Power Quality Services

Power Factor is a measure of system electrical efficiency

The total Electrical Power (Kilo Volt Amperes or kVA) used in an electrical system by an industrial or commercial facility has two components:

• Productive Power (Kilowatts or kW) which produces work.

and

• Reactive Power (Kilo Volt Amperes Reactive or kVAR) which generates the magnetic fields required in inductive electrical equipment (AC motors, transformers, inductive furnaces, ovens, etc.)

Reactive Power produces no productive work. Because the inductive electrical equipment employing magnetic fields requires this Reactive Power, which produces no productive work, the Total Power (kVA) provided by the generating source must be greater than the Productive Power.

The ratio of Productive power (kW) to Total Power (kVA) is Power Factor. It is a measure of system electrical efficiency in an alternating current circuit, and is represented as a % or a decimal.

The relationship between kVA, kW and kVAR is non-linear and is expressed:

In almost all industrial facilities where Demand charges apply, utility companies charge a penalty for poor power factor.

While there are many reasons to correct poor power factor we find that in general a facility will correct it’s Power Factor for one or more of the following reasons:

• To Reduce Utility Power Billing.

Most, if not all Hydro Utilities charge a penalty for poor power factor. In a lot of cases this penalty can be quite substantial (20-40% of the Demand Charges plus high I2R Losses). Therefore, one of the main reasons for correcting your facilities power factor is to eliminate needless financial losses.

• To Increase System Capacity.

Improving the Power Factor releases system capacity and permits additional loads (motors, lighting, etc..) to be added without overloading the system. In a typical system with a .80 P.F., only 800kW of productive power is able to be used out of 1000kVA installed. By correcting the system to unity (1.0 P.F.), the kW = kVA. Now the corrected system will support 1000kW, versus teh 800 kW at the .80 P.F. uncorrected condition; an increase of 200kW of productive power.

• Improved System operating characteristics.

A good power factor (.95) provides a “stiffer” voltage, typically a 1-2% voltage rise can be expected when Power Factor is brought to +/- .95.

Improving Power Factor will also lower losses in the distribution system of the facility since losses are proportional to the square of the current.

Regardless of why you improve the electrical systems Power Factor it will provide a combination of all the things listed above.

There are various improvements that can be made within an electrical system which will assist in the improvement of the facilities Power factor.

The replacement of existing motors with more energy efficient ones; ensuring motors are properly sized for their application and duty cycle; the use of high power factor lighting ballasts; and the Installation of Power Capacitors to name a few.

The installation of Power Capacitors is by far perhaps the most straight forward and most cost efficient method of improving a facilities Power Factor.

Ideally, to derive the maximum benefit, properly sized static capacitor banks should be located as close as possible to the offending loads.

Most times however, this may not be the most cost effective method of correction due to the sheer number of static capacitor banks that are required in a large electrical environment.

A more economical method is the installation of a centrally located automatic switching power factor correction bank (APFC). This capacitor bank is controlled by a microprocessor-based relay, which continually monitors the reactive power requirements. The relay then connects or disconnects capacitors to supply capacitance as needed.

Often times, we can take advantage of the use of both types of correction. Static capacitor banks can be placed at strategic locations through out facility while implementing an APFC to handle the remaining requirements for capacitance. Generally, APFCs are placed centrally at the main electrical panel.

Regardless of which strategy is implemented careful planning should be undertaken to ensure maximum benefit is derived from the correction. Other factors that need to be taken into consideration during this planning phase is Harmonics on the electrical system as well as know future expansions to the facility.

To best explain how Demand Billing works lets start with some real world readings and examine them as they are used by a utility when billing you. Then we will look at ways to possibly decrease the amount we have to pay for our power through the use of Power Factor Correction equipment.

Example

At the end of the billing period the utility records the following Peak reading numbers at your facility:

kW = 100
kVA = 150

Power Factor = kW / kVA = 100 / 150 = 0.67 or 67%

These are the readings that the utility will use to determine your Demand Billing.

You will have to check with your local utility to determine their rate schedule, but for our example we will use $9.00/kW.

The general billing practice (with few exceptions) are to apply their Demand rates to either:

• A. 100% of the kW reading.

or

• B. 90% of the kVA reading.

Whichever is the GREATEST!

Do not be misled by the term “Billed kW” as may be listed on your utility bill. This term is used only to indicate the reading value that rates are being applied to, not whether its kW or kVA as they would have you believe.

Although it seems to infer that the demand is based on kW, this number may in fact be either the kW reading or 90% of the kVA reading depending on which is largest and being used for billing purposes.

Example: Hydro One – Conditions of Service 2011 brochure

(Above link opens in new window)

2.4.1.2 Components of Distribution Rates – (found on Page 60)

Hydro One Distribution Service Rates include a monthly service charge component and a volume-based component. For Demand Billed Customers, the volume Rate is a per kW charge. The billing demand shall be taken as 90% of the kVA or 100% of the measured demand in kW, whichever is greater. For Energy Only Customers, the volume Rate is a per kWh charge. The monthly service charge component is designed to recover some common costs of Distribution Services that are independent of electricity use. All other Distribution Service costs are recovered through the volume Rate.

To determine what value rates will be applied to in our example lets have a look

100% of kW = 100

90% of kVA = 150 x 0.9 = 135

As we can see, 135kVA is the greater number, thus rates will be applied to this reading.

So, our Demand Billing (or Billed kW as it may be called) for this period would be: 135 x $9.00 = $1,215.00.

Reducing Costs

Now that we know how much we are billed and on what readings we are billed lets look to see if we can reduce this cost.

