PV Grid Integration: Backgrounds, Requirements, and SMA Solutions
PV Grid Integration: Backgrounds, Requirements, and SMA Solutions
PV Grid Integration: Backgrounds, Requirements, and SMA Solutions
PV Grid Integration
CONTENT
Introduction
1.
1.1
1.2
6
6
7
2.
2.1
2.2
2.3
2.4
2.5
2.6
2.6.1
2.6.2
2.6.3
2.6.4
Feed-in management
Active power reduction in case of overfrequency
Provision of reactive power
Dynamic grid support
Inverter and plant certification
SMA product solutions
Centralized plant concepts
Decentralized plant concepts
Remote power limitation
Remotely controlled reactive power setpoint and reactive power control
3.
3.1
3.1.1
3.1.2
3.1.3
3.2
3.2.1
3.2.2
3.2.3
3.3
Basic requirements
Active power reduction in case of overfrequency
Connection criteria and permissible unbalanced load
Grid and plant protection
Supplementary requirements
Provision of reactive power
Three-phase feed-in
Remote power limitation
SMA product solutions
8
8
8
9
10
11
11
12
12
13
13
14
14
15
16
17
18
18
20
20
21
4.
4.1
4.2
4.3
FNN recommendation and EEG transition period for simplified feed-in management
Design according to the 70 percent option
Retrofitting older PV plants
22
23
24
24
5.
25
6.
7.1
7.2
7.3
7.4
7.5
7.6
26
26
29
31
33
33
34
Note: Current information on this topic can be found on the Internet at www.SMA-Solar.com/gridintegration
Introduction
The subject of grid integration coupled with renewable power generation is playing an increasingly
important role. The powerful growth in Germanys
photovoltaic capacity is attracting considerable
attention and rightfully so: According to data by
the Federal Network Agency, a total of nearly 25
gigawatts of PV power has already been installed
in the grid since the end of 2011. The PV plants
consequently supply more than 16 large nuclear
power plants under ideal irradiation conditions. The
optimum integration of this decentralized and variable power generation capacity into the existing
distribution grid (designed for unidirectional flows
of power) is as crucial as it is pressing for that very
reason.
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Fig. 1: Solar power can already entirely cover the noon-time peak in the German power distribution grid on sunny days
PV grid integration
1 Renewable
4 European
PV grid integration
Feed-in management
01/01/2009
Remote setting
of various
reactive power values
04/01/2011
Dynamic grid support
Certication
Fig. 3: Chronological sequence of the requirements for the BDEW medium voltage directive
PV grid integration
Fig. 4: Among others, the reactive power may be regulated as a function of the supplied active power
10
PV grid integration
11
The inverters of the Sunny Central CP production series already fulfilled all requirements of the
medium voltage directive including full dynamic grid
support upon their market introduction.
12
PV grid integration
2.6.4 Remotely controlled reactive power setpoint and reactive power control
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Fig. 6: Remote power limitation with the SMA Power Reducer Box
13
14
PV grid integration
Fig. 7: The frequency/active power characteristic curve in accordance with the VDE code of practice: The power is limited from 50.2 Hz.
15
Fig. 8: The VDE code of practice allows a maximum of 4.6 kVA of apparent power from single-phase inverters per phase
16
PV grid integration
< 184 V
> 253 V
> 264.5 V
< 47.5 Hz
> 51.5 Hz
Reconnection limits:
Voltage greater than 195.5 V and less than 253 V
Frequency greater than 47.5 Hz and less than
50.05 Hz
Fig. 9: Another option includes the communication-based coupling of three single-phase Sunny Mini Central inverters
17
Far more PV plants can utilize the existing infrastructure of the low-voltage grid by means of inverters
with reactive power capability; as a result, the supply of reactive power is also now required on this
voltage level. Background: The feed-in of active
power into the low-voltage grid with its predominantly ohmic properties generally results in an increase
of the voltage at the feed-in point. In the case of long
network feeder, an additional aspect is that the voltage must already be set higher on the transformer
side in order to ensure that the lower voltage threshold of 207 V is still maintained at the consumer.
