High Speed Biased Differential relay block DTH31

Features and technical specifications:

1. High speed.
2. Low burden.
3. Immunity to magnetising inrush.
4. Immunity to transients and surges
5. Compact in size.
6. Single pole version with separate flags for differential/highset operation available.                                                                                                                                                                                                                                                                                            This Motor Protection relay block is easily insert in your schematic diagram and connect as you like way.                                                                                                                                                                                                                                                        Biased to provide stability during heavy through-faults, the relays utilise second harmonic restraint to prevent operation by normal
   magnetising in-rush currents produced when the transformer is energised. In addition, type DTH/DTTM relays employ fifth harmonic by-pass circuit to avoid possible maloperation under over-excited conditions. An instantaneous highset circuit overrides the biased differential circuit to clear heavy internal faults in about one cycle. Type DTH 31/DTTM 11 is applicable for two-winding transformers.
   Extremely low burdens are achieved by the use of input devices which convert current to voltage (transactors). Static circuitry is
   employed throughout, and a single attracted armature unit provides the output. These relays have the advantage of small dimensions and increased reliability over their electro-mechanical equivalents. Ideally, the CT primary rating should agree with the protected power transformer’s full load rating, and with the transformation ratio. This ensures the secondary currents flowing in the interconnecting pilots are balanced and matched with the relay rating. When interposing CTs are used for ratio correction, the main CT secondary should preferably be star connected and the phase angle correction whenever necessary should be adopted on the
   interposing CT by connecting them in star/delta. Figure 1 block schematic diagram shows a typical application with a three-phase two-winding transformer. Input currents I2 and I1 from the power transformer line CTs are added vectorially in the centre tapped restraint bias transactor T1.B Three taps in each half of the transactor primary enable bias settings of 15%, 30% and 45% to be
   obtained. The output of T1 is full wave rectified and smoothed to obtain the restraint bias voltage level VB. The centre tap of T1
   is connected to the differential circuit which B comprises transactors T2 , T3 and current transformer T4 connected is series. A tuned circuit which includes the secondary of T2 is arranged to resonate at the second harmonic frequency. The output of this circuit is
   rectified and smoothed to obtain the harmonic restraint voltage level VH. In addition, outputs of transactor T3 and current transformer T4 are rectified and smoothed to obtain the differential voltage VD and the highset voltage level VO respectively.
   The greater of the two restraining voltage levels VB and VH is detected in one comparator and compared in magnitude with the differential operating voltage level VD in a second comparator stage. When the operate voltage exceeds the restraining voltage by more than a preset amount, the second comparator produces an output to operate the common relay drive circuit. The highset voltage level VO operates the relay drive circuit if the differential current exceeds ten times the rated current.