Systems Operating at High DC:AC Ratios
WHITE PAPER
Abstract
Recent trends in the PV industry have been towards an increasing DC:AC ratio,
where the AC conversion capacity attached to a PV system is significantly
smaller than the nameplate DC value. As the cost of the DC components
continues to decrease, and the need to reduce grid disturbances due to
the intermittent behavior of conventional PV systems, this trend will likely
accelerate even further. For this trend to continue significantly beyond the
DC:AC ratios of 1.4 being used today however, several hardware limitations
on conventional systems will have to be resolved. This paper describes a cost
competitive, and highly redundant electrical topology where these hardware
limitations have been eliminated.
In addition, for bi-directionally oriented systems using dual MPPT or microinverter based solutions and high DC:AC ratios, a second major issue results
from preferential clipping due to the combination of fixed AC limits and the
imbalance of DC energy presented by the system. This results in additional
energy losses at high DC:AC ratios. The tenK electrical topology presented
within also eliminates this source of energy loss.
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Introduction
The most important point to observe when considering DC:AC ratios when sizing PV systems, is that two productivity measures exist
in the industry. The DC productivity (kWh AC/kW DC) is the AC energy production normalized to the DC system nameplate sizing. If
there is no AC power limit on the system, this is often the critical metric used. The AC productivity (kWh AC/kW AC) is the AC energy
production normalized to the AC system sizing. For cases where the AC system power is limited (typical), this is often the most critical
metric, depending on the value placed on the kWh AC, the fixed project costs, etc.
Due to the intermittency of PV, the relationship between the DC and AC productivity is very different from conventional energy
sources. Figure 1 is an illustration of the relationship, based on various DC:AC ratios. At the right side of the graph, the DC
productivity is limited by the total amount of sunlight available, so in this case is limited to a capacity factor of about 14%. Because
the system is limited by the total radiation available, a low DC:AC ratio (more AC capacity) cannot increase the net system output,
thus the AC capacity factor falls with DC:AC ratio.
As the DC:AC ratio is then increased, the AC capacity factor then begins to climb, at the expense of the DC capacity factor, since the
under-sizing of the AC conversion capacity on the system is basically limiting the AC output power whenever the DC power available
exceeds the AC output (ignoring DC:AC efficiency in this example). In PV, as the upper limit of the DC output is capped with a higher
DC:AC ratio, the impact on DC productivity is initially very little, since the only capping is near solar noon on a few of the best days.
As the DC:AC ratio continues to increase, the impact on DC productivity is larger, however the AC capacity factor can be driven up
very quickly to over 30%, when moving toward the left of Figure 1. At the extreme left, this is basically a very large PV array (large DC
nameplate) with a very small AC conversion capacity. The sun comes up in the morning and it turns on at its maximum, and shuts off
in the evening, limited only by the total hours where some sunlight is available (extremely stable, but very expensive).
Since the AC productivity is the (or one of the) critical parameters for many systems, selecting a high DC:AC ratio is an advantage.
However when optimizing for the lowest cost of energy, the total system cost is also critical so having an extreme DC:AC ratio is not
economical, as installing a very large DC array and clipping most of the DC energy away has a poor economic value (unless the PV DC
costs drop much further). Thus, optimizing financial returns based on the entire system economics, at various DC:AC ratios will define
the optimum design point.
Effect of DC Array Configuration (i.e., Module Tilt, Positioning)
When considering various DC:AC ratios as design options for optimizing system economics, it is also important to realize that some
particular DC array configurations are preferred over others (orientation and placement of the modules). As noted previously, if one
is to select a high DC:AC ratio, the lost DC energy will be mostly near solar noon (for a south facing array) and during direct beam
illumination (i.e., summer sunny days in the northern hemisphere). To minimize the DC productivity losses with higher DC:AC, one
should select a DC panel configuration that has a flatter daily profile (early to rise, late to fall, low peak) and responds strongly to
diffuse illumination.
To illustrate, Figure 2 is an example of a shading tolerant panel configuration where the array is comprised of 25° facing south
modules and 15° facing north modules. Observe that in the summer, the north facing modules produce 90% of the energy of the
25° tilted south modules but only rise to 70% of the power, due a much higher productivity in the early AM and late PM. Thus
one could use a much higher DC:AC ratio with the north facing modules in the summer, with much less clipping of the DC power.
Likewise, during diffuse periods, the north modules have nearly the same productivity as the south facing modules. Higher tilt, east/
west configurations also have a lower daily energy profile and are becoming much more common in Europe. Higher tilt, south facing
modules generally have a shorter day, higher peaking profile, making this configuration less desirable for high DC:AC applications.
Redundant Array Electrical Topology
Before detailing out the limitations of existing systems with high DC:AC ratios, an introduction to a tenK PV electrical topology is
required. The electrical architecture of the tenK system is illustrated in Figure 3. tenK modules as shown interconnect all cells in
both series and in parallel, such that any cell-level shading or soiling, broken cells or interconnects allow current to flow around a
partially contributing cell (as opposed to it being a constraint). A set of highly redundant DC:DC converters are used on each module
along with a very low cost current return backsheet to form a complete module circuit where current can take many paths through
a module for superior reliability and minimal cell-to-cell current dependencies. Figure 4 is an illustration of an extreme shading test,
showing the effectiveness of the design as it relates to cell-to-cell shading.
