SHORT CIRCUIT STUDY M ARIFIN Moh.arifin85@gmail.com SHORT CIRCUIT Objectives Short circuit study can be determine: 1. Determine short circuit current level on very panel 2. Determine size busbar kA rating and CB kA rating SHORT CIRCUIT Parameter of short Circuit current calculation (IEC60909) 1. Initial Symmetrical Short-Circuit Current (I"k) This is the rms value of the AC symmetrical component of an available shortcircuit current applicable at the instant of short-circuit if the impedance remains at zero time value. Where Un = Voltage at the short circuit location Zk = the equivalent impedance at the fault location 2. Peak Short-Circuit Current (ip) This is the maximum possible instantaneous value of the available short-circuit current SHORT CIRCUIT Where k = a function of the system R/X ratio at the fault location, according IEC 609090 Paragraph 4.3.1.1 or paragraph 4.3.1.2 3. Symmetrical Short-Circuit Breaking Current (Ib) This is the rms value of an integral cycle of the symmetrical AC component of the available short-circuit current at the instant of contact separation of the first pole of a switching device. Far from Generator fault SHORT CIRCUIT Near to Generator Fault Where µ and q are factors that account for AC decay µ = factor µ, depend on the minimum time delay t min and the ratio l”kG /IrG. For detail refer to IEC 60909-0 Page 109 q = factor q, depend on the minimum time delay t min and the ratio PrM /P. For detail refer to IEC 60909-0 Page 113 I”kG = Initial symmetrical short circuit of Generator IrG = Rated current Generator PrM = Rated Active Power in MW P = number pole of motor SHORT CIRCUIT 4. Steady-State Short-Circuit Current (Ik) This is the rms value of the short-circuit current, which remains after the decay of the transient phenomena. where, λ = factor λ , depend on the Xdsat and the ratio l”kG /IrG. For detail refer to IEC 60909-0 Page 117 5. DC component (Idc) Short Circuit mean value between the top and bottom envelope of a short-circuit current decaying from an initial value to zero of short circuit current SHORT CIRCUIT Where, I”k = initial symmetrical short-circuit current; f = is the nominal frequency; t = is the time; R/X = R/X ratio at the fault location, according IEC 609090 Paragraph 4.3.1.1 or paragraph 4.3.1.2 6. The thermal withstand short-circuit current capability(Ith) The joule integral ݅ ଶ ݀ ݐis a measure of the energy generated in the resistive element of the system by the short-circuit current. where, I”k = initial symmetrical short-circuit current; m = the factor for the heat effect of the d.c. component n = the factor for the heat effect of the a.c. component SHORT CIRCUIT SHORT CIRCUIT SHORT CIRCUIT SHORT CIRCUIT SHORT CIRCUIT Characteristic short circuit current: 1. Far from Generator Short Circuit short circuit during which the magnitude of the symmetrical a.c. component of the prospective (available) short-circuit current remains essentially constant (see figure 1) 2. Near to Generator Short Circuit short circuit to which at least one synchronous machine contributes a prospective initial symmetrical short-circuit current which is more than twice the machine's rated current, or a short circuit to which asynchronous motors contribute more than 5 % of the initial symmetricaf short- circuit current I”k without motors (see figure 2) SHORT CIRCUIT SHORT CIRCUIT SHORT CIRCUIT Type of Short Circuit Fault: 1. 