Smart Grid Communications

Power Systems

110 volts vs 220 voltsHistorically, the type of electricity delivered to homes and businesses was first DC but then changed to AC electricity. The standard voltage level started at 110V, went to 240V, back to 110V, and then to 220V. The frequency started at 60Hz and then went to 50Hz in most areas.

Early in the history or electricity, Thomas Edison's General Electric Company was distributing DC electricity at 110 volts in the United States. Then Nikola Tesla devised a system of three-phase AC electricity at 240 volts. He had calculated that 60Hz was the most effective frequency.

Tesla later compromised to reduce the voltage to 120 volts for safety reasons.

Tesla's AC system became the standard in the United States. Meanwhile, the German company AEG started generating electricity andbecame a virtual monopoly in Europe. They decided to use 50Hz instead of 60Hz to better fit their metric standards, but they stayed with

120V.

Unfortunately, at 120V 50Hz AC has greater losses and is not as efficient as 60HZ. Due to the slower speed 50Hz electrical generators are 20% less effective than 60Hz generators. Electrical transmission at 50Hz is about 10-15% less efficient. 50Hz transformers require larger windings and 50Hz electric motors are less efficient than those meant to run at 60Hz. They are more costly to make to handle the electrical losses and and 50Hz electric motors are less efficient than those meant to run at 60Hz. They are more costly to make to handle the electrical losses and

the extra heat generated at the lower frequency.

Europe stayed at 120V AC until the 1950s, just after World War II. They then switched over to 230V for better efficiency in electrical transmission. Great Britain not only switched to 230V, but they also changed from 60Hz to 50Hz to follow the European lead. Since many

people did not yet have electrical appliances in Europe after the war, the change-over was not that expensive for them.

The United States also considered converting to 220V for home use but felt it would be too costly, due to all the 120V electrical appliances people had. A compromise was made in the U.S. in that 240V would come into the house where it would be split to 120V to power most

appliances. Certain household appliances such as the electric stove and electric clothes dryer would be powered at 240V.

The voltage and frequency of AC electricity varies from country to country throughout the world. Most use 230V and 50Hz. About 20% of the countries use 110V and/or 60Hz to power their homes. 240V and 60Hz are the most efficient values, but only a few countries use that

combination.

Single Phase vs Three Phase1. Rotating magnetic field is needed to turn the motors on which is

easily produced using 3 phase but with single phase we need a capacitor to produce second phase which increases cost and size.

2. The constant power is delivered in 3 phase but in 1 phase the power is not constant.

Fig. 1. A typical bus of the power system.

Applying KCL to this bus results in

(1)

I=YV

** * *

i1 1

S

This is an equation with complex numbers.

Sometimes we would like an equivalent set of real

power equations. These can be derived by defining

n n

i i i ik k i ik kk k

V I V Y V V Y V

power equations. These can be derived by defining

ik ik ik

i

Y G jB

V

�

�

jRecall e cos sin

iji i i

ik i k

V e V

j

�

* *i

1 1

1

S ( )

(cos sin )( )

ikn n

ji i i ik k i k ik ik

k k

n

i k ik ik ik ikk

P jQ V Y V V V e G jB

V V j G jB

i1

i1

Resolving into the real and imaginary parts

P ( cos sin )

Q ( sin cos

n

i k ik ik ik ik Gi Dik

n

i k ik ik ik ik

V V G B P P

V V G B

)k Gi DiQ Q