AZT 3/0
AUTONOMOUS IDMT O/C BACKUP PROTECTION
 
The electronic autonomous IDMT back up protection type AZT 3/0 has an especial and important object. Namely, it is possible to occur an abnormality which causes a breakdown of the auxiliary (battery) power supply of an electric work, e.g. of a transformer substation. In this case, the protections in the substation lose their ability to operate. If a fault occurs during this time, the protections do not operate, or because of lack of the tripping energy the protection operation is not followed by a CB trip. It causes serious damage of the primary devices on an unlucky case. The device type AZT 3/0 eliminates this dangerous situation.
 
AZT 3/0
 
Application field
 
The AZT 3/0 device may be used as a back-up of the existing main protections advantageously if the dc supply of an
electric work will break off partially or entirely, since it safely gives trip even in this case, and also stores the trip energy for CB trip. The IDMT characteristic may be used for protection even in meshed networks as weII. If the device is applied as a zero sequence back up protection for a transmission line in an effectively earthed network it may be ordered with so-called "two-phase-operation block" as an option.
 
Main features
 
  • External energy source is not necessary to operate the AZT 3/0 device since its supply is got from the CT circuits (self-supply)
  • The device contains an energy storage of capacitors
  • The energy is charged by CT circuits during fault, and it serves to trip the CB
  • The device has an inverse definite minimum time characteristic, so its time delaying is about inversely proportional with the current
  • The characteristic may be shifted parallel to the direction of the time axis
  • The three phase O/C relays with maximum selection and the built-in energy store is able to trip a CB with three phase drive as well
  • The device of the same design may be applied as a zero sequence back up protection. Then the device connection will be changed only
  • It may be ordered with so-called "two-phase-operation block" as an option.
 
Operation of the device
 
Operation of the AZT 3/0 device can be followed in Figure 2. It contained 6 auxiliary CT's, 3 of them (CT4, CT5, CT6) supply the IDMT O/C protection, the other 3 (CT1, CT2, CT3) charge the energy storage capacitor bank. Two CTs are
connected to each input terminals of the device.
On O/C picking up, the first three auxiliary CTs start to charge the energy storage capacitor bank until it reaches the limit voltage. The second three auxiliary CT's feed on the one hand the ac/dc supply unit which serves the own inner energy supply, and on the other hand the I> O/C relay and the [t1] and [t2] timers. If the O/C relay with maximum selection picks up, it starts the [t1] inverse electronic timer and then the [t2] independent one which are connected seriesly. [t2] has a setting range of 0 to 8 s. [t2] shifts the inverse characteristic parallel on the time axis. The [t1] characteristic is seen in Figure 1.
If the voltage relay of the energy storage capacitor bank is in reset condition, and [I>] O/C relay picks up, through [LA] logical gate, [L1] LED gets a signal, [SZ1] counter steps and [r1] relay operates which makes possible to start the capacitor bank charging. While [I>] O/C relay does not pick up, [r1] relay makes a short circuit on CT1, CT2, CT3 so the power consumption of the protection is low. On starting the charging, the power consumption will be increased until the capacitor bank is charged to 300 V.


Figure 1.

When this limit voltage is reached, the voltage relay connected along the capacitor bank picks up, [r1] relay resets, and stops the bank charging. Then the protection power consumption will be low again, due to the short-circuit of CT1, CT2, CT3. Picking up of [U>] voltage relay gives a permission signal to the [LB] trip logic gate. If [t1] and [t2] timers also run down, they also give permission to the [LB] gate. Due to the two permission signals, [L2] LED lights, [SZ2] counter steps, and [r2] relay operates. Then [J] contact gives an external alarm, and [R], [S] and [T] contacts discharge the capacitor bank charge to the proper trip coils of CB. So the CB trips.
The stored energy of the capacitor bank is 20,25 Joule. This energy is multiple enough to be tripped three phases of any CB drive.
After an operation, the LED annunciation's will disappear due to the self-supply system, so neither signal self-holding nor acknowledgement can not be realised. But the two built-in counters give information to check the proper operation of the device during a fault by considering the starting and tripping numbers. The alarm given by the [J] relay contact is suitable to use as a local and remote signal, and in a SCADA system.
If the AZT 3/0 device is used as a zero sequence back up protection of a transmission line in an effectively earthed network, its external connections have to be changed only. Of course, the setting equations will be also changed. They are acquainted in the handling manual.
The CT circuit consumption of AZT 3/0 device depends on the connection (normal or zero sequence), and the ratio of the six built-in auxiliary CT's. The ratio may be changed in M=1:2 rate by inner changeover.
For safety point of view and in order to avoid the energy loss, it is advisable to choose the secondary wires of the external connection circuits as short as possible and to leave the series CB auxiliary contacts. It is also advisable to put the AZT 3/0 device in the supplying CT cabinet and to apply direct wiring without cable ducts and distribution frame.


