Need of Lightning Arresternseadmin
High voltage power systems experience over voltages, which are generated through occurrence of faults, switching operations and lightning discharges. The duration of the over voltages vary from a few micro seconds to few seconds depending on the type of surges. Similarly, the magnitude of over voltages varies from 1.5 to 4 times of the normal operating voltages. Under these severe overvoltage conditions, the insulation of the power system/equipments undergoes stresses that could lead to catastrophic failure. Hence it is imperative that the power system equipments are protected from these over voltages at the time of occurrence. Using a device with variable impedance with respect to voltages can provide the protection of power equipments from overvoltage. This kind of overvoltage protection device is connected in parallel to the system/equipments to be protected.
The requirement (Characteristics) of LA is:
- It should take no current during normal power frequency condition. The break down strength should be above the normal power frequency voltage and permissible over voltage.
- Transient Over voltages of value more than insulation flash over or break down level should be diverted to earth.
- The discharge current should not damage the arresters. Lightning arrester should be in a position to absorb the energy without getting damaged.
- The voltage across arrester during discharge (residual voltage) should neither be too low nor too high.
- Normal condition should be restored soon after the surge has been diverted.
Different types of lightning arresters (LA) used in practice are
- Rod-gap LA
- Sphere-gap LA
- Horn-gap type LA
- Modified horn-gap LA
- Expulsion type LA
- Lead oxide type LA
- Thyrite type LA
- Valve type LA
- Metal-Oxide Surge Arrester (MOSA) or gapless LA
Usually, metal oxide (Zinc Oxide) type of Lightning Arrester (LA) is used for protection of system/equipments from overvoltage.
Selection of Lightning Arrester Voltage Rating:
Operating Voltage (COV & MCOV)
The continuous operating voltage (C.O.V.) is the normal sinusoidal power frequency voltage across the arrester terminal continuously. The maximum continuous operating voltage (M.C.O.V.) is the maximum permissible value of a sinusoidal power frequency voltage which may be continuously applied between the arrester terminals.
The MCOV and COV of commercial arresters usually bear a margin of 5 to 10% depending upon the manufacturer. Note that the margin is given based on the harmonic content in the system voltage.
The rated voltage is the maximum power frequency voltage that is applied in the operating duty test for 10 seconds (IEC:99-4).
Rated voltage = (Max. System Voltage) x (Ground Fault Factor)
Ground Fault Factor
- For effectively/Solidly grounded system = 0.8
- For ineffectively grounded system = 1.0
As in the case of COV & MCOV estimation, the margin in the rated voltage calculation is considered based on the harmonic content in the system voltage. However in any case, the margin should not be more than 10%. Use of higher rated arrester increases the capability of the arrester to survive on the power system, but reduces the margin of protection for a specific insulation level.
The arrester selection must strike a balance between arrester survival and equipment protection. Note that the rated voltage is the reference parameter to establish the power frequency voltage versus time characteristic of the arrester (IEC 99-4).
Standard Lightning Impulse
The wave shape of the standard impulse used is 1.2/50 µA.
CONDITION MONITORING OF LA
As per the standard, various techniques are available for the health/condition monitoring of LAs in service. Some of the techniques are mentioned below:
Off Line Arrester Field Testing
Insulation Resistance Measurement
Leakage Current Measuring
Standard Hipot Tester
On Line Arrester Condition Monitoring
AC Leakage Current Meter
Third Harmonic Current Measurement
Partial Discharge Detection
Off Line Arrester Field Testing
Off line field testing of arrester is required if an arrester has been removed from its service location or if it is still in the circuit but has been de-energized for some time. The main issue with off line testing is that to effectively assess an arrester condition, it must be energized near or above its operating voltage.
Surge counters count impulses at currents above certain amplitudes or above certain combinations of current amplitude and duration. If the interval between discharges is very short (less than 50 ms), surge counters may not count every current impulse (and this is quite often the case in multi stroke flash events). Some counters require power follow current that is generally present through Silicon Carbide arresters, and may not count the short impulse currents through metal-oxide arresters.
Depending on the operating principle and sensitivity of the counter, it may give an indication about over voltages appearing in the system, or it may provide information on the number of discharges corresponding to significant arrester energy stresses.
Surge Counter mounted on an arrester pedestal
Typical Surge Counter Face with Analog Leakage Meter and electromechanical counter
Insulation Resistance measurement:
Usually the health of an LA is monitored by periodically measuring the insulation Resistance (IR). The measurement of IR value of an LA apparently gives an indication of degradation due to ingress of moisture. In addition to this, some of the high voltage LAs are provided with an Ammeter connected in series to indicate the total leakage current flowing through the LA while in service.
The measurement of Insulation Resistance of LA does not provide any significant information about the health/degradation of metal oxide elements while in service. The insulation resistance of an LA may remain high even though the LA might be on the verge of failure due to various reasons including the ingress of moisture. Hence, the value of IR of an LA cannot be taken as a criterion for accurate monitoring the condition of an LA.
Leakage Current Measuring:
The optimum off line field test is to apply an AC voltage to the arrester and measure the leakage current. As with the on line monitors, the only leakage current that matters is the resistive leakage current. The total current that is predominately capacitive is not a good indicator of an arresters condition. Therefore any equipment used must be able to discern the total current from the resistive current.
