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Theory Of Harmonic Filtering

Harmonic source

Harmonic is sine-wave components of any electrical quantities, and its frequency is an integer multiple of the fundamental frequency.

In the power system, the generation of harmonics is introduced because the current and the voltage is not in linear relation on a nonlinear load, thus came the harmonic current. With the circuit rich in semiconductor such as switching operation of thyristor and nonlinearity of diode and thyristor, large amount of harmonics are introduced in power system equipment such as converter and inverter.

All the non-linear loads generate harmonic currents, and the typical equipments are: switch-mode power supply (SMPS), electronic fluorescent ballasts, DC motor drives, uninterruptible power supply (UPS), magnetic iron core equipment and certain appliances such as television sets.

Classification of harmonic

1, The harmonic number h must be an integer multiple of the fundamental frequency.

2, Inter-harmonics and sub-harmonic: Under certain power system conditions, there will be current fluctuations, thus introducing current component of non-integer multiple of the fundamental frequency, known as fractional harmonics or inter-harmonics. When Inter-harmonics’ frequency are lower than the fundamental frequency, they are also known as sub-harmonics.

3, Transient harmonic: In transient process a clear distortion with high-frequency components exists, but these high-frequency components are not consider harmonics. Harmonics must appear in the steady state condition, and its frequency must be integer multiple of the fundamental frequency whose waveform is continuous and lasting at least a few seconds, while transient phenomena usually disappear after a few cycles.

4, Short-time harmonic: Short-time current impact, for example, the inrush current of transformer, contains short-time harmonic and inter-harmonic, which should be distinguished from the steady-state and quasi-steady-state harmonics.

5, Notch: When converter device commutated, commutation notch or gap will appear in the voltage. Although this distortion is periodic, but is not treated as harmonic problems.

Basic concepts of Harmonics

1, each harmonic component has a different frequency, amplitude and phase angle.

2, When h is odd it is called "odd harmonics" and otherwise "even harmonics." In a balanced three-phase system, due to symmetry, even harmonics are eliminated, only odd harmonics exists and odd harmonics can caused more harm to the system than even harmonics.

3, h = 3k +1, k = 0, 1, 2 ... is called positive sequence harmonic whose phase shifting direction is the same as fundamental sequence; h = 3k +2 is called negative sequence with contrary shifting sequence; h = 3k +3 is called zero sequence, whose three-phase share the same phase.

4, The definition of harmonic content is, Where Currents are all in RMS.

5, The definition of ITHD, Where M is set at 25 in GB.

6, After introduction of harmonic, the Power Factor becomes , so it is relevent with distortion.

7, Harmonic currents will generate a non-sinusoidal voltage drop through the transformer impedance, resulting in voltage distortion.

8, The power supply network in one user may be affected by another adjacent user’s harmonics.

typical harmonic source

In general, there are three main categories of current harmonic sources, in terms of its nonlinear characteristics:

1, ferromagnetic saturation type: such as transformers, reactors, etc., their ferromagnetic saturation properties are nonlinear.

The magnetic flux density at rated voltage of transformer is chosen close to the inflection point of saturation curve, so when the voltage exceeds the rated value, the harmonic current of the transformer voltage rises rapidly. With the increase in operating voltage at light-load period, the degree of core saturation becomes deeper, and therefore increases the harmonic content. Where, Y-shaped-neural-line-connected connection contains a lot of 3rd order harmonic. While △ connection or Y-shaped-neutral-line-unconnected connection can greatly prevent the flow of zero sequence harmonic currents.

2, Power Electronic switching: The main AC-DC converter, bi-directional thyristor controlled switching devices, PWM based inverter and switching power supplies and other power electronic devices.

High capacity power converter is the most significant non-linear loads. The amplitude and order of the harmonic is relevant to the pulse number and control angle. While Inverter mainly produce 5th ,7th order harmonics.

3, arc-type: AC electric arc furnaces, welding machines, and fluorescent lamps and other exchanges.

These devices, even powered by ideal sinusoidal voltage supply, its current will always be rich in harmonic content. The amount of harmonic current is determined by its own characteristics and working conditions as well as the voltage applied to it, and is irrelevant with power system parameters, and are often treated as harmonic current source.

harmonic current content of Electric arc furnace (%)

Harm of Harmonics

Before power electronic equipment are widely used, we have some knowledge and some research of the harm harmonics may incur, but not enough attention was given to the harmonic pollution.

In the past four decades, rapid growth of various power electronic devices worsen the harmonic pollution to the grid, but until faults begin to show correlation with the harmonics, did the potential severity of damage caused by harmonic recognized.

Harmonics can cause the following harm:

1, Increase business operating costs

Harmonic currents will "travel back" through the transformer to the high-voltage power grid, resulting in increased power line losses, power equipment heating, thereby increasing the electricity bill.

