SOURCES OF HARMONICS

Conventional electromagnetic devices as well as semiconductor applications act as sources of harmonics. Conventional electromagnetic devices include stationary transformer as well as rotating machines. Harmonic generation in these machine depends on the properties of the materials used to construct them, different design constraints and considerations, operating principle and of course load environment. Beside these arcing devices produces considerable amount of harmonics. Other than conventional devices, semiconductor based power supplies, phase controllers, reactors, etc are used enormously in modern power system network and they are contributing huge amount of harmonics to the power system. In electric power system, main sources of harmonics may be classified as follows:

1. Magnetization non-linearities of transformer
2. Rotating machines
3. Arcing devices
4. Semiconductor based power supply system
5. Inverter fed A.C. drives
6. Thyristor controlled reactors
7. Phase controllers
8. A.C. regulators

Above mentioned sources are described in the following sections.

1) MAGNETIZATION NON LINEARITIES OF TRANSFORMERS

Transformer magnetic material characteristic is non linear. This non linearity is the main reason for harmonics during excitation. Sources of harmonics in transformer may be classified into four categories as follows:

1. NORMAL EXCITATION: Normal excitation current of a transformer is non sinusoidal. The distortion is mainly caused by zero sequence triplen harmonics and particularly the third present in the excitation current. Presence of the electric path like air, oil or tank for zero sequence components can be used to reduce those harmonics. Their high reluctance tends to reduce them. Delta connection of poly-phase transformer is very effective to reduce triplen harmonics provided the three phase voltages are balanced.

2. SYMMETRICAL OVER EXCITATION: Transformers are designed to make good use of the magnetic properties of the core material. When such transformers are subjected to a rise in voltage, the cores face a considerable rise in magnetic flux density, which often causes considerable saturation. This saturation with symmetrical magnetizing current generates all the odd harmonics. The fundamental component is not a problem and all triplen harmonics can be absorbed by delta connection in balanced system. The harmonics generated by symmetrical over excitation are odd harmonics (like 5, 7, 11, 13, 17, 19. . . . etc) i.e. those of orders 6k ±1, where k is an integer.

3. INRUSH CURRENT HARMONICS: When a transformer is switched off, sometimes there exists a residual flux density in the core. When the transformer is re-energized the flux density can reach peak levels of twice the maximum flux density or more. It produces high ampere-turns in the core. This causes magnetizing currents to reach up to 5–10 per unit of the rated value, which is very high as compared to the normal values of a few percentage points. This is known as inrush current. This causes generation of enormous second order harmonic component in the transformer current.

4. D.C. MAGNETIZATION: Under magnetic imbalance, the shape of the magnetizing characteristics and the excitation currents are different from those under no load conditions. When the flux is unbalanced, the core contains an average value of flux (φdc), which is equivalent to a direct component of excitation current of the transformer. Under such unbalance conditions, the transformer excitation current contains both odd and even harmonic components.

2) ROTATING MACHINE

Rotating machines also act as source of harmonics in power system. Causes of harmonics generation in rotating electrical machines are classified into following categories:

1. MAGNETIC NONLINEARITIES OF THE CORE MATERIAL: is a nonlinear magnetization characteristics of the core material causes harmonic generation.

2. NON UNIFORM FLUX DISTRIBUTION IN AIR GAP: often it is assumed that the air-gap flux distribution is uniform and the operating principles of rotating machines are discussed based on this assumption. But in most of the rotating machines, flux distribution in air-gap is not uniform which leads to harmonics production.

3. SLOT HARMONICS: slots are inevitable in rotating machines. Alternate presence of slot and teeth changes the reluctance of the magnetic flux varies in similar type of alternating fashion. This variation acts as a reason for harmonic generation. Harmonics produced due to pitch factor and distribution factor.

4. DESIGN PARAMETERS LIKE PITCH FACTOR AND DISTRIBUTION FACTORS: HARMONIC generation in synchronous generator depends on the different design factors like pitch factor and distribution factors.

