These figures speak for themselves: it means saving hundreds of thousands of tons of fuel and making several power plants and hundreds of transformer rooms available.

Thus in the case of low power factors utility companies charge higher rates in order to cover the additional costs they must incur due to the inefficiency of the system that taps energy.

It is a well-known fact that electricity users relying on alternating current – with the exception of heating elements – absorb from the network not only the active energy they convert into mechanical work, light, heat, etc. but also an inductive reactive energy whose main function is to activate the magnetic fields necessary for the functioning of electric machines.

The power factor is thus the ratio between active power and apparent power (vectorial sum of active and reactive power), an indicator of the quality of a facility’s electric system since the lower the power factor is, the higher the inductive reactive component will be in relation to the active component.

It is possible to produce reactive energy, where necessary, by installing power capacitors or automatic power factor correction systems. Capacitors absorb a current that is 180% out of phase with the inductive reactive current; the two currents are algebraically summed together so that circulating upstream from the point of installation of the capacitor is a reactive current that is equal to the difference between the inductive and capacitive currents.

The exchange occurs between the capacitor and user; this is why we say that the capacitor supplies reactive energy to the user.

Power flows with and without power factor correction
 

Theoretically speaking, when you must choose where to locate the capacitive power the most appropriate solution from a technical standpoint would be to assign each load its own power factor correction capacitor, to be switched on together with the machine.

In practice, however, this entails excessive costs and technical problems in most cases, since it requires the installation of a larger number of low-power capacitors distributed in many different points, which cannot be effectively monitored over time; plus little benefit is to be derived from reducing losses in the cables, negligible compared to those in the power transformer. Therefore, this solution is only feasible in large facilities or where there are very high power loads.                             

The most appropriate power factor correction system thus consists in the installation of an automatic capacitor bank on the bus bars of the distribution panel and, if necessary, fixed capacitor banks for correcting the power factor of the transformer, asynchronous motors and any loads absorbing considerable quantities of reactive power.

The automatic system of the capacitor bank has the task of switching in the necessary capacitance according to the load requirements at each given moment.

• At each moment optimized cosφ
• Soft switching without transients
• Flicker eliminated

The real-time power factor compensation equipment of series Activcomp offers a solution. In these assemblies the classic components controller and air contactor are substituted by a combination of corresponding high speed controller and thyristor power modules type Activcomp . This system reacts immediately on load fluctuation and reactive power surges will be neutralised in the supply network. The power factor is optimised at each moment, on cycle to cycle basis, typical response time less than 20 ms. The negative effects described above are reduced to a minimum. This gives the consumer not only the advantage of a stable supply ratio, but the ability to minimise energy distribution and reduce costs. To increase the switching performance to an optimum, the control signal for the steering of the capacitor banks can alternatively be given directly by the logic control of bigger loads.

Advantages of Thyristor Switched Over Electro-Mechanical Compensation System ?

Fast and Accurate Compensation

The Activcomp is a Transient Free Fast Compensation System. It is used for Power Factor Correction & Harmonic Filter. The compensation is based on averaging the FFT analysis of each cycle, resulting in more accurate compensation, even in the presence of harmonics.

Simultaneous Group Connection

When load changes require connection or disconnection of more than one step, the Activcomp controls the switching of as many steps as required at precisely the same time. Simultaneous connection or disconnection provides the following benefits :

• Fast full compensation.
• For Eg., with 1:2:2 system configuration and groups 1 & 2 are connected. When 1 more step is required, group 3 will 
  be connected simultaneously while group 1 is disconnected.
• Real binary sizing - 1:2:2 is exactly the same as 1:1:1:1:1.

Transient-free Switching

Electronic switching technology prevents any transients typically associated with conventional capacitor switching. This is extremely important in sites with sensitive electronic equipment, such as hospitals, data centers and facilities.

Fixed Capacity & Filter Characteristics

The capacity of the Activcomp capacitors is virtually permanent over the years, which prevents the need to replace capacitors. Moreover the tuning frequency remains constant over time, which allows system performance to remain at the highest possible level.

There is an ongoing cumulative reduction of capacity in electro mechanically switched PFC systems due to the effect of transients during connections and disconnection. This can be detrimental to detuned electro mechanically switched systems where the changes in ratio between the capacitors/reactors shift the resonance frequency, which may result in resonance. The Activcomp prevents these conditions.

Long Life and Reduced Maintenance Costs

Neptune Activcomp reduces site maintenance costs by increasing the lifetime of:
• Switching elements
• Capacitors
• Sensitive electronic equipment

Capacitor Duty Cycle-SCAN Mode

The unique SCAN feature protects the Activcomp’s capacitors & reactors, increases their life span. Simultaneous connection and disconnection of steps in FIFO (First In First Out) manner is shown in diagram on the previous page.

The scan feature reduces the average current in the capacitors and the reactors and therefore providing the following advantages:

• Reduces substantially the increase of temperature in these elements resulting in longer life expectancy of the
   inductors and capacitors.
• Reduces the effect of over-current and over voltage caused by the harmonics on the capacitors and inductors.
• The tuning frequency of the de-tuned filter or tuned filter is stable due to the fact that the capacitor  value (micro farad) 
  doesn't change due to the low temperature achieved by the scan mode.

