ENGINEERING ARTICLES

Let us consider the case of two platinum electrodes dipped in dilute sulphuric acid solution. When a small potential difference is applied across the electrodes, no current is found to flow. When, however, the applied voltage is increased, a time comes when a temporary flow of current takes place. The H +  ions move towards the cathodes and O -  ions move towards the anode and are absorbed there. These adsorbed ions have a tendency to go back into the electrolytic solution, thereby leaving them as oppositely-charged electrodes. This tendency produces an emf that is in opposition to the applied voltage which is consequently reduced. "This opposing emf which is produced in an electrolyte due to the absorption of gaseous ions by the electrolyte from the two electrodes is known as the back emf of electrolysis or polarization." The value of this back emf is different from different electrolytes. The minimum voltage required to decompose an electrolyte is called ...

PRIMARY CONTROL : Primary control consists of changing a generating unit’s power versus the frequency, according to its static generation characteristic as determined by the speed governor settings. The objective of primary control is to re-establish a balance between generation and demand within the synchronous area at a frequency different from the nominal value. This is done at the expense of the kinetic energy of rotating masses of generating sets and connected motors. The primary control action time is 0 to 30 s after disturbance of the balance between generation and demand. Under normal conditions the system operates at nominal frequency, maintaining the condition of equality of generated power and demand. Each disturbance of this balance, due to, for example, disconnection of a large generating unit or connection of a large load, causes a change in frequency. At first, the frequency varies rapidly, practically linearly, and attains the maximum deviation from the...

A power system can be referred to as an islanding system when it is disconnected from other systems and does not exchange power through tie-lines. Due to the fact that frequency is the same within the entire system, frequency control in an islanding system can be achieved in a relatively simple way. The turbine speed governors should be provided with supplementary elements that change the settings according to frequency variations. After a load change, the frequency reaches a new steady-state level, different from the initial value. The frequency (or rotational speed) error causes an additional integral term to generate a signal which modifies the value of the power setting of a generating unit. With a sufficiently large number of generating units supplied with control systems of this type, the power system yields such a change in the generated power that frequency returns to its initial value. This concept of frequency control can be referred to as decentralized since it...

The main difference between a STATCOM and an SVC is the way they operate: a STATCOM works as a controllable voltage source while an SVC works as a dynamically controllable reactance connected in parallel. Compared with an SVC, a STATCOM offers the possibility of feeding the grid with the maximum available reactive current even at low voltage levels, this is possible because in every equilibrium condition the injected reactive power varies linearly with the voltage at the Point of Common Coupling (PCC). In contrast, for an SVC there is a quadratic dependence of the reactive power on the voltage at the PCC which means that to inject the same reactive power it is necessary to install an SVC with a nominal capacity higher than that of a STATCOM. With regard to the maximum transient capacitive current it is observed that in an SVC the capacitive current is limited by the size of the capacitor and by the magnitude of the AC voltage. In the case of a STATCOM the maximum capa...