In our example rates were applied to 90% of the kVA reading because it was larger. Ideally, we would liked to have rates applied to our kW because it is the smaller reading number. If we could do that then we could realize a reduction in our utility billing. Just how much?:

Cost if rates were applied to the kW reading: kW x $9.00/kW = 100 x $9.00/kW = $900.00

Potential reduction to the Demand portion of our electrical bill if rates were applied to the kW reading:

$1,215.00 – $900.00 = $315.00

So what can we do to our electrical system so that rates are applied to the kW reading? The most economical way of doing this is through the installation of Power Factor Correction Capacitor Banks. Power Factor correction has been around as long as electricity has. It is the means through which we can tweak our electrical system to make it more energy efficient, and by doing so we can ensure that we are paying the least possible cost for our power.

By correcting the Power Factor here to above 90% ensures that no additional expenses are being incurred.

For more information please feel free to contact us.

When the line voltages applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings will result. A small percentage voltage unbalance will result in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages.

Should voltages be unbalanced, the rated horsepower of the motor should be multiplied by the factor shown in the graph below to reduce the possibility of damage to the motor. Operation of the motor at above a 5 per cent voltage unbalance condition is not recommended.

Alternating current, polyphase motors normally are designed to operate successfully under running conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1 per cent. Performance will not necessarily be the same as when the motor is operating with a balanced voltage at the motor terminals.

Example:

With voltage of 460, 467 and 450, the average is 459, the maximum deviation from the average is 9, and the Percent Unbalance = 100 x (9 4 459) = 1.96

Some users of medium voltage motors argue the fact that an optimized surge protection arrangement eliminates the risk of motor failure resulting from switching surges. The issue really is whether the cost of surge protection is economically feasible compared to the risk of motor switching surge failure. The other argument is the type of protection arrangement that should be used.

The motor manufacturer cannot be expected to admit that his motors are generally susceptible to switching surges, likewise switchgear manufacturers are also unlikely to recommend surge protection for motors in the facility and remain price competitive with their products. It is therefore left to the user to decide whether or not to install surge protection.

Laboratory tests can provide totally unrealistic results, particularly if high frequency circuit parameters are not realistically simulated. These guidelines for motor surge protection are based on many field tests involving actual motor installations.

The objective here is to provide general guidelines concerning the application and configuration of surge protection. Protection is generally recommended for all applications where reasonable doubt exists as to the ability of a given motor to survive switching surges over it’s life span. Consideration must be given to economic factors in the sense that the cost of the surge protection does not out weigh the cost of related failures which could otherwise be expected.

It is important to note that an inter-turn failure is generally associated with a localized low-energy discharge which is unlikely to cause an immediate coil-to-coil-slot failure on the motor. However, once the turn insulation has been pin-holed, subsequent switching operations may cause further breakdown at a much lower voltage. This process may be repeated over a considerable period of time before a major coil insulation failure occurs. Optimized surge protection is therefore also aimed at eliminating further failures in a coil which was previously slightly damaged, but which has not yet failed.

Cost Implications of a Switching Surge Failure

Risks in terms of the total cost associated with failure of a specific motor should always be determined by the user. The following factors – which are not always obvious – should also be taken into consideration:

• Production Loss
• The quality of the rewind (i.e., whether the impulse withstand voltage will be the same as for the original motor or whether the risk of a second failure will be higher.
• The cost of a rewind, taking into account cost differences between a “normal” rewind and a “superior” rewind, which could be quite substantial.
• Long term insulation degradation due to localized non-catastrophic inter-turn faults, which may eventually cause failures of several motors over a relatively long period, but which can not always be reversed once the first series of failures has drawn attention to the need for surge suppression.

Type of Switchgear and Switching Duty

Vacuum Switchgear

It is commonly accepted that surge protection should be provided in all cases, regardless of motor size, impulse withstand characteristics or switching duty. This applies to vacuum contactors, circuit breakers, and claims by manufacturers.

Air & SF6 Switchgear

For special design motors where impulse withstand is at 4pu at 0.2 usec. surge protection is not required in the following cases:

• Low switching duty i.e. typically one or two switching operations per week.
• If the motor is not used for essential processes.
• If the motor is not used for “jogging” or similar applications.
• If the total cost of a failure is relatively low compared to the cost of the surge protection

For standard design motors surge protection is not required in the following cases:

• Low switching duty i.e. not more than a few siwtching operations per month.
• If the motor is not used for essential or semi-essential processes.
• If the cost of a failure is relatively low compared to the cost of surge protection.

These guidelines should be reviewed and consideration given to a possible installation of surge arrestors at the panel where the switching device of the motor is located.

Line Reactors

Input to Inverter / Drive

On the input of an electronic controller, line reactors protect sensitive electronic equipment from electrical noise created by the drive or inverter (notching pulsed distortion, harmonics). They also protect the controller from surges or spikes on the incoming power lines, as well as reduce harmonic distortion. They help to meet the requirements of IEEE 519.

Multiple Controllers

Multiple drives or inverters on a common power line require one reactor per controller. Individual reactors provide filtering between each controller (reduce crosstalk) and also provide optimum surge protection for each unit. A single reactor serving several controllers does not provide adequate protection or filtering when the system is partially loaded.

Load Reactors

Output of Inverter / Drive

The use of reactors between a controller and a motor (or other load) protects the controller from either a short circuit in the load, or a surge in output current. It accomplishes this by limiting surges.

Reactors also reduce operating temperature and audible noise in motor loads. Harmonic compensation of all Guard-AC reactors allows standard units to be used here with confidence. They improve waveform integrity, thus enhancing motor performance and system efficiency.

Multiple Motors

When more than one motor is controlled by a single drive, a single reactor can typically be used between the controller and the motors, as illustrated. The reactor should be sized based on the total motor/load horsepower.