3-Phase Fault 2. LLG Fault 3. LL Fault 4. LG Fault SHORT CIRCUIT Type of Short Circuit Fault SHORT CIRCUIT IEC Short Circuit terminology: 1. Peak Current/electrodynamics withstand/making capacity (Icm) : peak value of the short circuit. can be easily see from the wave. Panel, ACB and VCB usually has this rating value. MCB, MCCB rarely has this rating plate. 2. Breaking Current (Icu/Ics) is to represent rms value of the SC when the one of the CB pole open. Only CB has this value and their value is determined by the breaking time. All breaker MCB, MCCB, ACB, VCB has this rating value. Icu is for ultimate breaking current meanwhile Ics is for symmetrical. Icu must be same/higher than Ics. 3. Thermal withstand current/Short time withstand current (Ith) is value to represent thermal damage caused by high SC current for some period of time. usually 1s for LV-Switchboard/CB and 3s for HV Switchboard/CB. Panel, ACB and VCB usually has this rating value. MCB, MCCB rarely has this rating plate. 4. DC current/Idc is to represents dc current level. near generator dc current will be high therefore usually CB near generator required dc short circuit evaluation. SHORT CIRCUIT Device capability Vs short circuit calculation Equipment Component Device capability Short Circuit Calculation SWITCHYARD Busbar Short circuit withstand current (Ithr)3 sec (Tkr) Ith HVCB Rated breaking current Ibsym Rated making current Ip Busbar Short circuit withstand current (Ithr)3 sec (Tkr) Ith GCB Rated breaking current Ibsym %Idc Idc Closing & latching capability Ip GCB SHORT CIRCUIT Device capability of short circuit calculation Equipment Component Device capability Short Circuit Current (Etap) MV / LV Busduct Busbar Short circuit withstand current (Ithr)3 sec (Tkr) Ith Rated Peak Withstand Current Ip Busbar Short circuit withstand current (Ithr)3 sec (Tkr) Ith VCB Rated Making Current Ip Ultimate breaking current (Icu) or Service breaking current (Ics) Ibsym MV Switchgear Icu must be same/higher than Ics Fuse Rated breaking current Ibsym SHORT CIRCUIT Device capability of short circuit calculation Equipment Component Device capability Short Circuit Current (Etap) LV Switchgear Busbar Short circuit withstand Ith current (Ithr)1 sec (Tkr) ACB Rated short circuit ultimate breaking capacity (Icu) or Service breaking capacity (Ics) Ibsym Icu must be same/higher than Ics Short circuit withstand current (Icw) Ith Short Circuit Making Capacity (Icm) Ip MCCB Service breaking capacity Ibsym (Ics) or ultimate breaking capacity (Icu) MCB Service breaking capacity Ibsym (Ics) or ultimate breaking capacity (Icu) SHORT CIRCUIT Equipment Contribute the Short Circuit Current: 1. Power Grid Parameter Correlation Impact SC MVAsc ↑MVAsc↑SC ↓MVAsc↓SC I”k X/R ↑X/R↑SC ↓X/R↓SC Ip, Idc Remarks Typically 14 (value x/r for testing HVCB) SHORT CIRCUIT 2. Generator Parameter Correlation Impact SC Remarks MW ↑MW↑SC ↓MW↓SC I”k kV ↑kV↓SC ↓kV↑SC I”k PF ↑pf↓MVA↓SC ↓pf↑MVA↑SC I”k X”d ↑X”d↓SC ↓X”d↑SC I”k X/R ↑X/R↑SC ↓X/R↓SC Ip, Idc Typically X/R for generator >40 X”d Tolerance ↑X”d Tolerance↑SC ↓X”d Tolerance↓SC I”k Typically X”d tolerance are 5% or 10% or 15% (for safety factor only, if not have