EENG 105 Topic 6: Understanding Power Quality PDF

EENG 105 TOPIC 6: ENGR. ERNICK R. ROMEN Faculty, CvSU EE Objectives: To understand the concept of what specifically becomes a power quality problem To have a common understanding of power quality and its attendant terminology To...

EENG 105 TOPIC 6: ENGR. ERNICK R. ROMEN Faculty, CvSU EE Objectives: To understand the concept of what specifically becomes a power quality problem To have a common understanding of power quality and its attendant terminology To establish collaboration among involved parties in dealing with PQ problems Power Generation System  A SIMPLE SECONDARY SYSTEM INDICATING THE VOLTAGES NOMINAL SYSTEM VOLTAGE SERVICE VOLTAGE NOMINAL UTILIZATION UTILIZATION VOLTAGE VOLTAGE Power Quality The quality of the voltage, including its frequency and the resulting current, that are measured in the Distribution System during normal conditions Power Quality Power quality addresses problems that deal with electromagnetic compatibility. Electromagnetic Compatibility “The ability of a device, equipment or system to function satisfactorily in its electromagnetic environment, without inducing intolerable electromagnetic disturbances in anything in that environment.” PQ is all about electromagnetic compatibility Equipment operates within its environment without causing disturbance either to itself or other equipment, or back to utility source End-use equipment (load) rating matches supply voltage rating and variations Mismatch can be avoided right from the design stage PQ is all about electromagnetic compatibility Utility service standards must be considered before purchase of end-use equipment for compatibility of ratings A mismatch is costly in terms of bridging incompatibility Power Quality Problem "Any power problem manifested in voltage, current, or frequency deviation that results in failure or mis-operation of utility or end-user equipment."  Power Quality Illustrated Failure Normal Failure PQ Problem Illustrated Fault/s anywhere in the network may result to power disturbances of varying magnitude and duration depending on the type and proximity of fault. Power quality issues may be viewed from three different perspectives: ❑ End-user ❑ Utility ❑ Equipment Manufacturer End-user Concerns: ✓ Types of disturbances which may affect equipment operation ✓ Adverse impact of power disturbance on electrical equipment ✓ Over-all impact of power disturbance on plant operation Utility Concerns: ✓ Origin and cause of power disturbance ✓ Effect of disturbance on its customers and in the electrical system ✓ Minimize occurrence of power disturbances ✓ Provide reliable power quality service within the prescribed limit Equipment Manufacturers: ✓ Equipment’s rating compatibility ✓ Tolerance of equipment on power disturbance Power Quality Solution – Joint Effort Solution to PQ problems is not the responsibility of only one party. The solution is a concerted effort between the power supplier, the electricity user, and equipment manufacturer. Parameters and Limits Power quality begins with knowing the characteristics of the distribution utility, its capabilities and limitations. Customers must take note of the following parameters in determining the appropriate equipment and system design for their facilities. Parameter Limit/s Frequency shall be 60 Hz, with allowable Frequency Variations variation of 59.7 to 60.3 Hz Plus/Minus 10% of RMS Voltage Secondary Nominal Voltages Service Voltages 230 Volts, 400 Volts, 460 Volts Primary Nominal Voltages 13.2 kV, 13.8 kV, 34.5 kV, 69 kV, 115 kV Voltage THD Within 5% Voltage Unbalance Within 2.5% Reference: Philippine Distribution Code Parameters and Limits Effect of Utility Variations on Equipment Ratings Equipment must operate satisfactorily within the -10% to +10% voltage variation of the Distribution Utility's nominal service voltage Voltage Variation Lower Limit Upper Limit Nominal Service Voltage: 230 V 207 volts -10% 253 volts +10% Ex. Equipment Rating: 200 V 180 volts 3% 220 volts 27% Ex. Equipment Rating: 220 V 198 volts -6% 242 volts 15% Nominal Service Voltage: 460 V 414 volts -10% 506 volts +10% Ex. Equipment Rating: 440 V 396 volts -6% 484 volts 15% Ex. Equipment Rating: 480 V 432 volts -14% 528 volts 5%  VOLTAGE LIMITS ALLOWED US BY THE ENERGY REGULATORY BOARD B. COMMERCIAL AND INDUSTRIAL CUSTOMERS Allowable Limits Nominal Voltage Minimum Maximum (-10%) (+10%) 230 V 207 V 253 V 460 V 414 V 506 V 34.5 kV* 31.05 kV 37.95 kV *For primary metered customers  FORMULA FOR COMPUTING VARIATION FROM NOMINAL VOLTAGE Measured/Given Nominal % VAR. Voltage - Voltage = X 100% Nominal Voltage  SAMPLE COMPUTATIONS OF VOLTAGE VARIATION EXAMPLE 1: Given: Solution: Vmin = 215V %Var. 215 - 230 X 100% = - 6.52% = 230 EXAMPLE 2: Given: Solution: Vmax = 250V %Var. 250 - 230 X 100% = + 8.70% = 230 COMMON VOLTAGE PROBLEMS 1. Undervoltage (Low Voltage) 2. Overvoltage (High Voltage) 3. Voltage Unbalance (Imbalance) 4. Voltage Sag (Dip) 5. Voltage Swell 6. Voltage Fluctuation (Flicker)  UNDERVOLTAGE Refers to a measured voltage having a value less than the nominal for a period of time greater than 1 minute.  