Electrical Systems

Facility Electrical Systems and Understanding Electric Power

Basic Electrical Systems in Our Buildings

• Electricity is the flow of electrical energy through some conductive material, such as our power cords or distribution wiring, and powers equipment such as lights and air conditioners to do useful work in our building or facility.

• Electricity has two current types: AC (alternating current) and DC (direct current). – AC is the power we get from the standard

outlets in our office building or other facility, and

– DC is the power we get from batteries, or from power supplies inside our computers, copiers and FAX machines.

• Solar cells, fuel cells, and most wind generators produce DC as their initial output.

• In AC systems, the voltage changes directions 50 times per second, moving first positive and then negative. This is called 50 Hertz, and is the basic frequency of most countries power systems. In the US and North America they use 60 Hz electric power. The AC voltage changes as shown in Figure 1.

• AC power exists because it has advantages for the power company since they can step up the voltage to transmit and distribute it to our buildings and facilities; and this reduces the transmission and distribution losses from the resistance in the power lines.

• AC also makes it easier to build large, high efficiency motors, and to have high efficiency lighting such as fluorescent lights. Using transformers, the AC can also easily be changed to different voltages for different uses in our homes and facilities.

Phases and Voltages in AC Power Systems

• The number of phases in an AC power system is given by the number of different sine waves that make up the power.– A single phase system has only one sine wave,

whereas a three phase system has three sine waves.

• Typical voltages in the European area are:220 volts single phase (240 volts for Cyprus)380 volts three phase (415 volts for Cyprus)

Single Phase AC Electrical Systems

• This is a power system having only one sine wave, and the frequency of that sine wave is 50 Hz.

Fig. 1: Single phase voltage

• For example, in most homes or small buildings where only single phase AC systems supply the electric power, the voltage is 220 volts. It is brought to the building with 3 wires, one being a neutral or ground, and the other two having a voltage difference of 220 volts.In England and Cyprus this voltage difference is 240 volts.

Three-Phase AC Electrical Systems

• This is a power system having three sine waves, and the frequency of each sine wave is 50 Hz. The three sine waves are separated in time with a phase angle of 120 degrees. See Figure 2.

• For another explanation see: http://www.howstuffworks.com/power1.htm

Figure 2: Three-phase voltage

• In new and recent installations, the most popular systems are called 4 wire grounded wye systems. – For most larger buildings and facilities it is a

220/380 volt system. – The low voltage part is 220 volts single

phase. – The high voltage part is 380 volts 3 phase. – In a 4 wire grounded wye system the high

voltage is 1.73 (the square root of 3) times higher than the low voltage.

Grounded Wye Three Phase Systems

Grounded Wye Three Phase Systems (cont)

• This system is flexible since it has the ability to handle single phase plug loads or lighting circuits that operate at 220 volts, from the same system that feeds the 3 phase circuits for motors, equipment for heating, air conditioning, elevators, and industrial machinery.

380 volt, four wire wyesystem

3 Φ (phase)Y system

Neutral

Ground

A

B

NG

C

system3balancedain0=NIV220=3/380=NLV

V380=LLV

Φ

Power in Simple AC Circuits and Systems

• In DC circuits, and in AC circuits with only resistors in them, if we substitute the voltage from Ohm’s Law (V=IR) into the expression for power, the result is:

P = VI = I × R × I = I2 R• Example - Find the power dissipated by a 100

ohm resistor with a current of 1.2 amps through it.

• Solution -P = I2R = (1.2)2 x 100 = 1.44 x 100

= 144 watts

Power in General AC Circuits and Systems

• Most of our AC power is in circuits and systems that have other physical effects in them besides the effect of a resistor.

• One of these effects is from an inductor – a device often made with wire wrapped around an iron core.

• This effect shows up in transformers, induction motors, and magnetic fluorescent ballasts, for example.

• Power in these AC systems is much more complicated.

Power Triangle

• We sometimes show this power relationship in the form of a triangle, called the Power Triangle, to show this more complicated relationship.

kW

kVAkVAR

• In the power triangle, the horizontal leg is the real power in kW. – Real power does real work, as in an

induction motor with its shaft work.

• The vertical leg of the triangle is the reactive power in kVAR – kilovolt amperes reactive. – The reactive power is physically present, but

it does not do real work. – In the case of an induction motor, the

reactive power is the power that magnetizes the motor windings, and helps the motor start and develop running torque.

• The hypotenuse of the triangle is the total or apparent power in kVA. All electric systems have capacities that are rated in kVA.

• Commonly we hear someone talk about a 500 kW distribution panel, but this is not correct. It is a 500 kVA distribution panel.

• The power triangle is a right triangle, and the relationship between the kW, the kVAR, and the kVA is given by the Pythagorean Theorem:

kW2 + kVAR2 = kVA2

• Also, in this triangle, the ratio of kW to kVA is given the name Power Factor.

PF =kW/kVA

• For a DC power system there is no reactive power, and thus no power triangle.

• In addition, for DC power systems the power factor is always 1.0, or 100%.

• In AC power systems, pure resistive loads such as incandescent lights, resistance space heaters, resistance water heaters and electric ovens all have power factors of 1.0 or 100%.

Example

An AC induction motor has the following power triangle. Verify the relationship between kW, kVAR and kVA. What is the power factor of the motor?

40 kW

50 kVA30 kVAR

SolutionkW2 + kVAR2 = kVA2

(40)2 + (30)2 = (50)2

1600 + 900 = 25002500 = 2500

Yes, the relationship is verified.The power factor of the motor is found from the ratio of kW to kVA:

PF = kW/kVA = 40/50 = 80%

Power in Single Phase Systems

• Most residential and small commercial buildings only have single phase AC 220 volt power available. – Devices like full house air conditioners,

electric hot water heaters, and electric clothes dryers are almost always operated on 220 volts AC, single phase.

Power in Single Phase Systems

• For single phase AC systems, the equation for the electric power used is:

P = V × I × PF

Where P = real power in watts

V = voltage in volts

I = current in amperes

PF = power factor

ExampleFind the real power drawn by a 220 volt AC, single phase electric resistance water heater, if it is drawing 20 amps of current.

Solution:Since the water heater is an electric resistance device, its power factor is 100%. Thus, the real power drawn is:

P = V × I × 1.0 = (220)(20)(1.0) = 4400 watts= 4.4 kW

Power in Three Phase Systems

• Almost all electric motors over 2 kW, as well as all large pieces of electrical equipment in a facility, run on three phase power.

• The equation for the power drawn by a general three phase load is:

P = √3 × V × I × PF watts

Example

A three phase, 380 volt electric motor draws 100 amperes, and has a power factor of 87%. How much real power is the motor using?

SolutionP = √3 × V × I × PF watts

= 1.732 x 380 x 100 x 0.87 watts= 57,259.9 watts= 57.3 kW

References for Basic Electrical Systems

• Tom Igoe, Interactive Telecommunications Program, New York University, http://www.itp.nyu.edu/tigoe, Brooklyn, NY, 2003.

• Motors and Voltages, Cowern Papers, Motorsanddrives.com.

• A+ Certification: The Basics of Electronics Adapted From: A+ Certification For Dummies, 2nd Edition, Volts, Amps, and Watts: What are they?

• Energy Education: Concepts and Practices, Wisconsin Energy Education Program, KEEP.

• John Fetters, An Introduction to Electricity for Energy Managers, Energy User News, August 2002

• Guide to Energy Management, 4th Edition, Capehart, Turner and Kennedy, Fairmont Press, Lilburn, GA, 2003.