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-----Original Message-----
From: Rebecca Van [mailto:rvan@tnag.net]
Sent: Sunday, October 25, 2009 1:46 PM
To: Monty
Subject: How PV grid-tie inverters can zap utility power factor -
Photovoltaics World
How PV grid-tie inverters can zap utility power factor by Gerritt Lee,
contributorThe rush to harness energy from the sun to make electricity has
inevitably fueled the development of large industrial-grade grid-tie
inverters (GTI) that convert DC from photovoltaic (PV) panels into AC power
for commercial use. Compared to their residential forerunners that generated
only a few kilowatts (kW) of power, the mammoth systems of today are
designed to put out upwards of 100kW. While these super-sized powerhouses
can dramatically lessen reliance on utility power, there is something else
they do that may surprise you.Customers with PV systems that have displaced
a good portion of utility power and whose loads have a sizable reactive
component can actually see utility power factor deteriorate after their
system is connected to the grid. Low power factor presents a heavier
generation and transmission burden on the power grid and also deposits a
larger carbon footprint. Because of this, most tariffs have provisions
allowing the utility to charge a penalty for low power factor. But it is
possible to address this problem.Background on commercial powerWe begin with
a basic discussion of the different kinds of commercial power. One is called
real power because it is directly related with doing real and useful work
such as heating and lighting. Resistive loads are consumers of real power,
whose common unit of measure is the kilowatt (kW). In a resistive load, both
supply voltage and load current are "in-phase" such that the peaks and
troughs of their sinusoidal waveforms coincide and constructively work
together.Another kind of power is reactive power that is present in loads
with inductive or capacitive elements in them such as motors and lamp
ballasts. Unlike real power, reactive power behaves differently by storing
energy in the electric and magnetic fields of the reactive load and
releasing it back to the grid on each AC cycle, made possible because supply
voltage and load current are 90-degrees out-of-phase with each other. The
unit for reactive power is the kilo-volt-ampere reactive (kVar).The
relationship between real and reactive power is represented by the right
triangle shown in Figure 1 where real power falls on the horizontal axis and
reactive power rises on the vertical. The vector sum of real and reactive
power is called apparent power, whose magnitude is the length of the
hypotenuse and whose common unit of measure is the kilo-volt-ampere (kVA).
Seen another way, apparent power is the total burden placed on the grid by
the load from both its real and reactive parts. Figure 1. Relationship
between real, reactive and apparent power.In Fig. 1, the cosine of the angle
f (a fraction between 0 and 1) represents power factor at the fundamental
power line frequency -- commonly known as displacement power factor
associated with linear loads where a phase shift exists between supply
voltage and load current. A lagging power factor is associated with an
inductive load, while a leading one is indicative of a capacitive load.Power
factor can also be affected when harmonic components of the fundamental
frequency are present in magnitude to distort the waveform of the load
current. True power factor is then a combination of simple phase
displacement and harmonic distortion. Because of the technical complexities
associated with harmonic distortion, the remaining discussion will be
focused solely on displacement power factor.The power factor represents the
degree to which the load is resistive with the ideal occurring when f is
zero degrees and power factor equals one, or unity. At unity power factor,
apparent power equals real power, reactive power is zero and the load is
purely resistive in nature. If the load also contains reactive elements (as
most do), reactive power will be non-zero and apparent power will exceed
real power as Fig. 1 shows. Therefore, given two loads each consuming the
same amount of real power, the one with the higher power factor will be more
efficient and draw less circulating current than the other with the lower
power factor.The inefficiencies associated with low power factor require
larger power plants and bigger transmission lines to generate and deliver
the higher currents. For this reason, the utility must set minimum power
factor standards in accordance with applicable tariffs to mitigate the
problem. Non-conforming users with sub-par power factor may be penalized
with higher rates. Figure 2. Utility power factor without grid-tie inverter
(GTI).Putting it all togetherHaving the basics covered, let's focus our
attention on Figure 2, which shows an example of a commercial load connected
to the grid through the electric meter. The meter shows that the load draws
1000kW of real and 450kVars of reactive power, which results in a
respectable power factor of 0.91 as indicated by the triangle in Fig. 2.In
Figure 3, we have the same configuration except this time a large GTI unit
displaces 50% of utility real power. In other words, the grid and GTI each
feed 500kW to the load. The meter now registers 500kW of real power and the
full 450kVars of reactive power as well. Crunching the numbers again, you
find the power factor has dipped to 0.74. What caused this plunge of 19%,
you may ask?Remember that power factor can be decreased in a couple ways.
One is by holding real power (kW) constant while increasing reactive power
(kVars). The other way is to keep reactive power constant and trim real
power, which is precisely what is happening here. The missing piece of the
puzzle is the fact that GTIs are designed to operate with a unity power
factor output and be synchronized with grid frequency. Recall that a system
with unity power factor exhibits 100% real and 0% reactive power. So the GTI
does not contribute any reactive kVar production to the equation, which
leaves the utility to bear the entire burden. The reason for the mysterious
drop in power factor now becomes clear. Figure 3. Utility power factor with
grid-tie inverter (GTI).What can be done to ensure that power factor falls
within acceptable utility limits? In reality, PV power output is not
constant but fluctuates throughout the day depending on the sun's position
and cloud cover. Because of the inversely proportional relationship between
PV output and utility power factor just explained, the latter will also
fluctuate throughout the day. However, if average power factor falls below
utility standards, the initial power factor must be boosted to compensate
for the loss. Since most loads are inductive in nature, capacitor banks can
be employed to cancel some of the negative effects of inductive reactance,
since opposing reactances offset.In the example of Fig. 3, suppose the
utility requires a minimum power factor of 0.85. What must occur to boost
power factor from the sub-par 0.74 to the minimum 0.85? For this to happen,
the meter must see 500kW of real power and 310kVars reactive. Doing the
math, we find that the initial power factor must be raised from 0.91 to
0.96. The reader is left to ponder the details of this conclusion,
summarized in the table below.ConclusionFollowing on the heels of the
residential bandwagon, large GTI systems are becoming more prevalent today
as large businesses embrace sustainability and take greater ownership of
their energy costs. The availability of export energy credits from
feed-in-tariffs provides an income stream to help defray the large capital
expenditure. Following all the hard work and money invested to make your PV
system just right, getting cited for sub-par power factor would cast a gray
cloud over an otherwise sunny day. But armed with the foregoing knowledge,
you are now prepared for the solution. Comparison of utility power factor
with and without grid-tie inverter. *Fig.2 **Fig.3BiographyGerritt Lee
received his BSEE degree from the U. of Hawaii and is a registered
professional engineer employed by Hawaiian Electric Company; e-mail
gerritt.lee@heco.com. Photovoltaics World Article Categories:
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