High Voltage Direct Current (HVDC)
Page 4: AC / DC Conversion
Alternating Current (AC)
Direct current (DC) is a steady stream of electricity. Alternating current (AC), on the other hand, fluctuates rapidly, reversing direction many times per second.
Alternating current (AC) is generated in sine waves, because of the circular motion of generators, and the need to be able to power motors (which are basically generators in reverse). If one (single phase) generator is used to generate the AC electricity, the generated electricity has particular instants of time in which there is no voltage, when the voltage to push current is changing directions.
Figure 4.1 Single phase AC voltage.
But if three generators are used that are slightly out of phase (one-third cycle apart each), or if a three-phase generator is used (with three separate windings that are one-third cycle apart), then there is always voltage between different conductors.
Figure 4.2 Three phase AC voltage.
Figure 4.2 above shows the oscillating voltage of three AC phases on the time scale of the first phase A. The following graph shows that time scale:
Figure 4.3 AC phase voltage cycle. [WECC]
The AC phase voltage is zero at Time = 0. The sine of zero is zero (Time = 0 corresponds to a phase angle of zero).
The AC phase voltage rises to maximum at Time = 1/4th cycle (which corresponds to phase angle 90 degrees or π/2 radians). Then it goes down to zero at Time = 1/2 cycle (phase angle 180 degrees or π radians).
After it is half way through a cycle, the AC phase voltage becomes negative (points in the opposite direction, creating pressure for the current to change direction).
Two-thirds way through the cycle (phase angle 270°), voltage is the most negative (reverse direction flow pressure is maximum).
At 360° the cycle is complete and will start over (no flow pressure, reverse flow pressure stopped, will resume forward flow pressure).
Phase B is 120 degrees out of phase with phase A, and phase C is 120 degrees out of phase with phase B. Each phase is 120 degrees and 240 degrees out of phase with the other two phases, since each phase starts 1/3 way through another phase.
Three Phase AC Connections
There are two types of three phase AC systems: Delta and Wye. With delta systems, any two conductors span a phase. For wye systems, any two conductors span two phases, with allowance for a fourth wire to use one phase. The fourth wire is neutral / ground, and only used for single phase loads. If there are no single phase loads, the neutral / ground is not needed.
Figure 4.4 Three phase AC Delta and Wye schematic. [WECC]
The different systems, delta and wye, result from how the wire windings are wound in generators and transformers. As shown above, for delta systems the windings are relative to each other, one winding skipping to another one which goes back to the intervening one and starts again, while for wye the start of each winding joins at a common neutral with the other end at the terminal for each phase.
To convert a delta system to a wye system, a transformer may be used, by simply winding one side of the transformer with the phase windings feeding each other sequentually (delta side), and the other side windings (wye side) separate for each phase and each connected to a neutral / ground.
An important difference between delta and wye transmission is that they are slightly out of phase (by 30°) for the same power. That will be discussed later in this page.
Diode / Rectifier
A diode is a semiconductor device that only allows electricity to travel in one direction. The symbol for a diode is a triangle pointing in the direction that electricity is conceptually allowed to flow:
Figure 4.5 Diode symbol.
The diode illustrated in Figure 4.5 above would allow electricity to travel from left to right, but not from right to left.
Converting AC to DC is called rectification, and a device that converts AC to DC is called a rectifier. If we use a single diode as a rectifier, only half the electricity is converted, because AC reverses directions and the diode will not let electricity come back in the other direction, losing half of the power (referred to as half wave rectification):
Figure 4.6 Half wave rectification.
To convert all of the AC power to DC requires more than one diode, and two conductors on the DC side, for electricity to flow in one direction in a DC conductor and flow in the opposite direction in the other DC conductor. One of the DC conductors is labeled plus +, the other labeled minus , signifying that electricity flows in opposite directions (one direction in one conductor, the other direction in the other conductor).
Full wave rectification converts all of the fluctuating AC power to two-conductor DC, and requires four diodes connected in a bridge:
Figure 4.7 Full wave rectifier, diode bridge. [Wykis]
The AC circuit pushes current onto one of the DC conductors, and pulls current from the other DC conductor, then changes to pulling and pushing, then pushing and pulling again, etc. The resulting DC voltage is shown in this graph:
Figure 4.8 Full wave rectification.
The resulting DC voltage is not constant. It consists of pulses that drop very low twice per cycle. These pulses correspond to the absolute values of the AC voltage.
This example was for only one AC phase. When rectification is done for all three phases, the pulsing evens out more and does not drop low.
Diodes that have a gate are called thyristors (or valves). The gate turns on the thyristor. Normally, a diode automatically lets any electricity through that flows in one direction. A thyristor also only lets electricity flow in one direction, but it must be turned on to do that.
While a diode has two leads (or conducting surfaces) for the electrical flow, a thyristor additionally has a third lead to receive a signal to instruct it to open. The electrical symbol for a thyristor is therefore the same as for a diode but with a line sticking out of the diode. That line may be transverse or diagonal, and may be bent. There is no single convention for how to draw that line that sticks out of the thyristor.
Like a diode, a thyristor will only let electricity flow in one direction. But unlike a diode, it must be turned on. If a thyristor is not turned on, no electricity will flow through it, no matter what direction the electricity is trying to flow.
Thyristors are made of layers of semiconductor materials that are triggered to turn on (allow electricity to flow through it) when a relatively weak electrical or optical signal is transmitted to the thyristor. Sending the electrical or optical signal to the thyristor is referred to as firing the thyristor (like igniting a spark plug in an internal combustion engine, but with less signal power).
Figure 4.11 Material cross section of the thyristors pictured above.
Once the thyristor is fired, it allows electricity to pass through it in the predefined direction until voltage drops to zero and turns negative, at which point the thyristor automatically turns off. Then, it will not allow any more electricity through until it is fired again.
Note that while it is possible to turn on a thyristor, turning off the thyristor can only happen automatically by voltage becoming negative. Thus a thyristor can be turned on with a gate signal, but cannot be turned off. It can only turn off by itself, when voltage turns negative in the predefined (allowable) direction.
Thyristors are used instead of diodes for rectification of three phase AC. Following is a bridge of six thyristors to convert 3 phase AC to DC:
Figure 4.12 Six pulse thyristor bridge.
This illustrates three phase AC from the left converted to DC (two conductors) on the right. First the AC power goes through a transformer that sets the proper voltage, then each phase of the AC that is at the proper voltage is connected to two of the six thyristors.