AC equipments are subjected to occasional lightning surges, switching surges and power frequency overvoltages. Upto voltage levels of 345 kV lightning surges are more dangerous and after that switching surges which are consided to be 2.7 p.u. wheareas lightning level is around 900 kV. Each equipment has certain assigned withstand levels as:
- Basic Lightning Impulse Withstand Level.
- Switching Impulse Withstand Level.
- Power Frequency Voltage Withstand Level.
Basic impulse insulation levels are reference levels expressed in impulse crest volatge with a standard wave not longer than 1.2/50 ms wave. Apparatus insulation should be greater than basic insulation level. The basic impulse insulation level are given below:
|Reference Class kV||Standard Basic Impulse Level kV|
As per study conducted, the causes of electrical disturbances, from maximum to minimum, in a typical network are:
|1. Satic High and Low Voltage:||33 %|
|2. Direct Lightning / Indirect Lightning and Related Disturbances:||30 %|
|3. Internal Generated Surges:||22 %|
|4. Harminics and Grid Generated Noise:||12 %|
|5. EMI / RFI:||02 %|
|6. NEMP:||01 %|
Also the reasons of system failure are:
|1. Fire:||05 %|
|2. Sundry:||05 %|
|3. Poor Quality:||07 %|
|4. Improper Handling:||17 %|
|5. Natuaral and Artificial Electrical Disturbences:||66 %|
So electrical disturbances are a major cause of system failure and if the equipments are not protected against these abnormal voltages, they may cause damage to the insulation and can result in shutdown of the system. The protection against lightning and other surges are divided into two classes:
External Protections: These are used to protect equipment against external overvoltages, mainly due to lightning and are further subdivided into shilding methods and non-shielding methods. In shielding method arc path is not allowed to form between the line conductor and ground, there by giving inherent protection. Use of ground wire on power lines and lightning conductor for protection of building are the example of shielding method. We will discuss only lightning conductor in subsequent pages.
In non shielding method an arc path is allowed between the line conductor and ground structure but methods are provided to quench the arc.
- Internal Protections: These includes lightning and surge protection and equipotential bonding
Effects of Voltage Surges:
- Direct strike on buildings without an external lightning protection installation: It results in fire hazard due to specific energy charge. If an electric current flows as a consequence of a lightning, energy is converted, heating the part of the building through which the current is conducted to earth. In addition material may be melted or ejected at the foot of the strike.
- Direct strike on an overhead HV Line: It causes voltage surges due to maximum lightning impulse current. For a lightning strike on an overhead line, the impedance at the first instant is determined by the surge impedance of the line. U = ZW × I/2. This impedance ZW is usually in the region of 400 to 500 Ω for a single conductor.
- Direct strike on LV Line: It causes voltage surges and partial lightning currents due to maximum lightning current. The precondition of a direct strike on a LV OH lines are not the same as for direct strike on HV OH lines. The fundamental difference here compared to HV line is the proximity to the building through which the conduction of partial lightning current is possible.
- Direct strike on a building with an lightning protection system w/o equi-potential bonding: It causes overvoltages and insulation brekdown due to maximum lightning impulse current. If the lightning currents are diverted to earth, there will be a voltage rise at the earthing point of the installation. This will be carried into the house via the equipotential bonding system. Adequate equipotential bonding for lightning protection prevents damage to the electrican installation. U = i × Rearth.
- Lightning strike to a tree (close strike): It results in coupling of lightning current into earth lines, inductive and galvanic coupling and fire hazard, if the strike is close enough. If current flows as a result of lightning strike to a tree, energy is converted and result is fire hazard. In addition, current diverted to earth can be coupled into the earth lines or the nearby building.
- Inductive coupling: It results due to maximum steepness (di/dt) of the lightning current. A magnetic field forms around every conductor through which current flows. If conductor loops are located in the vicinity of a conductor in which lightning current is flowing, a voltage will be induced.
- Switching actions: It causes overvoltage on network lines due to energy being returned to the network. Switching actions occur almost everywhere work is done with electrical energy, but especially vulnerable are areas where large inductive loads are switched for example motors, transformers, chokes, welding equipments and long runs of fluorescent tubes.