Easy to Use and Maintain

The advanced DSP and microprocessor-based controller, with its large full graphic LCD display, provides easy-to-use operation. The controller includes a complete electrical measurement system, which can replace a facilities main monitoring meter. The controller operates the BIT (Built In Test), which reports system or network conditions. The optional PowerlQ software can remotely control all Activcomp operation and display additional system power information.

 

Power Factor Corrections Capacitors.

Unlike most electrical equipment, power factor correction capacitors, each time they are energized, continuously operate at full load or at loads which differ from this value only as a consequence of variations in voltage and frequency.

Overstressing and overheating shorten the life span of the capacitor. For this reason the operating conditions (temperature, voltage and current) must be carefully controlled in order to obtain optimum results as respects the lifespan of the capacitor.

Voltage
The capacitors are produced in accordance with reference standards EN 60831-1/2 which regulate their manufacturing, testing, installation and application and which indicate the following maximum values for over voltage applicable to the capacitors:

Over voltages in excess of 15% should not occur more than 200 times in the lifespan of the capacitor. Often when there is the presence of overload conditions during service, in the presence of a moderate harmonic load for example, it is common to use oversized capacitors in terms of voltage.
 

In such cases the output power at operating voltage will be reduced with respect to that of the rated load. It is advisable to evaluate the reduction experienced in the output power on the basis of the correlation between the operating voltage and the rated voltage.

The following table indicates the output power of a 100 kvar capacitor used on a 400 V network with a rated voltage of 450 V, 500 V  and 550 V.
 

Temperature

The temperature of the capacitor during operation is the parameter that, along with the voltage, has the greatest influence on the life span of the capacitor. It is important that the capacitor always be placed in a position where the cooling air can freely circulate, avoiding the radiant heat of hot surfaces of other components. When the capacitors are placed in closed cabinets it is necessary to have air vents which allow for an easy exchange of air between the interior and exterior of the cabinet. On the other hand, when the degree of protection of the cabinet does not permit this exchange of air, the internal spaces must be much larger and the positioning of the capacitors must be carefully studied to permit the necessary channels which allow for the circulation of cooling air. In this case the forced cooling air will have to be provided by suitable fans. As a rule the temperature of the cooling air inside the cabinet should not differ by more than 5ºC with respect to the external air at the control panel.

Cooling air temperature

This is the temperature of cooling air measured at the hottest point of the bank of capacitors, under working conditions, halfway between two capacitors or on the surface of one of these.

Categories of ambient air temperature

This represents the range of cooling air temperatures in which the capacitor is designed to operate. As a rule there are 4 categories represented by a number and a letter or by two numbers as shown in the table.
 

The first number represents the minimum temperature of the cooling air at which the capacitor can be energized (-25ºC) on request -40ºC. The letter or the second number represents the upper limit of the temperature range and precisely the max. value indicated in the table.

Residual voltage

This is the voltage that remains after disconnecting the capacitor from the network. This voltage must be eliminated in order to avoid dangerous conditions for the operator. All three-phase capacitors are equipped with discharge devices, that reduce the residual voltage to a value of minimum 75 V after 3 minutes.

It is important to bear in mind that the capacitors cannot be energized if there is a residual voltage of more than 10% across them. Particular attention must therefore be applied in harmonizing the capacitor discharge times with the response times of the control devices (Regulators). In cases where the lag time of the regulators are shorter than the discharge time of the capacitor, it is necessary to provide additional discharge devices until the connection occurs with a residual voltage of less than 10%.

To reduce the residual voltage to 50 V in 20 seconds in batteries with a power less than or equal to 20 kvar at a voltage of 400 V, use 3 metal oxide resistors of 68 kohm, 4W in a delta connection.

Max current

The capacitors are made to function conforming to standards EN 60831 - 1/2 continuously at an effective value at last of 1.3 times the value of the current at rated voltage and frequency. Bearing in mind the capacitance tolerance, the maximum current can arrive to 1.5 In, the value to which it is necessary to refer in the scaling of the line of control and protection devices. This overcurrent factor can be determined by the combined effect of harmonics, overvoltages and capacitance tolerance.

Max inrush current

Transient overcurrents having elevated amplitudes and high frequencies occur when the capacitors are switched in to the circuit. This is especially true when a bank of capacitors are put in a parallell connection with other already-energized capacitors. It may therefore be necessary to reduce these transient overcurrents to acceptable values both for the capacitor and the contactor used by connecting the capacitor using suitable devices (resistors or reactors) in the power circuit of the battery. The peak value of the overcurrent caused during maneuvering operations must be limited to a maximum value of 100 In (crest value of the 1st cycle).

