Sizing and Selection of Grounding Transformers­ Decision Criteria George Eduful Godfred Mensah Electricity Company of Ghana Electricity Company of Ghana P.O. Box 5278, Accra-North, Ghana P.O. Box 5278, Accra-North, Ghana System Planning Division System Planning Division georgeeduful@yahoo.com Abstract- godmens@ieee.org Within a period oHwo years, the Electricity t:ompany of Ghana (ECG) lost a total of six grounding transformers in a particular substation. The situation created a lot of instability and resulted in huge productivity losses to both the company and its customers. The failures were believed to be related to wrong selection of grounding transformer rating. However, using the concept of capacitive charging current of a system, it was found that the short time rating of the grounding transformers were rightly selected. Analysis of the phenomenon strongly linked the damages to protection deficiency. This paper duty transformer of equal kVA rating. For this reason, grounding transformers are often not sized by "kVA" but by their continuous and short time current ratings. They are usually oil immersed and may be installed outdoor. Grounding transformer is used for direct grounding or through a current limiting resistor. Zero sequence impedance of grounding transformer is quite low, but it can be increased if the purpose is to limit current through the transformer during earth fault. The reasons for limiting current may be: discusses analysis of the problem and proposes decision criteria a. for selecting a grounding transformer. b. Keywords: Grounding transformer, Capacitive charging current, Zero sequence impedance, short time rating current I. To reduce transient over voltage incursion from phase-to-earth fault. To reduce mechanical stresses apparatus carrying fault currents. in circuits and As a rule of thumb, grounding transformers are designed INTRODUCTION A proposal for a change in specification of grounding transformer was presented in response to persistent failure of grounding transformer in a particular substation of the Electricity Company of Ghana. Among others, the proposal with a continuous current rating equal to approximately 10% of its short-time rating. For example, a grounding transformer rated 1000A for 10 seconds may carry 100A (10% of 1000A) continuously. In practice, the size of a grounding transformer is based on capacitive charging current of a system. This is suggested a reduction in flow of earth fault current from 3180 because the charging capacitive current is the lowest level of seconds to 10 minutes. be effectively reduced. A to 1245 A and an increase in short time rating from 10 The role of grounding transformer in power systems is so critical that issues relating to its quality and reliability are earth fault current at which system transient overvoltage can As discussed above, grounding transformers can safely carry about 10% of it short time rated load. Temperatures treated with the utmost seriousness. As a holistic approach to during its continuous rating should not damage the windings. of grounding transformer in power systems to put the subject short duration currents. Temperatures that cause excessive broader context. Based on technical analysis, it was proposed temperature for the windings in direct contact with the oil solving the problem, the report first looks at the basic concept in perspective. Thereafter, the proposal is examined in a that the existing grounding transformer specification be maintained. This paper presents report of the analysis and proposes decision transformer. criteria for selecting a grounding Heating of grounding transformers are caused by random gas development in the oil should be avoided. The should not exceed 140°C. For this reason, Bucholz relay and temperature protection are provided. Neutral C.T is also installed at neutral point of grounding transformers to ensure that in an event of severe earth-fault, it signals the appropriate II. BASIC CONCEPT OF GROUNDING TRANSFORMER earth-fault relay to initiate tripping to protect the transformer. IN POWER SYSTEMS III. Grounding transformer is used to provide a ground path to an ungrounded delta connected system. As a short-time rating device, its size and cost are less compared with a continuous With the DISCUSSION OF PROPOSAL brief overview of the general concept the proposal in detail. 978-0-9564263-4/5/$25.00©2011 IEEE of grounding transformers in power system, we now examine 45 Proposal 1: Reduce the thermal stress on network components, and hence failure rates, by reducing currents that flow during earth faults from the current maximum of 3180A to 1245A. ................it is being proposed that the existing zero sequence impedance of 19.2Q be changed to 50Q. The zero sequence capacitance of transformer is negligible. However, for over headlines, zero sequence capacitance can be high if considerable lengths are involved. As a general rule, the following approximate capacitance values are used: Transformer Although the proposal did not give detail on the technical Over headline consideration that influenced the choice of the 1245A, it is a general knowledge that zero sequence impedance determines the value of earth-fault current. The desired value of the zero Co = O.OlflFltransformer Co = 0.00625 flFlkm As indicated above, value of 3Ico is critical for sizing and selecting grounding transformers. For good approximation of sequence impedance is dependent on the system