Protect Circuit Breakers from Close-In Faults

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A Close-In Short Circuit Can Cause Serious Out-Rush Currents from a station capacitor bank. Whether the short circuit is at thebus or at any line emanating from a station, the resulting out-rushcurrents have high-frequency and high-magnitude components,depending on the size of the bank.
An out-rush reactor, connected in series with the capacitor bank,has been a common method used to protect substation general-purposebreakers from failure caused by attempting to switch excessivefault currents. Without the out-rush reactor, out-rush current froma close-in fault can significantly exceed the general-purposebreaker capacity for the current-times-frequency (I×f)product, with potentially disastrous results. THE PROBLEM WITH OUT-RUSH REACTORS
Typically, an out-rush reactor is specified to bring the currentmagnitude and frequency within the general-purpose substationbreakers' capability. The device itself is not expensive, but thecivil works to accommodate it can be a significant part of acapacitor-bank installation. The photo below shows an out-rushreactor installation associated with a 110-MVAR, 230-kV project.
The problem is that a short circuit between the capacitor bank andthe out-rush reactor can effectively ground one end of the out-rushreactor. Breakers face significant stress when attempting tointerrupt the short-circuit current associated with the smallinductance of an out-rush reactor.
The process of interrupting the current through the out-rushreactor is highly complex, but fortunately can be understood insimple terms. Fundamentally, the successful interruption of a faultinvolves the dielectric strength of the breaker contacts beingstronger than the voltage (stress) across the breaker contacts.Otherwise, the distance between the breaker contacts is arced overand no interruption occurs. Because the breaker contacts are inmotion, this strength-versus-stress process is dynamic.
As the breaker attempts to interrupt the current, a race startsbetween the change in dielectric strength between the breakercontacts, or the rate of rise of dielectric strength (RRDS), andthe increase in voltage difference between the breaker contacts, orthe rate of rise of the recovery voltage (RRRV). If the RRRV isgreater than the RRDS, the breaker may fail to interrupt thecurrent.
Some investigators believe this is what happened in an incidentreported by The Toronto Star on Jan. 31, 2007, where a fault at a capacitor bank resulted in astation fire and the “temporary interruption of about 1500 MWof capacity.” Breaker failures, particularly of oil breakers,can be quite damaging and dramatic events. ANALYTICAL CONSIDERATIONS
The RRRV is largely a function of the capacitance to ground at thereactor side of the breaker and the inductance of the out-rushreactor. The smaller the capacitance to ground, the higher theRRRV.
In the case where there is no intentional additional capacitance atthe location, a small stray capacitance to ground exists of theorder of 400 pF for a typical 230-kV installation. This capacitanceforms a resonant circuit with the out-rush reactor developing ahigh-frequency voltage on the reactor side of the breaker. Theresulting RRRV can be several times higher than the capability of astandard special-purpose breaker. To bring the RRRV to within thebreaker capability, the typical solution is the installation offixed capacitors of nanoFarad size connected from the high side ofthe out-rush reactor to ground. For reference, the capacitance of a110-MVAR, 230-kV bank is 5.5 µF.
Because no out-rush reactor is installed in series with the fixedcapacitors, and the I×f product happens to be primarily afunction of the inductance between the point of short circuit andthe fixed capacitors, we are right back at the starting situation— a capacitor bank without an out-rush reactor. The I×fprimary dependence on inductance can be readily seen from thedefining equation:
This equation is a direct derivation from the expressions for peakcurrent and frequency shown in IEEE Standard C37.012-1979. In thisequation, V LL is the bus nominal line-to-line voltage in volts, n is the numberof capacitor banks in parallel, and L EQ is the intrinsic inductance of the bank and the series combinationof the inductances from the capacitor bank and the line and/or buswork to the point of the short circuit.
We can use the equation to calculate the I×f product for ashort circuit at the bus 50 ft (15 m) away from a capacitor bank.Assuming 10 µH for the capacitor inherent inductance,0.285-µH per ft of bus for a typical 230-kV substation andoperating the bus at 245 kV, we have an I×f product of 1312-kAkHz. The I×f product capability of general-purpose breakers is20-kA kHz, according to C37.06-2000, and 110-kA kHz, according tothe draft of C37.06-2007.
