The following points highlight the five main devices used for protection from over-voltage. The devices are: 1. Surge Arresters and Transient Voltage Surge Suppressors 2. Isolation Transformers 3. Low-Pass Filters 4. Low Impedance Power Conditioners 5. Utility Surge Arresters.
Device # 1. Surge Arresters and Transient Voltage Surge Suppressors:
Arresters and TVSS devices protect equipment from transient over-voltages by limiting the maximum voltage, and the terms are sometimes used interchangeably. However, TVSSs are generally associated with devices used at the load equipment. A TVSS will sometimes have more surge limiting elements than an arrester, which most commonly consists solely of MOV blocks. An arrester may have more energy handling capability; however, the distinction between the two is blurred by common language usage.
The elements that make up these devices can be classified by two different modes of operation, crowbar and clamping.
Crowbar devices are normally open devices that conduct current during over-voltage transients. Once the device conducts, the line voltage will drop to nearly zero due to the short circuit imposed across the line. These devices are usually manufactured with a gap filled with air or a special gas.
The gap arcs over when a sufficiently high over-voltage transient appears. Once the gap arcs over, usually power frequency current, or “follow current,” will continue to flow in the gap until the next current zero. Thus, these devices have the disadvantage that the power frequency voltage drops to zero or to a very low value for at least one-half cycle. This will cause some loads to drop offline unnecessarily.
Clamping devices for ac circuits are commonly non-linear resistors (varistors) that conduct very low amounts of current until an over-voltage occurs. Then they start to conduct heavily, and their impedance drops rapidly with increasing voltage. These devices effectively conduct increasing amounts of current (and energy) to limit the voltage rise of a surge. They have an advantage over gap type devices in that the voltage is not reduced below the conduction level when they begin to conduct the surge current. Zener diodes are also used in this application.
Example characteristics of MOV arresters for load systems are shown in Figs. 3.17 and 3.18. MOV arresters have two important ratings.
The first is maximum continuous operating voltage (MCOV), which must be higher than the line voltage and will often be at least 125 percent of the system nominal voltage. The second rating is the energy dissipation rating (in joules). MOVs are available in a wide range of energy ratings. Figure 3.18 shows the typical energy- handling capability versus operating voltages.
Device # 2. Isolation Transformers:
Figure 3.19 shows a diagram of an isolation transformer used to attenuate high frequency noise and transients as they attempt to pass from one side to the other. However, some common mode and normal mode noise can still reach the load. An electrostatic shield, as shown in Fig. 3.20, is effective in eliminating common mode noise.
However, some normal mode noise can still reach the load due to magnetic and capacitive coupling.
The chief characteristic of isolation transformers for:
1. Electrically isolating the load from the system for transients is their leakage inductance.
2. High frequency noise and transients are kept from reaching the load, and any load generated noise and transients are kept from reaching the rest of the power system.
3. Voltage notching due to power electronic switching is one example of a problem that can be limited to the load side by an isolation transformer.
4. Capacitor switching and lightning transients coming from the utility system can be attenuated, thereby preventing nuisance tripping of adjustable speed drives and other equipment.
5. An additional use of isolation transformers is that they allow the user to define a new ground reference, or separately derived system. This new neutral-to-ground bond limits neutral-to-ground voltages at sensitive equipment.
Device # 3. Low-Pass Filters:
Low-pass filters use the pi-circuit principle to achieve even better protection for high-frequency transients. For general usage in electric circuits, low-pass filters are composed of series inductors and parallel capacitors. This LC combination provides a low impedance path to ground for selected resonant frequencies. In surge protection usage, voltage clamping devices are added in parallel to the capacitors. In some designs, there are no capacitors.
A common hybrid protector combines two surge suppressors and a low-pass filter to provide maximum protection. It uses a gap type protector on the front end to handle high energy transients. The low-pass filter limits transfer of high frequency transients. The inductor helps block high frequency transients and forces them into the first suppressor. The capacitor limits the rate of rise, while the nonlinear resistor (MOV) clamps the voltage magnitude at the protected equipment.
Device # 4. Low Impedance Power Conditioners:
Low impedance power conditioners (LIPCs) are used primarily to interface with the switch mode power supplies found in electronic equipment. LIPCs differ from isolation transformers in that these conditioners have a much lower impedance and have a filter as part of their design.
