Continued from part 1: Defining Size and Location of Capacitor in Electrical System (2)
Content
- Size of circuit breaker (CB), fuse and conductor of capacitor bank:
- A. Thermal and magnetic setting of a circuit breaker
- B. Fuse selection
- C. Size of conductor for capacitor connections
- Size of capacitor for transformer no-load compensation:
- Sizing of capacitor for motor compensation:
- Placement of power capacitor bank for motor:
- Location 1 (the line side of the starter)
- Location 2 (between the overload relay and the starter)
- Location 3 (the motor side of the overload relay)
- Placement of capacitors in distribution system:
- Common capacitor reactive power ratings
Size of CB, Fuse and Conductor of Capacitor Bank
A. Thermal and Magnetic setting of a Circuit breaker
1. Size of Circuit Breaker
1.3 to 1.5 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors
- 1.31×In for Heavy Duty/Energy Capacitors with 5.6% Detuned Reactor (Tuning Factor 4.3)
- 1.19×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)
- 1.12×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor (Tuning Factor 2.7)
2. Thermal Setting of Circuit Breaker
1.5x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors
3. Magnetic Setting of Circuit Breaker
5 to 10 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors
Example: 150kvar,400v, 50Hz Capacitor
- Us = 400V, Qs = 150kvar, Un = 400V, Qn = 150kvar
- In = 150000/400√3 = 216A
- Circuit Breaker Rating = 216 x 1.5 = 324A
- Select a 400A Circuit Breaker.
- Circuit Breaker thermal setting = 216 x 1.5 = 324 Amp
Conclusion: Select a Circuit Breaker of 400A with Thermal Setting at 324A and Magnetic Setting (Short Circuit) at 324A
B. Fuse Selection
The rating must be chosen to allow the thermal protection to be set to:
1.5 to 2.0 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors.
- 1.35×In for Heavy Duty/Energy Capacitors with 5.7% Detuned Reactor (Tuning Factor 4.3)
- 1.2×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)
- 1.15×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor(Tuning Factor 2.7)
For Star-solidly grounded systems:
Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics).
For Star -ungrounded systems:
Fuse > = 125% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics).
Example 1: 150kvar,400v, 50Hz Capacitor
- Us = 400V; Qs = 150kvar, Un = 400V; Qn = 150kvar.
- Capacitor Current =150×1000/400 =375 Amp
To determine line current, we must divide the 375 amps by √ 3
- In (Line Current) = 375/√3 = 216A
- HRC Fuse Rating = 216 x1.65 = 356A to
- HRC Fuse Rating = 216 x 2.0 = 432A so Select Fuse Size 400 Amp
Problems with Fusing of Small Ungrounded Banks
Example: 12.47 kV, 1500 Kvar Capacitor bank made of three 3 No’s of 500 Kvar single-phase units.
- Nominal Capacitor Current = 1500/1.732×12.47 = 69.44 amp
- Size of Fuse = 1.5×69.44 = 104 Amp = 100 Amp Fuse
If a capacitor fails, we say that It may approximately take 3x line current. (3 x 69.44 A = 208.32 A).
It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x 69.44 A = 208.32 A). The capacitor case will rupture long before the fuse clears the fault.
C. Size of Conductor for Capacitor Connections
Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in accordance with NEC Article 460.8 (2005 Edition).
Size of capacitor for Transformer No-Load compensation
Fixed compensation
The transformer works on the principle of Mutual Induction. The transformer will consume reactive power for magnetizing purpose. Following size of capacitor bank is required to reduce reactive component (No Load Losses) of Transformer.
| Selection of capacitor for transformer no-load compensation | |
| KVA Rating of the Transformer | Kvar Required for compensation |
| Up to and including 315 KVA | 5% of KVA Transformer Rating |
| 315 to 1000 KVA | 6% of KVA Transformer Rating |
| Above 1000 KVA | 8% of KVA Transformer Rating |
Sizing of capacitor for motor compensation
The capacitor provides a local source of reactive current. With respect to inductive motor load, this reactive power is the magnetizing or “no load current“ which the motor requires to operate.
1. If no-load current is known
The most accurate method of selecting a capacitor is to take the no load current of the motor, and multiply by 0.90 (90%).
Example:
Size a capacitor for a 100HP, 460V 3-phase motor which has a full load current of 124 amps and a no-load current of 37 amps.
Size of Capacitor = No load amps (37 Amp) X 90% = 33 Kvar
2. If the no load current is not known
If the no-load current is unknown, a reasonable estimate for 3-phase motors is to take the full load amps and multiply by 30%. Then multiply it by 90% rating figure being used to avoid overcorrection and overvoltages.
Example:
Size a capacitor for a 75HP, 460V 3-phase motor which has a full load current of 92 amps and an unknown no-load current.
No-load current of Motor = Full load Current (92 Amp) x 30% = 28 Amp estimated no-load Current.
Size of Capacitor = No load amps (28 Amp) X 90% = 25 Kvar.
