Voltage Regulation Using Electromagnetic Relays

Introduction

A very famous and conventional method to implement ac regulation is, using Electromagnetic relays to change the taps of multi-tap Transformer. This technique is commonly known as “Online tap changing using Electromagnetic relays”. This is one of the oldest technique to implement ac regulation and still it is used with many domestic appliances. In this technique, a multi-tap transformer is used, where each tap gives unique turns ratio. So, based on the selection of tap, output voltage can be changed, because output voltage depends on the turns ratio of the transformer. That is why, by selecting appropriate tap, a multi-tap transformer can be used to step up and step down the applied voltage. This feature of multi-tap transformer can be used to maintain a constant voltage level at the output. If output voltage gets reduced, then by selecting appropriate high turns ratio tap, output voltage can be restored. Similarly, if output gets high then by selecting low turns ratio tap, output can be maintained constant. But this will require infinite taps, which is not practically possible. However, by providing finite number (usually six to eight) of taps, output voltage can be maintained in a certain range. That is why, some equipment and instruments which are very sensitive to the voltage level cannot be regulated by this technique. For most domestic appliances, this technique will be sufficient, as output will remain close the desired value 220 V.

Now the question is, “How to select(change) appropriate tap of multi-tap transformer to get desired output voltage?”. There are many ways to do that. In early 20^th century, when electronics was not advance, manual changing of tap was required. Obviously, A person had to observe the output voltage constantly and tripping of the circuit was required to change the tap. The same situation is still present in some distribution transformer. Later, by the introduction of Electromechanical relays, the automatic tap selection becomes possible. At a time, only one tap is selected. But, electromechanical relays had reliability issue. Electromechanical relay was replaced by the solid state electromagnetic relay which has no moving part and has longer life. Early, electromagnetic relays were controlled by purely electronic circuit i.e. output voltage was compared with reference some reference voltage using op-amps, which makes regulatory circuit very complex.

In this technique of ac regulation, electromagnetic relays are used for the selection of specific tap of multi-tap transformer. Relays are controlled by the micro-controller. STM32F407 microcontroller is used in this technique. Optocouplers are provided for the isolation of microcontroller with high voltage circuit. Output ac voltage are step down using PT, rectified using bridge capacitor circuitry and fed to the microcontroller. In this way, controller senses the output, compares with reference voltage, controls relays by giving signals to relay triggering circuit and regulates output. This technique has lot of switching transients as the output can be changed at any instant without detecting zero crossing. The problem of transients has been solved in a new similar technique which is discussed in chapter 6. Moreover, this technique has certain pros and cons, which is discussed when comparative analysis of different techniques has been performed. The detailed description of designed circuit, Simulations, hardware, software Implementation and Power quality analysis is provided in the upcoming sections of this chapter.

Electromagnetic Relay

Electromagnetic relay is an electrically operated switch, which operates on the principle of electromagnetic induction. The current flowing in one circuit causes opening and closing of another circuit. A typical relay consists of a coil, sometimes called solenoid and three terminals, common pole (COM), normally open (NO), normally closed (NC). When no current is flowing in the coil, there is no electromagnetic induction and common terminal is internally connected with NC terminal. This is the normal or non-triggering state of relay. When current flows through the coil, electromagnetic induction occurs and COM terminal gets connected with NO terminal due to magnetic force. This is called operated or triggered state of the relay. Such type of relay is called single pole double through (SPDT) relay and can be used for make and break of circuit. The current through coil is controlled by external circuit and a dc voltage source of usually 12 to 24V.We are using 12V dc SPDT relay. A typical SPDT electromagnetic relay is shown in figure 1.

Triggering circuit for Relay

To control the operation or state of relay, an external control circuitry is required. This circuitry control the flow of current through the coil of relay and hence change the state of relay. Figure 2 shows the triggering circuitry for the relay. As we are using microcontroller for the control of whole circuit, so we designed a circuit such that it can trigger the relay by the pulses generated by controller.

 Figure 2: Triggering circuit for Electromagnetic relay