Standard Hipot Tester:
An alternative to the optimum piece of test equipment is to use a standard Hipot tester. Again this can only be accomplished if the arrester Uc rating is below the maximum voltage of the hipot tester. If an AC hipot tester is used, the best the most effective means of assessing the arresters condition is to determine at what voltage the arrester starts heavy conduction. This is also referred to as measuring the arrester Vref. Vref is a term used to quantify the level where an arrester conducts 1-5 ma of resistive current. The method is to energize the arrester until it conducts approximately 1mA. If this level is 5-15% above the Uc rating then the arrester is most likely a good arrester.
On Line Arrester Condition Monitoring
It is desirable to check the emendation of surge arresters at regular time intervals, by measuring the resistance component of the continuous leakage current in service without de-energizing the arrester.
To monitor the healthiness of the lighting arrester, each arrester shall be provided with surge monitor. Surge monitor shall be designed to record directly the number of surges handled by the lightning arrester on a cyclometric counter and also indicates the leakage current passing through the lightning arrester on an ammeter continuously.The design of surge monitor shall be such that in the eventuality of its failure, thelightning arrester base should automatically be connected to the earth system.
Nowadays various devices like Doble’s LCM ll, Siemens’ACM are available in the industry for condition monitoring of Lightning Arrester.
AC Leakage Current Meter:
AC leakage meters are generally an accessory of surge counters. When the readout is an analog meter, the current being read is the total current of the arrester. The total leakage current of the arrester is a combination of the capacitive current and resistive current through the disks and over the external housing of the arrester. If the arrester is equipped with a special ground terminal that isolates the internal from external currents, then just the total internal current can be monitored without the interference of the external surface leakage current. Another advantage of this arrangement as shown in figure is that no insulators are needed on the bottom of the arrester if monitoring the external surface leakage current is not necessary. For very tall and/or arresters in a high seismic region this could be quite an advantage.
Third harmonic resistive leakage current:
More recent vintage and design surge counters with third harmonic current sensing offer significantly more information on the condition of the arrester than earlier generations of surge counters that were designed for SiC (Silicon Carbide) arresters.
The surge amplitude and time are recorded along with leakage current data. From the total current the device calculates the third harmonic of the current. A voltage-current characteristics of a typical metal oxide LA when a sinusoidal voltage is applied to it, is shown in the figure below.
The nonlinear characteristics of Zinc Oxide blocks, then introduce a third harmonic resistive current in the leakage current. This current component is therefore generated by the arrester itself and will be an indicator of changes in the non-linear characteristics of Zinc Oxide blocks for a period of time due to ageing phenomenon.
The resistive current consists of fundamental, third harmonic, fifth harmonic and seventh harmonic components. The harmonic contents depend on the magnitude of the resistive current and on the degree of non-linearity of the voltage current characteristics of zinc oxide blocks. The third harmonic is the largest harmonic contents of the resistive current and most commonly used for monitoring purposes.
Further, it is not only the measurement of third harmonic resistive current for one time, but also a data base for this resistive leakage current are to be built up for monitoring the periodical changes due to normal/abnormal ageing phenomena. Sudden rise in third harmonic resistive current or very high value of third harmonic resistive current indicates degradation of Zinc Oxide blocks and calls for a corrective actions required to be taken in advance in order to prevent a catastrophic failure of LA in service.
Thus, Leakage current monitor is used for measuring the third harmonic resistive components (THRC) of leakage current of LA.
Partial Discharge Detection:
During the life of gapless arresters, the internal components are continually exposed to stresses that can lead to partial discharge. Most arresters with internal air volume (porcelain housed and hollow core designs) will experience partial discharge during rain, fog and sometimes snowy conditions. It is an acceptable condition in most arrester designs for this to occur. However during dry periods, arresters should not experience partial discharge. Partial discharge within an arrester can lead to dialect failure of insulating materials. Because internal partial discharge (PD) in an arrester is an undesirable condition, detection systems have been developed to locate internal PD and give arrester users the ability to proactively mitigate the issue. Fortunately for arrester users interested in this type of assessment it is very similar to undesirable conditions in other high voltage equipment. This means the same equipment can be used for more than just arrester assessment. Because this type of assessment equipment is not just a special arrester too, there is a wider array of on line and field oriented PD detection equipment. The IEC and IEEE standards both require that no more than 10 pico-coulombs (pC) be present in the arrester.
This form of arrester condition assessment is a very fast and effective. Within seconds, an infrared detector can determined if there is a critical arrester condition to be concerned with when entering a substation. If an arrester is in a long term failure mode and is nearing its end of life, there is a high probability that it will be hot. A hot arrester can be detected from a hundred meters away with even the simplest of infrared detecting equipment.
Arrester condition assessment is still a developing area with many options available and may more opportunities ahead of us. As the smart grid concept evolves, these assessment tools will become mandatory and not just for critical areas.
We, NS Engineers have expertise in providing services for Testing, Commissioning, condition monitoring, predictive maintenance, Power system Analysis, Relay Coordination Study, Arc Flash Study and Sub Station Automation Solutions. In case of any such requirement contact us on email@example.com