To provide protection within a specific zone within a switchboard i.e. Internal busbars and switching devices. To be fast acting & avoid nuisance tripping (CT saturation). Traditionally high impedance type systems used within MV switchgear. Simple principle of operation: Based on Kirchoffs first law. Matched CT’s are located on all circuits within the protected zone. Each phase monitored individually by a relay element. Provides protection against phase-to-phase and phase-to-earth faults. Under normal conditions the sum of the CT currents = zero. Under fault conditions equilibrium is lost resulting in a trip signal. Relay trips CB’s in protected zone. Relay command time 15-20mS + 60-75mS CB trip time = 75-95mS
First conceived over 80-years ago, wide spread usage for over 50-years. Question: Why was busbar differential protection necessary? Switchgear was unreliable and prone to failure Mechanical failure (switching/racking mechanisms, interlocks, corrosion) Insulators and Insulating material failure over time (oil, bitumen, paper, solid insulation). Occurrence of PD due to poor manufacturing quality Ingress of pollution & rodents. Internal failure would usually lead to total destruction of the switchgear and could easily kill operators in the vicinity. Often does not cover cable terminations, due to CT locations. Rapid disconnection times were needed to limit damage to switchgear & substations. Switchgear failure modes were unpredictable & very dangerous! Internal arc fault containment testing not conceived until 1970’s.
Utilities clients: Unwilling to break with tradition Unaware or unconvinced of new technologies / concepts Manufacturers have not been proactive in introducing new technologies Industrial clients: As above The use of BDP is heavily influenced by engineers migrating from the Utility industry.
Siemens. Innovation for generations.
Busbar Differential Protection in MV Switchgear
Alternative Solutions
ARC (Light) detecting systems: Light + current sensing prevents nuisance tripping 10mS command time Monitors entire cable compartment, not restricted by CT locations. Optical sensors in all compartments
Protection Relay
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Pressure detecting systems Similar to above 15mS command time ≈
ARC eliminators, 5mS operation IS limiters, 1-2mS operation
Internal faults have all but been eliminated within modern type tested switchgear. IEC 62271-200 arc fault containment tested equipment ensures that switchgear is able to contain internal arcing faults in a safe and controlled manner. IEC 62271-200, defines new levels of personnel safety. Pressure wave peak occurs 10ms, therefore mechanical damage cannot be avoided. Minimum use of insulation reduces fire damage after 100mS BDP CT’s are problematic within modern switchgear BDP schemes are expensive to implement ≈
BDP does not protect the most vulnerable part of the switchgear – the cable terminations
Busbar Differential Protection in MV Switchgear CONCLUSIONS In the event of an internal fault IEC 62271-200 switchgear is capable of containing a fault safely. The need to install BDP or other high speed ARC detection & switching devices is questionable in this circumstance. Allow OC and EF protection to trip CB’s. 400-500mS absolute worst case scenario. In the case BDP or high speed ARC detection devices being used extensive damage will still occur within the switchgear. Therefore they serve little purpose in conjunction with TTA’s.
High speed detection devices only need to be deployed if 1s IAC equipment is not used.
Best practice adopted within all AIS & GIS switchgear products: Use of type tested switchgear, IAC tested to 1.0s to IEC 62271-200 All switching devices repeat type tested within the panel All conductors bare copper air or gas insulated. Minimum use of insulation materials: Creepage & clearances distances maintained under all conditions Reduces possibility for PD & disruptive discharges Minimum fire load under fault conditions Use of proven vacuum interrupters within AIS & GIS Vacuum interrupters to IEC 62271-100 class E2, M2, C2 Class PM partitions class & loss of service continuity category LSC2B All mechanisms are ultra reliable, designed & tested to IEC standards Positive indication of switching device positions / status to IEC standards Inherent fool proof interlocking ensures safe & reliable operation.
GIS 8DA10 GIS: single pole encapsulated construction, prevents phase-to-phase faults NX PLUS C and 8DH10: silicon rubber insulated busbars. NX PLUS C and 8DJ/H10: sealed for life, no gas work onsite.. ever IP65 Hermetically sealed construction, independent of climate. IP65 plug-in cable terminations cable terminations. Completely maintenance free for life.
