In all industrial electrical distribution systems, the major loads are resistive and inductive. Resistive loads are incandescent lighting and resistance heating. In case of pure resistive loads, the voltage (V), current (I), resistance (R) relations are linearly related, i.e. V = I x R and Power (kW) = V x I Typical inductive loads are A.C. Motors, induction furnaces, transformers and ballast-type lighting. Inductive loads require two kinds of power: a) active (or working) power to perform the work and b) reactive power to create and maintain electro-magnetic fields. Active power is measured in kW (Kilo Watts). Reactive power is measured in kVAr (Kilo Volt-Amperes Reactive). The vector sum of the active power and reactive power make up the total (or apparent) power used. This is the power generated by the SEBs for the user to perform a given amount of work. Total Power is measured in kVA (Kilo Volts-Amperes)
The active power (shaft power required or true power required) in kW and the reactive power required (kVAr) are 90° apart vectorically in a pure inductive circuit i.e., reactive power kVAr lagging the active kW. The vector sum of the two is called the apparent power or kVA, as illustrated above and the kVA reflects the actual electrical load on distribution system. The ratio of kW to kVA is called the power factor, which is always less than or equal to unity. Theoretically, when electric utilities supply power, if all loads have unity power factor, maximum power can be transferred for the same distribution system capacity. However, as the loads are inductive in nature, with the power factor ranging from 0.2 to 0.9, the electrical distribution network is stressed for capacity at low power factors.
E.g., Real Power=100kw; Apparent Power = 142 KVA; Power Factor =100/142 = 0.70
As the load on the motor comes down, the magnitude of the active current reduces. However, there is no corresponding reduction in the magnetizing current, which is proportional to supply voltage with the result that the motor power factor reduces, with a reduction in applied load. Induction motors, especially those operating below their rated capacity, are the main reason for low power factor in electric systems. Inductive Loads such as electric motors & transformers, High intensity discharge lights, VFDS, Rectifiers etc. The total or apparent power required by am inductive device is composed of the following: – Real power measured in kW – Reactive power or the non working power caused by magnetizing current required to operate the device measured in kVAR Reactive power required by inductive loads increases the apparent power (measured in kVA) in the distribution system. The increase in reactive power & apparent power reduces the power factor.
I L = P/√3 V L kVA = kW /Cos φ
Pf plays a very important role in energy management since power consumption mainly depends on pf Synchronous condensers or phase advancers
From the above table, it may be noted that required capacitive kVAr increases with decrease in speed of the motor, as the magnetizing current requirement of a low speed motor is more in comparison to the high speed motor for the same HP of the motor. Since a reduction in line current, and associated energy efficiency gains, are reflected backwards from the point of application of the capacitor, the maximum improvement in overall system efficiency is achieved when the capacitor is connected across the motor terminals, as compared to somewhere further upstream in the plant's electrical system. However, economies of scale associated with the cost of capacitors and the labor required to install them will place an economic limit on the lowest desirable capacitor size.
The solution to improve the power factor is to add power factor correction capacitors (see Figure) to the plant power distribution system. They act as reactive power generators, and provide the needed reactive power to accomplish kW of work. This reduces the amount of reactive power, and thus total power, generated by the utilities.
Selection and location of capacitors Direct relation for capacitor sizing. kVAr Rating = kW [tan φ1 – tan φ2] where kVAr rating is the size of the capacitor needed, kW is the average power drawn, tan φ1 is the trigonometric ratio for the present power factor, and tan φ2 is the trigonometric ratio for the desired PF. φ1 = Existing (Cos-1 PF1) and φ2 = Improved (Cos-1 PF2) Example: The utility bill shows an average power factor of 0.72 with an average KW of 627. How much kVAr is required to improve the power factor to .95 ? Using formula Cos Φ1 = 0.72 , tan Φ1 = 0.963 Cos Φ2 = 0.95 , tan Φ2 = 0.329 kVAr required = P ( tanφ1 - tanφ2 ) = 627 (0.964 – 0.329) = 398 kVAr
After improvement the plant had avoided penalty and the 1500 kVA transformer now loaded only to 60% of capacity. This will allow the addition of more load in the future to be supplied by the transformer.