report test actual X”d generator ) X’d ↑X’d↓SC ↓X’d↑SC Ik X0 ↑X0↓SC ↓X0↑SC Unsymmetrical SC (Ground Fault) X2 ↑X2↓SC ↓X2↑SC Unsymmetrical SC (Ground Fault) NGR ↑R↓SC ↓R↑SC Unsymmetrical SC (Ground Fault) Typically X”d generator is 10-20% Solid Grounded, HRG, LRG SHORT CIRCUIT 2. Generator SHORT CIRCUIT 2. Generator SHORT CIRCUIT 3. Motor Parameter Correlation Impact SC Remarks kW ↑kW↑SC ↓kW↓SC I”k kV ↑kV↓SC ↓kV↑SC I”k PF ↑pf↓MVA↓SC ↓pf↑MVA↑SC I”k LRC ↑%LRC↑SC ↓%LRC↓SC I”k Typically 500% - 800% ↑X/R↑SC ↓X/R↓SC Ip, Idc Typically 3.8 ( for Pf starting 25%) Number of pole X/R SHORT CIRCUIT 3. Motor SHORT CIRCUIT 3. Motor SHORT CIRCUIT 4. Lump Load Parameter Correlation Impact SC Remarks Load Type ↑load motor↑SC ↓load motor↓SC I”k MVA ↑MVA↑SC ↓MVA↓SC kV ↑kV↓SC ↓kV↑SC %LRC ↑%LRC↑SC ↓%LRC↓SC I”k Typically 500% - 800% X/R ↑X/R↑SC ↓X/R↓SC Ip, Idc Typically 3.8 (Pf starting 25%) SHORT CIRCUIT 4. Lump Load SHORT CIRCUIT 4. Lump Load SHORT CIRCUIT Equipment Can Reduce the Short Circuit Current: 1. Transformer Parameter Correlation kV ↑kV↓SC, ↓kV↑SC MVA ↑MVA↑SC ↓MVA↓SC Z ↑Z↓SC, ↓Z↑SC Z Tolerance ↑Z Tolerance↑SC ↓Z Tolerance↓SC X/R ↑X/R↑SC ↓X/R↓SC Impact SC Remarks Based on standard IEC 76 = ±10%. Ip, Idc SHORT CIRCUIT 1. Transformer SHORT CIRCUIT 1. Transformer SHORT CIRCUIT 2. Cable Parameter Z (R, X and Length) Correlation ↑Z↓SC, ↓Z↑SC Impact SC Remarks SHORT CIRCUIT 3. NGR Parameter Z Correlation ↑Z↓SC, ↓Z↑SC Impact SC Ground fault Remarks reducing ground fault effectifically SHORT CIRCUIT 4. Current Limiter This device not reduce short circuit current, but this device can detect and cut short circuit very fast the SC before their peak value. 5. VSD Some VSD have capability to block current the contribution motor to upstream system. It will depends on thyristor principle work. Capabilities thyristor to black current called k factor. Parameter Correlation HP ↑HP↑SC, ↓HP↓SC kV ↑kV↓SC, ↓kV↑SC PF ↑pf↓MVA↓SC ↓pf↑MVA↑SC K factor ↑k↑SC, ↓k↓SC Impact SC Remarks SHORT CIRCUIT 4. VSD SHORT CIRCUIT Device Capability Evaluation 1. HVCB SHORT CIRCUIT Device Capability Evaluation 2. LVCB SHORT CIRCUIT Device Capability Evaluation 3. Switchgear SHORT CIRCUIT Tools Short Circuit of ETAP 1. Info 3 1 2 SHORT CIRCUIT Tools Short Circuit of ETAP 1. Info 1. SC at terminal motor will be calculated and indicated 2. Can be adjust to get more accurate result 3. Pre start voltage have small effect the calculation. Bigger pre start voltage will have impact on bigger short circuit SHORT CIRCUIT Tools Short Circuit of ETAP 1. Standard 2 1 4 3 5 SHORT CIRCUIT Tools Short Circuit of ETAP 1. Standard 1. Select standard based on Standard Equipment (IEC or ANSI) 2. C factor, it have effect the initial symmetrical current (for detail see IEC 60909 table 1, page 40). 3. IEC give us option to calculate SC based on max through fault current or total fault current. Total fault current will get a more conservative value. Application the Max through fault current is optimal cost within select GCB, select bigger the contribution SC between of generator and system side. 4. IEC give 3 option to calculate X/R. it will effect within calculate Ip SC. Method B will increase 1.15 than method A. Method C is recommended in meshed networks (see IEC 60909-1) SHORT CIRCUIT Tools Short Circuit of ETAP
