What are non-linear loads and why are they a concern today? |
|
A load is considered non-linear if its impedance changes with the applied voltage. The changing impedance
means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal
voltage. These non-sinusoidal currents contain harmonic currents that interact with the impedance of the power distribution
system to create voltage distortion that can affect both the distribution system equipment and the loads connected to
it.
Harmonic problems are now common in not only industrial applications but in commercial buildings as well.
This is due primarily to new power conversion technologies, such as the switch-mode power supply (SMPS), which can be found
in virtually every power electronic device. The SMPS is an excellent power supply, but it is also a highly non-linear load.
Their proliferation has made them a substantial portion of the total load in most commercial buildings.
|
What problems do non-linear loads and harmonics create? |
|
Most power systems can accommodate a certain level of harmonic currents but will experience problems when
they become a significant component of the overall load. As these higher frequency harmonic currents flow through the power
system, they can create problems such as:
- Overheating of electrical distribution equipment, such as cables, transformers, standby generators, etc.
- Overheating of rotating equipment, such as electric motors
- High voltages and circulating currents caused by harmonic resonance
- Equipment malfunctions due to excessive voltage distortion
- Increased internal losses in connected equipment resulting in component failure and shortened lifespan
- False operation of protection equipment
- Metering errors
- Lower system power factor preventing effective utilization
- Voltage regulator problems on diesel generators
- Inability of automatic transfer switches to operate in closed transition
|
Can't equipment manufacturers design their products to be free of harmonics? |
|
Yes they can, but lowering the current distortion levels at the input to the SMPS in a computer will add to
the cost of the computer. This is not a step that computer manufacturers wish to take because of the continuous and intense
cost cutting in the computer industry. Actually it is less costly overall to provide a harmonic mitigating transformer to feed
several hundred computers than it is to improve the operation of the SMPS in each computer. This is especially true when we
consider that the added cost of the improved SMPS will reappear every three years when a new computer system is purchased.
|
What is a harmonic mitigating transformer and how is it different than a K-rated transformer? |
|
Harmonic Mitigating Transformers, or HMTs, are specifically designed to minimize the voltage distortion and
power losses that result from the harmonics generated by non-linear loads such as personal computers. K-Rated transformers, on
the other hand, are simply designed to prevent their overheating when subjected to heavy non-linear loading but do very little
to reduce the harmonic losses themselves and as for voltage distortion, they perform virtually no better than conventional
delta-wye transformers.
|
Are there standards that can help in addressing harmonics? |
|
The standard most commonly applied to the control of harmonics in Power Systems is IEEE standard 519,
'IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems'. This standard recommends
maximum acceptable limits for both voltage and current harmonics to prevent problems that can result from heavy non-linear
loading. The limits for harmonic currents are designed to minimize the amount of voltage distortion these currents would
produce in the power system.
|
What is a Variable Frequency Drive and how does it generate harmonics? |
|
A Variable Frequency Drive (VFD) is a solid state
device that converts utility power to a variable voltage and frequency in order to control the speed of a
3-Phase Induction Motor. By controlling the motor's speed, both energy savings and
better motor control can be achieved.
Figure 4.1 shows a typical VFD schematic diagram. The front-end rectifier and its DC bus smoothing capacitors
make the VFD a non-linear load since it will draw current in a non-sinusoidal manner. The characteristic harmonics generated by
a diode bridge rectifier will follow the relationship below:
h = np +/- 1, where: h = the harmonic number n = any integer p = the pulse number of the rectifier
Most VFD's use a 3-phase, 6-pulse (p = 6) rectifier which results in currents of harmonic number 5, 7, 11,
13, 17, 19, etc. being generated. When dual rectifiers are used and phase shifted by 30° a 12-pulse scheme is created. 12-pulse
VFD's will only have residual amounts of 5th and 7th harmonics since substituting p = 12 in the above equation results in
harmonics 11, 13, 23, 25, etc. Other multipulse schemes such as 18 and 24 can be used to reduce harmonics further.
|
What is the LINEATOR AUHF and how does it treat VFD harmonics? |
|
The LINEATOR a purely passive device consisting of a revolutionary new inductor combined with a relatively
small capacitor bank. Its innovative design achieves cancellation of all the major harmonic currents generated by VFD's and
other similar 3-phase, 6-pulse rectifier loads. By reducing current harmonic distortion to < 8% and often as low as 5%, the
LINEATOR matches 18-Pulse VFD performance in a smaller footprint, at lower cost and with higher efficiency.
|
Is the LINEATOR compatible with all VFD's? |
|
The standard LINEATOR AUHF Type D is designed to reduce the harmonic currents generated by an AC PWM
Variable Frequency Drive equipped with a 6-pulse diode bridge rectifier. This includes a VFD that uses an SCR bridge for
pre-charge purposes. It is compatible with all PWM AC Drive configurations. For thyristor bridge (or SCR) applications, such as
DC Drives and industrial rectifiers, a Type T LINEATOR should be selected. The Type T unit is designed to accept the phase back
angle introduced by the thyristor operation. Reduction of current distortion will be slightly less than that achieved with a
Type D unit operating on a diode bridge but still will achieve < 8% ITHD at full load operation.
|
Can I use a single LINEATOR to supply multiple VFD's? |
|
A single LINEATOR can be used to supply multiple VFD's but in such an application all downstream loads must
be VFD's. The trapezoidal output voltage of the LINEATOR, although ideal for a VFD application, is not suitable for fixed speed
motors or other linear loads.
|
What if my VFD is equipped with a by-pass? |
|
On Variable Frequency Drive (VFD) installations designed or specified with a bypass circuit, it is
recommended that the LINEATOR AUHF be installed within the bypass circuit. When the LINEATOR is left in the circuit with the VFD
bypassed, its through impedance will cause a voltage drop at the motor. This typically will not be enough to under excite or
completely 'starve the motor of voltage' but will cause the motor to draw more current to compensate for the lower voltage. This
could lead to slightly higher slip, heavier losses, lower torque and the potential for overheating. If it is determined that the
motor can accept the voltage drop, then the LINEATOR can be left in the circuit when the VFD is being by-passed.
|
Can I get a computer simulation to demonstrate compliance with IEEE Std 519 or simply to show the
LINEATOR's effectiveness in a specific application? |
|
In many applications, it is desirable to perform computer simulations prior to the VFD installation in order
to ensure that the design will meet harmonic limits as defined in IEEE Std 519 or some other harmonic standard. In order to
provide this service to our customers, Mirus has developed a custom computer simulation program, known as 'SOLV', which has been
field proven to provide an accurate prediction of performance provided proper system information is applied.
|
