Distributed asset 1 which requires large capital investment

Distributed
Volt-Watt Control of Low Voltage Power Networks in the Presence of High PV
Penetration

Distributed Energy Resources (DER)
like solar Photovoltaics (PV) continue to
make inroads into low and medium voltage networks because of increasing green
energy demand and favorable government policies.  In the US, higher PV penetration is planned as
utility asset 1 which requires large capital investment and continuing
operating expense. Advanced smart inverter capabilities e.g. volt-var control, volt-watt
control, and fixed power factor (PF) control by the utility company have all facilitated
high penetration from DERs into the distribution feeders. However, high power
injection from these DERs can present a challenge for the Distribution Management System (DMS) causing
power quality and reliability issues. For instance, PV sources connected far
from the substation experience higher voltage problems which can cause serious
system damage 2-3. Therefore, voltage control is one of the fundamental
issues that need to be resolved to get
maximum benefit from high PV penetration. Advanced control functions such as volt-watt
and volt-var controls have been stated in California Rule 21 to mitigate the
problem 4. However, there is still no standard selection criteria available for
choosing the control parameters to curtail/increase active and reactive PV power
injection while maintaining fairness to all PV owners in terms of revenue
earned.

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ANSI C84.1-2011 suggests that
voltages must be maintained in the range of 0.95 ~ 1.05 pu throughout the network. During high PV penetration and low
loading periods, reverse power flow can cause overvoltage problems in the low
voltage (LV) feeders. To address the overvoltage issues, LV transformer tap
settings, voltage regulator operations 5, reactive power absorption by PV
inverter 6-7, use of battery energy storage 8, and active power curtailment
2, 5 have been proposed in the literature. This paper proposes a unique
active power curtailment scheme using volt-watt control while maintaining fairness
to all roof-top PV owners or prosumers. A
distributed voltage control scheme is proposed which does not require a costly
central communication infrastructure. Instead, it uses the concept of
micromanagement of each smart PV inverter with minimal information exchange
from the nearby inverter units only.

A simple radial feeder connected
with a DG is shown in Fig. 1, where

 is the line impedance,

 and

 are
the PCC and substation voltages, respectively and ? is the phase angle between

BHC1 Fig. 1: Single line diagram of a simple radial distribution
system.

the two buses. If

 and

 are the net active and
reactive powers injected at the PCC, then power flow from the PCC can be
calculated using the following equations –

 

(1)

 

(2)

In low voltage distribution
networks, R/X ratio is generally high, so that the reactance of the network may
be neglected

. Assuming the phase
angle difference

 to be
small, the change in voltage due to power injection at certain times can be
formulated as eqn. (3) 9.

 

(3)

Any
substantial amount of power injected by the DG can result in voltage rise/drop
throughout the network, especially in a weak distribution feeder where line
impedances are higher. This voltage variation also depends on some other
factors like DG size and location, load profile, capacitor bank size and
location, and additional voltage regulation methods applied in the network.

The
PV inverter is normally operated at unity power factor until the PCC voltage
reaches the maximum permissible value,

 without violating any network constraints. For
the rooftop PV inverter units operating at unity power factor, (P(t) = P1(t),
Q(t)= 0) eqn. (3) can be reduced to eqn. (4).

 

(4)

Any
increment of injected active power from this point must be compensated for by
reactive power absorption (-Q injection). If the bus voltage remains unaffected
(

,
then from eqn. (5)

 

(5)

the reactive power requirement at the corresponding bus can
be calculated to maintain the voltage at

 for any further increment in the injected
active power (

). The reactive power requirement
can be formulated as-

 

(6)

From (6), as the R/X
ratio increases, a higher amount
of reactive power absorption is required to prevent overvoltage. This might require
higher inverter kVA rating as well as result in higher line losses in the feeder,
and lower power factors at the substation. So, reactive power absorption as a
means to allow higher active power injection is not an attractive option for overvoltage
mitigation. However, active power curtailment (APC) is more effective due to
the stronger relationship between voltage
and active power in LV systems.
The active power output of the PV inverter
can be a function of the PCC voltage which follows the volt-watt (V-W) profile
shown in Fig. 2.

Here,

(t)
is the maximum power generated by the PV array for a given solar irradiance,

 is the slope factor
(kW/V), and 

is the minimum permissible output which is dependent on the
load connected. The inverter output follows maximum power point tracking
until the critical bus voltage (

) is reached.

Fig. 2: V-W profile of
a smart inverter.

The active power is then curtailed
according to slope

  once the bus voltage exceeds

. If the voltage at the PCC exceeds the upper voltage limit

 , the active power
output is reduced to the minimum,

 which serves the
household loads only.