Thumb Rule:
It is widely accepted to use a thumb rule that Motor compensation required in kvar is equal to 33% of the Motor Rating in HP.
Placement of Power Capacitor Bank for Motor
Capacitors installed for motor applications based on the number of motors to have power factor correction. If only a single motor or a small number of motors require power factor correction, the capacitor can be installed at each motor such that it is switched on and off with the motor.
Required Precaution for selecting Capacitor for Motor:
The care should be taken in deciding the Kvar rating of the capacitor in relation to the magnetizing kVA of the machine.
If the rating is too high, It may damage to both motor and capacitor.
As a general rule the correct size of capacitor for individual correction of a motor should have a kvar rating not exceeding 85% of the normal No Load magnetizing KVA of the machine. If several motors connected to a single bus and require power factor correction, install the capacitor(s) at the bus.
Where do not install Capacitor on Motor:
Do not install capacitors directly onto a motor circuit under the following conditions:
- If solid-state starters are used.
- If open-transition starting is used.
- If the motor is subject to repetitive switching, jogging, inching, or plugging.
- If a multi-speed motor is used.
- If a reversing motor is used.
- If a high-inertia load is connected to the motor.
Fixed power capacitor banks can be installed in a non-harmonic producing electrical system at the feeder, load or service entrance. Since power capacitor banks are reactive power generators, the most logical place to install them is directly at the load where the reactive power is consumed.
Three options exist for installing a power capacitor bank at the motor.
Location 1 (The line side of the starter)
Install between the upstream circuit breaker and the contactor.
This location should be used for the motor loads with high inertia, where disconnecting the motor with the power capacitor bank can turn the motor into a self excited generator, motors that are jogged, plugged or reversed, motors that start frequently, multi-speed motors, starters that disconnect and reconnect capacitor units during cycling and starters with open transition.
Advantage
Larger, more cost effective capacitor banks can be installed as they supply kvar to several motors. This is recommended for jogging motors, multispeed motors and reversing applications.
Disadvantages
- Since capacitors are not switched with the motors, overcorrection can occur if all motors are not running.
- Since reactive current must be carried a greater distance, there are higher line losses and larger voltage drops.
Applications
- Large banks of fixed kVAR with fusing on each phase.
- Automatically switched banks
Location 2 (Between the overload relay and the starter)
Install between the contactor and the overload relay.
- This location can be used in existing installations when the overload ratings surpass the National Electrical Code requirements.
- With this option the overload relay can be set for nameplate full load current of motor. Otherwise the same as Option 1.
- No extra switch or fuses required.
- Contactor serves as capacitor disconnect.
- Change overload relays to compensate for reduced motor current.
- Too much Kvar can damage motors.
FLA (New) = P.F (Old) / P.F (New) x FLA (Name Plate)
Application:
Usually the best location for individual capacitors.
Location 3 (The motor side of the overload relay)
Install directly at the single speed induction motor terminals (on the secondary of the overload relay).
- This location can be used in existing installations when no overload change is required and in new installations in which the overloads can be sized in accordance with reduced current draw.
- When correcting the power factor for an entire facility, fixed power capacitor banks are usually installed on feeder circuits or at the service entrance.
- Fixed power capacitor banks should only be used when the facility’s load is fairly constant. When a power capacitor bank is connected to a feeder or service entrance a circuit breaker or a fused disconnect switch must be provided.
- New motor installations in which overloads can be sized in accordance with reduced current draw
- Existing motors when no overload change is required.
Advantage
- Can be switched on or off with the motors, eliminating the need for separate switching devices or over current protection. Also, only energized when the motor is running.
- Since Kvar is located where it is required, line losses and voltage drops are minimized; while system capacity is maximized.
Disadvantages
- Installation costs are higher when a large number of individual motors need correction.
- Overload relay settings must be changed to account for lower motor current draw.
Application
Usually the best location for individual capacitors.
Placement of capacitors in Distribution system
The location of low voltage capacitors in Distribution System effect on the mode of compensation, which may be global (one location for the entire installation), by sectors (section-by-section), at load level, or some combination of the last two.
In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant.
A. Global compensation
Principle
The capacitor bank is connected to the bus bars of the main LV distribution board to compensation of reactive energy of whole installation and it remains in service during the period of normal load.
Advantages
- Reduces the tariff penalties for excessive consumption of kvars.
- Reduces the apparent power kVA demand, on which standing charges are usually based
- Relieves Reactive energy of Transformer , which is then able to accept more load if necessary
Limitation
- Reactive current still flows in all conductors of cables leaving (i.e. downstream of) the main LV distribution board. For this reason, the sizing of these cables and power losses in them are not improved by the global mode of compensation.
- The losses in the cables (I2R) are not reduced.
Application
- Where a load is continuous and stable, global compensation can be applied
- No billing of reactive energy.
- This is the most economical solution, as all the power is concentrated at one point and the expansion coefficient makes it possible to optimize the capacitor banks
- Makes less demands on the transformer.
B. Compensation by sector
Principle
Capacitor banks are connected to bus bars of each local distribution Panel.