Each module is then connected in parallel to a common low-voltage DC bus. Also residing on the DC bus are a set of inversion units
also operating in parallel (using a proprietary DC voltage control method to avoid communication between units). Each inversion
unit may pull energy from the DC bus and deliver it to the grid, depending on the individual settings and DC voltage on the bus.
Low illumination efficiency is greatly improved since energy from any module can flow through any inverter (and only the inverters
needed are operated at low illumination). This electrical topology also yields a very efficient module due to the low parasitic losses
and minimal edge spacing due to the low voltage of each module, and also a module that can tolerate variable illumination between
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cells. For reliability and minimum field service, complete redundancy is achieved in the entire system from the cell to the grid.
PV System: DC vs. AC Capacity Factors
“Hours in the Day” Limit
(On/Off PV System)
40%
30%
20
15
20%
Incident
Radiation Limit
10%
0%
25
10
DC:AC Ratio
AC Capacity Factor
50%
5
0
0%
5%
10%
DC Capacity Factor
AC CapFactor
15%
DC:AC Ratio
Figure 1. The relationship between DC (kWh AC/kW DC) and AC (kWh AC/kW AC) productivity (stated as annual capacity factors)
in Minneapolis, MN. As the DC:AC ratio is increased and some of the DC energy is clipped at the high productivity periods, the
DC productivity falls, however the AC productivity increases. As the DC:AC is increased further, the amount of DC energy lost
increases, causing the rate of increase in AC productivity to slow as the DC:AC ratio is increased further. A DC:AC ratio of 2.5
corresponds to a DC capacity factor of 12% and an AC capacity factor of 30%.
Designs for Tolerating High DC:AC Ratios
For a conventional PV system, the design requirements are that any component included on the DC side of the array, must be able to
withstand 156% of the short-circuit current of the array. Consider as an example, where a DC:AC ratio of 2 is to be considered for a
1 MW AC array (i.e., a 2 MW DC system). This requirement forces the entire array to be able to support 156% times the short-circuit
current of a 2 MW DC array, including all DC wiring capacity as well as the 1 MW AC inversion unit must have a greatly oversized
set of DC circuits to withstand an abnormal short-circuit event in the inverter. All of this oversizing of the DC side vs. the allowed
maximum AC power limit adds costs to the system.
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Figure 2. Examples of various DC energy production profiles to illustrate how to optimize systems for high DC:AC ratios. The upper
graph is a summer trace for PV modules tilted at 25° south and another group at 15° north. Observe that the 15° north modules
can produce 90% of the energy of the 25° south facing modules, but with only 70% of the peak DC power. Thus, if the system is
capped at 70% of the peak power, the south facing modules would effectively produce less energy than the north facing modules.
The second graph illustrates a diffuse day, where the 15° north facing modules produce the same amount of energy as the 25°
south facing modules, since the sky is nearly uniformly illuminated under these conditions.
RAIS PV Architecture
VOC = 0 ; JSC = 0
DC Bus
Optional
Storage
Redundant
Inversion
Grid
Figure 3. The RAIS electrical architecture, illustrating the modules with cells connected both in serial and parallel, the highly
redundant electronic DC:DC converters integrated into each module “circuit”, the low-voltage parallel DC bus, and the redundant
inversion units operating on the same DC bus. No communications is required between any of the units on the DC bus, for greatly
improved reliability. An optional storage system is also shown which can also operate directly on the DC bus.
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Figure 4. A shading test illustrating an opaque block across the module eliminating 22% of the optical energy, and 25% of
the module power (this extreme block results in a 3% loss due to the electrical topology of the module, vs. nearly 100% on a
conventional module under similar conditions). The thermal images illustrate the temperature uniformity (no hot spot or other
deleterious effects of partial shading as exists with other solar electrical topologies).
For the tenK electrical topology of Figure 3, the electronics within each module has been certified to withstand the entire DC shortcircuit of the PV system within each module (proprietary, highly redundant cell and electronic architecture within module), and also
is certified to export a limited amount of current, which can be at or below the maximum PV power without consequence, as the
module simply moves off the peak power point within the module if the peak output current limit is reached. This “extra” PV current
during short-circuit conditions can never exist in the DC wiring or DC:AC conversion process.
The following describes how the system of Figure 3 operates:
1)
At times when the system is not limited in power output, the modules all produce the maximum current
possible at whatever system voltage is presented to the modules (30-55V DC).
2)
At times when the system is not limited in power output, the redundant inverters pull current to maintain
the system voltage between 50-52V DC. In this case, the inversion units set the system voltage to a
targeted value, not the modules. Also note the inversion units do not need to manage MPPT, since the
modules deliver maximum power at any voltage. If one attempted to run MPPT with multiple inverters on
a parallel bus, each inverter would not be able to distinguish changes in array power levels from changes
in power level of other inverters – destabilizing the entire system each time an inverter changed its peak
power point.
