Overview

Hilkar Medium Voltage Capacitor Banks are complete, factory-assembled systems designed for reliable reactive power compensation and easy installation. Each system integrates capacitor units, harmonic filter reactors (when required), switching and protection equipment, and a dedicated control system. Their modular design enables flexible indoor or outdoor installation, providing efficient power factor correction and improved power quality for a wide range of medium-voltage applications.

System Features

Hilkar MV Capacitor Banks are complete, factory-assembled systems that include:

Ready-to-install, stand-alone design
Earthing switch for safe capacitor isolation and grounding
Iron-core or air-core harmonic filter reactors, or air-core inrush current limiting reactors
Optional low-inductance damping resistors for high-pass filters
Vacuum contactors and capacitor fuses for reliable switching and protection
Automatic Power Factor Correction (APFC) controller
Overvoltage and overcurrent relays, current transformers (CTs), and voltage transformers (VTs)
Touch-safe design
Optional double-star configuration with unbalance protection
Operating voltage from 2.4 kV to 36 kV, 50/60 Hz, up to 200 kV BIL
Smoke detector for enhanced safety
Modular design suitable for indoor or outdoor installation

Hilkar metal-enclosed MV capacitor bank with segmented gray enclosure, inspection windows and service doors

Rating cards for a 22 kV, 2 x 1500 kVAr metal-enclosed capacitor bank with IP55 protection

Applications

Power Factor Correction
Harmonic Filtering
Overvoltage Mitigation
Reduction of System Losses

Enclosure Types

Hilkar MV Capacitor Banks are available in metal-enclosed or ISO containerized configurations to suit site requirements, installation conditions, and environmental applications.

Standards: IEEE Std. 1036, IEC 60871-1

ISO container-enclosed MV capacitor bank configuration

Metal-enclosed MV capacitor bank front view

Metal-enclosed MV capacitor bank with control cubicle and four capacitor sections, angled view

MV capacitor bank with control panel front view

Outdoor MV capacitor bank enclosure unit

Capacitor Bank Protections

VCB: Vacuum circuit breaker
DS: Disconnector with earthing switch
CT: Current transformer
VT: Voltage transformer
27: Undervoltage relay
59: Overvoltage relay
50/51: Overcurrent relay
50N: Neutral current relay (unbalance)
VC: Vacuum contactor
F: Fuse
L: Inrush current limiting (or harmonic filter) reactor
Q: Capacitor bank
I_n: Capacitor rated current

Select the protection fuse current as I_f ≅ 2 × I_n.

Set the ANSI 51 (time overcurrent) relay to 4–6 × I_n with a 0.1 s time delay for short-circuit protection.

Set the ANSI 50 (instantaneous overcurrent/overload) protection to 1.3 × I_n with a 4 s time delay.

Set the 50N relay (overload protection) with a 4-second delay at 0.05 × I_n.

Use the delta and star connection formulas to calculate the resistor R (kΩ) needed across the capacitor. This resistor drops the voltage of a capacitor bank with capacitance C (µF) below 75 V after 10 minutes (600 seconds).

Delta Connection

R = \dfrac{600}{C} \times \dfrac{1}{\ln\!\left(\dfrac{\sqrt{2}\,U}{75}\right)}

Star Connection

R = \dfrac{1800}{C} \times \dfrac{1}{\ln\!\left(\dfrac{\sqrt{2}\,U}{75\sqrt{3}}\right)}

R = Discharge resistor value (kΩ)
C = Capacitance per phase (µF)
U = System line-to-line voltage (V)
I_n = Capacitor rated current (A) (used for protection settings, not in the resistor calculation)
75 V = Maximum residual voltage after 10 minutes (600 s) according to common capacitor discharge requirements.

Inrush Current Calculation (Single Capacitor Bank)

The inrush current when energizing a capacitor bank depends on the system impedance and the capacitor bank characteristics. To prevent excessive switching currents, a current-limiting reactor may be connected in series with the capacitor bank.