The
impact of different PV inverter active power injection with unity power factor
on any bus voltages can be measured by a sensitivity matrix 9. For a
distribution feeder with N-PV connected inverter buses, the sensitivity matrix
is formed using eqn. (6).

 

(6)

Each
element (Smn) in the inverted submatrix is interpreted as the
variation that would happen in the voltage profile in a certain bus m in case of active power injection in
bus n.

Figure
3 depicts a residential test feeder where the LV network is connected through
a distribution transformer. If V-W profile shown in Fig. 2 is followed during
high penetration low loading condition, unfair PV power curtailment can occur
especially at the remote end of the feeder to prevent overvoltage.
Coordinated control for APC is proposed in 10, where the sensitivity matrix
is used to find optimum PV power curtailment while maintaining fairness to
the PV owners.

 

Fig.
3: A residential test feeder

Fig.
4: Voltage profile at the PCC of each house

Equal
loss of revenue, equal percentage revenue reduction 11, and minimization of
the standard deviation of active power
curtailment 12 have been proposed to address the issue. However, effective,
and fair treatment of all prosumers with
different generation capacity throughout the network still remains to be solved.
This paper proposes a distributed V-W control of the PV inverters of different
capacities which enables them to communicate with neighboring units only. To
take fairness into consideration while curtailing their active power injection
during overvoltage conditions, a linear constrained optimization problem is
formulated as below-

where,
function

 represents
cost for line losses in the network,

 stands for possible cost of active power
curtailment, and

 represents cost for voltage deviation. Here,

and

 denote
weights of the cost functions towards the minimization problem. In addition,
voltage limit constraint for each bus, active power backwards flow constraint
in each line, and a local voltage sensitivity matrix are considered. Finally,
the results obtained for distributed control will be compared with
central/coordinated control to evaluate the performance.

References:

1   B. Kaun,
“Cost-Effectiveness of Energy Storage in California: Application of the EPRI
Energy Storage Valuation Tool to Inform the California Public Utility
Commission Proceeding R. 10-12-007”, Technical Update-3002001162, June
2013.

2   R. Tonkoski, D. Turcotte and T. H. M.
EL-Fouly, “Impact of High PV Penetration on Voltage Profiles in
Residential Neighborhoods,” IEEE Trans. on Sustainable Energy, vol.
3, no. 3, pp. 518-527, July 2012.

3   R. A. Shayani and M. A. G. de Oliveira,
“Photovoltaic Generation Penetration Limits in Radial Distribution
Systems,” in IEEE Trans. on
Power Systems, vol. 26, no. 3, pp. 1625-1631, Aug. 2011.

4
   Rule 21 smart inverter working group
technical reference. Available online: http://www.energy.ca.gov/electricity
analysis/rule21/

5   C. L. Masters,
“Voltage rise: The big issue when connecting embedded generation to long 11 Kv
overhead lines,” Power Eng. J., vol. 16, pp. 5–12, 2002

6    M. H. J. Bollen
and A. Sannino, “Voltage control with inverter-based distributed generation,” IEEE
Trans. Power Delivery, vol. 20, no. 1,
pp. 519–520, Jan. 2005.

7   J. C. Vasquez,
R. A. Mastromauro, J. M. Guerrero, and M. Liserre, “Voltage support provided by
a droop-controlled multifunctional inverter,” IEEE Trans. Ind. Electron.,
vol. 56, no. 11, pp. 4510–4519, Nov. 2009

8   Y. Ueda, K.
Kurokawa, T. Itou, K. Kitamura, K. Akanuma, M. Yokota, H. Sugihara, and A.
Morimoto, “Advanced analysis of grid-connected PV system’s performance and
effect of batteries,” Elect. Eng. Japan, vol. 164, pp. 247–258, 2008

9   S. Conti, A.
Greco and S. Raiti, “Voltage Sensitivity Analysis in MV Distribution
Networks,” in Proceedings of the 6th WSEAS/IASME  International Conference on Electric Power
Systems, High Voltages, Electric Machines, 2007

10 R. Tonkoski, L. A. C. Lopes
and T. H. M. El-Fouly, “Coordinated Active Power Curtailment of Grid
Connected PV Inverters for Overvoltage Prevention,” in IEEE
Trans. on Sustainable Energy, vol. 2, no. 2, pp.
139-147, April 2011.

11 A. Latif, W. Gawlik, P.
Palensky, “”Quantification and Mitigation of
Unfairness in Active Power Curtailment of Rooftop Photovoltaic Systems Using
Sensitivity Based Coordinated Control.” Energies 9(6): 436.

12 M. G. Kashani, M. Mobarrez
and S. Bhattacharya, “Smart inverter volt-watt control design in high PV
penetrated distribution systems,” 2017 IEEE Energy
Conversion Congress and Exposition (ECCE),
Cincinnati, OH, 2017, pp. 4447-4452.

 

 

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