3)
A narrow but finite range of voltage setpoints are used in each of the inverters, for those inverters with
setpoints low in the allowable range, they will operate at full power since they cannot maintain the system
voltage setpoint. For inverters with higher setpoints, they will pull smaller amounts of current (if any), and
will actively adjust the amount of current pulled to maintain a constant system voltage (50-52V DC).
4)
The inverter setpoints change daily, to wear-level all inverters.
5)
When the last inverter reaches a point of saturation, the system voltage will then begin to rise, at which
point one or more of the modules will then reduce its current output to maintain the system voltage
of 55V DC. At this point in time (when all inverters are saturated), the modules then control the system
voltage, not the inverters).
Figure 5 is an illustration of the daily output of a commercial 13.2 kW AC sub-array (24 redundant inverters in this sub- array, and
part of a larger 99.0 kW AC array) when operating in saturation over a portion of the day. Observe the rise and fall show no transition
periods when inverters are coming on or off, or the transition from when the inverters are controlling the DC voltage setpoint vs. the
modules taking control when the inverters saturate.
Observe that in this topology, the inversion units are never exposed to the full PV short-circuit current, only the current limited
output of each module. When using very high DC:AC ratios (i.e., when all inverters are saturated), the modules limit the system
current, thus allowing the wire and inversion system sizing to be designed to the AC power output limit (no cost penalty), rather than
the wiring and inversion DC circuits having to match the installed DC capacity (see wire sizing comparison between tenK systems and
600/1000V DC conventional systems).
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Figure 5. An example of a 24 redundant inverters (13.2 kW AC) operating with a 14 kW DC array, illustrating the rise and fall in
the production through the day, and also the period of inverter saturation. At the left and right, the system voltage is controlled
by the inverter group, allowing more inverters to engage and operate as the power climbs and falls (note there are no transitions
visible – it is not detectible when units come on or off). At the saturation point, all 24 inverters are running all-out, and the
system voltage is held at 55V DC by the constant voltage operation of some or all of the modules. Observe also how smooth the
cap line is as well.
Clipping in Bi-Directional Arrays (e.g., East-West Arrays)
Another severe issue in bi-directional arrays which are AC power limited and operating at relatively high DC:AC ratios, is related to
how the AC power is summed and combined in conventional DC systems, or even in micro-inverter based systems. For bi-directional
arrays where one group of panels is tilted in one direction and another group is tilted in another direction (an example would be an
east-west system where a group of panels is tilted east and another group is tilted west), due to the mis-alignment each of these two
groups must be tracked on a separate maximum peak power tracking loop (MPPT), using either two central or string inverters, or
inverters with two DC inputs and separate MPPT loops. Another option is to use all micro-inverters, since each inverter then tracks
MPPT separately.
Consider the case of a 1 MW AC limited system operating at a DC:AC ratio of 2.0 (i.e., 2 MW DC nameplate). First consider the case of
two separate inverters, 500 kW AC each and an east-west system. By mid-morning – the sun alignment is high on east facing panels
and the power may be 800 kW DC, but still poorly aligned on the west facing panels (say 300 kW DC). Since there is 1100 kW DC of
total power available, the system should be delivering at the peak of 1 MW AC. However, since each inverter (or MPPT tracker clips
individually) clips at 500 kW AC (ignoring DC:AC efficiency for this simple example), and the other is operating at 300 kW AC, for a
total of 800 kW AC – well below the maximum energy available. This same issue exists on dual-MPPT tracking systems since the DC
sizing is limited to about ½ of the total AC output, in order to minimize the cost of the DC side of the unit. Micro-inverters AC undersized to the same limits suffer the same issue, where the east facing inverters are saturated and the west facing inverters are not –
thus the total AC energy falls short.
Consider now the tenK topology of Figure 3, where the DC from east and west facing modules are combined on the DC bus before
the current is delivered for inversion. Since the DC sources are combined before the AC limit is imposed, current from any module
can flow through any inverter to fulfill the total maximum AC level. Thus, in the example above, an equal number of east facing and
west facing modules have their DC combined such that the sum total of the 800 kW DC is combined with the 300 kW DC, the entire 1
MW AC can be delivered (only limited by the sum of the AC inversion capacity).
Conclusion
By limiting the DC current output at the source of the PV, tenK electrical topology allows the use of much larger DC:AC ratios than
are possible with conventional technologies without severe cost penalties for oversizing the DC systems. In addition, in bi-directional
arrays such as east-west systems, the tenK electrical topology eliminates the early saturation that takes place when one DC current
input is largely imbalanced from the other.
About tenKsolar
tenKsolar, Inc., provides a photovoltaic solar solution that delivers on the promise of the lowest cost of solar electricity, while at the
same time improving power density, safety, longevity and bankability of photovoltaic systems. Since its founding in 2008, tenKsolar
has been a leading innovator in the delivery and implementation of photovoltaic solar systems for commercial customers. More
information about tenKsolar is available online at www.tenKsolar.com
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TKS WP 50005.02