotter, or a cabinet with restricted airflow, needs review.
• Non-linear loads: uninterruptible power supplies, variable-speed drives and large banks of electronic equipment draw distorted current that adds heating.
• Planned additions: a new floor, an extra air-conditioning unit or a server room expansion should be known before the rating is fixed.
• Installation constraints: floor space, access for maintenance and cable routes can favour one configuration over another.

What to collect from each site

1. Connected and peak operating load, with the largest motor start.
2. Recorded incoming voltage per phase over at least a week.
3. Supply arrangement: three-phase or single-phase, neutral and earthing condition.
4. Installation location, ambient conditions and access.
5. Known load changes over the planning period.

In India, IS 9815 covers servo-controlled voltage stabilizers and is a common reference in specifications. The applicable standard and the acceptance checks for a programme are agreed in the technical scope.

Where to go next

• Servo Voltage Stabilizers: typical ratings and specification rows.
• Corporate multi-site power quality: how a multi-site programme is reviewed.
• Discuss a multi-site requirement: share site data for an engineering review.