To calculate the cos phi of the plant, it is necessary to have data on the consumption of Active Energy (kWh) and Reactive Energy (kVArh), or Active Power (kW) and Apparent Power (kVA).
These values can be found in the electricity bill or through a network analysis.
If in possession of kWh and kVArh
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To define the required reactive power of an automatic PFC system, it is essential to have:
• Active Power (kW)
• Initial Cos φ (also deduced from the Active and Reactive Energy consumed, see above)
• Desired Cos φ
Formula: Q = P * k
• Q: Necessary reactive power (kVAr)
• P: Active Power (kW)
• K: Cos φ coefficient from table >> attachment 1
Example
Plant with active power 650 kW and initial Cos φ 0.75, to be brought to 0.95.
What is the necessary reactive power?
500 kW *0.553 (coefficient K for cos phi from 0.75 to 0.95)
= 276 kVAr
It is advisable to oversize the necessary reactive power by 15-20% in order to maintain an average Cos φ of 0.95 even with load variations and/or future extensions.
Compensation of MV/LV Trafo
For economic reasons, it is advisable to compensate the reactive power that the Transformer absorbs for the magnetisation of the core and for the winder reactors.
The choice of Reactive power can be made based on table >> attachment 2
Compensation of Asynchronous motors
The reactive power necessary for the power factor correction of Asynchronous Motors is chosen from the following table
>> attachment 2
It is always advisable in these situations to take into account the possible self-excitation of the capacitors, which is why the installation of an automatic PFC system rather than a fix one is preferred.
It is always advisable to take into account possible operation of the Motor as a self-excited generator, and this can result in voltages that are considerably higher than those of the network.
The current of the C.T. must be chosen according to the current circulating in the plant regardless of the power in kVAr of the power factor correction and the available power. A good compromise is to choose a T.A. that has a current that is double what normally circulates in the plant. Remember that the sensitivity of the controller can control up to a minimum of 5% of the current of the C.T. Example: available power 150 KW - circulating power 110 KW (about 165 A in 400 V 50 Hz networks), choose a T.A. with variable size from 300 to 350 A.
The C.T. must be positioned upstream both of the loads and of the power supply to the PFC system. Ultimately, the C.T. must be able to “feel” both the inductive load and the capacitive power of the capacitors.
For ease of installation, in our systems the C.T. is inserted in phase R and in the connection to the power factor correction system respects phase sequence R-S-T-. In reality, the C.T. can be positioned on any of the three phases. The selected phase MUST be anchored on terminal R of the PFC switch.
Please see the instructions in the Use and Maintenance Manual, which can be downloaded from the Download section.
The Controller does not switch on
• Check the internal fuses.
• Check that the connector is properly inserted.
• Check the voltage of the power terminals.
The capacitor banks cannot be inserted
• Check the internal fuses.
• Check that the connector is properly inserted.
• The C.T. is inserted on cables that power the PFC system. It does not “feel” the inductive current of the load.
• Make sure that the cos φ on the display is < than the set value.
• Make sure that the Δkvar is higher than the value of the first bank.
Example
cos φ indicated on the display 0.72
Δkvar 3.7
First PFC bank 5 kvar
The system will not connect.
The value of cos φ remains unchanged when banks are inserted
• The C.T. is inserted only on the load, it does not “feel” the capacitor capacitance.
• The auxiliary fuses or those of the capacitor bank power are interrupted.
The Controller inserts all banks but the value of cos φ does not reach the established set point.
• The auxiliary fuses or those of the capacitor bank power are interrupted.
• System power is lower than necessary.
• Check the PFC bank current, it may have lost capacitance.
A01 undercompensation
• The capacitor bank power fuses are interrupted.
• System power is lower than necessary.
• Check the PFC bank current, it may have lost capacitance.
A02 overcompensation
• The C.T. is inserted on phase R but the cable is anchored on a different phase (S or T).
• You have chosen to position the C.T. on a different phase (S or T), but it is still not anchored on terminal R of the PFC switch.
• The actual cos φ of the plant is > of the established set point. (presence of resistive loads - plant capacitors - photovoltaic)
A03 current too low
• The C.T. records a current lower than 5% of its primary current.
• The C.T. circuit is interrupted.
• The C.T. is inserted on a line that power the PFC system.
• The C.T. is faulty.
A12 maintenance (PCRL…), A20-21-22 maintenance (PCRJ..)
The system continues regular operation. Please see the relative addendum in the respective manuals.
If it is true that, in large power plants, it is more probable to find the presence of harmonics, it is not true that in utilities with small loads their presence is to be entirely excluded.
