analysis of voltage problems white

VOLTAGE TERMINOLOGIESVOLTAGE TERMINOLOGIES

1. NOMINAL SYSTEM VOLTAGE1. NOMINAL SYSTEM VOLTAGEThe voltage by which a portion of the system is designated, and to which certain operating characteristics of the system are related. Each nominal system voltage pertains to a portion of the system bounded by transformers or utilization equipment.

2. SERVICE 2. SERVICE VOLTAGEVOLTAGEThe voltage at the point where the electrical

system of the supplier and the electrical system of the user are connected.

Taken from ANSI C84.1-1995 [Electric Power Systems and Equipment - Voltage Ratings (60 Hz)]

VOLTAGE TERMINOLOGIESVOLTAGE TERMINOLOGIESVOLTAGE TERMINOLOGIESVOLTAGE TERMINOLOGIES

3. UTILIZATION VOLTAGE3. UTILIZATION VOLTAGE

The voltage at the line terminals of utilization equipment.

4. NOMINAL UTILIZATION 4. NOMINAL UTILIZATION VOLTAGEVOLTAGEThe voltage rating of certain utilization

equipment used on the system.The voltage at which the operating and performance characteristics of equipment are referred.

Taken from ANSI C84.1-1995 [Electric Power Systems and Equipment - Voltage Ratings (60 Hz)]

NOMINAL SYSTEM

VOLTAGE

SERVICE VOLTAGE

UTILIZATIONVOLTAGE

A SIMPLE SECONDARY SYSTEM A SIMPLE SECONDARY SYSTEM INDICATING THE VOLTAGESINDICATING THE VOLTAGES

A SIMPLE SECONDARY SYSTEM A SIMPLE SECONDARY SYSTEM INDICATING THE VOLTAGESINDICATING THE VOLTAGES

NOMINAL UTILIZATION

VOLTAGE

VOLTAGE LIMITS ALLOWED US BY THE VOLTAGE LIMITS ALLOWED US BY THE DISTRIBUTION CODEDISTRIBUTION CODE

VOLTAGE LIMITS ALLOWED US BY THE VOLTAGE LIMITS ALLOWED US BY THE DISTRIBUTION CODEDISTRIBUTION CODE

± 10% from nominal measured at the Connection Point (Metering Point)

230 V 207 V 253 V

460 V 414 V 506 V

Nominal VoltageAllowable Limits

Minimum(-10%)

Maximum(+10%)

34.5 kV* 31.05 kV 37.95 kV

*For primary metered customers

VOLTAGE LIMITS ALLOWED US BY THE DISTRIBUTION CODE

VOLTAGE LIMITS ALLOWED US BY THE DISTRIBUTION CODE

FORMULA FOR COMPUTING VARIATION FROM NOMINAL VOLTAGE

FORMULA FOR COMPUTING VARIATION FROM NOMINAL VOLTAGE

% VAR. =

Measured/GivenVoltage

NominalVoltage

X 100%

NominalVoltage-

SAMPLE COMPUTATIONS OF VOLTAGE VARIATION

SAMPLE COMPUTATIONS OF VOLTAGE VARIATION

Given:

Vmin = 215V

Solution:

%Var.215 - 230

230= X 100% =

EXAMPLE 1:

Given:

Vmax = 250V

Solution:

%Var.250 - 230

230= X 100% =

EXAMPLE 2:

- 6.52%

+ 8.70%

COMMON VOLTAGE PROBLEMSCOMMON VOLTAGE PROBLEMSCOMMON VOLTAGE PROBLEMSCOMMON VOLTAGE PROBLEMS

1. . Undervoltage (Low Voltage)Undervoltage (Low Voltage)

2. . Overvoltage (High Voltage)Overvoltage (High Voltage)

3. . Voltage Unbalance (Imbalance)Voltage Unbalance (Imbalance)

4. . Voltage Sag (Dip)Voltage Sag (Dip)

5. . Voltage SwellVoltage Swell

6. . Voltage Fluctuation (Flicker)Voltage Fluctuation (Flicker)

Refers to a measured voltage having a value less than the nominal for a period of time greater than 1 minute. Typical values are 0.8 - 0.9 p.u.

UNDERVOLTAGEUNDERVOLTAGE


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:Causes:

Causes Of Low Primary Supply VoltageCauses 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


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

Effects:Effects:


Load shifting or load splitting

Reconductoring

Correction of loose connections

Raising of transformer tapping

Installation of line capacitors and AVRs

Typical Solutions:Typical Solutions:

Upgrading of source transformer

Refers to a measured voltage having a value greater than the nominal for a period of time greater than 1 minute. Typical values are 1.1 - 1.2 p.u.

