POWER QUALITY SOLUTIONS

POWER QUALITY SOLUTIONS

By Engr. Aneel Kumar   April 28, 2015   No comments:

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 reduce or eliminate the effect of that medium.

4- Treat the symptoms of the power quality problems by use of power conditioning equipment. It provides essential protection against disturbances. Power conditioning equipment include devices that reduce or eliminate the effect of a power quality disturbance. It can be used to condition the source, the transmitter, or the receiver of the power quality problems. The equipment can be divided into ten categories, surge suppressors, noise filter, isolation transformer, low-voltage line reactors, various line voltage regulators, motor-generator sets, dual feeders with static transfer, uninterruptible power supplies, harmonic filters and Dynamic voltage restorer (DVR).

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POWER QUALITY PROBLEMS IEEE

POWER QUALITY PROBLEMS IEEE

By Engr. Aneel Kumar   April 16, 2015   No comments:

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 to 1 minute’s time. For swells, high-impedance neutral connections, sudden (especially large) load reductions, and a single-phase fault on a three-phase system are common sources. more »

5. WAVEFORM DISTORTION

There are five primary types of waveform distortion:

1. DC offset
2. Harmonics
3. Interharmonics
4. Notching
5. Noise
more »

6. VOLTAGE FLUCTUATIONS

A voltage fluctuation is a systematic variation of the voltage waveform or a series of random voltage changes, of small dimensions, namely 95 to 105% of nominal at a low frequency, generally below 25 Hz. more »

7. FREQUENCY VARIATIONS

Frequency variation is extremely rare in stable utility power systems, especially systems interconnected via a power grid. Where sites have dedicated standby generators or poor power infrastructure, frequency variation is more common especially if the generator is heavily loaded. more »

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NULL METHOD OF MEASUREMENT

NULL METHOD OF MEASUREMENT

By Engr. Aneel Kumar   April 09, 2015   No comments:

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.

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ANALOGUE INSTRUMENTS

ANALOGUE INSTRUMENTS

By Engr. Aneel Kumar   April 09, 2015   No comments:

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 spaced. The voltmeter shown has a range 0–100 V, i.e. a full-scale deflection (fsd) of 100 V. A nonlinear scale is shown in Fig. 1(b) where the scale is cramped at the beginning and the graduations are uneven throughout the range. The ammeter shown has a fsd of 10 A.

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WHAT IS GROUNDING

WHAT IS GROUNDING

By Engr. Aneel Kumar   April 09, 2015   No comments:

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 conductive objects so that currents, due to faults or lightning, do not result in voltages sufficient to cause a shock hazard. Proper grounding facilitates the operation of the overcurrent protective device protecting the circuit.

EQUIPMENT AND BUILDING PROTECTION:

Equipment and building protection is provided by low impedance grounding and bonding between electrical services, protective devices, equipment and other conductive objects so that faults or lightning currents do not result in hazardous voltages within the building. Also, the proper operation of overcurrent protective devices is frequently dependent upon low impedance fault current paths.

ELECTRICAL NOISE REDUCTION:

Proper grounding aids in electrical noise reduction and ensures:

1. The impedance between the signal ground points throughout the building is minimized.
2. The voltage potentials between interconnected equipment are minimized.
3. That the effects of electrical and magnetic field coupling are minimized.

Another function of the grounding system is to provide a reference for circuit conductors to stabilize their voltage to ground during normal operation. The earth itself is not essential to provide a reference function. Another suitable conductive body may be used instead.

The function of a grounding electrode system and a ground terminal is to provide a system of conductors which ensures electrical contact with the earth.

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TYPES OF SUBSYNCHRONOUS RESONANCE INTERACTIONS

TYPES OF SUBSYNCHRONOUS RESONANCE INTERACTIONS

By Engr. Aneel Kumar   April 01, 2015   No comments:

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:

1. Induction generator effect
2. Torsional interaction
3. 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 frequencies, growing subsynchronous currents can be expected. This is the condition known as the induction generator effect. Should this condition occur, the generator may experience subsynchronous torques at or near a natural shaft frequency, which may cause large and sustained oscillations that could be damaging to the shaft.