2, Additional heating of transformer

Harmonic current and consequential voltage distortion will increase iron loss and copper loss of transformer. At the same time, the transformer’s eddy current loss will grow proportional to the square of amplitude of harmonic current. So harmonic currents will cause transformer overheating, vibration and noise increasing, insulation aging, thus seriously shorten the lifetime of the transformer, lower the reliability of power supply and may caused serious consequences to the process of production.

3, Causing resonance

Network contains a large number of capacitive and inductive loads in series or in parallel, which constitute a number of series or parallel resonant conditions. If the harmonic current components overlap in one of the particular frequency, harmonic current may be amplified by the phenomenon of resonant, resulting in over-voltage or over-current, endangering the safety operation of power systems and causing transmission and distribution accident.

4, Damage to generators and motors

Generator and motors are more vulnerable to the harmonic voltage distortion. Harmonic voltage is applied to the rotator of the generator or stator windings of motor. Then harmonic flux will show up in the rotator or in the core, thereby reducing the efficiency of power generation and consumption. In more serious occasions, the harmonic oscillation will produce turbine torque vibration, causing mechanical resonance, resulting in Deformation and fatigue of turbine’s blade. Motors will have over-heating problems when the voltage distortion reach 8% to 10%.

5, the impact on relay

Relay protection devices are crucial to ensure the safety operation of the grid, while harmonic current can easily cause relay Malfunction. In particular, relay has been widely used in the microprocessor-based protection and integrated automation devices, their malfunction may bring the whole plant’s grid into collapse, causing widespread power outages accidents.

6, Measurement error

Harmonic will affect the accuracy of measuring instruments, causing measuring error and leading to incorrect measurement and decision.

7, Circuit breaker rekindling

When short-circuit or overload occured, circuit breaker need to break the current. But when the harmonic is serious, breaker’s breaking capacity will be greatly reduced, thus re-igniting the arc, even causing circuit breaker exploding, which tremendously affect the short-circuit protection of power grid.

8, lower quality of production line

Resonant current will cause additional vibration to motors and other equipment on a production line, increasing the production error and reducing production precision.

9, Affect digital and analog communication systems

When the transmission lines lie in parallel or lie in close proximity with analog signals and digital communication lines, the harmonics in the transmission lines will interfere with the communication system, thereby reducing the accuracy of information, resulting in errors or noise. This can lead to communication or relay system malfunction, which can be a serious threat to the control system.

Harmonic treatment

Since the 1970s, power electronics technology has been growing rapidly. its application in power systems, industrial, transportation and family are increasingly widespread, and the consequential harmonics are causing more harm than ever. Most countries are aware of the harmonic problems and held a number of international academic conferences on harmonics treatment, and national standards and international standards are being established and revised to coerce limitation of harmonics in power system.

China has strengthened the harmonic monitoring, management and governance by issuing national standard GB/T 14549-93 "power quality - the utility grid harmonics." The standard has given the allowance of the injection of harmonic current values for different voltage levels.

Method of harmonic treatment

1, Passive filter compensation device

Passive filter compensation device can filter harmonic and compensate reactive power at the same time, which is low-cost, simple, easy to maintain and widely used.

Passive filter compensation device can be divided into the following categories:

1.1, Single-tuned narrow passband filters, with good filtering effect, little loss, easy tuning, is the most widely used type. Because a single-tuned filter uses fewer components, which means lowering cost in investment and maintenance.

1.2, Double-tuned narrow passband filter can replace two single-tuned filter, only one reactor to withstand all the impulse voltage, but the wiring is complex and the tuning difficult, so it is used only in the high-voltage systems.

1.3, First-order high-pass filter: its power loss for fundamental current is too great, so it is generally not used.

1.4, Second-order high-pass filter, its passband is very wide, filtering effect good, and both the resonant point and Q factor can be tuned, and accidental resonance and amplification are prevented, therefore it could be used as low-pass filter.

1.5, Third-order high-pass filter is generally used for electric arc furnace filtering.

1.6, "C"-type high-pass filter can be used for electric arc furnace filtering. It is particularly effective for 2nd harmonic.

2, active harmonic filter device

Active Power Filter comprise of power electronic components and DC energy storage components. The systems generate a harmonic current with the same frequency, magnitude, but opposite phase of the harmonic currents on the power grid thus offsetting harmonic.

Features of active Power Filter are:

2.1, APF is not only able to filter harmonics, but also is able to inhibit flicker, to compensate reactive power.

2.2, APF's filtering characteristics is irrelevant with system impedance, thus eliminating the risk of resonance with the system impedance;

2.3, APF can automatically track for changes in the harmonic and filter accordingly, with a high degree of controllability and rapidity.

2.4, APF use d-q-0 transformation algorithm, which suit for any non-sine non-symmetrical three-phase circuit, can be used for measurement under various condition.

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