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Harmonic Distortion

Prior to the 1960's electrical systems did not experience Harmonic distortion issues. This is becasue harmonics are produces by digital switching of a signal, such as voltage. Digital electronics, suchs computers and printers are non-linear loads. Non-linear loads can cause disrupt to an electrical systems.

In a typical AC power system, the current fluctuates sinusoidally at a specific frequency, usually at 50 or 60 hertz. When we connect a linear electrical device or load to the system, it will draw current (sinusoidal) at the same frequency as the voltage, generally out of phase. However, that is not the case when you connect a non-linear device or load to an electrical power system.

Non-sinusoidal complex waveforms are constructed by “adding” together a series of sine wave frequencies known as “Harmonics”.

    Harmonics:
  1. Harmonics is the generalised term used to describe the distortion of a sinusoidal waveform by waveforms of different frequencies.
    2. Harmonic Distortion:
  2. In a power system, harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. If the fundamental frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the third is 180 Hz, etc.
    3. Harmonic Distortion:
  3. Non-linear loads produce currents and voltages with frequencies that are integer multiples of the 50 or 60 Hz fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics.

This is because non-linear loads create current harmonics through their actions. Moreover, when we connect non-linear devices like a rectifier to a system, it will draw current that is not ordinarily sinusoidal. More so, the current waveform can become somewhat elaborate, depending on its interaction with other components in the system and the type of device or load. No matter the complexity, current waveforms, according to Fourier series analysis, are deconstructable into basic sinusoids. These sinusoids, begin at the power system’s fundamental frequency and occur at integer multiples of the fundamental frequency.

    Common Disturbances associated with harmonics
  1. Overloads the neutral conductor
  2. Overloads, vibration and premature ageing of generators,transformers, motors,etc
  3. Overloading and premature ageing of capacitors
  4. Distortion of supply voltage
  5. Disturbance on communication networks
Harmonic Frequency

Total Harmonic Destortion:

  1. Harmonics are a mathematical model of the real world.

  2. • Harmonics are a mathematical model of the real world.

  3. A non-sinusoidal periodical function can be represented as the sum of :-

    A sinusoidal term of the fundamental frequency Sinusoidal terms (harmonics) whose frequencies are multiples of fundamental frequency A DC component where applicable

  1. Where:
  2. \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Voltage Ratings:

For a given design, the rated voltage is limited by the flux that is capped by the field current. The rated voltage is also limited by the windings insulation breakdown limit.

Current Ratings:
Third Harmonic

harmonic currents created by computers cause the operation of this system to change. Computers generate a substantial amount of 3rd harmonic current. Due to the mathematical phase properties, third harmonic currents add instead of cancel out on the neutral wire (see Fig.1). T herefore, in a building with a large number of Personal Computers installed, the neutral wire can carry much higher currents than it was designed for. In fact, the harmonic current alone in the neutral wire can be larger than the full rated current of the power wiring. This is the most critical problem relating to harmonics and PCs. Note that the data above shows that while it is unlikely for the neutral current to exceed the phase current, the neutral current can reach the phase current value in a PC environment. For this reason it is essential that neutral undersizing never be used in an office environment.

Linear & Non-Linear Loads

Total Harmonic Destortion:

  1. Harmonics are a mathematical model of the real world.

  2. • Harmonics are a mathematical model of the real world.

  3. A non-sinusoidal periodical function can be represented as the sum of :-

    A sinusoidal term of the fundamental frequency Sinusoidal terms (harmonics) whose frequencies are multiples of fundamental frequency A DC component where applicable

Test Values
Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.1.1.6.1)
  2. olt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.2.1.1.1.6.2)
  3. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.1.1.6.3)
  4. Tap connections are left as found unless otherwise specified. (7.2.1.1.1.7)
  1. Where:
  2. \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Voltage Ratings:

For a given design, the rated voltage is limited by the flux that is capped by the field current. The rated voltage is also limited by the windings insulation breakdown limit.