Figure 2.
 
Technical data
 
Rated current, In
 1 A, 5 A or as requested
Rated voltage, Un (for "two phase operation block") 100-110 V, 200-220 V or other
Rated frequency

50-60 Hz
Overload capacity, thermal, continuous
1 s		 1,2xIn
50xIn (In=5 A)
Dynamic current limit 100xIn
Overload capacity in VT circuit 1,2xUn
Auxiliary CTs, ratio factor M 1 or 2
O/C relay [I>], setting range 1/M(1,5 to 3,48) In
O/C relay [I>], setting range at zero sequence connection 1/M(1,5 to 3,48) In
O/C relay, accuracy
 ± 5%
O/C relay, resetting ratio £85%
Consumption in CT circuits, in normal and tripping condition, if
In=1 A M=1
In=1 A M=2
In=5 A M=1
In=5 A M=2
0,6 VA
1,2 VA
2,0 VA
4,0 VA
Consumption in CT circuits, just before whole charging condition, if
In=1 A M=1
In=1 A M=2
In=5 A M=1
In=5 A M=2
1,5 VA
3,0 VA
5,0 VA
10,0 VA
Consumption in CT circuits, in normal and trippingcondition, at zero sequence connection, if
In=1 A M=1
In=1 A M=2
In=5 A M=1
In=5 A M=2

1,8 VA
3,6 VA
6,0 VA
12,0 VA
Consumption in CT circuits, before whole charging condition, at zero sequence connection, if
In=1 A M=1
In=1 A M=2
In=5 A M=1
In=5 A M=2

4,5 VA
9,0 VA
15,0 VA
30,0 VA
Controlled CB tripping coil, rated voltage 220 V (other value on request)
Protection time delaying t1+ t2
t1 inverse timer characteristic
t2 inverse timer, accuracy		 acc.to Figure 1.
± 5%
t2 independent timer
setting range
at zero sequence connection
steps
accuracy
3-8 s
0-8 s
0,5 s
± 50 ms
Under voltage relay of "two-phase-operation block", setting values, if
Un = 100-110 V
Un = 200-220 V
30 V and 40 V
60 V and 80 V
Consumption in VT circuits £1 W/phase
External blocking or external zero sequence O/C relay, dc auxiliary supply voltage 220 ± 20% or
110 ± 20%
Consumption DC circuit £1 W
Output relay contacts (J, R, S, T), electrical data
rated switching voltage
continuous load current
making current
DC breaking current at 220 V DC
at pure resistive load
at inductive load of L/R = 40 ms
250 V
8 A
16 A

0,25 A
0,14 A
Permissible ambient temperature -20o do 60o C
Insulation test (IEC 255) 2 kV, 50 Hz
5 kV, 1.2/50 µs
Disturbance test (IEC 255) 2.5 kV, 1 MHz
 
Design
 
The device is seen in the photograph. It is switchboard mounted type design. It is placed into a closed steel case painted to flatted black. Its dimensions are as follows: width: 323 mm, height: 432 mm, depth: 124 mm. External connection of the device are 21 pcs terminals with front connection. Its weight is 9 kg. The enclosure protection of the device is IP 40.
 
Data to be ordered
 
  • Rated current (1A, 5A or other)
  • Auxiliary DC voltage (220 V, 110 V)
  • "Two-phase-operation block" option (if it is necessary)
  • O/C relays, reset ratio (if it deviates from 0,85)
  • Rated voltage of "two-phase-operation block"
  • CB trip coil, rated voltage (if it deviates from 220 V DC)