The advantages of PF improvement by capacitor addition a) Reactive component of the network is reduced and so also the total current in the system from the source end. b) I2R power losses are reduced in the system because of reduction in current. c) Voltage level at the load end is increased. d) kVA loading on the source generators as also on the transformers and lines upto the capacitors reduces giving capacity relief. A high power factor can help in utilizing the full capacity of your electrical system. Cost benefits of PF improvement While costs of PF improvement are in terms of investment needs for capacitor addition the benefits to be quantified for feasibility analysis are: a) Reduced kVA (Maximum demand) charges in utility bill b) Reduced distribution losses (KWH) within the plant network c) Better voltage at motor terminals and improved performance of motors d) A high power factor eliminates penalty charges imposed when operating with a low power factor e) Investment on system facilities such as transformers, cables, switchgears etc for delivering load is reduced.
Location of Capacitors The primary purpose of capacitors is to reduce the maximum demand. Additional benefits are derived by capacitor location. The Figure indicates typical capacitor locations. Maximum benefit of capacitors is derived by locating them as close as possible to the load. At this location, its kVAr are confined to the smallest possible segment, decreasing the load current. This, in turn, will reduce power losses of the system substantially. Power losses are proportional to the square of the current. When power losses are reduced, voltage at the motor increases; thus, motor performance also increases. Locations C1A, C1B and C1C of Figure indicate three different arrangements at the load. Note that in all three locations extra switches are not required, since the capacitor is either switched with the motor starter or the breaker before the starter. Case C1A is recommended for new installation, since the maximum benefit is derived and the size of the motor thermal protector is reduced. In Case C1B, as in Case C1A, the capacitor is energized only when the motor is in operation. Case C1B is recommended in cases where the installation already exists and the thermal protector does not need to be re-sized. In position C1C, the capacitor is permanently connected to the circuit but does not require a separate switch, since capacitor can be disconnected by the breaker before the starter. It should be noted that the rating of the capacitor should not be greater than the no-load magnetizing kVAr of the motor. If this condition exists, damaging over voltage or transient torques can occur. This is why most motor manufacturers specify maximum capacitor ratings to be applied to specific motors. The next preference for capacitor locations are at locations C2 and C3. In these locations, a breaker or switch will be required. Location C4 requires a high voltage breaker. The advantage of locating capacitors at power centres or feeders is that they can be grouped together. When several motors are running intermittently, the capacitors are permitted to be on line all the time, reducing the total power regardless of load. From energy efficiency point of view, capacitor location at receiving substation only helps the utility in loss reduction. Locating capacitors at tail end will help to reduce loss reduction within the plants distribution network as well and directly benefit the user by reduced consumption. Reduction in the distribution loss % in kWh when tail end power factor is raised from PF1 to a new power factor PF2, will be proportional to …. Check the equation in the slide
Capacitors for Other Loads The other types of load requiring capacitor application include induction furnaces, induction heaters and arc welding transformers etc. The capacitors are normally supplied with control gear for the application of induction furnaces and induction heating furnaces. The PF of arc furnaces experiences a wide variation over melting cycle as it changes from 0.7 at starting to 0.9 at the end of the cycle. Power factor for welding transformers is corrected by connecting capacitors across the primary winding of the transformers, as the normal PF would be in the range of 0.35. Performance Assessment of Power Factor Capacitors Voltage effects: Ideally capacitor voltage rating is to match the supply voltage. If the supply voltage is lower, the reactive kVAr produced will be the ratio V12 /V22 where V1 is the actual supply voltage, V2 is the rated voltage. On the other hand, if the supply voltage exceeds rated voltage, the life of the capacitor is adversely affected. Material of capacitors: Power factor capacitors are available in various types by dielectric material used as; paper/ polypropylene etc. The watt loss per kVAr as well as life vary with respect to the choice of the dielectric material and hence is a factor to be considered while selection. Connections: Shunt capacitor connections are adopted for almost all industry/ end user applications, while series capacitors are adopted for voltage boosting in distribution networks. Operational performance of capacitors: This can be made by monitoring capacitor charging current vis- a- vis the rated charging current. Capacity of fused elements can be replenished as per requirements. Portable analyzers can be used for measuring kVAr delivered as well as charging current. Capacitors consume 0.2 to 6.0 Watt per kVAr, which is negligible in comparison to benefits. Some checks that need to be adopted in use of capacitors are : i) Nameplates can be misleading with respect to ratings. It is good to check by charging currents. ii) Capacitor boxes may contain only insulated compound and insulated terminals with no capacitor elements inside. iii) Capacitors for single phase motor starting and those used for lighting circuits for voltage boost, are not power factor capacitor units and these cannot withstand power system conditions.