Where:

U = Phase-to-neutral voltage (V)
X_c = Phase-to-neutral capacitive reactance (Ω)
X_L = Total inductive reactance between the capacitor connection points (Ω)
Q, Q₁, Q₂ = Capacitor bank reactive power (kVAr)
S_SC = Short-circuit power at the capacitor connection point (kVA)
I_N = Rated capacitor bank current (A_rms)
I_SC = Short-circuit current at the capacitor connection point (A_rms)

To limit the capacitor switching inrush current to:

I_C \leq 100 I_N

A current-limiting reactor should be connected in series with the capacitor bank. The reactor inductance is selected to reduce the peak inrush current while maintaining acceptable voltage drop and normal operating performance. This improves capacitor lifetime, minimizes contactor wear, and reduces electrical stress on switching devices.

\begin{aligned} I_C &\approx I_N\sqrt{\dfrac{2S_{SC}}{Q}} \\ &= \sqrt{2\,I_{SC} \times I_N}\;(\mathrm{A}) \end{aligned}

symmetrical peak-current estimate

\begin{gathered} L = \dfrac{U^2}{2\pi f}\left[\dfrac{200}{Q} \cdot \dfrac{10^6}{S_{SC}}\right] (\mu\mathrm{H}) \\[6pt] {\small U\,[\mathrm{V}],\; f\,[\mathrm{Hz}],\; Q,\, S_{SC}\,[\mathrm{VA}]} \end{gathered}

Example Calculation

Given:

Capacitor bank rating, Q = 200 kVAr
System voltage, U = 5 kV (line-to-line)
Transformer apparent power, S = 1000 kVA
Transformer impedance, Z_SC = 5%

Step 1 – Calculate the capacitor rated current

I_N = Q/(√3 × U) = 200/(√3 × 5) = 23 A_rms

Step 2 – Calculate the short-circuit power

S_SC = S/(Z_SC/100) = 1000/(5/100) = 20,000 kVA

Step 3 – Calculate the capacitor bank inrush current

I_C = I_N√(2S_SC/Q) = 23√((2 × 20,000)/200) = 23√200 = 325 A

Step 4 – Verify the inrush current limit

100 I_N = 100 × 23 = 2300 A

Since:

I_C = 325 A < 2300 A = 100 I_N

Result:

The calculated inrush current is well below the recommended limit of 100 × I_N. Therefore, an inrush current limiting reactor is not required for this application.

Inrush Current Calculation (Parallel Capacitor Banks)

When an additional capacitor step is energized while n capacitor steps are already in operation, the resulting inrush current is calculated as follows.

Capacitor Step Current Relationship

Q (kVAr) = U (kV)² · C (µF) · ω · 10^-3 = √3 · U (kV) · I_N (A)

Where:

Q = Capacitor step reactive power (kVAr)
U = Grid line-to-line voltage (kV)
C = Capacitance of the capacitor step (µF)
ω = Angular frequency (rad/s)
I_N = Rated current of the capacitor step (A)

\omega = 2\pi f

Initial Inrush Current Without Reactor

The peak inrush current when the (n+1)th capacitor step is energized is:

I_C = √(2/3) · U · n/(n+1) · √(C/l)

Where:

I_C = Peak initial charging current (A)
n = Number of energized capacitor steps before switching
l = Inductance of busbars and cables between capacitor steps (µH/m)

Resonance Frequency

The natural resonance frequency of the capacitor switching circuit is:

f_r = 1/(2π√(l · C))

Where:

f_r = Resonance frequency (Hz)
l = Circuit inductance (H)
C = Equivalent capacitance (F)

Inrush Reactor Calculation

To limit the peak inrush current to:

I_C ≤ 100 I_N

The required series inrush reactor is:

L (µH) = (2 × 10⁶)/3 × Q/(2πf) × (n/(n+1))² × 1/I_C²

Where:

L = Required inrush reactor inductance (µH)
f = System frequency (Hz)
Q = Capacitor step power (kVAr)
I_C = Allowed peak inrush current (A)

Inrush Current With Reactor

If a series inrush reactor with inductance L is installed, the expected peak inrush current becomes:

I_C = √((2 × 10⁶)/3 × Q/(2πf) × (n/(n+1))² × 1/L)

This calculation is used to select the appropriate inrush current limiting reactor for medium-voltage capacitor banks with multiple parallel steps.

