Each 7TM70 unit has
– five rectifier units to be used for five CTs.
– five output relays each having two contacts. Each relay to be used for one feeder for tripping the lock out relay/ circuit breaker.
The 7TM70 units are placed in a casing type – 7XP20. One casing can take maximum four 7TM70 units. The 7XP20 casing is flush mounting device.One 7SJ62 relay is used per Bussection for CT circuit supervision. The relay is mounted in Bus PT panel.
F. Comparison between High and Low impedance scheme :
Please find below the comparison between High and low Impedance schemes:
1. In High Impedance schemes, all Main CT must have same ratio and separate CT cores are required for Busbar protection.This is not a mandatory requirement for Low Impedance scheme.
2. Special CTs (Class PS) required for high impedance due to high burden of relays.Normal protection class CTs suited for Low impedance schemes.
3. Low Impedance scheme relays can provided fault disturbance records,communication etc which are not available in high impedance scheme relays.
4. In high impedance scheme, higher currents(1/5A) are switched/ connected in parallel across all feeders. However for low impedance scheme very low current is used i.e 100m A.
The 7SS60 relay also provides CT circuit supervision scheme.The supervision scheme operates from the same current input ‘Id’ of the 7SS60 relay.However it operates on a very low setting range.
In case any of the CT is open circuit or the wiring between the CT and the 7SS60 is disturbed (open) then the CT circuit supervision picks up and gives an alarm. During this stage the Busbar protection is blocked i.e. if during this stage any internal fault occurs, then the busbar protection will not get activated. This is provided as a safety measure to prevent any maloperation which can result into tripping of all feeders. At the same time input connection to the ‘Id’ coil of 7SS60 is shorted. This is done to prevent the affect of any high voltages (due to open circuit ) to 7SS60 relay. Thereafter the operator /maintanence engineer has to check each and every circuit / tighten all connections and then reset the Busbar Protection for normal operation.
The lowest possible setting available for CT circuit supervision is = 10% of nominal current This setting is not suitable for feeders with lower CT ratios i.e. <150 A. For open circuit of the CT of these feeders, the CT circuit supervision of 7SS60 may not operate.
Further during the commissioning of HCU Panels at M/s BPCL we were informed that the Busbar protection should not be blocked in case of CT circuit supervision operates.Though it is not recommended we have provided the following modifications in the scheme.
An additional contact of 7SJ62 relay is connected in series with the 7SJ62 coil.
Thus when 7SJ62 relay operates,
Thus with the above modifications, The Busbar protection remains blocked for certain time and then resets and then again gets blocked . Thus a toggling scheme is provided where in the for few secs the scheme remains blocked and for few secs therafter remains ‘on’ and ths cycle is repeated continuously until the CTs are checked /connections are checked/tightened. Once all the CT and connections are healthy the 7SJ62 relay will not pick up and will not block the Busbar protection.
If toggling scheme is to be switched to non toggling i.e permanent blocking then the output
contact of 7SJ62 relay is to be programmed as latched type.
1. With Buscoupler Open:
This is simplest operation. If a fault occurs in Bussection A then the 7SS60 relay for that bussection will operate and trip the feeders on Bus A.
2. With Buscoupler Closed :
As you can see from the enclosed drawings, the CT connections are crossed for the Buscoupler i.e. the Buscoupler CT on bus B is connected to the Bussection A 7SS60 relay and vice versa. This is done to ensure correct tripping of Busbar protection relay for faults between the Buscoupler Breaker and the CTs.
For e.g: For a fault between the Buscoupler breaker and the CT T4-T6, the Busbar Protection relay for Bus A will operate and trip all feeders on Bus A including the Circuit breaker. In this case the Bus B continous to run in normal operation.
The 7SS60 relay scheme can be utilised in two ways:
1. With summated CT :
In this case the Main CT secondaries of all three phases is connected to a Summated CT. The summated CT acts also as a matching CT.Since all three phases are summated, only One 7SS60 relay is required.This scheme is used normally for 3.3KV to 110 kv voltage levels where the tripping is three phase.
2. With Matching CT :
In this case, each phase Main CT secondary is connected to a individual auxiliary CT.Since each phase circuit is used, three separate 7SS60 relays are required.This is scheme is used normally for higher voltage levels i.e 132 KV and above where the Circuit breakers are single pole operated.The auxiliary CT (matching or summated) have no. of turns on the primary side. These turns are used to provide the matching turns ratio for different Main CT ratios.Thus it is not required that ALL the Main CTs should have the same ratio for Busbar Protection as in case of high impedance scheme.Further you can also use the same CT for other relays and dedicated CT core for Busbar protection is not required.
Refer the enclosed scheme drawing. There are two bussections and for each bussection a separate Busbar Protection relay – 7SS60 is provided.The type of scheme used is summated principle wherein the CT secondaries of all three phases is connected to a summation CT. The output of the summation CT is then connected in parralle l and connected to the ‘Id’ input of the 7SS60 relay.The output of each summation CT is also connected to the rectifier unit 7TM70. The output of this 7TM70 unit is then connected in parallel and connected to the ‘Ir’ input of the 7SS60 relay.The relay now has both the inputs i.e ‘Id’ and ‘Ir’ and thus provides the Busbar protection as explained above.
Differential protection is based on the law of kirchhoff : in a healthy system, the sum of currents in a node must be zero. This is a ideal case. But in practice, CT errors and measuring errors need to be considered. For that reasons, the protection needs to be stabilized. The differential criteria and the restraint criteria are defined as follows
Differential Current : I diff = [I1 +I2+I3+……+IN] (vector sum)
Restraint Current : I restraint = [I1]+[I2]+…..+[IN] (magnitude sum)
7SS60 is a low impedance Busbar Scheme.
As can be seen from the enclosed drawing, the main CT secondary currents are not directly used (paralleled ) as in case of high impedance scheme. The secondary of each main CT is connected to a auxiliary CT (either summation or matching CT). The Auxiliary CT output which works on low current (100m A) is then connected in parallel for all feeders. The parallel connection is then connected to the ‘Id’ input of the 7SS60 relay. At this input the vector sum of ALL currents is given as a input to the relay. In ideal case as per kirchhoffs law this is almost zero.
Since for direct tripping of feeders, separate contact may be required from the busbar scheme. This is achieved through the 7TM70 relay. This relay has five coils (relays) each have two contacts. The 7SS60 output contact energises the coils (relays ) of 7TM70 relay which in turn are used for tripping the Circuit breakers directly or through lockout relays .
– It has rectifier units to rectify the CT currents.
– It has output contacts for direct tripping of circuit breakers/ lockout relays.