Harmonics are the main enemy of capacitors. If the THDi and THDv values are high, this significantly compromises the life of the System, compromising the customer's payback and creating considerable damage.
It is always therefore advisable to take note of the types of load present in the system, so as to evaluate installation of a Standard system (for networks with low harmonic content) or a "Detuned" system, or rather equipped with suitably tuned filter reactors, in order to protect the capacitors from harmonics and avoid the risk of resonance. A Network analysis is able to verify the presence or absence of such phenomena (see How important is a network analysis?)
To select the rated current of the circuit-breaker upstream of the PFC system (as well as the main system device), refer to the current absorbed by the PFC system in continuous service (Inc) at nominal voltage and frequency.
Standard CEI EN 60831-1 (CEI 33-9) prescribes that capacitors must be able to be fully operational with a current of effective value equal to 1.3 Inc, to take into account the combined effect of the harmonics and possible overvoltages. Moreover, since it is necessary to take into account tolerances of 10% in the capacitance values of this, it leads to an overcurrent equal to
1.1 x 1.3 Inc = 1.43 Inc
The rated current of the upstream switch, In, should therefore be greater than or equal to 1.5 Inc.
At the time of insertion, the capacitors absorb a current higher than the rated one. Calibration of the magnetic trip units of the upstream switch must take into account the insertion overcurrent.
Typically, magnetic trip units calibrated at 10 times the rated current of the battery are suitable.
* Inc is the rated current of the PFC system.
With regards to the choice of the cross-section of the connection cables between the upstream switches and the power factor correction systems, cables with a flow rate greater than the current value below must be selected:
1.5 Inc
Consult the main cable manufacturers’ catalogues when determining the appropriate cross-section, taking into account the type of installation and the possible coexistence in the same conductors on multiple circuits.
* Inc is the rated current of the PFC system.
The distances that the power factor correction system must remain at with respect to the walls of rooms and/or other electrical panels are:
• 40 cm from walls and other panels to ensure the correct flow of air inside the system, so as not to generate overheating during operation that would lead to premature ageing of internal components and which could also cause faults, short circuits, etc. in the components themselves.
• Furthermore, to ensure correct opening of the system door, obviously it is necessary to guarantee a distance that is equal to its width. System maintenance can be carried out by removing the drawers from the front. However, if access the back of the panel is desired for maintenance purposes, a distance of 80 cm must also be maintained on the back side compared to other electrical panels or to the walls of the room.
The power in kVAr of both a standard and especially a “Detuned” PFC system must ALWAYS be referred to the voltage of the network on which it will be installed. There is no technical basis for referring the power to the capacitor voltage. Doing so only creates confusion since, when referring the kVAr to a higher voltage, the power will be greater than that referring to the network voltage. However, once the System is installed, a % power deficit will be obtained as a function of the network voltage and that of the capacitors table >> attachment 3
Regardless of the voltage of the Capacitors, TELEGROUP always refers the power in kVAr to the network voltage.
The voltages of standardised worldwide Capacitors are 440 V, 460 V, 480 V, 525 V, 690 V and 800 V.
However, the voltage, even if it is a parameter of primary importance, does not represent the unit of measurement used for verifying the quality of a Capacitor. It is true that a 690V capacitor will be more robust than a 440V one, but it all depends on where the capacitor is operating.
On a standard 400 - 415 V (± 10%) network, with THD around 15% or less, it does not make sense to install a 500 or 525 V Capacitor, as a quality Capacitor with a voltage of 440 or 460 V is able to operate perfectly.
In parallel, on systems with a medium to high THD presence, it is absolutely not sufficient or technically correct to use Capacitors with higher voltage to face the harmonics but, as also required by IEC standards, it is necessary to install suitably tuned filter reactors.
See the List of typical cos phi >> attachment 4
Usually, the standard payback of a PFC system it’s no over 24 months from the installation.
See Power Factor Correction >> Introduction and benefits
See Capacitor Technology >> attachment 5
See Nitrogen Capacitors >> attachment 6
Network analysis, especially on complex and high-power plants, is undoubtedly an essential step for determining all values in order to size a Power Factor Correction system. In this case, in addition to the analysis of voltage, current, frequency and cos phi, it is possible to identify with certainty the THD values in voltage and current for the three priority harmonic orders (3rd, 5th and 7th), in order to evaluate the best type of system to propose, in addition to possible identification and measurement of the particular loads/machinery with high power, for which a distributed power factor correction is required. TELEGROUP he has been offering this service through its technology for end customers for years, especially in the heavy industry sector.
See Power factor correction in the presence of cogeneration systems >> attachment 7