If active power is to be fed in on the side of the
consumer now without absorbing power of a similar magnitude at the same time, the upper voltage
limit may be exceeded at the feed-in point (fig. 10).
However, inverters may lower the voltage at the
grid connection point by simultaneously consuming
lagging reactive power. Consequently, the code
Fig. 10: The required voltage setting may cause the maximum voltage to be exceeded at the feed-in point. The solution: lagging reactive power
18
PV grid integration
Fig. 11: Power generation units must absorb lagging reactive power above 50 percent of their nominal power in order to reduce the voltage
19
Fig. 12: Single-phase inverters can only be designed with a maximum of 4.6 kVA of total apparent power per phase in accordance with AR 4105
20
PV grid integration
Reactive powercapable
Smax [kVA]
Fulllment of
the basic
requirements
1.3/ 1.6
2.1
2/ 2.5/ 3
2.5/ 3
2)
3.6/ 3.8
2)
3/ 3.68 / 4/ 4.61)
2)
4.61)
With communicative
coupling
5.5/ 6
With communicative
coupling
With communicative
coupling
9/ 10/ 11
With communicative
coupling
STP 15000/20000TLEE-103)
15/ 20
STP 15000/20000TLHE-103)
15/ 20
Product identication
SB 1200/1700/2500/3000
SB 1300/1600TL-10
SB 2100TL
SB 2000/2500/3000HF-30
SB 2500/3000TLST-213)
SB 3300-11/3800-11
SB 3000/3600/4000/5000TL-213)
SMC 4600A-11
SMC 5000/6000A-11
SMC 7000HV-11
SMC 9000/10000/11000TLRP-10
STP 8/10/12/15/17000TL-103)
Feed-in management:
1)
Limitation to 4.6 kVA when selecting the country data record for Germany only
Compatible with the Power Control Module
21
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Fig. 13: Requirements of the EEG 2012 on the participation of new and existing PV plants in feed-in management
22
PV grid integration
Furthermore, the Bundestag agreed on a transitional period for PV plants 100 kWp in terms of
participation in the feed-in management as part of
the recent EEG amendment on March 29, 2012.
All plants commissioned in 2012 are required to
be capable of remotely limiting the active power
starting on January 1, 2013. The version with the
limitation to 70 percent for plants up to 30 kWp will
only be relevant as of this date.
Compared to the possible yield losses incurred by
capping feed-in capacity at 70 percent of generator nominal power, the installation of an additional
contactor (and ripple control receiver, if required)
is usually the cheaper option. Furthermore, SMA is
working on an inverter-integrated solution for active
power limitation.
17QMBOU
3FMBJT
Fig. 14: Remote controlled disconnection: The FNN recommendation for simplified feed-in management for PV plants 100 kWp
23
Based on todays perspective, the required retrofitting of older PV plants is not a problem. The
appropriate technology is available for plant and
distribution grid operators for plants with more than
100 kWp, meaning that retrofitting work could be
completed by mid-2012 without any difficulties. For
this reason, SMA has offered the Power Reducer
Box since the beginning of 2009 as a solution for
remote power limitation.
Note: The potential yield losses increase disproportionately as the size of the inverter becomes smaller.
If intermediate sizes are yielded by the design when
applying the 70 percent option, it is often not costeffective to choose the next smaller device version.
Normally, you should choose the next larger inverter
and limit it based on the corresponding parameterization to the precisely required power value.
24
PV plants between 30 and 100 kWp can be retrofitted in accordance with the FNN recommendation.
However, we expect to encounter more technically
demanding solutions for the feed-in management
before the retrofitting deadline at the end of 2013.
PV grid integration
25
26
PV grid integration
Fig. 15: Fluctuating, but always positive power pure active power
results when the current i and the voltage v are in phase
Fig. 16: The average value of the power is zero pure reactive power in
case of a phase shift of 90 degrees between i and v
27
It results from geometric subtraction and consequently does not amount to about five, but rather
approximately 31 percent of the apparent power
in this case (see example calculation). Viewed from
the opposite side, the apparent power is about
5.26 percent greater than the given active power at
a cos() of 0.95 (apparent power = active power
divided by displacement power factor, ref. example
calculation).