Example Calculation – Inrush Current for Parallel Capacitor Banks

Given:

Busbar and cable inductance: l = 0.5 µH/m
Distance between capacitor banks: 5 m
System voltage: U = 5000 V (line-to-line)
Number of energized steps before switching: n = 2
New capacitor step: (n+1) = 3
Capacitor step power: Q = 200 kVAr
Frequency: f = 50 Hz

Step 1 – Calculate Capacitor Rated Current

I_N = Q/(√3 × U)

I_N = 200/(1.73 × 5) = 23 A_rms

Step 2 – Calculate Capacitor Capacitance

C = √3(U × I_N)/(U² × 2πf)

C = (1.73 × 5000 × 23)/(5000² × 2π × 50) = 25.3 µF

Step 3 – Calculate Inrush Current Without Reactor

I_C = √(2/3) × U × n/(n+1) × √(C/l)

I_C = 0.81 × 5000 × 2/3 × √(25.3/(0.5 × 5)) = 8589 A_p = 8.59 kA_p

The allowable inrush current limit is: 100 I_N = 100 × 23 = 2300 A

Since: I_C = 8589 A_p > 2300 A

Result: A series inrush current limiting reactor is required.

Step 4 – Required Inrush Reactor Inductance

The minimum required reactor value is:

L (µH) ≥ (2 × 10⁶)/3 × (Q × 10^-3)/ω × (n/(n+1))² × 1/I_C²

ω = 2πf = 2π × 50 = 314 rad/s

L ≥ (2 × 10⁶ × 0.2)/(3 × 314) × (2/3)² × 1/2300² = 35.66 µH

The minimum required reactor inductance is: L ≥ 35.66 µH

Step 5 – Check with a 50 µH Reactor

If a 50 µH inrush reactor is installed:

I_C = √(2/3) × 5000 × 2/3 × √(25.3/50) = 1935 A_p

Comparison: 1935 A < 2300 A

Result: The 50 µH reactor successfully limits the inrush current below 100 × I_N.

Step 6 – Resonance Frequency with Reactor

f_r = 1/(2π√(L × C))

f_r = 1/(2π√(50 × 10^-6 × 25.3 × 10^-6)) = 4475 Hz

Final Results

Final Results
Parameter	Value
Capacitor step current (I_N)	23 A_rms
Capacitance per step (C)	25.3 µF
Initial inrush current (without reactor)	8.59 kA_p
Maximum allowed inrush current	2.3 kA_p
Reactor required	Yes
Minimum reactor inductance	35.66 µH
Selected reactor	50 µH
Final inrush current	1.94 kA_p
Resonance frequency	4.48 kHz

This confirms that the selected 50 µH inrush reactor provides adequate switching current limitation for the parallel MV capacitor bank.

Capacitor Bank Sizing Calculation

For capacitors, I_max = 1.3 × I_n

V_max = 1.1 V_n — 12 hours / day
V_max = 1.2 V_n — 5 min
V_max = 1.3 V_n — 1 min

The required capacitor bank rating is determined based on the desired capacitive reactive power compensation and the system voltage.

Where:

S = Transformer rated power supplying the capacitor bank (kVA)
S_SC = Short-circuit power of the transformer supplying the capacitor bank (kVA)
Z_SC = Transformer short-circuit impedance (%)
Q_s = Required capacitive reactive power compensation (kVAr)
U_s = System voltage (V)
Q_N = Required rated capacitor bank power (kVAr)