Harmonics in electrical system results in wave form distortion. In general harmonics are periodic disturbances in voltage and current. Any non-sinusoidal periodic wave form can be considered as combination of sine waves of certain frequencies, amplitudes and phase angle (fourier series). In simple terms Harmonics are multiple of the normal main power frequency, like "3rd order" harmonics has got frequency of 150 Hz and "5th order" has to 250 Hz frequency. Harmonics causes pollution in electrical systems, which may affect the equipment at much larger distances from origin and if not controlled can become expensive preposition.

In a wave shape only the fundamental component of voltage and current contribute to power translation. The harmonics only load the translation system and cause increase in apparent power of source. This non-active power due to harmonics is referred as distortion power.

Power factor correction capacitor banks of such loads are put under considerable stress by presence of harmonics.

• Static power converters (AC-DC conversion using SCRs)
  
• Static rectifiers
  
• Static frequency convertor
  
• Static uninterruptible power supplies
  
• Static induction regulators.

• Variable impedance loads, using electric arcs, arc furnaces, welding units, fluorescent tubes, discharge lamps, Light control, brightness etc.
  
  
• Loads using strong magnetising currents saturated Transformer, inductance furnaces, reactors etc.
  
  
• Office automation products like computers, UPS, printers and fax machine etc.

A non-linear load is a load for which the current consumption does not reflect the supply voltage although the load voltage source is sinusoidal, the current consumption is not sinusoidal.

High level of harmonic distortion can create stress and resultant problems for the utility's distribution system, the plant's distribution system, as well as the plants equipment. Equipment shutdown can result from too-high levels of harmonic distortion.

Following is the list of problems that enhanced harmonic distortion can create :

• The capacitors are subjected to a voltage overload and their lifetime is drastically reduced; in fact the voltage drop across the capacitor caused by each of the harmonic currents present must be added to the fundamental voltage. This over voltage is well above 10% when resonance is present, causing the premature dielectric breakdowns.
  
  

• Connection of the cap. causes a large increase in the voltage distortion factor THD(V).
  
  
• Motors; Reduced motor life, inability to fully load motor.
  
  

• The user transformers, wires and loads are affected by this increase in current, which leads to increased I2 R losses and eddy current losses in transformer. This reduces the capacity of transformer resulting into economic loss as well. Apart from this transformer may also be subjected to excessive overheating and saturation. 
  
  This will shorten the life or transformer. When transformer fails, the cost of loss of productivity during emergency repair time far exceeds the replacement cost of transformer itself.
  
  

• Interference in computers, microprocessor & solid state controlled equipment because of distorted 240 V supply. Increased neutral to ground voltage may cause hardware problems which may initially look like software problem. International standards recommend that voltage distortion for computer use be limited to maximum of 5% total harmonic distortion (THD-V) and the largest single harmonic not to exceed 3%.
  
  

• High voltage distortion shall also be encountered when shifting from mains to emergency generator as they offer high impedance than supply transformer.
  
  

• Random switching in deferential relays.
  
  

• Large current in neutral wires of power distribution system. This is a real fire hazard as usually phase wires are protected by circuit breakers or fuses.
  
  

• Poor power factor: As mentioned earlier the harmonic current caused by non-linear loads do not carry any real power (KW) even though they do increase the volt amperage (KVA). This lowers the power factor at (Pf=KW/KVA) the main distribution transformer. Utilities put penalties on consumers with low p.f, less than 0.9.
  
  

• Over heating in fuses resulting in false blowing. False / spurious operations of breakers, which may lead to variation in other characteristics.
  
  

• Higher rotation speed of the disks in energy counters due to additional torque resulting in measurement errors.
  
  

• Reduction of power generated by UPS.
  
  

• Communication / Telephone circuit interference because of induced harmonic noise.

Standards define a Harmonic as “one of the components obtained from the breakdown of a periodic wave (voltage or current) in the Fourier Series”. The order of a harmonic is further defined as “the ratio between the frequency of a harmonic and the fundamental frequency of the network”.

Power converters are being used more and more frequently in plants to control motors, medium-frequency ovens, uninterruptible power supplies, etc. These devices regulate the load by modulating the supply voltage, which results in the generation of current harmonics. As they circulate through equivalent impedances in the cables and mains, these harmonics distort the sine waveform of the supply voltage.

Characteristic harmonics are defined as “those harmonics produced by static ac/dc converters in ideal theoretical operation”.

The order h of characteristic harmonics is h=mp±1

where m is an integer other than 0 (thus 1, 2, 3, 4, ...) and p is the number of solid-state switches of the bridge. Therefore, a converter with six-phase reaction (p=6) generates characteristic harmonics of the 5th and 7th order (m=1), 11th and 13th order (m=2), 17th and 19th order (m=3), etc., whereas a converter with twelvephase reaction (p=12) generates characteristic harmonics of the 11th and 13th order (m=1), 23rd and 25th order (m=2), etc. Noncharacteristic Harmonics are defined as “the harmonics produced:

• by an unbalance in the energy network;
  
• by an asymmetric delay in the triggering angle of the converter;
  
• by non-linear devices (frequency changers, fluorescent lamps, arc furnaces, electric welders, saturations, …)”.

The parameter used to determine the level of harmonic distortion presents in an electrical network is THD% (Total Harmonic Distortion), defined as: