ENERGY UNIVERSITY

COURSE 2: POWER FACTOR AND HARMONICS

In electrical installations, power factor and harmonics means additional power losses and reduce energy reliability. These are important issues to consider for the management of electrical installations. Power factor correction and harmonic mitigation are intended to reduce power losses, reduce electricity bill, and generate the possibility to use the total system capacity.

Power Quality: In electrical systems, harmonic means alterations of the sinusoidal waveforms of voltage and current. Harmonics are superimposed waves with frequencies which are multiples of the power frequency. The multiplying factor is called harmonic order.

Harmonic currents are caused by nonlinear loads (when the current the load draws does not have the same waveform as the supply voltage) connected to the distribution system. The flow of harmonic currents through system impedances in turn creates voltage harmonics which distort the supply voltage. This result in disturbances of sensitive equipment, mainly related to the circulation of currents in the grounding conections.

The voltage fluctuation is the systematic variation of the voltage waveform or a series of random voltage changes of small dimension, namely 95 to 105% of nominal at low frequency.

The usual origin of voltage fluctuation disturbances are motor start up and welding

Power Factor: The active power P (kW) is the real power transmitted to loads. PF=kW/kVA. For sinusoidal or undistorted voltage and current a vector representation is useful. For most electrical loads like motors, the current I is lagging the voltage V by an angle PHI.

Reactive power = Magnetizing power. Is the power needed to produce the magnetic fields to enable the real work to be done.

Energy Efficiency: The maximum active power is transmitted to the load when voltage and current are undistorted and in phase.

Power factor correction and harmonic mitigation: Benefits- Reduced overloading on the electrical system, thereby releasing useable capacity. Reduced demand power. Reduced risks of outage. Extended equipment lifetime. Reduced electricity bills. Reduced power losses. Reduced cable size. Improved process quality. Improved business performance.

Mitigating power problems:

Capacitor Banks are the basic solution for power factor correction. The main objective is to avoid reactive energy penalties charged by the utility. Equipment may be connected at different levels in the electrical system:

- MV substation- LV main switchboard- LV secondary switchboard- Machine terminals

Compensation of an installation is determined in four steps:

- Calculation of reactive power

- Selection of compensation mode Global: Capacitor bank is connected at the supply end of the installation. Ideal for stable

and continuous loads. By sectors: Capacitor bank is connected at the supply end of the sector to be compensated.

Ideal for extended installations including workshops with varying load systems. Local: Capacitor bank is directly connected to the terminals of the machine. Is the best

technical solution because reactive energy is supplied where it is needed. The substantial problem is the additional cost represented in the use of multiple banks.

- Selection of compensation type Fixed: A constant level of compensation. Its connection might be manual, semi-automatic,

direct connection. These capacitors are applied at the terminal of inductive loads, at busbars supplying numerous small motors and inductive appliances, and/or in cases when the load factor is reasonably constant

Automatic by steps: Adapts the quantity of reactive power to the variations of the installation on order to maintain the targeted cos phi. Is applied at points in an installation where the active power and/or reactive power variations are relatively large. E.g. at the busbars of a main distribution switch-board. At the terminal of a heavily loaded feeder cable. When the kvar rating <= 15% of the supply transformer rating, a fixed compensation is appropriate. Otherwise, it is advisable to install an automatically-controlled bank of capacitors. For compensation of highly repetitive connection of capacitors is necessary and must be used static switches.

Dynamic: When fluctuating loads are present, and voltage fluctuations should be avoided. Associate a fixed capacitor bank and an electronic VAR compensator, providing either leading or lagging reactive currents.

- Consideration of harmonics: When capacitor banks are installed in presence of harmonics, two parameter shall be considered.

Active filters: Systems employing power electronics to provide the harmonic currents required by nonlinear loads. Avoid distortion on the power system. The active filter injects, in opposite phase, the harmonics drawn by the load, such that the line current remains sinusoidal.

Hybrid Filters: Systems including a passive filter and an active filter in a single unit. Cumulate the advantages of both technologies (high performance, cost effective).

Electronic compensators: Active or hybrid filters. Capable of compensating the fluctuations of reactive energy. Static VAR compensators (SVC) or hybrid VAR compensators (HVC).

Mitigating VSD (variable speed drivers) power problem: Capacitor-less (C-less) technology: Combined with an advanced control algorithm.

Decreases the THDi by 50% compared to traditional technology. Dedicated to centrifugal pumps, fans and HVAC machines.

AC-line or DC-link reactors (chokes) are commonly used with drivers up to about 500kW unit power in order to smooth the line current and so reduce the distortion. When a large number of drives are present within an installation, the use of AC-line or DC-link chokes for each individual drive is recommended. This measure increases the lifetime of the drives and enables use of cost effective mitigation solutions at installation level, such as active filters for example.

Multi-pulse arrangement: Usually for drives above 400kW, with the precondition of having a dedicated transformer directly supplied from the MV network, with a 3-winding arrangement. This limits the harmonic emission considerably and usually no further mitigation is necessary. Multi-pulse solutions are the most efficient in terms of power losses.

Active Front End (AFE): Is the best performing solution concerning harmonic mitigation with drives. It is an electronically controlled circuitry. It limits the THDi below 5%. Is not necessary a detailed system evaluation, so it made it a system easy to implement. And, power generation and power factor correction are inherent.