Video summary

Capacitor Banks Design and Operational Fundamentals Webinar

Main summary

Key takeaways

Educational

Main Ideas and Concepts Covered

Purpose of capacitor banks (VAR/PF compensation)

Capacitor banks are used for power factor (PF) correction by supplying reactive power (kVAR). They reduce the inefficient loading caused by inductive equipment such as motors, transformers, furnaces, and HVAC systems.

Power factor & power relationships (VAR compensation basics)

  • Active power (kW): useful work (measured with a watt meter).
  • Reactive power (kVAR): does not perform useful work; circulates between the source and the load and increases system burden.
  • Apparent power (kVA): vector combination of kW + kVAR.
  • Power factor (PF):
    • Defined as kW / kVA (equivalent to cos(θ)).
    • Low PF → higher kVA/current for the same kW, increasing inefficiency and typically costs/penalties.

Improving PF with capacitors

Adding power factor correction capacitors can raise PF substantially.

  • Example progression: 70% PF → ~95% PF for the same 200 kW demand.
  • Example impact: 284 kVA → 210 kVA
    • Leads to cost savings and reduced losses.

Why good power factor matters

  • Lower current → less heat loss in wiring → higher efficiency.
  • Lower stress on components → longer lifetime.
  • Avoid utility penalties for low PF.
  • Reduce electric bills through decreased system losses and better utilization.

Capacitor Bank Design Requirements (What’s Needed to Design a Bank)

Customer inputs/data needed (to reduce revisions and lead time)

Electrical requirements

  • Nominal voltage
  • Actual system voltage (line-to-line rated)
  • Required reactive power (in MVAr)
  • Frequency (50 Hz vs 60 Hz)
  • Connection type (examples referenced)
    • Ungrounded Y
    • Grounded Y
    • YY
    • Grounded YY
  • Insulation level (kV-BIL)
  • Capacitor unit size
  • Accessory choices / switching approach
    • Whether energized via capacitor switcher (cap switcher) or other device
  • Current limiting reactor (CLR)
    • For inrush and/or outrush control
  • Protection measurement devices
    • Potential Transformer (PT/VT)
    • Current Transformer (CT)
  • System protection elements
    • Arresters (example: one per phase + a fourth on neutral)
  • Safety equipment
    • Ground switch (and/or ground studs) to safely ground the bank before maintenance

Environmental & installation requirements

  • Altitude
    • Higher altitude reduces insulating performance and increases risk of issues like partial discharges/low dielectric gradient.
  • Ambient temperature
    • Below ~40°C: concerns tied to dielectric oil performance/operation in cold conditions.
    • Above ~45°C: derating considerations (reduced unit stress/current).
    • Limits cited:
      • Capacitor unit internal temperature continuous: ≤ 65°C
      • Maximum ambient temperature: ~55°C
    • Life expectancy note:
      • Capacitors generally do not see life reduction solely due to elevated temperature as expected; very cold operation is more problematic.
  • Wind and seismic requirements
    • GE typically provides foundations for ~90 mph wind.
    • If seismic requirements apply, additional foundation design and potentially outsourced reaction loading may be required.

Externally Fused Protection Schemes (Externally Fused Capacitor Banks)

What the externally fused concept includes

Externally fused designs use capacitors with external fuse links to provide open/fault isolation—without needing to re-terminate series strings like some fuseless approaches.

Typical externally fused ungrounded Y example (high-level structure)

  • Nominal voltage example: commonly 34.5 kV
  • Substructure: about one rack/substructure, ~10 ft tall
  • Three phase racks:
    • Each rack represents a phase (A/B/C)
    • Capacitor units arranged as series sections, with series and parallel groupings based on required MVAr.
  • Series arrangement example (one series section):
    • Example described: 12 capacitors total in a series structure.
  • Accessories mounted on the substructure:
    • Ground switch
    • Junction box including PT (neutral monitoring)
    • Arresters: three phase arresters + neutral arrester (4 total)

Lead time / standardization points

  • Standard externally fused catalog banks
    • Example lead time: ~16 weeks from order to shipment.
  • Drawing lead time
    • Standard: ~2 weeks
    • Custom: up to ~5 weeks
  • Benefits of standardized designs:
    • Faster proposals/drawings
    • Rolling spare inventory for standardized capacitor units
    • Shorter outage time after failures (when spares are allowed by system design)

How series sections affect design and cost

Core calculations and constraints

  • Line-to-line voltage relationship
    • Stated as: V(line-line) = unit voltage × √3 (with series-quantity context)
  • Voltage per series section limit
    • Example: series section voltage must be ≤ 24,900 V
  • Parallel energy limit
    • Example: parallel energy per series section per phase must be ≤ 9,300 kVAR
  • If limits are exceeded:
    • Add more series sections, which increases:
      • insulator cost
      • rack sizes
      • bus/pipe/cable connection changes

Multiple banks on the same bus

If multiple capacitor banks share a bus and no CLR is used for inrush/outrush, the parallel energy calculation must include all banks.

Determining number of units needed for protection coordination

Externally fused protection requires a minimum capacitor quantity to make protection logic meaningful:

  • 1 fuse operated → alarm
  • 2 fuses operated → trip breaker

Example minimum: at least 4 capacitors in series in some designs to achieve adequate protection behavior.