OVERVOLTAGEOVERVOLTAGE


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

Causes:Causes:

Failure of electronic devices

Shortened equipment life

Unwanted operation in some relays

Damage to capacitors

Effects:Effects:


Lowering of transformer tap-setting

Automatic or manual switching of capacitor banks during off-peak periods

Installation of additional pole grounding



The maximum deviation from the average of the three phase voltages, divided by the average of the three phase voltages, expressed in percent.

UNBALANCED VOLTAGEUNBALANCED VOLTAGE

FORMULA FOR COMPUTING VOLTAGE FORMULA FOR COMPUTING VOLTAGE UNBALANCE ON THREE PHASE SYSTEMSUNBALANCE ON THREE PHASE SYSTEMS

FORMULA FOR COMPUTING VOLTAGE FORMULA FOR COMPUTING VOLTAGE UNBALANCE ON THREE PHASE SYSTEMSUNBALANCE ON THREE PHASE SYSTEMS

% VOLTAGE UNBALANCE =

Maximum Deviationfrom the Average

Average VoltageX 100%

The Distribution Code prescribes an limit of 2.5% at the Connection Point during normal operating conditions.

SAMPLE COMPUTATION OF VOLTAGE UNBALANCE

SAMPLE COMPUTATION OF VOLTAGE UNBALANCE

% Unb.8.33V

228.33V= X 100% = 3.65%

Given:

Va = 220V, Vb = 230V, Vc = 235V

Required:

% Voltage Unbalance

Solution:

Vave220 + 230 + 235

3= = 228.33V

Maximum Deviation = 220 - 228.33 = 8.33V

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

Causes:Causes:UNBALANCED VOLTAGEUNBALANCED VOLTAGE

UNBALANCED VOLTAGEUNBALANCED VOLTAGE

Overheating of three-phase motors

Effects:Effects:

Shutdown of equipment due to operation of unbalanced voltage (zero-sequence) relay

Load balancing

Typical Typical Solutions:Solutions:


Installation of additional pole grounding

Installation of AVRs

MOTOR DERATING FACTOR DUE TO UNBALANCED VOLTAGE

MOTOR DERATING FACTOR DUE TO UNBALANCED VOLTAGE

Der

atin

g F

acto

r

0 1 432 5

1.0

0.9

0.8

0.7

Percent Voltage Unbalance

Taken from 14.35 of NEMA MG 1 - 1993

RMS voltage variation between 0.1 to 0.9 of the nominal voltage for less than 1 minute.

VOLTAGE SAG (DIP)VOLTAGE SAG (DIP)


Line-to-ground faults

Starting of large loads (such as motors, ACU and arc furnaces)

Loose connections

Causes:Causes:

Shutdown of sensitive loads

Effects:Effects:

Flickering or turning off of lights


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

Typical Typical Solutions:Solutions:

RMS voltage variation exceeding 1.1 p.u. for less than 1 minute.

VOLTAGE SWELLVOLTAGE SWELL


Line-to-ground faults

Switching-on of capacitor bank Dropping of large loads

Causes:Causes:

Failure of of electronic and computer devices

Shortened equipment life

Unwanted operation in some relays

Effects:Effects:

Damage to capacitors


Ensure of system neutral

Scheduling of capacitor switching

Typical Typical Solutions:Solutions: Eliminate causes of faults

Series of random voltage changes. The changes normally are between 95% to 105%.

VOLTAGE FLUCTUATIONVOLTAGE FLUCTUATION


Loads with significant current variations such as arc furnaces, sawmills and arc welders. Loose connections

Causes:Causes:

Effects:Effects: Annoying variation in light output (flicker)

from incandescent and discharge light sources Video output distortion



Reconductoring


Provision of separate source for the loads causing the problem

MISCELLANEOUSMISCELLANEOUS

TOPICSTOPICS

A. Voltage MismatchA. Voltage Mismatch

B. Power Factor Correction CapacitorsB. Power Factor Correction CapacitorsC. Improper Setting of Over- and

Under-voltage ProtectionC. Improper Setting of Over- and

Under-voltage Protection

D. Deficiencies in the Customer’s Distribution System

D. Deficiencies in the Customer’s Distribution System

COMMON PROBLEMS AT THE CUSTOMER SIDE


A.A. Voltage Mismatch Voltage MismatchSome 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.



B.B. Power Factor Correction Power Factor Correction CapacitorsCapacitors

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 .


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.



C.C. Improper Setting of Under- and Over-Improper Setting of Under- and Over-voltage voltage ProtectionProtection

Under- and over-voltage relays in customer installations are sometimes set at very narrow voltage ranges resulting in unnecessary trippings.


1. Adjust the relay settings to correspond to the actual operating range of the equipment being protected



D.D. Deficiencies in the Customer’s Deficiencies in the Customer’s DistributionDistribution SystemSystem

In some instances, the voltage problem is caused by problems on the customer’s electrical facilities, such as:

1. Loose connections2. Undersized, overloaded or overextended conductors3. Unbalanced distribution of single-phase loads


1. Correct the problem(s) and seek the services of an electrical consultant, if necessary.



HOW DO SUPPLY PROBLEMS CAUSE MOTOR FAILURE?


