Selecting the current rating for equipment 1. Rated current: the maximum continuous load current. Short-time withstand current: thermal withstand current term term is given for 1s or 3s short circuit current duration Equivalent 1s current stress, when and time t are nown: W Þ Þ 1s 1s R 1s t ; t / s energy is the same f the transient components are high, the equation becomes: Þ t 1s '' (m + m the effect of DC-component n the effect of decay in AC-component Std VDE 010 : m 0 0,8 n 0, 1 n) t/s m ~ r/x, t n ~ /, t 3. Pea withstand current: defines the mechanical strength requirements i dyn,5 term» 1,8 Ö term
Mechanical stresses of a short circuit F a m0 p i i a 1 F L i F Example: -phase short circuit, 10 A a 1,0 m ; F/L? i 1 L F L m0 i1 i p a 4p 10 p 5 15 m -7 10 ; -1 i 5 1 m i 10 6,5 Vs A Am æ çva è 5 A 1 m J s VAs m m ö s ø 1 m m ote: in the case of a 3-phase fault, the forces are 15% bigger
Grid short circuit power S 100 MVA, Transformer rating S 16 MVA, and reactance u x 10 %, secondary voltage 0 V, line length is 40 m and resistance and inductance r 0.3 W/m ja l 1.1 mh/m. Compute a) 3-phase short circuit current in A and B, b) the dynamic current rating of busbar A, and c) the equivalent 1s current for line A-B when the fault time is 0.6 second?. 100 V 0 V 40 m One-line diagram: S S u x A l, r X X T A R j X i B B Th? Th? etwor impedances (in 0 V) : X X R T j S u x s r S 0 W 100 40 0,3W 0 0,1 16 0,333 W W 1 W X i pf s l p 40 50 1,1 10-3 W 13,8 W
a) 3-phase short circuit current in points A & B : Thevenin s method : Th 0 3 A : B: Z Þ Z Th Th X Rj + Z + Th Th X T @ j(x,833 W 4,075 A + X T + X j ) 1 + j16,653 Þ Þ Z Th Z Th Th 1 + 16,653 @ 0,56 A @ 0,53 W b) Dynamic rating for busbar A : i dyn,5 term,5 4,075 A @ 10, A c) Thermal rating for line A-B, when t 0,6 s :, max s 4,075 A, max t/s 4,075 0,6 A @ 3,16 A
Switching devices Rated quantities : Rated voltages and insulation level Thermal 1s withstand current () Dynamic withstand current (dyn.5*) Maing capacity of short circuit current highest fault current pea value, that the switch can close Breaing capacity of short circuit current highest fault current, that the switch can interrupt usually given as: - ac-component rms value, and - dc-component %-share
1. Disconnector (isolator) in open-position a reliable open point when closed, can carry load and fault current no breaing or maing capacity. Switch-disconnector a disconnector able to brea load current & mae fault current used e.g. in MV overhead distribution networs Example: n 4 V n 630 A 1s 16 A ma 40 A (maing capacity) Capacity of disconnecting a transformer in no-load 16 A Capacity of disconnecting an OH-line in no-load 4 A Capacity of disconnecting an underground cable 5 A
3. Fuse-switch-disconnector A switch-disconnector equipped with HV fuses Operation of one fuse causes opening of the switch Þ prevents -phase operation 4. Circuit breaer switch able to brea and mae also the short circuit currents wors either automatically or manually controlled in the case of fault, tripped by a protective relay
Operation of a circuit breaer a) melted metal bridge b) stationary power arc c) moving power arc d) the arc voltage ~ time The breaing wor W must be ept small. W t ò 0 u v (t) i v (t) dt n order to help the interruption, the arc is: drawn longer divided into parts cooled down de-ionized (absorbing materials)
Some CB types air CB oil CB minimum oil CB Pressurized air CB SF 6 - CB vacuum CB
Current transformers n s p The current error F 100 % K n transformation ratio s secondary current p primary current i K p - 100 1 c n s T ò - The composite error e (K i i ) dt % Measurement transformers nstrument security current current, for which e c equals 10% F s nim F s nstrument security factor normal value 5 or 10 Protective transformers Accuracy limit factor F T defines the current, when e c 5% or 10% (5P tai 10P) typical values 5,10,15,0 or 30 (times the rated current) Example: 10 P 15 p T o p