Current Ratings:
Third Harmonic

harmonic currents created by computers cause the operation of this system to change. Computers generate a substantial amount of 3rd harmonic current. Due to the mathematical phase properties, third harmonic currents add instead of cancel out on the neutral wire (see Fig.1). T herefore, in a building with a large number of Personal Computers installed, the neutral wire can carry much higher currents than it was designed for. In fact, the harmonic current alone in the neutral wire can be larger than the full rated current of the power wiring. This is the most critical problem relating to harmonics and PCs. Note that the data above shows that while it is unlikely for the neutral current to exceed the phase current, the neutral current can reach the phase current value in a PC environment. For this reason it is essential that neutral undersizing never be used in an office environment.

Frequency (Hz):

The rated frequency of a synchronous machine depends on the power system to which it is connected. Once the operation frequency is determined, only one rotational speed in possible for the given number of poles.

Electrical Frequency
\(freq (Hz) =\large \frac{n_{m} \cdot P}{120} \)
  1. Where:
  2. \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Generator Harmonic Distortion:

The harmonic currents noted above are caused by harmonics in the stator winding, in the rotor, e.g., 5th- and 7th-order stator harmonics will produce 6th-order rotor harmonics, while 11th- and 13th-order stator harmonics will produce 12th-order rotor harmonics.

Triplen harmonic

currents may result in neutral current as much as 1.7 times the phase current. Harmonic currents also cause circulating currents in the primary windings of delta-wye connected transformers. Harmonic currents flow through the impedance in an electrical distribution system (primarily conducto

ReactivePower
  1. The most troublesome are

    3,5,7,11,13,23,25

    Beyond the 50th order harmonic currents are negligible

  3. Variable-speed drives
Impedance:

Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary winding of a transformer. The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.

The percentage impedance of a transformer (Z%) is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit. In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal. The total impedance is the vector sum of the resistance and the reactance. The impedance is measured by shorting the low voltage terminals. With low voltage windings shorted, a voltage at the rated frequency is applied to the high voltage windings until full load current is circulated in low voltage windings. The ratio of voltage applied to circulate full load current to the primary voltage is the percentage impedance of the transformer. The percentage impedance of the transformer is calculated as: Z%= (Impedance Voltage/Rated Voltage)*100 Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.

Voltage Taps:

Transformers can be placed into service at locations where the line voltages are either higher or lower than the rated voltage of a transformer's primary. This these types of locations also cause the secondary voltage to be proportionately higher or lower respectively. In order to compensate for this voltage difference, transformers secondary voltage can be adjusted to nominal levels by adjusting the transformer's primary winding's voltage Tap. Transformer voltage taps chang the voltage ratio of a transformer so that its secondary voltage stays at nominal. On large power transformers, taps on the primary are used to offset any higher or lower input voltages. These tap connections are usually set at the factory for nominal line voltage. If the voltage at the site is different, the taps are changed accordingly.

Temperature Rise:

Transformer temperature rise is defined as the average temperature rise of the windings above
the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.

Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.

K Factor:

Per the NEC, transformers feeding non-linear loads shall be K-factor rated.
K-factor is the ratio between the additional losses due to harmonics and eddy current losses at 60Hz.

Basic Impulse Level
(BIL)
Rated Voltage of windings Windings Bushings
Insulation Class Bil Insulation Class Bil
480V 1.2KV 10KV 1.2KV 10KV
4160V 5KV 30KV 5KV 30KV
13.8KV 15KV 95KV 15KV 95KV
34.5KV 34,500V 150KV 34,500V 150KV
Harmonic Distrotion

Total Harmonic Destortion:

  1. Harmonics are a mathematical model of the real world.

  2. • Harmonics are a mathematical model of the real world.

  3. A non-sinusoidal periodical function can be represented as the sum of :-

    A sinusoidal term of the fundamental frequency Sinusoidal terms (harmonics) whose frequencies are multiples of fundamental frequency A DC component where applicable

  1. Where:
  2. \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Voltage Ratings:

For a given design, the rated voltage is limited by the flux that is capped by the field current. The rated voltage is also limited by the windings insulation breakdown limit.