1. Undervoltage1. UndervoltageA 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. Overvoltage2. OvervoltageA 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.

3. Unbalanced Voltage3. Unbalanced VoltageUnbalanced 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.



This graph indicates the

percent temperature

rise vs. percent voltage

unbalance. To calculate the percentage of temperature rise, double

the square of the voltage unbalance

percentage.

Taken from the Sept. 1999 Power Quality Advisor (Supplement of EC&M Magazine)

4. Phase Loss4. Phase LossThis 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 Reversal5. Phase Sequence ReversalInterchanging 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.


ESTIMATING VOLTAGE INCREASE OR DECREASE WHEN CHANGING

TRANSFORMER TAPS

ESTIMATING VOLTAGE INCREASE OR DECREASE WHEN CHANGING

TRANSFORMER TAPS

TAP CHANGING OF TRANSFORMERS WITH FIVE (5) 2.5% TAPS


A. Raising of Tap-setting

Number ofSteps

Multiply ExistingVoltage by

1 1.025

Note: No. of steps refer to the number of tap changes from the original to the new tap position. For example, there are 3 steps from Tap 2 to Tap 5.

2 1.050

3 1.075

4 1.100

SCHEMATIC WIRING DIAGRAM OF DUAL VOLTAGE TRANFORMERS

SCHEMATIC WIRING DIAGRAM OF DUAL VOLTAGE TRANFORMERS

PRIMARY WINDING

SECONDARY WINDING

120/240 V

TERMINALBLOCK

JUMPER

TAPCHANGER

THESE TAPS ARE USED BOTH FOR

TAP A & TAP B SETTINGS

139/277 V

H1

H2

12345

B A

X1 X3 X2 X4

B. Lowering of Tap-setting

Number ofSteps

Multiply ExistingVoltage by

1 0.975

Note: No. of steps refer to the number of tap changes from the original to the new tap position. For example, there are 4 steps from Tap 5 to Tap 1.

2 0.950

3 0.925

4 0.900



Vp = Vs x --------240

18,920

TAP NO.

1

2

3

4

5

VOLTAGE RATING

20,420

19,920

19,420

18,920

18,430

TRANSFORMATION RATIO

a1=20,420/240

a2=19,920/240

a3=19,420/240

a4=18,920/240

a5=18,430/240

1. Assume the transformer is at Tap 4

a4 = ------Vs

Vp

Vs

Vp

240

18,920------ = -----

2. Assume the tapping is lowered to Tap 2

Vsnew= ----a2 = -----Vsnew

Vp

a2

Vp

Vsnew= ------------

Vs x --------240

18,920

--------240

19,920

Vsnew = Vs x --------19,920

18,920

Vsnew = Vs x 0.95

SAMPLE DERIVATION OF THE MULTIPLYING FACTORS

Tap No. Rating

1 34.4

2 33.3

3 31.6

Voltage Rating For Each Tap

TMC 1 Bank No. 1

Fuij Denki Seizo4 MVA

34.4 kV - 13.8kVDelta-Wye

Tap No. Rating

1 37.088

2 36.225

3 35.363

4 34.500

5 33.638

* 2.5% per tap

Voltage Rating For Each Tap

Tanza Bank No. 1

Elin5 MVA

34.5 kV - 13.8kVWye-Delta

SAMPLE NAMEPLATE RATING OF POWER TRANSFORMERS


BF-Parañaque Bank No.1

EFACEC

50/66.7/83.33 MVA

100 kV-34.5kV-13.8kV

Wye-Wye-Delta

6 114.125

7 112.750

8 111.375

9 110.00

10 108.625

11 107.250

12 105.875

13 104.500

14 103.125

15 101.750

16 100.375

17 99.000

* 1.25% per tap

Tap No. Rating

Tap No. Rating

1 121.000

2 119.625

3 118.250

4 116.875

5 115.500

On-Load-Tap-Changer (OLTC)



Baliwag Bank No. 1Takaoka10 MVA

69 kV - 13.8kVDelta-Wye

Tap No. Rating

1 72.45

2 70.73

3 69.00

4 67.28

5 65.55

* 2.5% per tap

HV Tap Changer



1 12.420

2 12.590

: :

7 13.460

8 13.630

9 13.800

10 13.970

11 14.150

: :

16 15.010

17 15.180

* 1.23% per tap

Tap No. Rating

LV OLTC

analysis of voltage problems white

Documents

nominal voltage

measured voltage

voltage terminologies1

voltage swell

equipment voltage ratings

service voltagethe voltage

undervoltage low voltage

dual voltage transformers