charging 3Ico value, we considered all cables and the overhead lines charging current before the zero impedance value can be capacitor bank of 1O.8MVar at the station. capacitive current. Therefore, it is necessary to determine the selected. The generally accepted criterion for determining the size of zero sequence impedance (Zo) is that length in the system. Also considered are transformers and a Based on equation (2), the zero sequence capacitance of the cables are calculated, see the table-I. SIC value used for the capacitance calculation is 3.5.The capacitance values of the transformer and the overhead lines approximate values as indicated above. At this condition, the destructive voltage build up on the charging capacitance of the un-faulted phases cannot occur [1, 2, 3, and 4]. Where, Xco is the line-to-earth capacitive reactance of the system. Stated in another way, the current in the zero sequence impedance IN during a line-to-earth fault is = ..fi(2 31 ro Therefore, = .J3 ( 2 x 1T x 50 x104.3 x 33 fx C 103 o X ELL) Amperes (1) 3Ieo = 1872.308 Amps 33000 IV. GROUNDING TRANSFORMER SELECTION CRITERIA SIC insulation shield, d is the diameter of the conductor, system Co is zero sequence capacitance of the system Based on equation (1), the system charging currents for the (33kV network) 1 can be calculated and 624 =30.5Q is dielectric constant, D is the diameter of cables over the frequency and ) =--x- Where, ELL is the system line-to-line voltage in kilovolts, system 103 .J3 x 1[ x the Accordingly, using equation (2), 3Ico for the 33kv system current (3Ico) is given as co on From Table 1, total zero sequence capacitance of the 33kV must be equal to or greater than three times the line-to-earth 31 based system is 104.3IlF. system charging current, 3Ico. According to [4, 5], during line-to-earth system, charging are hence, determine the appropriate zero sequence impedance. The charging current is calculated by summing the zero-sequence capacitance of all the cable and equipment connected to the system. Criterion 1: Based on the general rule that Z0 ::; X0' it can be said that grounding transformers with values of Zo up to 30.50 is appropriate for selection. In relation to the above criteria, the 500 zero sequence impedance value suggested by the proposal does not match the property of the system. Hence, the proposed 500 zero sequence impedance is not appropriate. Criterion 2: It appears that the existing specification of 19.40 at short time current rating 3180A also satisfies the The zero sequence capacitance of any type of cable can be calculated using the following formula: o C (2) fl 978-0-9564263-4/5/$25.00©2011 necessary to compute the values of transient over voltage under the existing specification and the calculated one (30.50 = 0.00736 x SIC j1F 1l000 D logd general criteria. However, to take an informed decision, it is IEEE at rating of 1872A). For comparative analysis, transient overvoltage for Zo=500 is also computed. 46 I Table 1: Components of the Substation Variables Diameter over insulation D (mm) Diameter over conductor d (mm) Number Length(J,.'ll) Capacitance in J1 Total capacitance (Co) in I! Cable Cable Cable (I x630) (3x240) 48 28.2 Cable (lx500) 62 for for Zo=19.40, Zo=300, Zo=500, Transformers Capacitors NA NA 52 NA NA NA 18 I-bank 32.6 21.1 NA NA NA NA 17.2 39.85 12.02 69.3 0.153311763 0.204498567 0.337224215 8.702845932 26.89527355 13.3776735 From ASPEN One-liner modeling of the substation, for Overhead line NA (10.8MYar) NA 0.430610236 NA NA NA 0.18 54.66854076 Criterion 3: The third criterion is to consider sensitivity of the relaying system and the thermal stress that will be imposed on the system in an event of earth-fault. At this Xo/X\=33.6177 Xo/X\=62.5156 stage, system engineers are normally guided by protection Xo/X1=168.732 philosophies. The general philosophy is that in an event of Where, Xo/X\ is the Thevenin's ratio of zero sequence fault, enough current should be allowed to flow such that protective devices can detect earth-fault current and trip off­ reactance to positive sequence reactance of the location of the line but not so much current to cause major damage. The transient over voltage is then calculated from the on ft. Using Zo = 19.40 will result in the following: grounding transformer. following relation [6]: Thermal stress rating of power system equipment depends Higher earth fault current and faster operating times a. for the existing IDMT protection schemes at the station. Effects of high earth current will affect; b. a. During a line-to-earth fault on one phase, the transient voltages on the healthy phases in relation to the Zo values are 0 b. Transient P.U 48.56 30 1.48 48.99 50 1.49 49.31 Cable and Overhead lines if the damage IDMT (kV) 1.47 Where IFfault curve of these equipment are lower than the Overvoltage value 19.4 • current, 1tnp=relay operation time. given in P.U and kV as: Zo Values in Grounding transformer 2 2 ifl t. . n xtd eSl .gn Irlp >Id eSl g J curve of protection scheme protecting these equipment. From IDMT protection schemes at the station, the protection curves are all far lower than the damage curve of the cables and feeders, see the Fig.1. As can be seen, the transient voltage values presented by the respective zero sequence impedances to the healthy phases under line-to-earth condition are lower for Zo=19.40 and for Zo=30.50 as compared to Zo=500. This confirms that Zo=500 does not satisfy the general condition of Zo:::Xco. V. CASE STUDY: THERMAL STRESS ANALYSIS ON THE RECENT GROUNDING TRANSFORMER FAILURE AT THE STATION This case uses typical earth-fault data, obtained from the transient protection relays and technical data as specified on the most regarding the selection of the grounding transformer can still thermal stress, if any, on the system during the recent failure However, relative to the closeness of the overvoltage values for Zo=19.40 and Zo=300, decision recent failed grounding transformer, to examine impact of not be made at this stage. at the station. 