The accuracy classes of CT:s Accuracy burden power factor Primary error angle error composite class x S cos f x p ± % ± min error % 0,1 0,5 1 0.8 ind for 0,05 0,4 15 S ³ 5 VA 0, 0, 8 - otherwise 1,0 0,1 5 1 1, 0,1 5 0, 0,05 0,75 30 0, 0,35 15-1,0 0, 10 1, 0, 10 0,5 0,05 1,5 90 0, 0,75 45-1,0 0,5 30 1, 0,5 30 1 0,05 3,0 180 0, 1,5 90-1,0 1,0 60 1, 1,0 60 3 0,5 1 0,5 3 - - 1, 3 5 0,5 1, 5 - - 5 P 1 1 1 60 - n - - 5 10 P 1 3 - - n - - 10 ote n equals to the accuracy limit factor
Equivalent circuit for a CT 1 Z s Z w m 1: L m Z b nternal burden S s Z s Z s impedance of the secondary winding Z w impedance of the secondary wiring Z b impedance of the secondary burden (relays etc.) A CT eeps its accuracy class when Z w + Z b is smaller than the rated burden Z impedance defined by S Z
Example : 100/1 A 10 P 15 current transformer nternal burden Rated burden Secondary cabling 0.5 VA 10 VA W f the burden by relays is 10 VA, what is the effect of cabling impedance to the accuracy limit current? The burden of secondary cabling at rated current: S F w T ' R F T SS + S + S S VA w S + S The burdens are resistive: 0.5 + 10 Þ F T ' 15 1.6 0.5 + + 10 Þ the accuracy limit current F T 1,6 A
Measurement of zero sequence current Sum connection : Large load currents cause measurement errors Short circuit currents and unequal saturation Þ apparent sum current created cable type CT: even 0.5 A currents detected 1 3 A S Sum connection Cable type CT
Voltage transformers (VT) the rated voltage factor maximum operating frequency voltage, that the VT must tolerate for a given time ( rated ) protective transformer (3 P or 6 P) must eep its accuracy up to the voltage rated Rated voltage factors of VT:s rated voltage time Primary connection and system earthing factor 1, continuous in general between the phases and between the star point and earth 1, continuous 1,5 30 s between phase and earth in solid earthed systems 1, continuous between phase and earth in other systems 1,9 30 s if automatic earth fault tripping 1, continuous between phase and earth in unearthed and compensated 1,9 8 h neutral systems, if no earth fault tripping The maximum errors allowed for VT:s in general Additional requirements for protective VT:s class voltage error angle error % min 0,1 0,1 5 0, 0, 10 0,5 0,5 0 1 1,0 40 3 3,0 - class voltage error angle error % min 3 P 3,0 10 6 P 6,0 40
The measurement of zero sequence voltage d L1 + (1+ a 3 0 + a L + ) ( 1 L3 + ) + 3 0 a 1Ð10
Voltage transformer ferroresonance initiated by some switching action is stopped, when more lines are connected phase voltages strongly distorted may cause high overvoltages damping using 50-60 W resistor in open delta
Equivalent circuit for a power transformer R X R 0 X 0 Example: A 16 MVA transformer X R r 0 (S 0 P P S S 0 0 3 1 W 0,0161 0,088 16 1 W 0,005 16 & 0,0055 p.u. P 5,5 W 7,4 W 3 hr ) z R X Þ x r x x S S + r 0,1p.u. z - r 0,09985» 1 0,0055 W 0,15 W 16 1 0,1 16 W,76 W 0,1p.u.
The ampacity of a power transformer
Cellulose fiber Composed of chains of molecules Electrical strength depends on the length of the chain Chains are broen by oxygen water and heat
Temperature distribution At rated load: q a ambient temperature Dq or 55 C Hg r 3 C The rate of life time decay (EC 354) the winding hot-spot temperature the relative rate of life time decay q h V q a ( q + Dq h or -98) /6 é1+ RK ê ë 1+ R ù ú û x + Hg r K y K R / P n n / P 0 y 1,6 x 0,8
Overload capacity (EC 354) ormal cyclic load current 1,5 hot-spot temperature 140 top-oil temperature 105 Long term overload capacity current 1,8 hot-spot temperature 150 top-oil temperature 115 Short term overload capacity current hot-spot temperature * top-oil temperature * 16 14 1 The rate of life time decay ~ load current Life time decay p.u. 10 8 6 4 0 0,9 1 1,05 1,1 1,15 1, Load current p.u.