ABB Power factor improvement Application guide

Low Voltage Products

Power factor improvement

Application guide

Table of contents

3 Basics of power factor

5 Options and solutions

6 Options and solutions, applications and installation

7 Applications and installation

9 Low voltage capacitor construction

11 Sizing capacitors

14 Harmonics

16 Harmonics filtering

17 Appendix

2 Table of contents | Power factor improvement

Basics of power factor

Motor

Basic concepts

Most load on an electrical distribution system can be categorized into three types - resistive, inductive and capacitive.

On modern systems, the most common load is the inductive type simply due to the nature of loads that consume electricity. Typical examples include transformers, fluorescent lighting and AC induction motors.

Active Power Reactive Power

Figure 1

Apparent

Power

A common characteristic of all inductive loads from doorbells to transformers is that they comprise a winding of some kind. This winding creates an electromagnetic field that allows the motor or transformer to function. A certain portion of electrical power goes to maintain this electromagnetic field.

All inductive loads require two kinds of power to function properly: – Active power (kW) which actually performs the work – Reactive power (kVAR) that sustains the electromagnetic field

One common example of reactive power can be seen in an unloaded AC motor. When all load is removed from the motor, one might expect the no-load current to drop near zero. In truth, however, the no-load current will generally show a value between 25% and 30% of full load current. This is because of the continuous demand for magnetizing current by any inductive load. Active power is the power indicated on a wattmeter. Apparent power is simply the vector sum or geometrical sum of reactive and active power (Fig. 2).

What is power factor?

Power factor (p.f.) is the relationship between working (active) power and total power consumed (apparent power). Essentially, power factor is a measurement of how effectively electrical power is being used. The higher the power factor, the more effectively electrical power is being used.

A distribution system’s operating power is composed of two parts: Active (working) power and reactive (non-working magnetizing) power. The active power performs the useful work... the reactive power does not. It’s only function is to develop magnetic fields required by inductive devices.

kW (active power)

(apparent po

er)

kvar (reactive power)

Figure 2

Generally, power factor decreases (angle Φ increases) with increased motor load. This geometric relationship of apparent power to active power is traditionally expressed by the right triangle (Fig 2) relationship of:

Cos Φ = p.f. = kW/kVA

Why improve low power factor?

Low p.f. simply means poor utilization efficiency. CosΦ varies between 0 and 1, hence a value between 0.9 and 1.0 is considered good power factor and essentially means that metered power and used power are almost equal. From a consumer’s perspective, it simply means you are using what you paid for, with minimal wastage. When p.f. is low, the utility must provide the non-productive reactive power in addition to productive active power. For the utility that means larger generators, transformers, conductors and other system devices that pushes up their own capital expenditures and operating costs, which they simply pass on to industrial users in the form of power factor penalties. Hence, improved power factor helps avoid those penalties.

Key advantage of improved power factor = $$$ savings!!

1. Good p.f. minimizes or eliminates utility p.f. penalties

2. Good p.f. helps improve operating life of equipment

3. Good p.f. expands system capacity, hence facilitating partial deferrment of capital expenditures at plant level.

Power factor improvement | Basics of power factor 3

Basics of power factor

Capacitor

Motor

WITHOUT CAPACITORS

Reactive Power

Motor

% Current

Power Factor cos-phi

Figure 3 illustrates the relationship of power factor to total current consumed. With a p.f. = 1.0 given a constant load, the 100% figure represents the required useful current. As the power factor drops from 1.0 to 0.9, power is used less effectively. Therefore, 10% more current is required to handle the same load. A power factor of 0.7 requires approximately 43% more current; and a power factor of 0.5 requires approximately 200% (twice as much!!) current to handle the same load.

200

150

100

1 0.9 0.8 0.7 0.6 0.

Fig. 3

How capacitors solve the problem of low p.f.

Low p.f. is a problem that can be solved by adding power factor improvement (PFI) capacitors to the plant distribution system. As illustrated in Fig. 4, capacitors work as reactive current generators “supplying” reactive power (kVAR) to the system. By generating their own reactive power, industrial users free the utility from having to supply it; therefore, the total apparent power (kVA) supplied by the utility will be less, which is immediately reﬂected in proportionately smaller bills. Capacitors also reduce the total current drawn from the distribution system and subsequently increase system capacity.

Capacitor rating

Power factor correction capacitors are rated in electrical units called “vars”. One VAR = one Volt Ampere of Reactive power. VARs are units of measurement for indicating how much reactive power the capacitor will supply. As reactive power is usually measured in thousands of vars, the preﬁx “k” (for “kilo”) is added to create the more familiar “kVAR” term. The capacitor kVAR rating shows how much reactive power the capacitor will supply. Each unit of kVAR supplied will decrease the inductive reactive power demand by the same amount.

Active Power

Available Active Powe

Utility

Motor Motor

WITH CAPACITORS

Utility

Fig. 4

Example (Fig. 5): A low voltage network requires 410 kW active power at full load, and the power factor is measured to be 0.70. Therefore, the system’s full load consumption of apparent power is 579.5 kVA. If 300 kVAR of capacitive reactive power is installed, the power factor will rise to 0.96 and the kVA demand will be reduced from

579.5 to 424.3 kVA. Thus, savings can vary from 20~30% or even more in some cases, which cumulatively translates to considerable money savings with the PFI equipment often paying for itself in as little as 6 months.

