MCVG 61 [PDF]

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Midos



Type MCVG 61 Voltage Dependent Overcurrent Relay



Features ● Site selection of characteristics for voltage restrained or voltage controlled overcurrent ● Wide setting range ● Green led start indicators to facilitate testing ● Low ac burden ● DC power supply operates over a wide input range ● Separate test mode with trip test feature ● Non volatile memory for led trip indication



Figure 1: Type MCVG 61 relay withdrawn from case.



Models available MCVG 61 versions are available for phase-neutral and phase-phase connection.



Application For backup overcurrent protection of generators, ordinary overcurrent relays are sometimes difficult to apply, due to the decaying characteristic of the fault current. The value of the fault current will be progressively reduced due to armature reaction, to a value less than full load current. Therefore, normal overcurrent relays, set above the load current or maximum permissible overload, cannot be applied to provide time delayed backup protection as they will not operate for these fault conditions. For successful application of generator backup protection, the operating characteristics of the relay



2



are required to be a function of voltage and current. There are two types of relay that are customarily used for these applications, namely voltage restrained and voltage controlled overcurrent relays. With voltage restrained overcurrent relays, when the voltage falls below a set value, the operating time of the overcurrent characteristic is continuously reduced with declining voltage. In voltage controlled overcurrent relays, the operating time characteristic is changed from the load characteristic to the fault characteristic when the voltage falls below a set level. The MCVG 61 is a three phase voltage dependent overcurrent relay with both voltage restrained and voltage controlled characteristic available from the same relay. The desired mode is selected by a switch on the front of the relay.



Description Type MCVG 61 relays consist of a microcontroller based overcurrent element (identical to MCGG relays, see R6054) with one element available for each phase. The current measurement is performed by an analogue to digital converter (ADC) which forms an integral part of the microcontroller, the program in the microcontroller constantly samples the output of the ADC and compares the value with a predetermined reference. If the output of the ADC remains above the reference value, then the microcontroller will give a trip signal after a time delay, which is determined by the algorithm for the selected time delay characteristic. When a trip signal is given, the trip led is set to ‘on’ and the state remembered in the non volatile EEPROM. This ensures that, if the power to the relay is lost, the trip



indication will revert to the last state on re-applying the auxiliary power. Voltage controlled and voltage restrained overcurrent is achieved by controlling the amount of current seen by the overcurrent detectors. Voltage Controlled Overcurrent Mode In this mode the MCVG 61 operates with the load characteristic above a set voltage level. Below this voltage the relay operates with the fault characteristic. Voltage controlled overcurrent is achieved by the use of voltage level detectors. The principle of operation is that, if the instantaneous amplitude of the input voltage exceeds a preset threshold setting during any half cycle, a timer t1 is started. If t1 times out before the signal has fallen below the reference voltage, the output of the latch is set high. This latch turns the analogue switch on, which diverts current from the overcurrent measuring circuit to give the overload characteristic. When the input voltage falls below the preset threshold setting during any half cycle, a timer t2 is started. If t2 times out before the signal has exceeded the preset reference voltage, the output of the latch is set low. Resetting the latch causes the analogue switch to turn off resulting in the measuring circuit giving the fault characteristics. Voltage Restrained Overcurrent Mode In the voltage restrained mode the MCVG 61 operates on the load characteristic above 90%Vn. Below that voltage the relay pick up is reduced, depending on the magnitude of the reduction in the relay voltage. Thus the relay has a load characteristic during healthy conditions and a family of characteristics during fault conditions, when the voltage is less than 90%Vn. Voltage restrained overcurrent is achieved by taking the TRMS value of the input voltage. If this voltage is between the limits of 0.9Vn>V>0.55Vn, a voltage proportional to the input voltage is applied to the analogue switch. This diverts a proportion of the input current from the measuring circuit resulting in a gradual transition from fault characteristic to overload characteristic with increasing voltage.



Power for the relay is supplied by a dc to dc converter which is designed to work over a large voltage range. This allows one relay to cover several battery voltages. The dc to dc converter gives totally isolated outputs giving a high degree of noise immunity to the relay. Relay Settings Separate setting switches are provided for each phase on the relay front plate. These are used to select the required time/current characteristic, voltage restrained or voltage controlled operation, overload and fault setting current, voltage controlled voltage setting and time multiplier settings.



