Surge arresters are voltage limiting devices used to protect electrical insulation from voltage spikes (surges) in a power system. The job of a surge arrester is to protect the system from damage due to overvoltage conditions.
In the past surge arresters were called Lightning Arresters, this name was based on their primary function of protecting electrical insulation from lightning strikes on the system. The more generic term ‘Surge Arrester’ is now used to encompass overvoltage conditions, which can occur from numerous other sources, such as switching operations and ground faults.
In modern era, gapless ZnO or zinc oxide surge (MOSA) arresters are mainly used for surge protection.
Surge arrester working principle
When a voltage surge traveling along the conductor reaches the point at which a lightning arrester is installed it breaks down the insulation of the arrester momentarily, allowing the voltage surge to discharge to ground. As soon as the system voltage drops below the predetermined value, insulation between the conductor and ground is restored and further current flow to ground stops. (Refer figure 2)
To perform this protective function satisfactorily, arresters must
- Not allow current to flow to the ground as long as the system voltage remains normal.
- Provide a path to ground, when the system voltage rises to a predetermined value above normal, to dissipate the energy from the surge without raising the voltage at which the circuit is operating.
- Stop the flow of current to ground, as soon as the system voltage drops below the predetermined value, and restore the insulating qualities between the conductor and ground.
- Not be damaged by the discharge and be capable of automatically repeating discharging process frequently when required in line with energy rating and as per operation duty test.
The performance of any arrester is dependent on a good connection to ground. Arresters will not function without a proper ground; they are totally useless. The arrester should be placed as close as possible to the equipment, that is to be protected and leads connecting arresters to ground should be kept as short as possible & without any loop.
What exactly does a surge arrester do?
- It does not absorb the lightning
- It does not stop the lightning
- It does divert the lightning to ground
- It does clamp (limit) the voltage produced by the lightning or switching (Refer Figure 3)
- It only protects equipment electrically in parallel with it.
Over voltage in power system
There are several instances when the elements of power system (e.g. generators, transformers, transmission lines, insulator are subjected to overvoltages i.e. voltages greater than the normal value. These overvoltages on the power system may be caused due to many reasons such as lightning, the opening of a circuit breaker, the grounding of a conductor etc.
Most of the overvoltages are not of large magnitude but may still be important because of their effect on the performance of circuit interrupting equipment and protective devices. An appreciable number of these overvoltages are of sufficient magnitude to cause insulation breakdown of the equipment in the power system. Therefore, power system engineers always device ways and means to limit the magnitude of the overvoltages produced and to control their effects on the operating equipment. These are known as a voltage surge or transient voltage.
According to the location of overvoltage, it can be divided into two categories, one is external overvoltage, and the other is internal overvoltage. The external overvoltage, also known as lightning overvoltage, is divided into direct lightning and induced lightning. The internal overvoltage mainly includes power frequency overvoltage, resonance overvoltage and switching overvoltage. The classification is shown in Figure 4.
According to the duration of overvoltage, it can be divided into temporary overvoltages and transient overvoltages. Temporary overvoltages refer to power frequency overvoltage with long duration, while transient overvoltages last for a short time, only a few mS, even µS level, which can be highly damped oscillation wave or non-oscillation wave.
Temporary overvoltages mainly include power frequency overvoltage and resonance overvoltage. The duration of these overvoltages is relatively long, which is related to the structure, capacity, parameters, operation mode, fault conditions of the power system, and device characteristics of various safety protection devices.
Transient overvoltages include switching overvoltage and lightning overvoltage. The duration of these overvoltages is relatively short. It can be subdivided into fast wave front overvoltage with wave front time between 0.1µS and 20µS and slow wave front overvoltage with a peak time between 20us and 5000us. The classification is shown in Figure 5.
The typical waveform of overvoltage is as follows, as shown in Figure 6.
Surge arrester general selection criteria
The objective of arrester application is to select the lowest rated surge arrester which will provide adequate overall protection of the equipment insulation – and have a satisfactory service life when connected to the power system. The arrester with the minimum rating is preferred because it provides the greatest margin of protection for the insulation.
A higher rated arrester increases the ability of the arrester to survive on the power system, but reduces the protective margin it provides for a specific insulation level. Both arrester survival and equipment protection must be considered in arrester selection. It is important to understand the Voltage Current Characteristics of ZnO Block of surge arrester.
Figure 7 below shows the V-I characteristics of ZnO element, which is divided into three regions, being low current region (A), operating region (B) and high current region (C).
The class of lightning arrester to be applied depends upon the importance and value of the protected equipment, its impulse insulation level and the expected discharge currents the arrester must withstand. Protection class as per IEC standard is given in table 1.
Overall surge arrester selection method is depicted in the figure 8 & margin of protection is illustrated in figure 9 respectively.
Surge arrester voltage selection criteria
Once the system voltages are understood, the next step in the selection process is to determine the system configuration to which the arrester will be applied. In other words, one must determine if it is a wye or delta system (star or delta). Also needed for selection is to know how the system neutral conductor is used in the circuit if there is one. The power source transformer and the neutral bonding scheme determine how high the line to ground voltage of the unfaulted phases will rise during a ground fault. Fortunately the number of system configurations are limited.
