Power Factor Correction (PFC) in solar power plants

The large diffusion of photovoltaic plants to reduce costs on industry electric bill brings the problem of integrating the plant with the existing PFC devices (or to be designed). A couple of information must be evaluated to avoid future frustation on expected savings or worse to pay penalties. PFC is useless in solar plants because the production is @cosφ=1 FALSE ! The production is @ cosφ=1 but if the loads are not compensated this will result in greater consumption of energy i.e. more withdrawal from the net (or less delivery in net-metering) The feeling that solar power is free is very far from reality, as you will know if you recently purchased, repaired or maintained a system. So why should we waste the energy produced ? A first peculiarity of the photovoltaic system is to generate only active power, decreasing its demand from the energy provider; The reactive power, however, remains unchanged because it is determined by loads connected to the plant, consequently decreasing the power factor and increasing the need for PFC So during design we have to take care of oversizing the reactive power required

Graphically the active power required from loads P1 is subtracted of the power generated by photovoltaic Pf with result P2 (active power absorbed from the distributor in the presence of photovoltaic system). The reactive power (Q) does not vary and consequently increases the phase angle φ2> φ1 while cos (φ) decreases To avoid undesiderable reactive currents on the net, distributors apply limits to power factor (typically from 0.90 to 0.95), charging excess in kvar to the customer A practical example Manufacturing company in Venice district. Data from the electrical bill:

• Voltage 15/0,4kV • Power 950kW (P) • Mean Active Energy consumption (per month) Ea = 103MWh • Mean Reactive power consumption (per month) Er = 90.5MVArh

Q1

P1

Pf

φ1 φ2

P2

Q2=Q1

May June July

Active power (kWh)

Reactive power

(kVArh)

Active power (kWh)

Reactive power

(kVArh)

Active power (kWh)

Reactive power

(kVArh) F1 86873 71570 F1 85913 71020 F1 87911 72443 F2 9856 10883 F2 9714 10501 F2 9920 11896 F3 6581 9947 F3 6415 6404 F3 6843 7005 Tot 103310 92399 Tot 102042 87925 Tot 104674 91344

(F1, F2,F3= time periods) We can calculate:

- 22

cosra

aavg

EE

E

+=ϕ = 0.76

- Montly penalty for Er exceeding 33% of Ea = 286 € (0,00704 €/kVarh) - Montly penalty for Er exceeding 75% of Ea = 90 € (0,00905 €/kVarh) - cosφ target = 0.97 - required reactive power Qr = 0.60 * P = 574kVAr

Installing a 300kW solar power plant we can evaluate using on-line databases(ex. http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php?lang=it) that average solar power production is 25MWh. Active power withdrawn from the distributor drops to 78 MWh but the power factor decreases

- 22

cosra

aavg

EE

E

+=ϕ = 0.66

- Montly penalty for Er exceeding 33% of Ea = 55 € (0,00704 €/kVarh) - Montly penalty for Er exceeding 75% of Ea = 619 € (0,00905 €/kVarh) - cosφ target = 0.97 - required reactive power Qr = 0.89 * P = 843kVAr

The second feature of photovoltaic systems is that the energy produced by solar panels is strongly distorted by the presence of harmonics. This is caused by inverters with switching technology present in the system The amount of harmonics generated is a function of the goodness of the installed inverters

Photovoltaic inverter THDi = 6,27%

0%

1%

2%

3%

4%

5%

6%

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29h

pu

What you need to know to design your PFC equipment Sizing: The following information are mandatory

Ø Active Energy withdrawn by loads Ø Reactive power needed by loadsi Ø Maximum Active power generated by the photovoltaic plant (summer peak) Ø Cos(ϕ) target

Model Due to the harmonic presence designer must take care in choosing the suitable capacitor type and equipment. Heavy duty capacitors and/or equipment with detuned reactors are strongly recommended

Ø Harmonic content must be measured through an electric net assessment

PFC Controller When solar plant produce more Energy that loads can withdraw (i.e. surplus Energy is delivered to the net) the controller must be able to operate on the 4 quadrants. Available Products from COMAR Condensatori SpA,

DMP-FTV - This series is equipped with heavy duty capacitors Max harmonic current distortion in the network THDI(r) = 40% Max harmonic current distortion on capacitors THDI(c) = 90%

AAR/100 – series with detuned reactors

Max harmonic current distortion in the network THDI(r) = 100% Max harmonic voltage distortion in the network THDV(r) = 3%

AAR/600 – series with detuned reactors Max harmonic current distortion in the network THDI(r) = 100% Max harmonic voltage distortion in the network THDV(r) = 6%

AAR/D20 – series with detuned reactors Max harmonic current distortion in the network THDI(r) = 100% Max harmonic voltage distortion in the network THDV(r) = 20%

INSTALLATION OF THE CURRENT TRANSFORMER (CT)

The measuring CT in service of the PFC equipment must be placed downstream of the solarplant input on the net. The target cosφ has to be set at 1.00

CT could also be placed upstream but with the following limits

If active power PG produced from the solar plant is closet o the active power needed by the loads PL CT will not measure the whole active power consumed but only the fraction withdrawn by the distributor PR = PL – PG, leading to the following malfunctions

o Low current fault

o Inaccurate current measurement

o Inaccurate bank insertion / disinsertion due to wrong calculation of harmonic distorsion