Various types of automatic power factor controls are available with relay / microprocessor logic. Two of the most common controls are: Voltage Control and kVAr Control Voltage Control Voltage alone can be used as a source of intelligence when the switched capacitors are applied at point where the circuit voltage decreases as circuit load increases. Generally, where they are applied the voltage should decrease as circuit load increases and the drop in voltage should be around 4 – 5 % with increasing load. Voltage is the most common type of intelligence used in substation applications, when maintaining a particular voltage is of prime importance. This type of control is independent of load cycle. During light load time and low source voltage, this may give leading PF at the substation, which is to be taken note of. KILOVAR Control Kilovar sensitive controls are used at locations where the voltage level is closely regulated and not available as a control variable. The capacitors can be switched to respond to a decreasing power factor as a result of change in system loading. This type of control can also be used to avoid penalty on low power factor by adding capacitors in steps as the system power factor begins to lag behind the desired value. Kilovar control requires two inputs - current and voltage from the incoming feeder, which are fed to the PF correction mechanism, either the microprocessor or the relay. Automatic Power Factor Control Relay It controls the power factor of the installation by giving signals to switch on or off power factor correction capacitors. Relay is the brain of control circuit and needs contactors of appropriate rating for switching on/off the capacitors. There is a built-in power factor transducer, which measures the power factor of the installation and converts it to a DC voltage of appropriate polarity. This is compared with a reference voltage, which can be set by means of a knob calibrated in terms of power factor. When the power factor falls below setting, the capacitors are switched on in sequence. The relays are provided with First in First out (FIFO) and First in Last Out (FILO) sequence. The capacitors controlled by the relay must be of the same rating and they are switched on/off in linear sequence. To prevent over correction hunting, a dead band is provided. This setting determines the range of phase angle over which the relay does not respond; only when the PF goes beyond this range, the relay acts. When the load is low, the effect of the capacitors is more pronounced and may lead to hunting. Under current blocking (low current cut out) shuts off the relay, switching off all capacitors one by one in sequence, when load current is below setting. Special timing sequences ensure that capacitors are fully discharged before they are switched in. This avoids dangerous over voltage transient. The solid state indicating lamps (LEDS) display various functions that the operator should know and also and indicate each capacitor switching stage. Intelligent Power Factor Controller (IPFC) This controller determines the rating of capacitance connected in each step during the first hour of its operation and stores them in memory. Based on this measurement, the IPFC switches on the most appropriate steps, thus eliminating the hunting problems normally associated with capacitor switching.