In summary, adding the out-rush reactor to mitigate the highI×f product of the capacitor bank requires the addition offixed capacitors to mitigate the high RRRV. However, the fixedcapacitors generate similar I×f products as the originalsituation, and these are in excess of the capability of thegeneral-purpose breakers. RISK ANALYSIS
The most relevant parameter of the I×f product is theequivalent inductance between the capacitor bank and the point offault. Assuming 0.6 mile (1 km) of reactance for the transmissionline, the I×f product will be within the general-purposebreakers' capability on a typical 230-kV substation, provided thatthe short circuit is farther than 0.16 miles (0.26 km) from thesubstation and the total installed capacitance is less than about660 MVAR. This is more than most existing installations, especiallyfor Florida Power & Light Co. (FPL; Miami, Florida, U.S.), wherethe largest installation totals 440 MVAR.
The risk occurs only when attempting to reclose the general-purposebreaker on a permanent fault, which is one that remains on thesystem after the breaker opens for the first time. The risk doesnot occur at the first attempt to clear a fault, because at themoment of clearing, typically about three cycles after faultinception, the out-rush transient has already abated.
For a 660-MVAR installation, the breaker peak current rating ismore limiting than the I×f, requiring the minimum distancefrom the substation to be about 0.29 miles (0.47 km). Theprobability of a fault closer than 0.29 miles from a substation isvery small.
For 19 years, the FPL Outage Data Bank has covered about 104substations (230 kV and 500 kV) and roughly 5600 miles (9012 km) oflines. It shows only 18 faults within 0.29 miles of a substation.The only permanent fault was on a line emanating from a generationstation, where the policy is always to block automatic reclosing,so FPL did not attempt to reclose it. These statistics help explainwhy there have been so few reports of breaker failures caused bycapacitive current out rush, even though FPL has some capacitorbank installations without out-rush reactors. A SIMPLE SOLUTION
With the facts noted previously, and the consideration that if theI×f product limitation for breakers specified in StandardC37.06 is correct, then in the event of close-in faults, theout-rush currents not only from capacitor banks but also fromCCVTs, breaker-bushing capacitors or stray-bus capacitance exceedthe capability of general-purpose breakers.
Therefore, the simplest solution is twofold: eliminate out-rushreactors to remove the risk of excessive RRRV and block automaticreclosing on faults close to the substation. This solution isreliable, robust and relatively easy to implement.
It would be prudent to suggest setting relays for 0.5 miles (0.8km) or even 1 mile (1.6 km). The table on this page shows thenumber of faults within 0.29 miles, 0.5 miles and 1 mile along withthe number of faults over the 19-year period. The farther from thesubstation the relay is set to block reclosing, the safer it is, inthe sense that it will block reclosing for more faults. Thetradeoff is that each blocked reclosing event will requirepatrolling of the line before re-energization. To assess the impactof additional patrolling, the table also shows that preventingautomatic reclosing for faults closer than 0.5 miles results in anaverage of 3.2 more such patrols a year. This should not break theoperations budget.
Except for possible replacement of some obsolete relays, thissolution has negligible cost. It is important to note that thefault-location function cannot rely on current magnitudes becausethese can vary. It is necessary that the fault location bedetermined by modern relays capable of accurate and robustfault-location identification.
J. “Joe” R. Ribeiro is staff engineer in the Transmission Services and Planningdepartment of Florida Power & Light Co. (FPL). He joined FPL in1984 after seven years with Niagara Mohawk (Syracuse, New York,U.S.), three with PTI (Schenectady, New York) and three withAmerican Electric Power Service Corp. (New York, New York), wherehe started his career as a power-system planner back in 1970.Ribeiro holds a BSEE degree from New York University and an MSEEdegree from Union College in Schenectady. He is a professionalengineer in the states of Florida and New York.
J_R_Ribeiro@FPL.com