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.

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:

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Energy Storage Valuation Tool to Inform the California Public Utility

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2013.

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4

Rule 21 smart inverter working group

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BHC1Some

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