5. ROTOR SALIENCY: rotor saliency brings the variation of reluctance in the magnetic path and reactance in electric path which contribute to the harmonic generation.

6. CRAWLING: it is a common problem faced by induction motors. During this fault, odd harmonics like 5th and 7th orders appear. Fifth harmonics rotates in the same direction as of the fundamental but 7th order harmonics rotate in opposite to this direction. It changes the operating characteristics of the motor. During crawling, 7th harmonics is dominating over the 5th harmonics and lowers the stable operating zone to one seventh speed of the rated value.

7. COGGING: It is a problem where induction motors fail to start at all. Harmonic production from the motor during this condition is different from the normal condition.

8. ROTOR MISALIGNMENT: rotor misalignment causes variation of flux linkage in each cycle of rotation contributing to harmonic generation.

9. MASS UNBALANCE: with the aging, mass unbalance is observed specially in the rotor side. This refers to the core property and adds in harmonic generation.

10. BAR BREAKAGE: bar breakage in an induction motor, either symmetrically or asymmetrically, is reflected in harmonic generation in rotor circuit as well as in stator side.

11. FRACTAL ERROR: fractal error in core material produces harmonics.

12. UNSYMMETRICAL FAULT: unsymmetrical fault is also a reason for harmonic generation related to negative sequence components.

3) DISTORTION CAUSED BY ARCING DEVICES

Arcing devices are very important source of power system harmonics. The voltage versus current characteristics of an electric arc in an arcing device are highly non linear. Arc ignition is equivalent to a short circuit current with decrease in voltage.

The voltage-current is controlled by the power system impedance. In respect of harmonic generation, arcing devices are divided into three main categories:

1. Electric arc furnace
2. Discharge type lighting
3. Arc welders.

4 )POWER SUPPLIES WITH SEMICONDUCTOR DEVICES

Semiconductor based power supply systems are the main sources of harmonics. Harmonics generated in power supply include integer harmonics, inter harmonics and sub harmonics. Frequencies and magnitudes of the harmonics depend on the type of semiconductor devices used in the power supplies, operating point, nature of load variation, etc.

5) INVERTER FED AC DRIVES

Application of AC drives has increased to a great extent, most of which are inverter fed AC drives. They use switching circuits using semiconductor devices like GTO, IGBT, etc. Pulse width modulation (PWM) has got very popularity in AC drive application. All these drives are sources of integer as well as fractional harmonics.

6) Thyristor Controlled Reactors

VAR compensators used in power system network are also source of harmonics. Different types of thyristor controlled reactors are used in power system like series controller, shunt controller, static VAR compensator (SVC), fixed capacitor thyristor controlled reactor (FCTCR), thyristor switched capacitor thyristor controlled reactor (TSCTCR). All these circuits are sources of harmonics in power system. Use of static synchronous generator (SSG), voltage source STATCOM, current source STATCOM, etc in power system are increasing rapidly. All these contribute harmonics of both integer and fractional type in power system. For example, SVC produces odd harmonics. Under perfectly symmetrical voltage conditions, triplen harmonics are kept out of the line by delta connection.

7) Phase Controller

For supply of stable and balanced three phase electric power, phase controller plays important role in power system. Phase controllers used in power system act as source of harmonics. Modulated phase control method is used in cyclo-converter. It performs static power conversion from one frequency to another frequency. Most of the cyclo converter waveforms contain frequencies which are not integer multiples of the main output frequency.

8) AC Regulators

AC regulators used in power system apply both off line and on line control technique for voltage regulation which result in harmonic generation. On line regulation technique distorts wave-shape more than off line regulation along with other power system disturbances like transients, DC offset, flicker etc. Thyristor controlled single phase or poly phase regulators using half wave, full wave or integral cycle control technique produce sub-harmonics and inter-harmonics in power system.