The most common configuration is the 4 wire solid multi-grounded neutral as shown in figure 10. This is also known as an effectively grounded system.
A common industrial and very common configuration is the 3 wire impedance grounded wye (or star). The reason for popularity of this system is that the fault current to earth is limited by the impedance. When low impedance is used, it can limit the fault current to levels that allow for lower fault current rated equipment to be used on the system. This is often a cost savings configuration. When the impedance is high, a Petersen coil is used, which can offer fault extinguishing capabilities without using breakers to break the fault. This is sometimes referred to as a resonant grounded system. (Refer figure 11).
A third common system configuration is an isolated or ungrounded system. This can be either delta or wye configured. Figure 12 shows these two systems.
A common transmission line configuration is the single grounded Wye as seen in Figure 13.
Determining phase voltage rise due to earth or ground faults
When a three phase power system experiences a fault to earth on any one of its phases, the two unfaulted phases experience an increase in the voltage between the phase and ground. Since arresters are most often applied between the phase conductor and earth, then they also see this increase in voltage across their terminals. This increase in voltage will remain across the arrester until a system breaker operates and breaks or interrupts the fault. This is a very significant event in the life of an arrester and must be accounted for during the voltage rating selection of an arrester.
The determination of a voltage rise during a ground fault is not an easy task if a precise value is desired. There are some rules of thumb and graphs that can be used, but these are quite crude and difficult at best to use.
For distribution systems where the system and transformer impedances are relatively unknown, a worst case scenario is used for each type of system. The voltage rise during a fault in these cases is determined by multiplying the line to ground voltage by a ground fault factor or earth fault factor.
Table 2 lists the ground fault factors used to determine the unfaulted phase voltage rise during a ground fault.
Note 1: Two factors may be used to measure this type of overvoltage.
- Coefficient of grounding (COG)
- Earth fault factor (EFF)
where V’LN is the maximum phase-to-ground voltage on the unfaulted phases during a fault, and VLN, VLL, are respectively the nominal phase-to-neutral and phase-to-phase voltages.
Obviously: EFF = 1.73 COG
Note 2: Mixed Configurations
It is also important to note that the grounding of the neutral at the source transformer is the configuration referred to in determining the voltage rise of the system.
For example as seen in Figure 14, a delta/delta transformer is tied to a solidly grounded wye system. In this case MOV1 should be sized for a solidly grounded system, and MOV 2 should be sized for an isolated ground system.
Using the TOV curve to select an arrester’s MCOV
After the system configuration and potential overvoltage is determined (refer figure 15), it must be compared to the arrester TOV curve. Figure 16 shows TOV curves of several types of arresters. Figure 17 shows a comparison of system overvoltage and arrester TOV capability.
In the example in Figure 16, the selected distribution arrester would not withstand an overvoltage of an ungrounded or delta system, but would withstand an overvoltage from a uni-grounded and multi-grounded system. However, if a gapped MOV arrester was selected, it could withstand even an ungrounded system overvoltage.
For distribution systems, the process of comparing the potential system overvoltage and the arrester withstand capability is seldom completed because the time of the overvoltage is unknown. Because of this issue, for all systems other than the multi grounded system, the MCOV or Uc of the arrester is selected to equal or exceed the line to line voltage.
For substation applications, the comparison of the potential system overvoltage and the arrester overvoltage withstand capability is essential in selecting the arrester MCOV or Uc. In the case of transmission systems and substations, the expected system overvoltage magnitude and duration are known quantities so this comparison is quite accurate.
Transmission line arresters
The selection of Transmission Line Arresters (TLA) MCOV rating or Uc rating is different than a distribution or substation arrester. In the case of TLA’s the objective is to only protect insulators from the undesirable back flash during a switching or lightning surge. Since overhead insulators are generally a self-restoring type of insulation it is not imperative to have the lowest possible clamping voltage for the arrester to mitigate flashover. Sometimes it is also desirable to size the arrester so that it does not absorb any significant energy during a switching surge. In this case increasing the MCOV or Uc rating is an effective means to do just this. However, if the TLA is being applied to mitigate switching surges, then the arrester MCOV should be similar to that of the substation arresters.
Calculation of MCOV & rated voltage of surge arresters: The calculations involved in determining the MCOV and rated voltage as well as selected values in utilities are given in table 3 for reference.
Conclusion
The system grounding configuration determines the overvoltages that can occur during a fault to ground. A single phase-to-ground fault shifts the ground potential at the fault location, depending on the severity of this shift on the grounding configuration. On a solidly grounded system with a good return path to the grounding source, the shift is usually negligible. On an ungrounded system, a full offset may occur and the phase-to-ground voltage on the unfaulted phases approaches the phase-to-phase voltage.
Dr. Rajesh Kumar Arora obtained the B. Tech. & Master of Engineering (ME) degrees in Electrical Engineering from Delhi College of Engineering, University of Delhi. He completed his PhD in grounding system design from UPES, Dehradun. He is also a certified Energy Manager and Auditor. Presently he is working in D&E (Design and Engineering) department of Delhi Transco Limited (DTL). His research interests include high voltage technology, grounding system, protection system, computer application and power distribution automation.