Current Ratings:
Third Harmonic

harmonic currents created by computers cause the operation of this system to change. Computers generate a substantial amount of 3rd harmonic current. Due to the mathematical phase properties, third harmonic currents add instead of cancel out on the neutral wire (see Fig.1). T herefore, in a building with a large number of Personal Computers installed, the neutral wire can carry much higher currents than it was designed for. In fact, the harmonic current alone in the neutral wire can be larger than the full rated current of the power wiring. This is the most critical problem relating to harmonics and PCs. Note that the data above shows that while it is unlikely for the neutral current to exceed the phase current, the neutral current can reach the phase current value in a PC environment. For this reason it is essential that neutral undersizing never be used in an office environment.

Frequency (Hz):

The rated frequency of a synchronous machine depends on the power system to which it is connected. Once the operation frequency is determined, only one rotational speed in possible for the given number of poles.

Electrical Frequency
\(freq (Hz) =\large \frac{n_{m} \cdot P}{120} \)
  1. Where:
  2. \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Generator Harmonic Distortion:

The harmonic currents noted above are caused by harmonics in the stator winding, in the rotor, e.g., 5th- and 7th-order stator harmonics will produce 6th-order rotor harmonics, while 11th- and 13th-order stator harmonics will produce 12th-order rotor harmonics.

Triplen harmonic

currents may result in neutral current as much as 1.7 times the phase current. Harmonic currents also cause circulating currents in the primary windings of delta-wye connected transformers. Harmonic currents flow through the impedance in an electrical distribution system (primarily conducto

ReactivePower
  1. The most troublesome are

    3,5,7,11,13,23,25

    Beyond the 50th order harmonic currents are negligible

  3. Variable-speed drives
Impedance:

Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary winding of a transformer. The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.

The percentage impedance of a transformer (Z%) is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit. In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal. The total impedance is the vector sum of the resistance and the reactance. The impedance is measured by shorting the low voltage terminals. With low voltage windings shorted, a voltage at the rated frequency is applied to the high voltage windings until full load current is circulated in low voltage windings. The ratio of voltage applied to circulate full load current to the primary voltage is the percentage impedance of the transformer. The percentage impedance of the transformer is calculated as: Z%= (Impedance Voltage/Rated Voltage)*100 Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.

Voltage Taps:

Transformers can be placed into service at locations where the line voltages are either higher or lower than the rated voltage of a transformer's primary. This these types of locations also cause the secondary voltage to be proportionately higher or lower respectively. In order to compensate for this voltage difference, transformers secondary voltage can be adjusted to nominal levels by adjusting the transformer's primary winding's voltage Tap. Transformer voltage taps chang the voltage ratio of a transformer so that its secondary voltage stays at nominal. On large power transformers, taps on the primary are used to offset any higher or lower input voltages. These tap connections are usually set at the factory for nominal line voltage. If the voltage at the site is different, the taps are changed accordingly.

Temperature Rise:

Transformer temperature rise is defined as the average temperature rise of the windings above
the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.

Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.

K Factor:

Per the NEC, transformers feeding non-linear loads shall be K-factor rated.
K-factor is the ratio between the additional losses due to harmonics and eddy current losses at 60Hz.

Basic Impulse Level
(BIL)
Rated Voltage of windings Windings Bushings
Insulation Class Bil Insulation Class Bil
480V 1.2KV 10KV 1.2KV 10KV
4160V 5KV 30KV 5KV 30KV
13.8KV 15KV 95KV 15KV 95KV
34.5KV 34,500V 150KV 34,500V 150KV
Visual and Mechanical Inspection
PROCEDURE:

NETA ATS

Rated Voltage of windings Windings Bushings
Insulation Class Bil Insulation Class Bil
480V 1.2KV 10KV 1.2KV 10KV
4160V 5KV 30KV 5KV 30KV
13.8KV 15KV 95KV 15KV 95KV
34.5KV 34,500V 150KV 34,500V 150KV

NETA ATS

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

NOTE: This category consists of power transformers with windings rated higher than 600 volts and low-voltage transformers larger than 167 kVA single-phase or 500 kVA three-phase.
Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition..
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed.
  5. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.2.1.1.2.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.
  6. Verify that as-left tap connections are as specified.
  7. Verify the presence of surge arresters.