Sources of over voltages power quality.pptx

Voltage source

1. SOURCE OF OVER VOLTAGES

2. Causes of Transient Over voltages Capacitance switching Lightning Ferro resonance

3. Capacitor switching Utility capacitor switching: While this is one of the most common switching events on utility systems , it is one of the main causes of oscilltory transients. This Transient propagate into the utility’s local power system ,pass through its distribution transformers ,and enter into the end-user’s load facilities. The extent of the transient’s energy is a function of the turn’s ratio of the transformer feeding the end-user facility.

4. Common symptom that directly relates to utility capacitor switching over voltages is that the resulting oscillatory transients appear at nearly identical times each day. This is because electric utilities , in anticipation of an increase in load, frequently switch their capacitors buy time clock.

5. There’s a potential side effect to this scenario: Magnification of the utility capacitor switching transients. this can occur when the end-user adds power factor correction capacitors at its facility. One way to counter this potential problem is fr the electric utility to use synchronous closing breakers or switches with pre-insertion resistors.

6. Appearing as an inductance in series with capacitors, these filters decrease transient voltages at the end-user bus to acceptable levels. This solution has multiple benefits: 1)correction for displacement power factor 2)control of harmonic distortion levels 3)limitation of magnified capacitor-switching transients To correct power system problem that affect susceptible electronic equipment , you must identify the source or sources of these problems.

7. Capacitor Switching Transients (With Power Factor Correction Capacitors) They are switched in and out depending on the level of support needed at any one time. When the utility energizes a discharged capacitor, the bus voltage will momentarily collapse. This occurs because the voltage across a capacitor cannot change instantaneously. The overshoot associated with this oscillation can result in a peak voltage that has a theoretical peak value of two times the maximum value of the 50 Hz/60 Hz sine wave. The same effect can occur when a capacitor is switched off, if re-strike occurs during the switching operation (see Figure 3.7).

8. Voltage Magnification: Transients of this magnitude and duration are usually not a problem on the utility system, but they can produce problems at a user facility. Severe over voltages can appear on facility capacitors through a phenomenon known as voltage magnification. The voltage at the end-user capacitor can be greater than the voltage at the utility capacitor. This translates to a peak voltage with a theoretical upper value of 400%, although this is rarely seen. Magnification occurs when the predominant frequency of the switching transient is approximately equal to the resonant frequency of the LV system With reference to Figure 3.7, this situation can be represented mathematically as

9. Voltage magnification occurs if f1=f2 The IEEE Standard for Shunt Power Capacitors, ANSI/IEEE Std. 18-1992 specifies that capacitors "may reasonably be expected to withstand" transient over voltages from 205 % to 354% of rated peak voltage, depending on the number of times a year the overvoltage occurs.

10. Generally speaking, the voltage magnification will not result in capacitor damage. The problem that usually occurs is the failure or mis -operation of sensitive loads in the facility where the low voltage capacitors are installed. Calculation:

11. Capacitor bank switching transients concern: Capacitor bank energizing transients are becoming increasingly more important installations in power systems. This is bank switching is one of the most frequent utility operations potentially occurring multiple times per day and hundreds of time per year throughout the system. Transient-related currents and voltages on a power system associated with utility capacitor bank installations include: Voltage transients at the capacitor bank substation and neighboring substations This may include phase-to-ground and Phase-to-phase over voltages due to voltage magnification. Power quality impact on sensitive customer loads due to variations in voltage when energizing capacitor banks. Capacitor bank energization inrush currents . Capacitor bank out-rush currents due to faults in the vicinity of capacitor banks.

12. Lightning Lightning is an electrical discharge in the air between clouds, between different charge centre within the same cloud, or between cloud and earth (or earthed object). When lightning strikes occur in or near an electricity distribution system, lightning currents are generated and conducted through the power system into connected equipment. Large impulsive transient over voltages are produced as a result of this current flow. Lightning can strike directly to the phase conductors of overhead power lines producing very high magnitude transient over voltages. Peak current can be up to 200 kA with voltages over 1 MV.

13. The mechanism of lightning: Lightning mostly occurs on summer days when the ambient temperature is high and the air is humid . because of the temperature difference , the moist air is lifted to higher altitudes with a considerable lower ambient temperature . The strength of this atmospheric field is on summer days in the order of 60 V/m and can reach values of 500 V/m on a dry winter day . The strong electric field initiates discharges inside the cloud, and a negative stream of electrons emerges as a dim spark called a stepped leader or dart leader that jumps in steps of approximately 30 meters and reaches the earth in about 10 milliseconds. The main discharge current can be 100kA or more.

14. Ferroresonance Ferro resonance is a special case of series LC resonance where the inductance involved is nonlinear and it is usually related to equipment with iron cores. Ferro resonance is a general term applied to a wide variety of interaction between capacitors and iron-core inductors that result in unusual voltage and/or currents. In linear circuits, resonance occurs when the capacitive reactance equals the inductive reactance at the frequency at which the circuit driven.

15. The phenomena of ferroresonance is a name given to a situation where the nonlinear magnetic properties of iron in transformer iron core interact with capacitance existing in the electrical network to produce a nonlinear tuned circuit with an unexpected resonant frequency. Ferro resonance is a mysterious phenomenon that seems to occur randomly, It generally occurs during a system imbalance, usually during switching, that places capacitance in series with transformer magnetizing impedance. This can result in high over voltages that can cause failures in transformers, cables, and arresters. Any system capacitance can be involved in ferro resonance, but the major concern is about underground cable capacitance .

16. In power system the ferroresonance occurs when a nonlinear inductor is fed from a series capacitor. The nonlinear inductor in power system can be due to: a)The magnetic core of a wound type voltage transformer. b)Bank type transformer c) The complex structure of a 3 limb three-phase power transformer (core type transformer) d) The complex structure of a 5 limb three-phase power transformer (shell- type transformer) The circuit capacitance in power system can be due to a number of elements such as: The circuit-to-circuit capacitance ,Parallel lines capacitance , Conductor to earth capacitance ,Circuit breaker grading capacitance , busbar capacitance ,Bushing capacitance

17. Some problems associated with ferroresonance are as follows: Overheating Audible Noise High over voltages and surge arrester failure

18. A typical case of ferroresonance found in transformer energizing ,mainly in delta- wye connections,when only one of the poles of the switch is closed , as shown in figure.3.10.

19. Ferroresonance is resonance phenomena between a capacitance and iron con which is nonlinear inductor. In power system the iron core inductor is usually the magnetizing branch of a transformer. The capacitance could be the natural capacitance of the overhead line (or underground cable). The series circuit is established during single phase switching condition (including the energization and deenergization switching) and its occurrence is dependent on the configuration of the circuit, such as the delta connection of a transformer winding an with certain length of lines.

20. Under normal operating conditions wL > 1/ wC ,However when wL =1/ wC ferroresonance occurs. In power systems the following circuit (shown in Fig 3,11) is Susceptible to cause the ferroresonance problem. The circuit can be simplified as following series resonance circuits in Figure 3.12.