ENGINEERING ARTICLES

There are four ways to solve power quality problems: 1- Design equipment and electrical systems to prevent electrical disturbances from causing equipment or systems to malfunction. Where, manufactures of sensitive equipment can reduce or eliminate the effect of power quality problems by designing their equipment to be less sensitive to disturbances. They can add some devices to their equipment according to situation, for instance a capacitor to provide temporary energy storage when the voltage sags are too low. They can also alter their equipment to desensitize it to power quality problem for example; they can design special K factor transformers that tolerate harmonics. 2- Analyze the symptoms of power quality problems to determine its cause and solution. It is important to determine source and type of power quality problems, the type of power quality problem and its cause often determine the solution. 3-Identify the medium that is transmitting the electrical disturbances and...

IEEE defined power quality disturbances into seven categories based on wave shape: 1. Transients 2. Interruptions 3. Sag/ Under voltage 4. Swell/ Overvoltage 5. Waveform distortion 6. Voltage fluctuations 7. Frequency variations 1. TRANSIENTS Potentially the most damaging type of power disturbance, transients fall into two subcategories: 1. Impulsive 2. Oscillatory more » 2. INTERRUPTIONS An interruption is defined as the complete loss of supply voltage or load current. Depending on its duration, an interruption is categorized as instantaneous, momentary, temporary, or sustained. more » 3. SAG/ UNDERVOLTAGE A sag is a reduction of AC voltage at a given frequency for the duration of 0.5 cycles to 1 minute’s time. Sags are usually caused by system faults, and are also often the result of switching on loads with heavy startup currents. more » 4. SWELL/ OVERVOLTAGE A swell is the reverse form of a sag, having an increase in AC voltage for a duration of 0.5 cycles...

A null method of measurement is a simple, accurate and widely used method which depends on an instrument reading being adjusted to read zero current only. The method assumes: (i) If there is any deflection at all, then some current is flowing; (ii) If there is no deflection, then no current flows (i.e. a null condition). Hence it is unnecessary for a meter sensing current flow to be calibrated when used in this way. A sensitive milli-ammeter or micro-ammeter with center zero position setting is called a galvanometer. Examples where the method is used are in the Wheatstone bridge, in the DC potentiometer and with AC bridges.

All analogue electrical indicating instruments require three essential devices: (A) A DEFLECTING OR OPERATING DEVICE: A mechanical force is produced by the current or voltage which causes the pointer to deflect from its zero position. (B) A CONTROLLING DEVICE: The controlling force acts in opposition to the deflecting force and ensures that the deflection shown on the meter is always the same for a given measured quantity. It also prevents the pointer always going to the maximum deflection. There are two main types of controlling device; spring control and gravity control. (C) A DAMPING DEVICE: The damping force ensures that the pointer comes to rest in its final position quickly and without undue oscillation. There are three main types of damping used; eddy current damping, air-friction damping and fluid friction damping. There are basically two types of scale; linear and nonlinear. A linear scale is shown in Fig. 1(a), where the divisions or graduations are evenly spac...

There are several important reasons why a grounding system should be installed. But the most important reason is to protect people! Secondary reasons include protection of structures and equipment from unintentional contact with energized electrical lines. The grounding system must ensure maximum safety from electrical system faults and lightning. A good grounding system must receive periodic inspection and maintenance, if needed, to retain its effectiveness. Continued or periodic maintenance is aided through adequate design, choice of materials and proper installation techniques to ensure that the grounding system resists deterioration or inadvertent destruction. Therefore, minimal repair is needed to retain effectiveness throughout the life of the structure. The grounding system serves three primary functions which are listed below. PERSONNEL SAFETY: Personnel safety is provided by low impedance grounding and bonding between metallic equipment, chassis, piping, and other c...

There are several ways in which the system and the generator may interact with subsynchronous effects. A few of these interactions are basic in concept and have been given special names. We mention three of these that are of particular interest: Induction generator effect Torsional interaction Transient torque Each of the above effects will be discussed briefly. 1) INDUCTION GENERATOR EFFECT: Induction generator effect (IGE) is caused by self-excitation of the electrical network. The resistance of the generator to subsynchronous current, viewed looking into the generator at the armature terminals, is a negative resistance over much of the subsynchronous frequency range. This is typical of any voltage source in any electric network. The network also presents-a resistance to these same currents that is a positive resistance. However, if the negative resistance of the generator is greater in magnitude than the positive resistance of the network at one of the network natural frequ...

Subsynchronous resonance is a condition that can exist on a power system where the network has natural frequencies that fall below the fundamental frequency of the generated voltages. Transient currents flowing in the ac network have two components; one component at the frequency of the driving voltages and another component at a frequency that depends entirely on the elements of the network. For a network with only series resistance and inductance, an isolated transient, such as switching a load, will consist of a fundamental component and a de component that decays with a time constant that depends on the LIR ratio of the equivalent impedance between source and load. Since loads are frequently switched on and off, the transient currents usually appear as random noise, superimposed on the fundamental frequency currents. The addition of shunt capacitors to the network result in new natural frequencies of oscillation that are always greater than the fundamental frequency. In network...