Selection of time characteristics The time/current characteristic selection is carried out by means of ) three switches identified by the ( symbol on the nameplate. Table 1 gives the basic operating characteristics and the settings of these switches. The time given by each of the operating time characteristics must be multiplied by the time multiplier to give the actual operating time of the relay. This control is marked xt = ∑ where ∑ is the sum of all the switch positions. The range of multiplication is 0.05 to 1.0 in steps of 0.025. This acts as a convenient time multiplier on the current dependent characteristics and gives the following ranges for the definite time characteristics:



Operating characteristics (seconds)



Time range (seconds)



2 4 8



0.1 to 2.0 in 0.05 second steps 0.2 to 4.0 in 0.1 second steps 0.4 to 8.0 in 0.2 second steps



(0)



Switch position (1)



Operating characteristic



● ●



Trip test



● ● ●



Standard inverse



sec SI



Very inverse



sec VI



Extremely inverse



sec EI



Long time earth fault



sec LT



● ● ● ● ● ● ● ● ● ● ● ●



Definite time 2 seconds



D2



Definite time 4 seconds



D4



Definite time 8 seconds



D8



● ● ● ● ● ● ● Table 1: Operating time characteristics with corresponding switch positions, where I is the current applied to the relay.



3



100



10 Definite 8 seconds



Operating time t (seconds)



Definite 4 seconds Longtime standby earth fault t= 120 I – K∆Is –1 Is



Definite 2 seconds



Standard inverse 0.14 t= I – K∆Is 0.02 –1 Is 1.0



Very inverse t=



13.5 I – K∆Is –1 Is



Extremely inverse 80 t= I – K∆Is 2 –1 Is 0.1 1.0



10



100



I – K∆Is Is



Figure 2: Time delayed overcurrent, operation time characteristic.



4



5



22



24



26



28



21



23



25



27



36



38



37



46



48



50



45



47



49



42



34



33



35



41



30



29



B



Short terminals break before (c).



Long terminals.



(b)



(c)



b



B



See Note 1



a



n



N



A



c



C



P2 S2



P1 S1



Vx –VE



+VE



IC



IB



IA



1



14



13



26



25



46



45



24



23



42



41



22



21



38



37



Case earth connection



Power supply circuits



>V



>V



>V



Is



Mode selection



Vs



Is



Mode selection



Vs



Is



Mode selection



Vs



Figure 3: Application diagram 10 MCVG 61 02 sheet 1 issue D. (Diagrams 10 MCVG 61 02 sheets 2 and 3 show variations).



2 CT connections are typical only.



CT shorting links make before (b) and (c) disconnect.



C



B



A



(a)



1 Earthing connections are typical only.



Notes:



Module terminal block viewed from rear (with integral case earth strap)



14



13



1



Case earth



C



A



t



∆Is



t



∆Is



t



∆Is



PhA



µC



PhA



µC



PhA



µC



Curve selection



Curve selection



Curve selection



Trip



Trip



Trip



I>Is



I>Is



Output circuits Ph



I>Is



Reset



RL1-2



RL1-1



RL1 2



30



29



34



33



35



Trip output contacts



Technical Data Ratings AC current (In)



1A or 5A



AC voltage (Vn)



110V or 63.5V



Rated frequency



50Hz or 60Hz



DC Auxiliary voltage (Vx)



24/54V or 110/250V



Burdens AC current



Less than 0.25V for 1A relays and less than 0.5VA for 5A relays at unity power factor and at rated current on any setting. The impedance of the relays over the whole of the setting range is less than 0.25Ω for 1A relays and less than 0.2Ω for 5A relays and is independent of current.



AC voltage



Less than 0.1VA at rated voltage



DC voltage



Relay rating



Burden



24/54V 110/250V



8.5W 11.5W



The figures above are maxima under quiescent conditions. With output element operated they are increased by up to a maximum of 3W. Setting ranges Voltage setting



55% to 90% Vn in steps of 5% Vn



Voltage control mode Current pick up level Is ∆Is



Is + K∆Is



K = 0 when VVs



5% to 240% In



in steps of 5% In



50% to 200% In



in steps of 10% In



Where V is the applied ac voltage Drop off to pick up ratio of under voltage detector



>107%



Voltage Restrain mode Current pick up level



Is + K∆Is K=1 K=



when V>90% Vn



(V/Vn – 0.55) ( 0.35 )



when 55% Vn