UNDERVOLTAGE Causes:  Overextended secondary lines  Undersized/overloaded secondary lines, transformer lead wires or service drop  Loose connections  Overloaded transformer  Poor/loose grounding or cut system neutral  Wrong tapping of dual voltage transformers in wye-wye banks (120/240V instead of 139/277V)  Low transformer tap-setting  Low primary voltage Causes Of Low Primary Supply Voltage  Loose connections on the primary  Overextended primary lines  Undersized primary conductors  Low substation bus voltage  De-energized substation/line capacitor banks  Disabled or defective OLTC of substation power transformer  Defective substation/line AVRs  UNDERVOLTAGE Effects:  Equipment malfunction  Dropout of motor controllers  Heating losses and speed changes in induction motors  Shutdown of electronic and computer eqpt.  Reduced output of capacitor banks  UNDERVOLTAGE Typical Solutions:  Load shifting or load splitting  Reconductoring  Correction of loose connections  Raising of transformer tapping  Upgrading of source transformer  Installation of line capacitors and AVRs  OVERVOLTAGE Refers to a measured voltage having a value greater than the nominal for a period of time greater than 1 minute. OVERVOLTAGE more than 1 minute  OVERVOLTAGE Causes:  High tap-setting of distribution transformer  High primary voltage  Loose or isolated system neutral/grounding  Wrong transformer connection, polarity or tapping  Single-phasing of open-wye, open-delta bank  OVERVOLTAGE Effects:  Failure of electronic devices  Shortened equipment life  Unwanted operation in some relays  Damage to capacitors  OVERVOLTAGE Typical Solutions:  Lowering of transformer tap-setting  Automatic or manual switching of capacitor banks during off-peak periods  Installation of additional pole grounding  UNBALANCED VOLTAGE Steady state quantity defined as the maximum deviation from the average of the three phase voltages, divided by the average of the three phase voltages, expressed in percent. UNBALANCED VOLTAGE t  FORMULA FOR COMPUTING VOLTAGE UNBALANCE ON THREE PHASE SYSTEMS Maximum Deviation % VOLTAGE UNBALANCE from the Average = X 100% Average Voltage Annex D of ANSI C84.1-1995 Recommends that supply systems should be designed and and operated to limit the maximum voltage unbalance to 3% when measured at the electric utility revenue meter under no-load conditions.  SAMPLE COMPUTATION OF VOLTAGE UNBALANCE Given: Required: Va = 220V, Vb = 230V, Vc = 235V % Voltage Unbalance Solution: Vave 220 + 230 + 235 = 228.33V = 3 Maximum Deviation = 220 - 228.33 = 8.33V % Unb. 8.33V = X 100% = 3.65% 228.33V  UNBALANCED VOLTAGE Causes:  Unbalanced secondary load  Loose connections  Loose neutral or insufficient grounding  Non-uniform transformer taps  Large difference in transformer impedances  One-phase out  De-energized capacitor units  Single-phasing of open-wye, open-delta bank  Unbalanced loading of the primary line  Defective AVR  UNBALANCED VOLTAGE Effects:  Overheating of three-phase motors  Shutdown of equipment due to operation of unbalanced voltage (zero-sequence) relay Typical Solutions:  Load balancing  Correction of loose connections  Installation of additional pole grounding  Installation of AVRs  VOLTAGE SAG (DIP) RMS voltage variation between 0.1 to 0.9 of the nominal voltage for less than 1 minute. VOLTAGE SAG (DIP) 1 minute or less  VOLTAGE SAG (DIP) Causes:  Line-to-ground faults  Starting of large loads (such as motors, ACU and arc furnaces)  Loose connections Effects:  Shutdown of sensitive loads  Flickering or turning off of lights  VOLTAGE SAG (DIP) Typical Solutions:  Increasing the size of conductor and transformers feeding loads with high inrush current  Reduced-voltage motor starters  Use of UPS or constant voltage transformers (CVTs) for sensitive electronic loads  VOLTAGE SWELL RMS voltage variation exceeding 1.1 p.u. for less than 1 minute. VOLTAGE SWELL less than 1 minute  VOLTAGE SWELL Causes:  Line-to-ground faults  Switching-on of capacitor bank  Dropping of large loads Effects:  Failure of of electronic and computer devices  Shortened equipment life  Unwanted operation in some relays  Damage to capacitors  VOLTAGE SWELL Typical Solutions:  Eliminate causes of faults  Ensure of system neutral  Scheduling of capacitor switching  VOLTAGE FLUCTUATION Series of random voltage changes. The changes normally are between 95% to 105%. VOLTAGE FLUCTUATION  VOLTAGE FLUCTUATION Causes:  Loads with significant current variations such as arc furnaces, sawmills and arc welders.  