TRANSMISSION AND DISTRIBUTION Surge protection of distribution equipment by Glen Medlin, e-LEK Engineering This article examines the over-voltage protection of distribution equipment such as pole-top transformers and cable circuits using gapped zinc-oxide surge arresters, and also considers the environmental benefits of using gapped arresters versus non-gapped arresters. Surge arresters are constructed from materials that exhibit a strong non-linear voltage-current relationship. This means that the resistance of the arrester should be dependent on the voltage applied across the block. To begin with, an ideal arrester should have infinite resistance when the voltage across it is normal and zero resistance when the voltage exceeds the turn on voltage. We do, however, live in a real world where this ideal voltage-current relationship is difficult to achieve. The first over-voltage protection devices w e r e s i m p l e s p a r k g a p s, w h e r e t h e spacing was designed so that the gap broke down during surge conditions. The main drawback of such a gap is it could not quench the power frequency arc and required some form of upstream protection to trip the line and remove the fault. Later, expulsion tubes were added to the gaps to provide a mechanism to interrupt the power frequency arc. However, these tubes had limited fault current extinguishing capabilities. The fast breakdown of simple spark gaps can also cause inter-turn faults in transformer windings due to the extremely high rates of voltage change associated with spark gap breakdown. The next innovation was the gapped silicon carbide arrester which used blocks of silicon carbide in series with a spark gap. Silicon carbide exhibited a non-linear voltage-current relationship, but at normal voltage, the resistance across the arrester element was too low and permitted hundreds of Amps to flow to ground (Fig. 1). A series gap was thus added so that the entire voltage appeared across the gap in normal conditions. When a surge occurred, the gap would break down and the surge would be conducted to ground through the silicon carbide blocks. After the surge passed, the silicon carbide blocks would limit the current to a value which the gap could naturally extinguish. This enabled the arrester to interrupt the power frequency current after a surge, regardless of the available fault current. An improvement on the arrester concept arrived with the discovery that zinc oxide (ZnO) had a more favourable non-linear voltage-current relationship than silicon carbide. Zinc oxide blocks could withstand the entire voltage with very little leakage Fig. 1: Comparison between V-I characteristics of ZnO and SiC [2]. Fig. 2: Construction and voltage distribution of gapped SiC, non-gapped MOV and gapped MOV [2]. current and this permitted the removal of the series spark gap, creating the so called gapless arrester or metal oxide varistor (MOV) (Fig. 1). However, the energize - May 2009 - Page 12 gapless arrester is required to operate at or below a specific voltage since should this voltage be exceeded, the arrester begins to conduct. Zinc oxide also has a positive TRANSMISSION AND DISTRIBUTION temperature coefficient, which means that the hotter it gets, the more conductive it becomes. This property can lead to a condition known as thermal runaway where the temperature of the arrester and the current through the arrester grow until failure occurs. Gapless zinc oxide arresters need to be sized to withstand temporary over voltage without entering thermal runaway. This requires compromise since, to withstand the temporary overvoltage, a higher rated arrester needs to be specified. However, a higher rated arrester has a higher discharge voltage and this erodes the protective margin, which is the difference between the basic insulation level (BIL) of the equipment and the peak discharge voltage of the arrester. The latest innovation in arrester technology combines the spark gap from the silicon carbide arrester with the zinc oxide elements of the gapless arrester. The arrester blocks and the spark gaps share the voltage in steady state conditions by using resistively graded gaps. This combination improves the performance of the arrester in impulse and temporary over-voltage conditions. Fig. 2 shows how the technologies were combined. Fig. 3: TOV capability of non-gapped MOV [3]. Steady state operation of gapped MOV arresters During steady state conditions where the voltage across the arrester is the normal system line to ground voltage, U n, the voltage is shared equally between the spark gap structure and the MOV disks. This allows a reduction in the number of spark gaps as they are now only required to withstand half the voltage compared to the silicon-carbide design. The number of MOV disks is also reduced since they are required to withstand half the voltage compared to gapless arrester designs. The voltage sharing is achieved by using silicon carbide grading rings. Temporary overvoltage ( TOV) operation of gapped MOV arresters When the voltage across the gapped MOV arrester is increased during TOV conditions, the voltage sharing between the MOV elements and the gap structure changes from the 50/50 distribution found during normal conditions. The voltage across the gap structure increases faster than the voltage across the MOV disks and this enables the arrester to withstand the TOV better than non-gapped designs. The change in voltage distribution is caused by the silicon carbide resistive grading rings responding differently to over-voltage when compared to the MOV elements. To explain, the non-linearity of the voltagecurrent characteristic of silicon carbide is lower than that of zinc oxide, which means that the resistance of silicon carbide, as a function of the applied voltage, decreases at a lower rate than zinc oxide. Therefore, as voltage across the gapped arrester increases, more voltage begins to appear Fig. 4: TOV capability of gapped MOV [3]. across the gap rather than the zinc oxide disks. The zinc oxide disks do not begin to conduct and therefore cannot enter thermal runaway. Fig. 3 shows the TOV capability of a non-gapped MOV arrester, while Fig. 4 shows the TOV capability of a gapped MOV arrester. Impulse operation of gapped MOV arresters The gapped MOV arrester design has fewer zinc oxide disks in series than the non gapped arrester of the same rating and this causes a desirable reduction is the discharge voltage for all discharge currents. The gapped MOV arrester also uses fewer spark gaps than the silicon carbide design and this causes a reduction energize - May 2009 - Page 13 in the front-of-wave sparkover value compared to silicon carbide