Eliminating Voltage Notching on the Distribution System

When Silicon Controlled Rectifiers (SCRs) are used in electrical controls, it is possible to experience line voltage distortion in the form of "notches" in the waveform. The types of equipment that utilize SCRs in converters or rectifiers, to change the ac line voltage to a dc voltage, and thus experience notching include DC motor speed controls and induction heating equipment.

Line notches are just that - an irregularity in the voltage waveform that appears as a notch as illustrated in Figure 1. They are typically present in the waveform during SCR commutation. Commutation occurs when an SCR in one phase is turned on to turn off an SCR in another phase. For this very small duration of time, a short circuit is created between the two phases. With a short circuit, the current increases and the voltage decreases. The decrease in voltage is defined as a line notch.

Figure 1

The notch can appear at any point during the half cycle since the point of commutation changes as the firing point of the SCR changes. Since the speed of a dc motor is a function of the voltage applied to the motor, at low speed the notch may appear near the end of the half cycle. At higher speeds, the notch may appear near the beginning of the half cycle. In the most severe cases, the voltage is reduced to zero, creating an extra zero crossover or point where voltage normally changes polarity. This extra zero voltage crossover causes the biggest problems.

Zero Voltage Crossing

During a normal cycle of sinusoidal voltage, the voltage crosses the "x" axis, or zero, at 0 degrees and again at 180 degrees. During normal conditions, there are two zero crossovers in each cycle. Some electronic equipment is designed to be triggered on the zero crossover or when the voltage is zero. This allows equipment to be activated without the surge currents or inrush currents that would be present if switched while voltage was present. Some equipment uses the zero crossovers for an internal timing signal. DC drives use the zero crossovers to determine when to fire the SCR. When two dc drives are operating on the same distribution system, they can disturb the electrical system so that one dc drive operating at low speed will affect the other dc drive that is operating at a higher or different speed. To prevent the "power cross talk" between dc drives, it is normal practice to install an isolation transformer ahead of each dc drive.

When notches are present, particularly in three phase equipment, we can experience extra zero crossovers. Instead of two zero crossovers in each cycle of voltage, we can actually experience four notches. These extra notches may tell other equipment to "turn on." That means the equipment may turn on at the wrong time resulting in damage.

Eliminated voltage notching

Eliminating voltage notching requires that the source of the notching (commutation) be isolated or buffered from other sensitive equipment using the same distribution system. Considering the DC drive or other SCR controller as a source of "notch voltage" and the characteristics of the distribution system, it is only required that other sensitive equipment not share the same voltage source or a single voltage source. The simplest method to use to eliminate the voltage notch is to isolate each piece of equipment with an input transformer. If the impedance of the distribution system is low, generally dc drives will not create a severe voltage notch that affects other equipment. However, if the impedance of the distribution system is high (soft line) than voltage notching will occur and likely impact other equipment. An alternate method can use line reactors to reduce the voltage notch and reduce the possibility of affecting other equipment. It is important to note that notches created by dc drives will normally not affect the dc drive that is causing the notching. It is other equipment that can be affected.

It is important that we understand and consider where other sensitive equipment connects to that same voltage source. In order to protect the sensitive equipment, we must reduce the notches before they get to that equipment. We will do this through the creation of a simple voltage divider network.

Figure 2

If we add impedance, in the form of inductive reactance, in series with the SCR controller, and between the controller and the point of other equipment connection, (point B) then the notch voltage will distribute itself across the new impedance (reactance) and the pre-existing line to source impedance. If the added impedance (L3=L1=L2) is one-half as much as was already present, then 1/3 of the notch voltage is dropped across the new impedance and two thirds still remains at the point of common connection with the other equipment.

Figure 3

If the new impedance is equal to the existing input impedance(L3 = L1 + L2), then the notch distributes equally across the two impedances. One-half the original notch voltage is now present at the point of common connection (B).

Notice that if the new impedance is added anywhere else but between the SCR controller and the sensitive equipment, it will have minimal impact on the notch voltage. Placing the reactance on the opposite side of point B offers no improvement to the notching problem.

Use a 3% impedance reactor to solve three phase voltage notching problems. Experience shows that 3% impedance is normally sufficient to reduce the notch voltage, at the point of common connection with other sensitive equipment, to about 50% or less of it's initial value (depth). This eliminates the multiple zero crossings and typically solves the interference problems with neighboring equipment. It is typically not recommended to use a 5% impedance reactor with SCR circuits because the reactor not only reduces the depth of the notch, but it also increases the notch width. Excess impedance could increase the notch width (time) too much causing problems in the controller itself. Increasing the notch width may be seen as a loss of line voltage by some sensitive equipment. If the impedance of the distribution system is low (1%), then a 3% impedance reactor will result in a very small notch depth. Low impedance or stiff distribution systems often have no notching problems. High impedance or soft distribution systems may require a larger value of impedance to reduce voltage notching problems. It is important to note that too much impedance can result in severe loss of input voltage starving the equipment which can cause under voltage tripping problems.

Voltage notching can introduce harmonics on the distribution system. It is important that distribution system be designed with the lowest possible impedance. Prior to the use of power electronic equipment, designing an electrical system with impedance served to reduce the short circuit current or fault current in a system. With the use of power electronic equipment and its self contained current limiting feature, the need for system impedances to control fault current has been reduced.