Choosing ungrounded Y vs grounded Y (cost/performance tradeoff)

  • Ungrounded Y vs grounded Y
    • For open-ended cases, GE “generally choose ungrounded Y” for cost reduction.
    • If required, grounded Y may be used using different detection methods.

Detection of unbalance / failure

  • Ungrounded Y, one series section
    • Detects unbalance by measuring neutral (zero-sequence voltage) via PT.
    • Neutral changes indicate failed phase/string → relay alarm/trip thresholds.
  • Grounded Y / two series sections
    • Uses CT monitoring based on neutral current unbalance rather than only neutral voltage.
    • CT rating requirements depend on line-to-line voltage relationships (noted sizing relationship).

Protection setpoint logic (example described)

For an example 345 kV grounded Y case using dual ratio PT:

  • Alarm/trip thresholds are derived from expected neutral voltages for scenarios such as 1 can out, 2 cans out, etc.
  • Example outcomes:
    • First alarm: based on neutral voltage ratio (relay alarm set ~70% of computed threshold).
    • Second point: halfway between first alarm and the “two cans out” neutral voltage threshold → trip breaker.

Fuse link sizing rule of thumb (as presented)

Fuse continuous current capability depends on connection type:

  • Grounded Y: fuse continuous current capability ≥ 135% of rated current
  • Ungrounded Y: fuse continuous current capability ≥ 125% of rated current

GE provides the fuse rating calculation; smaller fuse sizes may reduce cost if permitted by requirements.

Externally Fused Maintenance Fundamentals

Typical maintenance actions emphasized

Externally fused banks provide a visual indicator:

  • The fuse link presence/condition can be checked from the control house or site visit.
  • If a fuse has opened:
    • fault localization is easier
    • field service time is reduced

Benefits compared to fuseless designs

  • Externally fused designs avoid needing to rewire series connections when a unit fails.
  • Standardized parts enable faster replacement because spare/standard unit sizes are available.

Switching and safety equipment fundamentals

  • Cap switcher
    • Sized based on MVAr rating.
    • Closing resistor sizing and use of a current limiting reactor help manage switching surges.
  • Ground switch
    • Often a four-pole ground switch for neutral + phases safety scheme.
    • Interlock approach ensures the bank is de-energized before grounding.
  • Arresters
    • Installed as close as possible to protected equipment for best surge protection.

Spare units concept

Standard externally fused designs may include spare units mounted and provided as part of the bank package.

Fuseless Capacitor Banks (Internal Fault “Self-Protection” Concept)

How fuseless protection works

Fuseless designs rely on capacitor element behavior:

  • If dielectric failure occurs, energy causes arc puncturing and film/foil behavior that welds layers together.
  • The result: the entire series section becomes electrically shorted, limiting fault impact.

Protection/monitoring approaches mentioned

  • PT-based scheme (neutral/phase monitoring)

    • PT measures voltage drop across a low-voltage capacitor element for fault indication.
    • CT-based schemes were also discussed separately depending on system voltage/MVAr.

  • CT-based scheme

    • CT monitors current changes per string, offering more granular fault location.
    • Pros: control house can identify the exact string that failed → faster replacement.
    • Cons: many CTs can increase initial cost.

Tradeoff example

CT monitoring across neutral points can be cheaper, but it may be harder to identify a specific failing unit if multiple failures cancel out the imbalance signal.

Maintenance implication stated

Fuseless designs may take longer to identify the exact failed unit, especially with monitoring schemes that do not uniquely identify failure location.

Common Q&A Themes (Issues and Answers Presented)

  • Partial discharge at temperatures below -40°C
    • Speaker indicated it would be reviewed and a detailed answer posted later.
  • Inrush reactors / filter option
    • Answer: Yes, can be used to protect against switching surges; used in practice.
  • Harmonic resonance protection
    • Not typically provided as a dedicated resonance protection method; PT/CT monitoring is used for neutral-based protection.
  • How to select externally fused vs fuseless
    • Decision mainly based on cost and size.
    • Example guidance: smaller banks at 345 kV tend toward externally fused; higher voltages may benefit more from internally fused/fuseless designs.
  • Discharging provision for externally fused banks
    • Answer: Yes, via ground switch solutions (optionally rack shorting switches too).
  • Y vs Delta connection choice
    • Depends on voltage/application.
    • Lower voltages (≤ ~345 kV) more commonly use ungrounded Y.
    • Higher voltages (~345 kV / 49 kV and above) more commonly use grounded Y; reasons depend on system specifics.
  • If a fuse blows, does it trip the main breaker?
    • Generally:
      • one fuse → alarm
      • two fuses → trip breaker
  • Fuse rating selection
    • Speaker offered to post the fuse rating calculation method later.
  • Handling -90 PF compensation
    • Capacitors have ratings such as -40 to +40°C; applications at -90 PF can be reviewed after the call.
  • Arrester voltage rating vs capacitor voltage rating
    • Arrester MCoV should be based on system voltage requirements, not necessarily capacitor bank rating.
  • Where the “1.5 factor” comes from in fuse sizing
    • Comes from the fuse manufacturer/fuse selection calculation requirements.

Speaker / Sources Featured

  • Ashish Kok — Digital Marketing Specialist, Grid Solutions (webinar host/intro)
  • Dylan Kirkland — Lead Applications Engineer, GE Grid Solutions (main presenter)

Original video