Loose connections Effects:  Annoying variation in light output (flicker) from incandescent and discharge light sources  Video output distortion  VOLTAGE FLUCTUATION Typical Solutions:  Correction of loose connections  Reconductoring  Provision of separate source for the loads causing the problem MISCELLANEOUS TOPICS  COMMON PROBLEMS AT THE CUSTOMER SIDE A. Voltage Mismatch B. Power Factor Correction Capacitors C. Improper Setting of Over- and Under-voltage Protection D. Deficiencies in the Customer’s Distribution System  COMMON PROBLEMS AT THE CUSTOMER SIDE A. Voltage Mismatch Some customer equipment, usually motors, are rated either 220 volts or 440 volts. The motors can successfully operate in supply voltages of up ±10% of their rating (198V to 242V for 220-volt motors and 396V to 484V for 440-volt motors). Our 230- and 460- volts supply voltage, meanwhile, could reach 253V and 506V, respectively, which although still within the ERB limits, are already above the operating range of the motors. Solutions at the Customer Side: 1. Install a special transformer or voltage regulator for the equipment. Solutions at the Utility Side: 1. Adjust the tap setting of the source transformer after careful evaluation of the customer’s existing supply voltage.  COMMON PROBLEMS AT THE CUSTOMER SIDE B. Power Factor Correction Capacitors Most industrial and commercial customers install capacitors to improve the power factor of their system. However, capacitors also raise the system’s voltage and could cause overvoltages during light load/off-peak periods when switched-on permanently. Solutions at the Customer Side: 1. Manually de-energize the capacitors during off-peak periods. 2. Install a controlling device (voltage or power factor based) that would energize only the needed capacitor units during certain periods of the day.  COMMON PROBLEMS AT THE CUSTOMER SIDE C. Improper Setting of Under- and Over-voltage Protection Under- and over-voltage relays in customer installations are sometimes set at very narrow voltage ranges resulting in unnecessary trippings. Solutions at the Customer Side: 1. Adjust the relay settings to correspond to the actual operating range of the equipment being protected  COMMON PROBLEMS AT THE CUSTOMER SIDE D. Deficiencies in the Customer’s Distribution System In some instances, the voltage problem is caused by problems on the customer’s electrical facilities, such as: 1. Loose connections 2. Undersized, overloaded or overextended conductors 3. Unbalanced distribution of single-phase loads Solutions at the Customer Side: 1. Correct the problem(s) and seek the services of an electrical consultant, if necessary.  HOW DO SUPPLY PROBLEMS CAUSE MOTOR FAILURE? 1. Undervoltage A motor running at less than nameplate voltage tolerance will run hotter than when it has the correct voltage. This is because the motor tries to maintain torque and will draw more current as the voltage decreases. Lower voltage also means a slower rotation, which means less movement of cooling air. 2. Overvoltage A slight increase in voltage causes the actual current draw to decrease. However, the motor’s impedance is not a fixed value and can change markedly when voltages are outside design parameters. So when the impedance starts to drop (due to the effects of the increased voltage), current goes up (per Ohm’s Law). If the voltage continues to increase, the current will also increase, causing excessive heat buildup.  HOW DO SUPPLY PROBLEMS CAUSE MOTOR FAILURE? 3. Unbalanced Voltage Unbalanced supply voltage causes overheating of both the rotor and stator windings of three-phase induction motors. The relationship between voltage unbalance and overheating is exponential, not linear. A small amount of heat produces a large amount of excess heat. The effect of unbalanced voltage on polyphase induction motors is equivalent to the introduction of a “negative sequence voltage” having a rotation opposite to that occurring with balanced voltages. This negative-sequence voltage produces an air gap flux rotating against the rotation of the rotor, tending to produce high currents. HOW DO SUPPLY PROBLEMS CAUSE MOTOR FAILURE? 4. Phase Loss This is the most severe level of unbalance. When phase loss occurs while a motor is running at full load, the winding temperature soars as the motor attempts to maintain it torque. It’s likely the motor will stall, subjecting the windings to locked rotor currents. (A motor in locked rotor conditions acts like a low impedance load, drawing high currents.) Unless the motor is disconnected from the the supply, it will most certainly fail. 5. Phase Sequence Reversal Interchanging any two supply conductors will reverse the direction of rotation of a polyphase motor. For some process configurations, reversal in direction could severely damage connected equipment -- or worse, fatally injure people. Thank you and God bless! EE LANG MALAKAS!!