BUS BAR AND LBB PROTECTION (1).pdf

SUTANU BANERJEE S.D.E(E), CRITL DVC, MAITHON

JANUARY 2016

What is ‘Busbar Protection’ ?  Bus Bar Protection scheme aims to protect the

whole ‘Bus’ of a Switchyard by tripping of all the associated Bays (connected to that particular ‘Bus’) in case of fault in a ‘Bus’ itself, without waiting for tripping from other end/backward looking protection and thereby ensures ‘Faster’ clearance of the Bus fault.  In case of Multiple Bus arrangement it also ensures isolation of the faulty Bus only and thereby avoiding TPF at the station. It is beneficial for both Power System Stability and Commercial aspect.

Need for Busbar Protection Need For Busbar Protection  In its absence fault clearance takes place in zone II of distance relay by remote end tripping.  This means slow and unselective tripping and wide spread black out with commercial implications. Effect of delayed clearance  Greater damage at fault point  Chance of Failure of CT and other collateral damages  Shock to connected costlier equipments like Generator ,transformer.

Requirements of Busbar Protection  Must have very fast fault Clearance time.  Must be able to detect internal faults (sensitivity).  Must be absolutely stable to external faults

(stability).  Must be able to detect and trip only faulty part of Busbar system (selectivity)  Must be secure against mal operation due to auxiliary contact failure / CT secondary circuitry problem etc.

CAUSES OF BUS ZONE FAULTS  Deterioration of Insulating Material.  Flashover of insulators due to lightning or System

Over Voltages.  Wrong application/operation of /or failure to remove temporary earth connections.  Short circuits caused by birds, monkeys, snake etc.  Short circuits caused by construction machinery.

How it operates ? The basic operating principal of Bus Differential Protection is based on Kirchhoff's Current Law (KCL) principal i.e. sum of all the currents (entering or exit) in a particular node is Zero. Here the node is ‘Bus Bar’ itself and the associated currents are the currents of all the Bays connected to that particular ‘Bus’ . Although it is quite obvious that KCL is applicable for all the faults whether it is Bus fault or an external/through Fault, detection of Bus fault depends on the fact that all the fault currents are being measured by the CTs in case of a through fault and in case of a Bus Fault, the Bus fault current remains unmeasured causing B/D protection to operate. However, the most important factor here is the proper connection of the CTs (Both Polarity and Ratio) used for Bus Bar Protection

Busbar Protection Current distribution for a through Fault :

For an external fault at Fdr#5, B/D Relay current : I1+I2+I3+I4+ (-) I5 (or IF) = 0 (as I1+I2+I3+I4= I5 OR IF) (CONSIDERING ‘+’ VE SIGN FOR CURRENT ‘IN’ AND ‘-’ VE SIGN FOR CURRENT ‘OUT’ THROUGH A FFEDER ) THEREFORE THE RELAY WILL REMAIN STABLE DURING A THROUGH FAULT

Busbar Protection Current distribution for a Bus Fault :

For a Bus fault, B/D Relay current : I1+I2+I3+I4+ I5 ≠ 0 (as I1+I2+I3+I4 +I5 = IF ) (CONSIDERING ‘+’ VE SIGN FOR CURRENT ‘IN’ AND ‘-’ VE SIGN FOR CURRENT ‘OUT’ THROUGH A FFEDER ) THEREFORE THE RELAY WILL OPERATE SENSING FAULT CURRENT

Types of Busbar Protection Busbar protection may be divided into two groups  Low impedance scheme : Low impedance scheme uses the principal of Biased Differential Relay to attain stability in case of CT Saturation/ CT error/through fault. i. It may be Centralised Unit Scheme or ii. Distributed Peripheral Unit with Central Unit Scheme  High impedance Scheme: High impedance scheme uses a stabilizing resistor in series with basic O/C Relay to attain the stability for CT Saturation/ through fault.

DISTRIBUTED LOW IMPEDANCE B/B PROTECTION STRUCTURE :

HIGH IMPEDANCE B/B PROTECTION STRUCTURE :

Busbar Schemes used in DVC  In DVC both High Impedance and Low impedance

Busbar scheme had been adopted. Earlier only Electromechanical/Static type Relays for High Impedance Busbar protection were used mainly in the 220kV Switchyard of Power Houses and Substations. Now a days with the advent of the Numerical Relays Low impedance Busbar Protections had been adopted in the 400kV Switchyard and also in new 220/132 KV S/Stn such as Giridih (New), Dhanbad.  In the 400kV Switchyard Busbar protection Main-1 and Main-2 Protection philosophy had been adopted

Bus configuration and Protection Schemes Depending on the Bus configuration such as 1 Main Bus 1Transfer Bus (mainly in 132kV Switchyard), 2 Main Bus 1Transfer Bus (mainly in 220kV Switchyard, 400 kV MTPS New) and One and half Breaker system (400 kV Switchyards except MTPS New) , different CT secondary and DC Circuitry arrangement is required.  In case of Single Main Bus system, Bus Bar Protection scheme is less complicated in respect of both CT and DC circuitry as this does not require any selectivity for Bus Current measurement and tripping.  In case of Double Main Bus system, selectivity is required for detection/isolation of the faulty Bus from the healthy one. Separate CT switching Relays are required for this purpose. In case of Numerical Relay this function is in built, therefore, no separate Relay is required. However in both the cases Main Bus 1&2 (and also of TB side ) Isolator contacts (open and Close) are required for individual Bus current matching as well as tripping circuit. The scheme gets more complicated with Transfer Bus and Bus Section (multiple Bus coupler) arrangements.

