Total Power Factor by John Houdek

Total Power Factor Tutorial 

What’s new in Power Quality? Not the Goal! PQM-109

The goal of quality power for your facility or application is probably still the same:

·    Supply equipment with power that does not cause malfunction or damage

·    Eliminate penalties from provider or save energy costs

·    Comply with industry standards

 

Peace of Mind

What is new in power quality is the ability to model the solution before it is installed to prove the effectiveness of the solution giving you peace of mind.  This can be done at the power electronic level which is the most precise.   

Achieving the goal is sometimes difficult.  You may be faced with a decision between multiple solutions offered by well meaning manufacturers.  Which solution will truly solve your problem or meet your needs?

Now, your system can be modeled at the actual power electronic level to predict the performance of your myriad of possible solutions.  Using circuit simulation with a comprehensive power quality analysis can save you from making an expensive mistake by evaluating various alternative solutions before you buy them.  Like a test drive before you buy it… this can give you peace of mind.

What is the function of a line reactor – for thyristor bridge PQM-108

Line reactors are commonly used in adjustable speed drive applications to improve power quality.   They are connected in series with the input terminals of the drive to serve multiple functions. 

1)   Suppress voltage transients, protect thyristors,

2)   Increase effective input impedance resulting in lower drive input harmonic current distortion and system voltage distortion, reduce true rms current, improve total (true) power factor.

5% effective impedance (at full load) yields about 35% THD-I, 3% impedance at full load results in about 45% THD-i. Note: a drive system with a 5% impedance line reactor, operating at 60% of reactor current rating will look like a 3% impedance reactor.

3)      Reduce voltage notching upstream of the line reactor. Reactor serves (along with upstream impedance, ie: transformer) as part of a voltage divider network where the notch voltage is distributed proportionately across the two impedances. To solve voltage notching, the line reactor must be connected at the input terminals of the SCR converter (ie: DC drive) that is producing the voltage notches.

What is the function of a line reactor – for diode bridge PQM-107

Line reactors are commonly used in adjustable speed drive applications to improve power quality.   They are connected in series with the input terminals of the drive to serve multiple functions. 

1)      suppress voltage transients enabling drives with a dc bus capacitor to ride thru some system voltage disturbances, minimize O/V nuisance tripping, protect diodes from transients, protect dc bus capacitor,

2)      Increase effective input impedance resulting in lower drive input harmonic current distortion and system voltage distortion, reduce true rms current, improve total (true) power factor.

5% effective impedance (at full load) yields about 35% THD-I, 3% impedance at full load results in about 45% THD-i. Note: a drive system with a 5% impedance line reactor, operating at 60% of reactor current rating will look like a 3% impedance reactor. 

What is the best way to mitigate harmonics? PQM-106

There are number of types of equipment that will reduce harmonic distortion (tuned harmonic filters, line reactors, active harmonic filters, multi-pulse rectifiers, 5% THD harmonic filters, phase shifting transformers, etc).  Each has its merits in the proper circumstances. Each has its advantages as well as disadvantages.  Each has a typical performance and cost.  There is no single method that is best for all cases.  

This best solution will depend on the unique aspects of the power system, facility operations, type of loads and the target distortion levels.  In a facility with many loads, often the best technical and economical solution involves a combination of methods. An examination of the single line diagram supplemented by simulation of the power electronic loads can result in a determination of the best economical and technical solution to a harmonics concern.

Allied Industrial Marketing Power Quality Services

What harmonic current distortion should I expect from a VFD? PQM-105

An 18-pulse drive to typically will achieve about 4% to 6% THD-i when operating at full load (assumes balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced.

 

A 12-pulse drive typically achieves 12 – 15% THD-i when operating at full load, (assuming balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced. Filters are available for 12-pulse drives that will reduce current distortion to 5% or less.

A 6-pulse drive without a DC or AC inductor may be as high as 60-100% THD-i.  However a 6-pulse drive with 5% line reactor (or equivalent DC choke) achieves about 35% THD-i. at full load (45% at 60% load) and with 3% line reactor (or equivalent dc choke) achieves about 45% THD-i at full load (about 60% THDi at 60% load).  Filters are available for 6-pulse drives that will reduce current distortion to 5%, 8%, 10%, 12%.  