Identification
Symbol
Unit
Apparent power
[VA]
Active power
[W]
Reactive power
[var]
cos()leading / lagging
28
PV grid integration
EXAMPLE CALCULATION
40 Sunny Tripower 15000TL inverters feed into the grid with a displacement power factor of 1 and a
total active power of 600 kW. As an alternative, grid feed-in should take place with a displacement
power factor of 0.95. Which apparent, active, and reactive power results? Are the available inverters
sufficient?
The available active power P is 600 kW.
The following applies
to the apparent power S: S =
P
cos()
i.e.,
S=
600
= 631.57 kVA
0.95
Q = 6312 6002
i.e.,
Q = 197.2 kvar
Result
Due to the phase shift with a displacement power factor of 0.95, the inverters must provide additional
197.2 kvar of reactive power in addition to the 600 kW of active power. The geometric total indicates an apparent power of 631.6 kVA. The inverters and the downstream grid infrastructure must be
designed for this apparent power. 631.6 kVA of inverter power are consequently required to operate
at the same PV generator e.g., 42 Sunny Tripower 15000TL inverters (or 37 STP 17000TL inverters
as an alternative if this is more favorable in terms of the module configuration).
Information
When planning with a reactive power supply, the power of the PV generator must be evenly distributed
to the now greater number of inverters. The power of inverters with reactive power capability is generally to be regarded as apparent power and must always be indicated in VA in that context. Active and
apparent power have the same value in case of a displacement power factor cos() = 1 only, resulting
in the power of devices without reactive power capability being indicated in watt as has been customary up to now.
29
30
PV grid integration
31
MORE PV POWER INTO THE GRID VIA INVERTERS WITH REACTIVE POWER CAPABILITY
The voltage-lowering effect of reactive power depends on the design of the respective grid level (overhead line or underground cable, cable design): For example, the German high and extra-high-voltage
grid is almost exclusively characterized by reactive impedance due to the large proportion of overhead
lines and the big line distances, while the (ohmic) active resistance has a more significant share at the
medium and low voltage level.
Therefore, the supply of reactive power has significantly less effects on the voltage at the lower grid
levels than in the high-voltage grid. Instead, the supply of active power also causes a noticeable increase
of the voltage here. This is exactly why the provision of reactive power should also become mandatory
on the low voltage grid starting from a plant power of 3.68 kVA: While the voltage-regulating effect is
comparably small, compensation of the voltage increase caused by active power is still essential.
Example 1: Given a grid impedance angle of 30 degrees (typical low-voltage grid), the voltage increase from feeding-in
27 kW of PV power with a displacement power factor of 0.95lagging may be reduced by almost 20 percent (from 0.94 percent to 0.76 percent)
Example 2: Twice as many PV inverters may be operated when feeding in with a displacement power factor of 0.9lagging. This results in a maximum
active power of 163 kW, instead of 90 kW without reactive power an increase of approximately 80 percent
32
PV grid integration
The cost greatly varies from case to case: For example, a radio ripple control receiver will be used, even
in large PV plants, only if the grid operator wants to
remotely control the reactive power supply on short
notice. An SMA Power Reducer Box is also needed
in that case. If stabilization of the desired reactive
power is also required at the grid-connection point,
a PLC-based control solution, such as the SMA
Power Plant Controller, is needed. Inverters with
reactive power capability do not incur any additional costs because this functionality is available
in virtually all SMA devices as standard due to the
relevant connection directives.
Besides that, the changed dimensioning of the inverters has an effect on the total costs of the plant: Either
more or more powerful inverters are necessary in
order to facilitate feeding in the entire active power
of the PV generator with phase shift. However, this
makes up less than one percent of the plant costs for
a required displacement power factor of 0.95.
33
Fig. 17: The active power of the PV generator is maintained in full; however, the inverter must be dimensioned to the larger
apparent power
34
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