The filter is on the output side and protects against high frequency, source side, common mode, and normal mode disturbances (i.e., noise and impulses). Note the new neutral-to-ground connection that can be made on the load side because of the existence of an isolation transformer. However, low to medium frequency transients (capacitor switching) can cause problems for LIPCs: The transient can be magnified by the output filter capacitor.
Device # 5. Utility Surge Arresters:
The three most common surge arrester technologies employed by utilities are depicted in Fig. 3.23. Most arresters manufactured today use a MOV as the main voltage limiting element. The chief ingredient of a MOV is zinc oxide (ZnO), which is combined with several proprietary ingredients to achieve the necessary characteristics and durability. Older technology arresters, of which there are still many installed on the power system, used silicon carbide (SiC) as the energy dissipating non-linear resistive element.
The relative discharge voltages for each of these three technologies are shown in Fig. 3.24.
Originally, arresters were little more than spark gaps, which would result in a fault each time the gap sparked over. Also, the sparkover transient injected a very steep fronted voltage wave into the apparatus being protected, which was blamed for many insulation failures. The addition of a SiC non-linear resistance in series with a spark gap corrected some of these difficulties. It allowed the spark gap to clear and reseal without causing a fault and reduced the sparkover transient to perhaps 50 percent of the total sparkover voltage [Fig. 3.24(a)].
However, insulation failures were still blamed on this front-of-wave transient. Also, there is substantial power-follow current after sparkover, which heats the SiC material and erodes the gap structures, eventually leading to arrester failures or loss of protection. Gaps are necessary with the SiC because an economical SiC element giving the required discharge voltage is unable to withstand continuous system operating voltage. The development of MOV technology enabled the elimination of the gaps. This technology could withstand continuous system voltage without gaps and still provide a discharge voltage comparable to the SiC arresters [see Fig. 3.24(b)].
By the late 1980s, SiC arrester technology was being phased out in favour of the gapless MOV technology. The gapless MOV provided a somewhat better discharge characteristic without the objectionable spark over transient.
The majority of utility distribution arresters manufactured today are of this design. The gapped MOV technology was introduced commercially about 1990 and has gained acceptance in some applications where there is need for increased protective margins. By combining resistance graded gaps (with SiC grading rings) and MOV blocks, this arrester technology has some very interesting, and counterintuitive, characteristics.
It has a lower lightning discharge voltage [Fig. 3.24(c)], but has a higher transient over voltage (TOV) withstand characteristic than a gapless MOV arrester. To achieve the required protective level for lightning, gapless MOV arresters typically begin to conduct heavily for low frequency transients at about 1.7 pu. There are some system conditions where the switching transients will exceed this value for several cycles and cause failures. Also, applications such as aging underground cable systems demand lower lightning discharge characteristics.
The gapped MOV technology removes about one-third of the MOV blocks and replaces them with a gap structure having a lightning sparkover approximately one- half of the old SiC technology. The smaller number of MOV blocks yields a lightning- discharge voltage typically 20 to 30 percent less than a gapless MOV arrester. Because of the capacitive and resistive interaction of the grading rings and MOV blocks, most of the front-of-wave impulse voltage of lightning transients appears across the gaps. They spark over very early into the MOV blocks, yielding a minor spark over transient on the front.
For switching transients, the voltage divides by resistance ratios and most of it appears first across the MOV blocks, which hold off conduction until the gaps spark over. This enables this technology to achieve a TOV withstand of approximately 2.0 pu in typical designs. Additionally, the energy dissipated in the arrester is less than dissipated by gapless designs for the same lightning current because of the lower voltage discharge of the MOV blocks. There is no power follow current because there is sufficient MOV capability to block the flow. This minimizes the erosion of the gaps. In several ways, this technology holds the promise of yielding a more capable and durable utility surge arrester. Utility surge arresters are manufactured in various sizes and ratings.
The three basic rating classes are designated distribution, intermediate, and station in increasing order of their energy-handling capability. Most of the arresters applied on primary distribution feeders are distribution class. Within this class, there are both small block and heavy duty designs. One common exception to this is that sometimes intermediate or station class arresters are applied at riser poles to obtain a better protective characteristic (lower discharge voltage) for the cable.