Most part of the installation System can benefits from this arrangement, mostly the feeder cables from the main distribution Panel to each of the local distribution panel.
Advantages
- Reduces the tariff penalties for excessive consumption of kvar.
- Reduces the apparent power Kva demand, on which standing charges are usually based.
- The size of the cables supplying the local distribution boards may be reduced, or will have additional capacity for possible load increases.
- Losses in the same cables will be reduced.
- No billing of reactive energy.
- Makes less demands on the supply Feeders and reduces the heat losses in these Feeders.
- Incorporates the expansion of each sector.
- Makes less demands on the transformer.
- Remains economical
Limitations
- Reactive current still flows in all cables downstream of the local distribution Boards.
- For the above reason, the sizing of these cables, and the power losses in them, are not improved by compensation by sector
- Where large changes in loads occur, there is always a risk of overcompensation and consequent overvoltage problems.
Application
Compensation by sector is recommended when the installation is extensive, and where the load/time patterns differ from one part of the installation to another.
This configuration is convenient for a very widespread factory Area, with workshops having different load factors
C. Individual compensation
Principle
- Capacitors are connected directly to the terminals of inductive circuit (Near to motors). Individual compensation should be considered when the power of the motor is significant with respect to the declared power requirement (kVA) of the installation.
- The kvar rating of the capacitor bank is in the order of 25% of the kW rating of the motor.
- Complementary compensation at the origin of the installation (transformer) may also be beneficial.
- Directly at the Load terminals Ex. Motors, a Steady load gives maximum benefit to Users.
- The capacitor bank is connected right at the inductive load terminals (especially large motors). This configuration is well adapted when the load power is significant compared to the subscribed power. This is the technical ideal configuration, as the reactive energy is produced exactly where it is needed, and adjusted to the demand.
Advantages
- Reduces the tariff penalties for excessive consumption of kvars
- Reduces the apparent power kVA demand
- Reduces the size of all cables as well as the cable losses.
- No billing of reactive energy
- From a technical point of view this is the ideal solution, as the reactive energy is produced at the point where it is consumed. Heat losses (RI2) are therefore reduced in all the lines.
- Makes less demands on the transformer.
Limitations
- Significant reactive currents no longer exist in the installation.
- Not recommended for Electronics Drives.
- Most costly solution due to the high number of installations.
- The fact that the expansion coefficient is not incorporated.
Application
Individual compensation should be considered when the power of motor is significant with respect to power of the installation.
Common Capacitor Reactive Power Ratings
| Voltage | Kvar Rating | Number of Phases |
| 216 | 5, 7.5, 131/3, 20, 25 | 1 or 3 |
| 240 | 2.5, 5, 7.5,10, 25, 20, 25, 50 | 1 or 3 |
| 480 | 5, 10, 15, 20 25, 35, 50, 60, 100 | 1 or 3 |
| 600 | 5, 10, 15, 20 25, 35, 50, 60, 100 | 1 or 3 |
| 2,400 | 50, 100, 150, 200 | 1 |
| 2,770 | 50, 100, 150, 200 | 1 |
| 7,200 | 50, 100, 150, 200,300,400 | 1 |
| 12,470 | 50, 100, 150, 200,300,400 | 1 |
| 13,800 | 50, 100, 150, 200,300,400 |
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Jignesh Parmar
Electrical Middle management professional having more than 22 years rich and dynamic experience in Project Execution / Project Management / Designing / Maintenance diversifies from Electrical Power Transmission (400KV/220KV/66KV)- Distribution(11KV/220V) to Lifts-HVAC-Ventilation-Fire Fighting-Fire Alarm-Lifts-CCTV-Stack Parking Works (High Rise Buildings, Townships, Shopping Complex, Commercial Complex, School, Temple).Profile: Jignesh Parmar
In our plant site we found most of the capacitors terminal connection are burned installed inside site power distribution transformers 11/0.4KV,1.0MVA, 50HZ and one of these transformer is fired completely at the LV side.
Could you explain what the probable reasons?
BR
Eng. Ali
Dear Sir,
I want to know the standards regarding capacitor bank capacity and unknown load (A) i.e capacitor bank for Main DB in an industrial warehouse. we will give that warehouse for lease. industrial load is customer dependent. how will we set capacitor bank capacity for db that can supply 300A, 500A, 600A current and load is unknown.
Any electrical standards for that db regarding capacitor bank capacity
Good information.
How the location of Capacitor Bank between Starter (magnetic Contactor) and Overload relay calls for New setting(lower value than previous Setting) on Overload relay? OLR will sense current of motor only.Is it not applicable to Location 3?(cap bank directly across motor)
Kindly clarify.
regards
Useful practical info for B.Tech. Students
How much units consume by a 600 kvar capacitor bank.
450w load would have 600kvar capacitor
Hi I am Chartered Engineer ( India) in Elect Engg., BEE certified Energy Auditor, Presently associate with one of the leading Power Industry.
Very good information for Engineers