Power factor correction of electrical industrial systems with photovoltaic net metering/self consumption

Power factor correction of electrical industrial systems with photovoltaic net metering/self consumption

Introduction

Every industrial electrical system connected to the electricity grid draws active power and deploys reactive energy, which is typically of the inductive type. Adding a generation that supplies all or part of the active energy required by the consumption, not only modifies the withdrawal from the electricity grid but also the electricity bill.

Let‘s take as an example that the penalty is for a consumption of reactive energy greater than 33% of active energy. This condition is equal to imposing a minimum power factor cosφ of 0.95.

The penalties are then applied in function to the relationship between the consumption of reactive energy and the active energy, and not on the absolute values.

Why PV net metering/self consumption impacts on low power factor cosφ penalties

The penalty is “triggered” if during the billing period the relationship between the reactive energy Er and active energy Ea absorbed by the system exceeds the value of 0.33.

Let’s assume that the industrial electrical system simplified in Fig. 1 has Er/Ea = 0.33 consumption: the bill will not be weighed down by the penalties due to the low power factor cosφ.

rifasamento e fotovoltaico (1)

Now let’s assume that in such a system a net metering PV system (Fig. 2) is added which supplies, during the billing month taken into consideration, active energy Eafv and let’s assume that this energy is totally self-consumed on-site: the consumption bill will decrease, in the component that is “active energy taken from the grid”, in the amount Eafv,, yet at the same time there will be penalties due to low power factor cosφ because the ratio of active and reactive energy taken from the grid becomes greater than the permissible limit:

Er/(Ea – Eafv) which will be greater than 0.33 since the denominator decreased.

How to evaluate the energy exchanged on site and the additional power factor correction required

Not all active energy supplied by the PV system is exchanged on-site: it depends on electrical system’s load curve and the  photovoltaic system’s generation curve. To avoid penalties, you must take into consideration the month that PV system has greatest efficiency, that is, the month in which the self-consumed energy is greatest.

Empirically we can say that the power factor correction needed in a system equipped with PV net metering/self-consumption, so as not to receive penalties, is the sum of two addends:

– the reactive power needed to bring the system’s power factor cosφ up to 0.95 prior to

adding the PV system;

– the reactive power equal to half of the peak value of the PV system installed.

A particular situation takes place when the PV system supplies active energy that is greater than energy required by the loads: in this case the power factor correction will have to provide all the reactive power required by the system (which therefore must be measured).

Technical Problems

In order to properly and effectively implement the power factor correction in an electrical industrial system equipped with a PV net metering/self-consumption system we have not to consider the simple quantification of the power factor correction required. We must also take into account the following technical problems:

– If the system is powered in MV through a dedicated transformer, with harmonics in the

electrical current, in order to avoid the risk of resonances between the power factor

correction and the medium/low transformer, it is advisable to install a power factor

correction with harmonic block inductances. Since the PV inverter introduces a distorted

electrical current (today’s inverters have a 3% rated THDI), it’s important also

eliminate eventual PFC systems/capacitors of distributed power factor correction.

– If the system is powered in LV, it is advisable to install robust power factor correction

capacitors, preferably with bi-metalized paper (TC10, TC20).

– If the PV’s power is much lower than that of the passive part of the system, the TA

which allows the power factor correction to monitor the system’s cosφ, and operate

accordingly, must be installed upstream of the PV’s derivation. The power factor

correction must be set to operate in four quadrants.

– If the PV has output power comparable to that of the passive part of the system, the TA

must be installed downstream of the PV’s derivation. The cos(phi) target is set to 1, and

the power factor correction must have enough power on board to reach this value. It is

not necessary to set the power factor correction to operate in four quadrants.