2) TORSIONAL INTERACTION:

Torsional interaction occurs when a generator is connected to a series compensated network, which has one or more natural frequencies that are synchronous frequency complements of one or more of the torsional natural modes of the turbine-generator shaft. When this happens, generator rotor oscillations will build up and this motion will induce armature voltage components at both subsynchronous and supersynchronous frequencies. Moreover, the induced subsynchronous frequency voltage is phased to sustain the subsynchronous torque. If this torque equals or exceeds the inherent mechanical damping of the rotating system, the system will become self-excited. This phenomenon is called torsional interaction (TI).

The network may be capable of many different subsynchronous natural frequencies, depending on the number of lines with series compensation and the degree of compensation installed on each line. Moreover, switching of the network lines can cause these natural frequencies, as viewed from the generator, to change. The engineer must evaluate the network frequencies under all possible switching conditions to determine all possible conditions that may be threatening to the generators. Another condition that can greatly increase the number of discrete network subsynchronous frequencies is the outage of series capacitor segments. The series compensation in high-voltage systems usually consists of several capacitor segments that are connected in series, with each series segment consisting of parallel capacitors as required to carry the line current. This permits individual segments to be removed from service for maintenance and still permit nearly normal loading of the lines. However, individual segments can fail, thereby changing the network natural frequencies and greatly increasing the number of possible frequencies that can be observed from an individual generator. This increases the work required to document and analyze the network frequencies as seen by each generating station.

Another possible source of subsynchronous currents is the presence in the network of HVDC converter stations. The controls of these converters are very fast in their control of de power, but the controls can have other modes of oscillation that may be close to a natural mode of oscillation of a nearby generator. Systems that include HVDC converters also must be carefully checked to see if these controls might induce subsynchronous currents in the generator stators, leading to torsional interaction.

3) TRANSIENT TORQUES:

Transient torques are torques that result from large system disturbances, such as faults. System disturbances cause sudden changes in the network, resulting in sudden changes in currents with components that oscillate at the natural frequencies of the network. In a transmission system without series capacitors, these transients are always de transients, which decay to zero with a time constant that depends on the ratio of inductance to resistance. For networks that contain series capacitors, and will contain one or more oscillatory frequencies that depend on the network capacitance as well as the inductance and resistance. In a simple radial R-L-C system, there will be only one such natural frequency. If any of these frequencies coincide with the complement of one of the natural modes of shaft oscillation, there can be peak torques that are quite large and these torques are directly proportional to the magnitude of the oscillating current. Currents due to short circuits, therefore, can produce very large shaft torques both when the fault is applied and also when it is cleared. In a real power system there may be many different subsynchronous frequencies involved and the analysis is quite complex.

Of the three different types of interactions described above, the first two, IGE and TI, may be considered as small disturbance conditions, at least initially. The third type, transient torque, is definitely not a small disturbance and nonlinearities of the system also enter into the analysis. From the viewpoint of analysis, it is important to note that the induction generator and torsional interaction effects may be analyzed using linear methods. Eigenvalue analysis is appropriate for the study of these problems and the results of eigenvalue studies give both the frequencies of oscillation and also the damping of each oscillatory mode. The other method used for linear analysis is called the frequency scan method, where the network seen by the generator is also modeled as a function of frequency and the frequency is varied over a wide range of subsynchronous values. This requires that the generator be represented as a tabulation of generator impedance as a function of subsynchronous frequency, which must be provided by the generator manufacturer. This is considered the best model of the generator performance at subsynchronous frequencies, and is often the preferred method of analysis, with eigenvalue analysis used as a complementary check on the frequency scan results.

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SUBSYNCHRONOUS RESONANCE

SUBSYNCHRONOUS RESONANCE

By Engr. Aneel Kumar   April 01, 2015   No comments:

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 networks containing series capacitors, the currents will include oscillatory components with frequencies that depend on the relative magnitude of the transmission line Land C elements, but have frequencies that are below the system fundamental frequency.

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