What is a Detuned Power Factor (PF) Capacitor? PQM-104

Since power factor capacitors have capacitance reactance (ohms) that decrease as frequency increases, they will naturally offer a low impedance path to harmonics. If the power system voltage is distorted, then current will be driven into the capacitor, at each of the frequencies that are present in the harmonic voltage spectrum. Additionally, the capacitance (Farads) will combine with upstream circuit inductance (Henries) to form a parallel tuned circuit and a series tuned circuit, each with their respective natural resonant frequency. If harmonic voltage is present at one of these frequencies, then harmonic resonance can cause amplification of harmonic voltage (parallel resonance) or harmonic current (series resonance).  A detuning reactor is used to prevent harmonic resonance, by purposefully shifting the capacitor circuit resonant frequency beneath the lowest harmonic that is present. It also restricts the flow of harmonic current into the capacitor by increasing the capacitor circuit impedance relative to harmonic frequencies.  Power factor capacitor systems that use detuning reactors are sometimes referred to as anti-resonant capacitors, capacitors with harmonic blocking reactors, or simply detuned capacitors. 

Is it OK to add power factor capacitors to a power system that supplies motor drives? PQM-103

Adjustable speed motor drives (ASDs. VFDs, VSDs) are non-linear loads that produce harmonic current distortion during their normal operation. Harmonics are currents flowing at frequencies which are multiples of the fundamental frequency (ie: 50Hz, 60Hz).  Since capacitive reactance (ohms) decreases as frequency increases, capacitors provide a low impedance path for harmonic currents. This can result in the power factor capacitors absorbing significant harmonic currents and becoming overloaded, leading to premature failure.  Capacitance also combines with system impedance (ie: power transformer) to form a tuned circuit.  If this circuit is tuned near one of the harmonic frequencies present, then a resonance condition occurs and either harmonic current or voltage can be amplified.

When adding power factor capacitors to a power system shared by both across the line started motors and adjustable speed drives, there are two choices.  1) apply harmonic filters to the drives, sufficient to suppress the system harmonic voltage distortion  to negligible levels,  2) use a detuned capacitor system (consists of a detuning reactor in series with capacitor to force the tuning point below the lowest order harmonic).

How does source impedance affect harmonic distortion? PQM_102

If we consider an individual non-linear load such as a 6-pulse rectifier, the input harmonic current distortion is largely a function of effective source impedance. Generally speaking, the higher the total input impedance, the lower the harmonic current distortion.  However, there is a point of diminishing returns. Typical effective impedance values range from about 0.5% (corresponding to about 100% THD-i) to about 5% (corresponding to about 35% impedance). If a system had a total effective impedance of 10%, current distortion would be about 26% THD-I. But, if the impedance was due to AC reactance, then the corresponding ac voltage drop (about 10%) would be too high for this to be a practical solution. While nameplate impedance (%) is based on full rated current, effective impedance is based on actual circuit (fundamental) current.  If a 500KVA, 5% impedance transformer supplies power to a 300KVA motor drive, then although nameplate impedance states 5%, the effective impedance will be about 3% (5% times 300/500) and harmonic current distortion at full load will be about 45% THD-i.  Current distortion, as a percentage of fundamental current, will increase as load is reduced (lower effective impedance), although the rms value of harmonic current will decrease.

.

Where does harmonic voltage distortion come from? PQM_101

Voltage distortion is caused when harmonic currents flow through system impedance.  Simply put,  when a piece of electronic equipment draws harmonic current,  the harmonic currents flow though the power source impedance (X_L)  and voltage results at each harmonic frequency based on Ohm’s Law (amps times ohms). The sources of harmonic voltage distortion are those electrical loads the draw non-sinusoidal current. 

On a 60 Hz power system, 5^th harmonic current is current flowing at 300 (5 x 60) Hz.  Following ohms law, when harmonic current flows though upstream impedance it will cause a voltage drop at the same frequency.  5^th harmonic current will cause 5^th harmonic voltage on the power system that supplies this load.  It is common to all loads connected to the system. 

Utilities generate sinusoidal voltage, but power electronic loads in customer facilities distort both current and voltage. To reduce power system harmonic voltage distortion, one must typically identify the source of harmonic current and minimize harmonic current distortion at this point.