410 kW

= 15° = .96 P

= 45° = .70 P

579.5 kV

Fig. 5

424.3 kV

155.2 kV

Reduction

8.901

ravk

C 8.904 = 1ravk roticapa

C 03 = ravk roticapa

4 Basics of power factor | Power factor improvement

Options and solutions

Options for improving power factor

The three main options for PFI are as follows:

– Individual capacitor units – One capacitor unit for each

inductive load (in most cases a motor)

– Banks of capacitor units – Several capacitors grouped

in an enclosure that is connected at a central point in the distribution system. Fixed capacitor banks comprise multiple capacitors racked in a common enclosure with no switching while automatic capacitor banks, also called “cap banks” have capacitors in a common enclosure with contactor or thyristor (SCR) switched by a controller.

– Combination of above – Where individual capacitors are

installed on the larger inductive loads and banks are installed on main feeders or switchboards, etc.

Advantages of individual capacitors – Increased system capacity – When active power

compensation is closest to the load, it opens up system capacity and also minimizes line losses.

– Cooler operation – Voltage drops cause the current to

increase, thereby cumulative heat losses occur due to marginally higher current flow. When voltage drops are corrected/addressed closest to the load itself, such temperature issues are prevented right from the start.

– Simpler control – Motor and capacitor can be switched

ON and OFF together, which means simpler control logic and fewer control parts.

– Precise compensation – Since the individual capacitor is

sized to the specific load and switches together with that load, there is no chance of over compensation.

– Easier selection – Selection of individual capacitors is

simple and straight forward and requires no special calculations. See relevant charts on pages 11 and 13.

Advantages of fixed or automatic bank systems – More economical – Cap banks are more economical than

individual capacitor units when the key objective is to reduce utility power bills and/or reduce the current in primary feeders from a main generator or transformer. Being a single installation for power factor compensation simply adds to the convenience.

– Lower installation costs – The cost of installing one fixed or

automatic capacitor bank unit can be less than installing a number of individual capacitors next to each inductive load.

– Switching – Automatic capacitor banks can switch all or part

of the capacitance automatically depending on load requirements. This way, only as much power factor correction as needed for the given load is provided. (This switching capability is a primary advantage over fixed capacitor banks where over-capacitance, leading power factor and resulting overvoltages can occur should the load decrease.)

– Single point of control – It is easier to manage, monitor and

operate the process when the power factor inprovement (PFI) equipment is in one physical location.

Low voltage capacitor construction

Combination of individual, fixed and automatic cap banks

A combination of individual and centralized cap banks is often the best. Individual capacitors are installed on larger motors and banks are installed on distribution systems. To determine the total power factor correction required, then, you need to know the total kVAR requirement for the facility and the desired power factor. By referring to the Power Factor Correction Chart, (Table 2), one can calculate the capacitance needed. See selection instructions on the following pages for more details.

Power factor improvement | Options and solutions 5

Options and solutions, applications and installation

Logic of mechanical protection in QCap

A low-impedence fault rapidly leading to a short-circuit, can be protected by a fuse. A high-impedance fault on the other hand may not lead to a current large enough to trip a fuse, it does cause cumulative resistive heating which increases the pressure in the sealed can, at a rate directly proportional to the rate of heating. Likewise, cumulative heating caused by self-healing action also slowly increases the pressure. Thereby the pressure threshold can be reached either by the long-term accumulation of gas released by self-healings (normal end of life) and/or by a high-impedance fault as described above.

Capacitors special applications

Care should be taken when power factor correction capacitors

Cylindrical capacitors

Functionally very similar to enclosed capacitors (rectangular), cylindrical capacitors are different in terms of physical dimensions and arrangement of internal parts. ABB cylindrical capacitors, also called QCap. Cylindrical capacitors are packaged very conveniently and are best suited to plain and detuned cap banks from 100 to 300 kVAR at 480V or 600V, 60Hz, standard design for use in commercial market segments in Canada.

are used in the following applications: – Frequent starts, plugging and jogging applications – Regenerative loads where load may drive the motor (coasting, etc.) – Multi-speed motors – Motors involving open-transition reduced-voltage starting – Reversing starters that switch more than once per minute – Electronic thyristor (SCR) controlled softstarters

QCap mechanical protection feature

A very simple but uniquely effective design feature provides mechanical protection to each Qcap unit. The device terminals are internally connected using three notched wires that break together when the lid rises due to gas pressure. To ensure the disconnection works reliably, the wires are indirectly anchored to the lid at one end and to the can at the other. The groove is designed to support this. The lid itself is has two stable positions – normal and expanded. When pressure pushes up the lid, all three wires break and the capacitor is entirely disconnected.

SNAP on guard

Rigid connections

Locked by groove

SNAP actuated

Typical wiring diagram (detuned)

Plain autobanks are exactly the same as above, only without the reactors R1, R2 and so on.

6 Options and solutions, applications and installation | Power factor improvement

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