Type VAEM 22 Negative Potential Biasing Relay

Type VAEM 22 Negative Potential Biasing Relay

Type VAEM 22 Negative Potential Biasing Relay VAEM 22 Relay withdrawn from case Features l Simple and robust construction l High sensitivity Application The negative potential biasing relay type VAEM 22 is designed to prevent the failure of the fine wire coils of protective relays which can arise from electrolytic corrosion. In addition to preventing corrosion, the relay initiates an alarm, if an earthfault occurs on the dc system and also detects any deterioration of the insulation resistance of the dc system. On insulated battery systems, if the coil of a protective relay is at a positive potential with respect to earth, it is anodic to leakage currents and thus results in electrolytic corrosion. If moisture is also present any fine wire coil may be corroded until open circuit occurs. Hence batteries are sometimes earthed in order to prevent electrolytic corrosion. The negative pole cannot be earthed because of the corrosion risk, but solid earthing of positive pole could result in heavy current for a low resistance earthfault on the negative pole. Also, the positive pole cannot be monitored for earthfaults. Resistance earthing of the mid point of the battery enables both positive and negative bus wires to be monitored and the earthfault is limited, however, only part of the circuit is protected against corrosion. The only method of earthing of the battery which provides complete protection from corrosion, enables both bus wires to be monitored, and limits the earthfault current, is the use of a negative potential biasing relay. General description The connection and circuit diagram is shown in Figures 1 and 2. This is suitable for battery voltages upto 250 volts. Figures 1 and 2 show the method of detection using the principle of negative potential biasing, whereby an earthfault anywhere in the field circuit can be detected. The d.c. injection supply establishes a small bias on the alternator field circuit so that all points are negative with respect to earth. The auxiliary transformer used in the biasing circuit is rated at 240 volts ac 50 c/s supply. For systems upto 125 the secondary output is rectified to give a bias voltage of 30-35 on open circuit. For a 250 system, the bias voltage is 60-70.

The rectified output of a transformer fed from the system L.V. supply provides a biasing potential, which is connected with positive terminal to earth. The negative terminal is connected through a long core unit C to the positive pole of the field circuit. When a fault occurs, current flows in the long core unit, which in turn makes the relay A to operate to give a trip. The scheme will operate only if ac supply is available. If ac supply fails, relay B drop off and initiates an alarm. Operation The scheme will operate only if ac supply is available. If ac supply fails, relay B drops off and initiates an alarm. Technical data Supply rating a) Bias circuit - 240 volts 50 c/s. b) Battery voltage - 30 volts to 250 Bias voltage 32.5 ± 7.5% volts for 30 to 125 65.0 ± 7.5% volts for 125 to 250 Figure 1: Schematic of negative potential biasing relay type VAEM 22 Earth leakage Setting: 2.25 ± 10%m. amp. dc Burden Less than 6VA at 240V ac Contacts 2 N/O self reset 1 N/C self reset for ac failure alarm. Figure 2: Earth fault detection using negative potential biasing principle in dc system

Contact ratings Make and carry Make and carry Break continuously for 3 seconds AC 1250 VA with 7500 VA with 1250 VA with maxima of 5 A maxima of 30 A maxima of 5 A and 660 V and 660 V and 660 V DC 1250 W with 7500 W with 100 W (resistive) maxima of 5 A maxima of 30 A 50 W (inductive) and 660 V and 660 V with maxima of 5 A and 660 V Critical insulation resistance Battery Insulation resistance in K.ohms voltage for fault on negative bus Insulation The relay meets the requirements of IS 3231:1965/IEC 255-5 Series C-2 KV for 1 minute. Case 1 D drawout case suitable for flush or projection mounting. Information required with order 1. Battery voltage 2. Auxiliary ac voltage 3. Case mounting, flush or projection. 250 126 125 61 110 55.7 50 28 30 19 Note : For fault on positive bus the critical resistance for the operation of the relay is 6 k. ohm.

ALSTOM Limited Pallavaram Works: 19/1, GST Road, Pallavaram, Chennai-600 043. India. Tel: 91-044-2368621 Fax: 91-044-2367276 Email: plw.applications@alstom.sprintrpg.eml.vsnl.net.in. 1998 ALSTOM Limited Our policy is one of continuous development. Accordingly the design of our products may change at any time. Whilst every effort is made to produce up to date literature, this brochure should only be regarded as a guide and is intended for information purposes only. Its contents do not constitute an offer for sale or advice on the application of any product referred to in it. ALSTOM Limited cannot be held responsible for any reliance on any decision taken on its contents without specific advice. PR:065:0799:A Printed in India.