978-0-9564263-4/5/$25.00©2011 IEEE 47 10 HXXl 2 3 4 5 7 1� . -1-' 2 3 4 5 7 lOll 111191 r-- c;w.l�IId<\c>'OM� .., � +' __ 2 3 4 5 7 ; lCDX1 2 3 4 5 7 f-' � I c. r-� f::: ..i!���="""'_ko(lDl9J'5IP':zo:o:nocmls m _ f-' CmiU:b'�Cuwtr630"'JIlR'tmo!tlHI>91B · ,oo · , · t - '-1-' · · \ f · · , \ .t-\ , \ + \ . lr-'" 1\ \ $ ; - " �: 1-: 1+- + . , "------ , Condu:I><Oo""rC"",el><:�CU)Q.A:tom�HI>S1E �: f-' . + t:t , �c=:�c"",e ko(l0497S_736470c"* , r- " r- " ID f- llX1ll00010 , , , Ie: · I� , , , I,: ++ · + ++ m � + m .1-- ' " , m , .. , ,oo , , .. , (a)3X240 , ,� Cu , .. XLPE , , .. , , .. , � 3 4 5 7 I. 100 : 3 4 5 7 1000 CURR9(f(Aj : 3 4 5 (a)IX630 AI XLPE + 1 X500 Cu 1+ + .- . XLPE Figure 1. Protection curves for cables at the station 978-0-9564263-4/5/$25.00©2011 IEEE 48 VI. Calculated design stress: 2 I � x t 4800 X 10 230,400,000A2 S Frequent damage of grounding transformer at the station is attributed to inability of temperature protection system to Relay on Grounding transformer IN>140A, td= O.3Sseconds, Inverse Curve (LTI) IFI7.7SkA (anticipated assuming no CT saturation) Calculated thermal 177S0 2 detect overheating of the grounding transformers possibly curve: trip Long Time time=0.34secs, on the 2 x 0.34 = 107,12 1,2S0A grounding imposed on the system during the fault condition. Ideally, the fault stress should not damage the transformer. The high level of the fault current could be attributed to a short in the transformer winding due to insulation breakdown. Insulation breakdown might be due to the following: the total capacitive charging current of the system. To avoid transient over-voltages, grounding transformers must be sized Grounding transformers should be selected to limit phase­ to-ground fault current such that the thermal stress imposed on the system will be less than the equipment design stress. Grounding transformer should be selected such that in an event of fault, enough current will flow to allow protective device to detect ground fault. REFERENCES Inability of temperature protection system to detect transformer overheating possibly of from grounding the flow of 1977. the grounding transformers. [2] J.P. Nelson, "System Grounding and Ground Fault Protection in the Petrochemical Industry: A Need for a Better Understanding," IEEE Transactions on Industry Applications, vol 38, pp 1633-1640, NovlDec 2002. fault current. Our analysis was also extended to the previous failures at the station. It was confirmed that the thermal stress from the phase-to-earth faults were all far lower compared to the equipment designed stresses. Based on the above analysis, it obvious that the existing (Zo [1] J.R. Dunki-Jacobs, "The Reality of High-Resistance Grounding," IEEE Transactions on Industry Applications, vol IA-13, pp 469-475, Sept/Oct current exceeding the continuous rating of Poor CT sensitivity to the flow of ground­ 2. frequent neutral CT to ground fault current. exceeds the electrical system's charging current. grounding transformer is about 200% greater than the stress the rating of grounding transformer and poor sensitivity of so that the amount of the earth-fault current allowed to flow s As shown from the calculation, the design stress of the specification resulting from the flow of current exceeding the continuous Sizing of zero sequence impedance depends entirely on stress transformer due to the fault: 1. CONCLUSION = = = 19.4n at 3180A) has no connection with damages. The existing specification even provides room for future growth of the substation compared with the proposed rating of SO n at [3] W.C. Bloomquist, KJ. Owen and R.L. Gooch, "High-Resistance Grounded Power Systems - Why Not?" IEEE Transactions on Industry Applications, vol IA-l2, pp 574-580, Nov/Dec 1976. [4] D.S. Baker, "Charging Current Data for Guesswork-Free Design of High Resistance Grounded Systems," IEEE Transactions on Industry Applications, vol IA-15, pp 136- 140, Mar/Apr 1979. [5] B. Bridger, Jr., "High-Resistance Grounding," IEEE Transactions on Industry Applications, vol IA-19, pp 15- 21, JanlFeb 1983. [6] Electricity Company of Ghana Distribution Planning Manual, Revised Edition 2011. 1245A. Proposal 2: Prolong the life span of the grounding transformers by increasing the short time duration rating from the current 10 seconds to 10 minutes. Line-to-earth is undesirable condition and must not be allowed to persist for long time. Short-time rating is necessary to limit damage in an event that the system earth­ fault escalates into a double line-to-earth fault or the impedance of the transformer becomes shorted. The standard rating allowed for grounding transformer ranges from 10 to 60seconds. However, where grounding transformers are used to establish a neutral point to enable connection of phase-to­ neutral loads, continuous neutral current rating of the device is allowed because of the attendant load imbalance. 978-0-9564263-4/5/$25.00©2011 IEEE 49