Power Factor Improvement
• Power Factor is the ratio between the kW component and the kVA component of power, drawn by an electrical load• Where kW is the actual load power & kVA is the apparent load power• It is a measure of how effectively the current is utilized for doing useful work• In all industrial electrical distribution systems, the major loads are resistive and inductive• Typical inductive loads are A.C. Motors, induction furnaces, transformers and ballast-type lighting
a) active (or working) power to perform the work -- kWb) reactive power to create and maintain electro-magnetic fields -- kVARc) The vector sum of the active power and reactive power make up the total(or apparent) power used -- kVA
Causes of Low Power Factor1. Most of the motors used in industries are of induction type, which have low lagging pf2. Lamps, discharge lamps operate at low lagging pf3. Transformer system work at low lagging pf4. Load on the system is varying which results in decreasing or low pf
Disadvantages of Low Power Factor1. Lower the pf, higher is the load current required2. Lower the pf, kVA rating of the equipment has to be more which means the equipment has to be larger and expensive3. To transmit or distribute a fixed amount of power at constant voltage, the conductor will have to carry more current at LPF which means the conductor size has to be large4. Large current at LPF, makes I2R losses to increase which results in poor efficiency5. The lagging PF reduces the handling capacity of all elements of the system due to the reactive component of the current6. Large current at LPF causes greater voltage drops in alternators, transformers, transmission lines and distributors – poor voltage regulation
Power Factor Improvement – Some important points• The total current is less than the original circuit current• The active component remains the same before and after pf correction i.e., I cosφ1 = I’ cosφ2• The reactive component is reduced i.e., I’ sinφ2 = I sinφ1 – Ic• Active power remains unchanged due to pf improvement since I cosφ1 = I’ cosφ2 => VI cosφ1 = VI’ cosφ2• KVAR (after pf correction) = lagging KVAR (before pf correction) – leading KVAR (after pf correction)• The capacitance of the capacitor required to improve pf from cosφ1 to cosφ2 is given by: C = IC / [ ω*V]• The leading KVAR supplied by pf correction device is given by: kW[tanφ1 - tanφ2]
Advantages of Power Factor Correction• Reduction in Electricity Bill due to reduced maximum demand• No low power factor penalty charges• Improved Voltage Level in System due to elimination of inductive current• KW capacities of prime movers, alternators, transformers and transmission lines is increased• Reduction in System losses; Efficiency of every plant increases; Overall cost per unit decreases• Voltage regulation of transmission lines increases i.e., Voltage drop in the system improves• Distribution capacity is augmented• Premature failure of motors and other inductive equipment can be
• Capacitive power factor correction is applied to circuits which include induction motors to reduce the inductive component of current
CAPACITOR RATINGS FOR POWER FACTOR CORRECTIONBY DIRECT CONNECTION TO INDUCTION MOTORS
Power Factor Correction• Decide most desirable power factor and capacitor rating• Can be corrected by either using shunt capacitors or by using synchronous motors• Determine the reactive power of the load• Decide on the location of capacitors• Automatic control of power factor - APFC• Maintenance: - Current unbalance - Harmonics in the system - Check on the design - revisit
Power Factor Correction• Static Power Factor Correction– By connecting correction capacitors to the motor starters-- Static power factor correction should provide capacitive current equal to 80% of magnetizing current• Bulk Power Factor Correction– Total current supplied to the distribution board is monitored and accordingly switches the banks
SPF Correction with a Soft Starter• SPF Correction should not neutralize more than 80% of magnetizing current of the motor• SPF Correction should never be applied when the motor is controlled by a VFD or Inverter• SPF correction should never be applied at the output of a solid state Soft starter
• A chemical industry had installed a 1500 kVA transformer. The initial demand of the plant was 1160 kVA with power factor of 0.70. The % loading of transformer was about 78%. To improve the power factor and to avoid the penalty, the unit had added about 410 kVAr in motor load end. This improved the power factor to 0.89, and reduced the required kVA to 913, which is the vector sum of kW and kVAr.
Considerations of supply harmonics in selection of Capacitors• Supply side harmonics are caused by non linear loads• Variable speed controllers & switch mode power supplies• Result of supply harmonics is heating of capacitors and reduced lifeMitigation of supply harmonics:• Active Filters which are essentially harmonic compensators by means of an inverter• Passive filters are resistors, inductors & capacitors• Detuned filters are reactors which make the circuit very inductive for resonant frequency and very capacitive on fundamental frequency
Performance Assessment of Power Factor Capacitors• Voltage effects: Ideally capacitor voltage rating is to match the supply voltage• Material of capacitors: Power factor capacitors are available in various types by dielectric material used• Connections: Shunt capacitor connections are adopted for almost all industry/ end user applications, while series capacitors are adopted for voltage boosting in distribution networks• Operational performance of capacitors: This can be made by monitoring capacitor charging current vis- a- vis the rated charging current
Various types of automatic power factorcontrols are available with relay /microprocessor logicTwo of the most common controls are:Voltage Control and kVAr Control