Bus configuration and Protection Schemes (Continues)  In One Half Breaker System in DVC, Numerical Relays

with Low Impedance Busbar Scheme had been used. In DVC both Distributed Numerical Relay Busbar Scheme (400 kV MTPS New, 400 kV DSTPS, 400 kV KTPS Switchyard) and Centralized Numerical Relay Busbar Scheme (400kV RTPS S/Y) is in service. Also Two set of independent Busbar Scheme (Main-1 and Main-2) is used in 400kV/One and half Braeker Scheme.

1 MB AND 1TB RRANGEMENT

TRIPPPING IN CASE OF BUS FAULT

2 MB AND 1 TB CONFIGURATION

TRIPPING IN CASE OF BUS FAULT

ONE AND HALF BREAKER CONFIGURATION

TRIPPING IN CASE OF BUS FAULT

What is Main Zone and Check Zone Protection :  Main Zone Protection : In case of multiple Bus Bar arrangement each Bus comes under Main zone protection. The current of all the Bays associated with the Bus are added with proper polarity to form the Bus Differential protection of the said Bus. It uses Isolator Status(through auxiliary contacts of Isolator) for segregation of each Bus Zone. It isolate the faulty Bus only in case of Bus fault in Multiple Bus arrangement.  Check Zone Protection : Check Zone basically covers all the Buses under Bus Bar Protection Scheme. This is summation of currents of all the Bays connected to the multiple Buses. As the Check Zone does not require any selectivity, this is independent of Isolator Status (Auxiliary Contacts) and therefore more reliable though not selective . For this reason each Main zone Protection tripping is ‘AND’ed with Check Zone protection for individual BUS tripping. If only Check Zone Protection is in Service, in case of Bus fault it will initiate tripping to all the Buses creating TPF at the Station.

BUS DIFFERENTIAL ZONES :

WHAT IS ‘CT SWITCHING RELAY’ ?  In case Multiple Main Bus configuration like Two Main

Bus One Transfer Bus Scheme any Bay can be connected to either Main Bus-1 or Main Bus-2 .The same is true in the diverted condition also. Therefore, it is essential to identify in which Bus the particular Bay is connected and distribute the current accordingly in the Bus Differential Circuit and also initiating tripping accordingly. This is achieved by ‘CT Switching Relay’. CT Switching Relay is basically electrically ‘Set’ / ‘Reset’ type of Relay and Relevant Isolator Contacts are used to ‘Set’ / ‘Reset the Relay .

CT SWITCHING RELAY CONFIGURATION

BUS WIRE FORMATIN FOR CT CIRCUT

CT Supervision Relays  This is a three phase monitoring Relay/Feature used in Bus Differential protection scheme (for each and every Zone i.e main Zones and Check Zone) to avoid mal tripping and detection of any Open circuit/Short circuit in the CT Secondary Circuit. The Relay operates on the principal that in case of such fault in CT Secondary Circuit, there will be some mismatch current in that particular Bus current summation(which may or may not be as high as to operate the Protection). This mismatching current is utilized to detect any problem in CT Secondary Circuit with adequate Time Delay ( 5-8 Sec.) so that it does not interfere with the main protection (which also operates due to current mismatch) and also ensures sustaining nature of the fault. If CT supervision Relay operates, it will block the Bus Differential protection of that very Zone to avoid malt ripping.

COMBINED LBB & B/D SIMPLIFIED DC CIRCUIT

Generator Status after B/B Relay(96) optd :  In 210 MW Units though having Class ‘C’ Protection,

in case of B/B Protection operation Unit will trip through Class A Lock-outs  In 250 MW/500MW Units if Bus Bar Operates Class C Lock-outs operates so that the Unit may run in the House load . Note : In most of the Power Houses it was not possible to run the Unit only in House load condition (mainly due to turbine Over speeding), therefore , the original scheme had been modified and Class-A tripping had been provided in case of 96 Relay operation.

Breaker Failure Protection (LBB) In modern networks the critical fault clearing time may be less than 200ms. Hence, if the fault is not cleared due to failure of the primary protective relays or their associated circuit breaker, a fast acting back-up protective relay must clear the fault. LBB is a protection designed to clear a system faulty by initiating tripping other circuit breaker(s) in the case of failure to trip of the appropriate circuit breaker due to problem in the Breaker itself or the associated DC Circuit.

LBB TRIP LOGIC

LBB RELAY DWG IN MTPS U#5&6 (250MW)