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A L P H A T E C H N O L O G I E S
Broadband Powering
Methods: A natural
engineering choice 3767 Alpha Way, Bellingham, WA 98226 Tel: 360-647-2360 Fax: 360-671-4936
A L P H A T E C H N O L O G I E S
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Broadband Powering Methods:
A natural engineering choice
John Sams of Alpha Technologies examines the reason for a move up from 60V
line power supplies in the future.
Although current European safety regulations preclude the use of 90V line power supplies, this article
examines the reasons for considering a move up from 60V in terms of efficiency, and describes possible
design considerations.
The process of competitive selection in broadband powering techniques has produced a keen emphasis
on power supply cost, efficiency and reliability. Product differentiation is strategically important in the
power conditioning industry, and relies on the manipulation of key performance parameters that directly
affect either the equipment capital or ownership costs. Capital costs extend directly from the quantity
and type of components used. Reducing power processing component counts reduces cost, and typically
increases reliability.
Repair
Ownership costs include repair an replacement of components and the cost of the electricity used by the
power conditioner and its loads. These costs are inversely related to reliability and the units efficiency in
powering the desired loads. Thus, a technique that optimally reduces component counts and increases
efficiency should be the natural power technology of choice.
This article looks at efficiency and then compares several technologies for specific capital cost, reliability
and efficiency performances.
A power conversion devices influence on the cost of electricity to power the desired loads is related to
the following six key factors:
1. INPUT POWER QUALITY
The quality of the power drawn by the power conditioner is quantified by the measure of its input power
factor. This is commonly defined as the ratio of the real power consumed to the apparent power (VA)
measured at the input terminals of the device. The customer (the system operator in this case) only pays
the utility for the real power consumed at the watt-meter, even though the host utility has to generate the
transmission losses associated with the apparent power (VA). Thus, it is in the interest of the utility that
these two values are as equal as possible: a power factor of one.
Restrictions
To this end, the US domestic electric power industry and regulatory bodies are planning restrictions or
penalties on loads that present a low power factor or high harmonic current content. Such measures have
already been legislated in the European Union and are commonly written into service agreements by
some domestic utilities. Therefore, in the long term, a power conditioner should present a power factor to
the utility greater than 0.90 and an input current total harmonic distortion (THD) of less that 10 per
cent, for the best terms in negotiating an electric service rate schedule.
The start-up inrush current required by a conditioner design can also become a problem for the system
operator. Conditioner designs based upon a single stage transformer can draw up to 10 times their rated 3767 Alpha Way, Bellingham, WA 98226 Tel: 360-647-2360 Fax: 360-671-4936
A L P H A T E C H N O L O G I E S
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input current due to saturation in the transformer core while operation stabilizes. Capacitive input condi-
tioner designs can draw similar or greater amounts of current during start-up due to the charging of large
input rectifier capacitors. To the operator this can translate to nuisance circuit breaker trips and related
truck roles.
2. CONVERSION EFFICIENCY
The raw power conversion efficiency of a power conditioner is simply defined as the output power deliv-
ered to the conditioners output terminals divided by the input power delivered to the input terminals. For
cable television and broadband systems, the power conversion process is defined as the conditioning of a
utility sine wave of standard voltage to a wave form and voltage best suited for the coax plant, commonly
a 60 or 90 Volt trapezoidal wave form. Two principal methods exist for performing this process and are
known as single conversion and double conversion.
2.1 Single conversion design
Single conversion designs connect the plant load to the utility through a single stage of power condition-
ing, typically a ferroresonant transformer as illustrated below in Figure 1.
In the event standby power is required during
an outage, an artificial utility is created through
a battery string and coupled to the transformer
via a switching inverter. The inverter commu-
tates the battery across the transformer at the
utility line frequency. Thus the inverter is only
used during utility outages. The maximum effi-
ciency of such a unit typically 90 per cent at
full load.
Conversion efficiency over the load range of 25
to 85 per cent of rated full load is typically 75
to 89 per cent. It is recommended that the power supply installation be operated at approximately 85 per
cent of full load to approach maximum efficiency and provide reserve capacity for plant start-up and peak
load fluctuations.
An alternative single conversion design, based upon controlled ferroresonance (CFR), improves efficiency
vs loading performance, output voltage regulation and transient load response, input frequency tolerance
and input power factor. CFR is created by adding a voltage controlled inductance across the ferros ;tank
circuit capacitor. Implementing output voltage control in this manner eliminates the need to operate the
transformer in saturation to achieve regulation, increasing efficiency particularly under higher load condi-
tions. The maximum efficiency of such a unit is typically 94 per cent at full load.
Superior characteristics
Conversion efficiency over the load range of 25 to 85 per cent of rated full load is typically 82 to 93 per
cent. This is performed without sacrificing the superior characteristics of ferroresonant technology. Thus,
a minor increase in complexity (cost), provides a wider dynamic range for cost efficient operation. The
trapezoidal output wave form of ferro-based designs typically produces a plant power factor of 0.85 to
0.92 The actual value is related to power supply loading and the coax loop resistance of the powered
plant segment. As loading or the loop resistance increases, power factor will also increase. 3767 Alpha Way, Bellingham, WA 98226 Tel: 360-647-2360 Fax: 360-671-4936
A L P H A T E C H N O L O G I E S
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2.2 Double conversion design
Double conversion requires the addition of a second
power processing stage to isolate completely the
load from the utility line. This is implemented by
placing a battery string between an input rectifier
and an output inverter, as illustrated in Figure 2.
The power conditioner output is now independent of
the utility line condition.
However, an efficiency penalty is incurred as each
stage typically operates at 91 per cent efficiency at
100 per cent of rated full load, the combined effi-
ciency is then 83 per cent. Conversion efficiency
over the load range of 25 to 85 per cent of rated full
load is typically 69 to 82 per cent.
Additionally, the double conversion implementation requires twice as many electronic components as
compared to the single conversion approach. This is due to the addition of the rectifier. Another contrast
is that the principal output stage (electronic inverter) must be connected directly to the outdoor plant
and must operate continuously, increasing fatigue and vulnerability to surge and lightening.
Tailored
Double conversion power conditioners also produce a trapezoidal wave form similar to the single conver-
sion designs. However, the crest factor of the wave form can be more closely tailored to and ideal wave
form, particularly at lower load levels. Thus power factors of 0.92 to 0.94 can be achieved.
3. PLANT SIDE POWER FACTOR COSTS
Power factor (PF) is a dimensionless unit that is a
measure of the ratio between the real and apparent
power consumed by an electric load. In a classic
sense, the power actually used by a device is
defined as real or resistive, the result is heat, RF,
light or whatever is desired as output from an elec-
trical device. Commonly, a device will also present
a reactive (inductive or capacitive) load along with
its resistive load.
Thus, the vector magnitude of a complex, quantity (P resistive + i P reactive) is defined as the appar-
ent power flow. Note that a vector magnitude is defined as the square root of the quantity of the resis-
tive component squared plus the reactive component squared (%(P resistive 2 + I p reactive 2)). The rela-
tionship is illustrated in Figure 3.
Using the following variable conventions and identities:
Note: Vector quantities printed in bold upper case.
Figure 2: Double conversion
Figure 3: Resistive and reactive loads
S = Apparent Power = Iac * Vac
(1)
Q = Reactive Power = Vac * I
reactive
(2)
P = Real Power = Vac * I
real
(3)