arresters. There is generally a slight increase in the 50 Hz sparkover compared to silicon carbide, which is a result of the superior TOV capability of the gapped arrester. The front of wave sparkover is a result of the spark gap and the values are a function of the design of the spark gap. The voltage sharing between the zinc-oxide disks and the spark gap result in lower sparkover values during impulse conditions since it has fewer gaps and a lower voltage across it at the instant of sparkover compared to silicon carbide. Fig. 5 shows the voltage and current discharge characteristics for gapped silicon-carbide, non-gapped MOV and gapped MOV. It can be seen TRANSMISSION AND DISTRIBUTION For example, a new 22 kV transformer with no excess arrester lead length and protected by a 24 kV rated gapless surge arrester will have 65% protective margin for a 40 kA 8/20 µs current impulse, assuming a BIL of 150 kV. This may appear sufficient, but two factors will tend to reduce this margin: Excess arrester lead length If t h e ov e r h e ad l i n e i s take n to th e transformer bushing first, and then to the arrester terminal, then the inductive volt drop across the connection lead will add to the discharge voltage of the transformer. The ground connection of the surge arrester is, in almost all cases, in series with the arrester and thus also adds to the discharge voltage. The inductive volt drop is a function of the inductance per unit length, length of the connection lead and the rate of change of current as per the well known formula: (2) Where L is the inductance of the connection wire (determined primarily by diameter and length) For example, if we have a 1 cm diameter aluminium surge arrester lead of 1 m length, then the approximate volt drop for an 8/20 µs 40 kA current surge will be 5,2 kV. Faster surges will increase the volt drop further. For our previous example, this additional voltage will drop the protective margin to 56%. In general, this effect becomes more significant on the lower voltage classes as the inductive volt drop is small in comparison to the discharge voltage for higher voltages. Fig. 5: Discharge current and voltage characteristics for Gapped SiC, non-gapped MOV and gapped MOV [2]. that the peak discharge current is lower for the gapped MOV compared to the older technologies. Applications of gapped MOV arresters The gapped MOV arrester has application in both the overhead and underground distribution network, where it will provide lower discharge voltages and hence better protective margin, and an improved TOV withstand capability compared to gapless MOV arresters. Overhead distribution equipment protection Overhead distribution equipment, is exposed to substantial surge events originating from direct and induced lightning strikes. As such,it will benefit substantially from the improved protection margin offered by gapped MOV arresters. The protection margin is given by [2]: (1) Equipment age As cellulose transformer insulation ages, it tends to show a decrease in the impulse insulation strength. Studies [5] show that the decrease is approximately 1% per year of operation. For example, a 30 year old 22 kV pole top transformer with a 1 m surge arrester lead and a 24 kV rated gapless arrester would have 9% protective margin left. If this transformer was protected by a gapped arrester of the same rating, then the protective margin would be 26%. Underground systems Water treed XLPE cables are sensitive to impulses and it is critical to keep the magnitude of impulses entering these cables as low as possible. If surge arresters are only applied at the overhead line to cable termination, then the protective margin is given by Eqn. 2, except that the discharge voltage used is twice the 10 kA arrester discharge voltage due to voltage doubling effects. If arresters are placed at the termination and the remote open end, then Eqn. 2 is used, but the discharge voltage is the 10 kA discharge voltage of the arrester at the termination plus half the energize - May 2009 - Page 14 remote end 1,5 kA discharge voltage. For example, for a 22 kV system protected by a gapless arrester at the termination only, then the margin of protection is only 3%. Using a gapped arrester of the same rating would improve the protective margin to 28%. A 15 kV rated gapped arrester on the same system would improve the protective margin to 95% without causing TOV problems. Other benefits Reduction in inventory Due to the improved TOV and discharge voltage characteristics of gapped surge arresters, it is possible to use the same arrester on 11 and 22 kV resistively earthed systems without significantly impacting the 11 kV margins of protection or 22 kV TOV capability. A single 15 kV rated gapped arrester would provide 86% (down from 94%) and 194% (up from 65%) protective margin on 11 and 22 kV systems respectively. TOV capability of the 15 kV gapped arrester on 22 kV systems is 22,6 kV for 24 hours. Reduced environmental impact Gapless surge arresters typically dissipate 50 mW per kV [6] rated voltage so a 24 kV rated arrester dissipates 1,2 W continuously. Conservative estimates place the number of pole top transformers in South Africa at approximately 325 000. If we assume they are all three phase 22 kV, then the upper limit for continuous surge arrester losses is approximately 1,2 MW. Gapped surge arresters typically dissipate 12 times less power than gapped surge arresters the equivalent loss would have been 100 kW. References [1] D A Gonzales, J A Bonner, K Argiropoulos: “Benefits of gapped MOV arrester to improve system reliability and extend equipment life”, 12th International Conference on Electricity Distribution, CIRED, pp. 2.14/1 – 2.14/5, Vol. 2, 1993. [2] D C Henr y, H E Fletcher: “Protection of underground circuits with gapped MOV technology offers improved margins of protection”, presented at 43rd Annual Power Distribution Conference, October 23rd, 1990 [3 Cooper Power Systems publication I235-5: “UltraSIL Housed VariSTAR Surge Arresters 5 kA and 10 kA Class 1 IEC 60099-4 (IEC99-4) for MV Systems to 36 kV”. [4] Cooper Power Systems publication 235-99: “UltraSIL Polymer-Housed Evolution (10kA) Surge Arresters”. [5] H Z Ding, Z D Wang, P N Jarman: “Effect of ageing on the impulse breakdown strength of oil-impregnated pressboard used in power transformers”, 2006 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 497 – 500. [6 Cooper Power Systems publication B23508033: “Taking Protection to the Extremes: UltraSIL Polymer-Housed Distribution-Class Surge Arrester”. [7] 2008 Eskom Annual Report, retrieved from www. eskom.co.za, May 2009. Contact Glen Medlin, e-LEK Engineering, Tel 012 349-2220, [email protected] v
