Busbar Fault Detection Systems

The busbar protection shall have two, independent, fault detection systems. The measuring arrangements within the fault detection system shall be effectively duplicated at 400 kV and arranged in a twoout- of-two tripping logic. The overall tripping logic shall be such that both fault detection systems must operate before correct tripping of circuit-breakers can occur (refer to Figures 1, 2 & 3). Each fault detection system shall provide fully discriminative current differential protection for phase-to-phase and phase-to-earth faults occurring within the busbar zone associated with any busbar section. The measuring relays shall comply with the requirements of IEC 255-6. If a biased ifferential principle is used, the protection shall also comply with the requirements of IEC 255-13. At least one fault detection system shall discriminate between the busbar sections of a protected busbar station and both fault detection systems shall discriminate between faults on the protected busbar station and faults elsewhere on the primary system. The fault detection system discriminating between sections of busbar, shall be known as the discriminating system and the protected sections of busbar shall be known as iscriminating zones. The other fault detection system shall be known as the check system and, for the case of single check zone substations, shall discriminate between faults within and external to the protected busbars of the complete substation. Where necessary, multiple check zones may be specified on the contract. For high impedance circulating current schemes, voltage limiting devices shall be provided if the peak voltage developed across the high impedance relay circuit during a busbar fault condition is greater than 3 kV. The characteristic provided shall be declared by the manufacturer.

Current Transformer Supervision

Open-circuited busbar protection current transformer windings or connections shall be detected by supervision of the current transformer secondary wiring circuits. CT supervision is required in both the check and discriminating zones. The CT supervision relay shall be capable of detecting `spill' current in all three separate phases. The CT supervision shall be arranged to give an alarm only and not to short-circuit the bus-wires. The fault detection and supervision system shall be capable of withstanding this condition continuously for the secondary current which corresponds to the maximum primary load condition. In this situation, security against maloperation of the protection is provided by the non-operation of the other fault detecting system. Initiation of the alarm shall be time delayed such that it is not annunciated for genuine busbar faults nor for external faults. A time delayed output covering the range 1.0 - 10.0 s shall be provided. Where this is not the case a suitable external timer shall be provided.

Protection System:-

The overall protection system shall comprise two separately energised independent tripping systems. Each tripping system shall be initiated for both check and discriminating zone fault detection systems in a twoout- of-two tripping logic, and operate the appropriate circuit breakers via a per circuit trip relay(s).These trip relays shall be of the high burden electrically reset latching type, and shall be provided with automatic resetting facilities Where due to burden limitations of the fault detection relay output contact, high burden trip relays cannot be driven directly, then a high speed, double pole switched, self reset trip repeat relay(s) with hand resetoperation indicator, and may be applied to operate the trip relay(s). Provision shall be made within the first tripping system for the circuit breaker fail protection to initiatebacktripping of all contiguous circuit breakers, see Figure 2. Discriminative tripping (and backtripping from circuit breaker fail protection) shall be achieved utilizingdisconnector auxiliary switches or repeat relay contacts .The operation of any single trip relay shall not result in the tripping of more than one circuit. When a busbar fault occurs, all circuit-breakers connected to the faulted busbar shall be tripped simultaneously, whether they can feed fault current or not. The rating of all busbar tripping contacts,connections and fuses/MCBs shall be suitable for meeting this requirement. Typical tripping logic diagrams for the first and second tripping systems are shown in Figures 2 and 3 respectively.

Inputs

Power Supply:-

The tripping systems shall be designed to operate from a 110 V (nominal) D.C. battery Each tripping system shall be provided with a separately fused supply fed from differentbatteries. If an auxiliary energising supply is required by a fault detecting system, it shall be taken from the tripping system supply to which the fault detecting system is connected.

Current Transformers:-

A typical current transformer arrangement, depicting signals to the busbar protection system is shown in Figure 1. For high impedance circulating current protection, class `X' Type `B' CTs shall be provided for both discriminating and check zones. For low impedance schemes alternative CT cores may be suitable and shall be specified by the manufacturer.Separate current transformers shall be utilized for each of the check and discriminating systems. There shall be no disconnector auxiliary switches or repeat relay contacts in the current transformer circuits of the check system. When disconnector auxiliary switches or repeat relays are used for switching current transformer circuits,they shall also be used to short to earth and disconnect the current transformer secondary wiring from any common A.C. bus wiring when the primary circuit is isolated. Check zone current transformers shall be provided on outgoing or incoming circuits.

Bus Section and Bus:-

Coupler circuit-breakers shall not have check zone current transformers, except where multiple check zones are specified. Discriminating zone current transformers shall be provided on all outgoing and incoming circuits. Overlapping discriminating zone current transformers shall also be provided on Bus Section or Bus Coupler bays, (seeFigure 1). In order to prevent incorrect operation of the busbar protection under certain fault conditions within the overlapping zone, auxiliary switches shall be provided in the CT circuits associated with busbar coupler and busbar section disconnectors. Suitable interlocking arrangements shall be provided for the busbar coupler and busbar section circuit breakers and their respective disconnectors to ensure no section of busbar is unprotected during switching operations. The secondary windings of each three phase set of current transformers shall be starred at the current transformer terminal block normally located at the switchgear. The position of the star point, with respectto the protected busbar.

Contact Initiations:-

(i) Primary disconnector auxiliary switches or repeat relay contacts shall be provided to satisfy the busbar protection system requirements stated in Section 3.1.3 and 3.2.2 and shown in Figures 1,2 and 3.The operating sequence of disconnector auxiliary switches or disconnector repeat relay contacts used in current transformer circuits and in 110 V D.C. tripping circuits shall be such that the auxiliary switches or repeat relay contacts operate:

(a) Before reaching the pre-arcing distance on closing the disconnector.(b) After the pre-arcing distance has been exceeded on opening the disconnector. Combinations of normally open and normally closed auxiliary switches or repeat relay contacts from the same disconnector shall `break' before `make' both for opening and closing the disconnector. Where repeat relay contacts are used in lieu of disconnector auxiliary switches, the repeat relays shall be of the latched type. The position of the primary plant contacts with respect to the repeat relay contacts shall be monitored such that any discrepancy from the normal relative positions will cause an alarm to be annunciated. The relay energising supply shall be supervised. Loss of this supply shall initiate an alarm. It shall be possible to determine whether the relay is simulating an open or closed disconnector, by observation of a suitable indicating device on the relay.

(ii) An input shall be provided from the circuit breaker fail protection of each circuit breaker to initiate back-tripping of all other circuit breakers associated with the same busbar zone(s) via the busbar protection first tripping system.

D.C. Initiations:-

The protection system shall be provided with inputs to receive D.C. signals for the following:-Trip relay reset (110 V D.C.).

Outputs

Trip Outputs:-

The trip relays of each tripping system shall be provided with output contacts for initiation of the following:-

(i) Tripping the associated local circuit-breaker(s).(ii) Circuit-breaker fail protection.(iii) Intertripping to the associated remote circuit-breaker(s). (Two channels each being double poleswitched for feeder circuits). (iv) Unblocking/accelerating or unstabilizing of feeder protection.

Facilities shall be provided to select or deselect direct tripping of remote circuit breakers from intertripping, unblocking, acceleration or unstabilising, as part of the busbar protection tripping scheme. Selection of intertripping, Unblocking/acceleration, or unstabilizing direct from the busbar protection triprelays shall be as specified by NGC on particular contracts. Where direct tripping is deselected, then tripping of the remote circuit breaker(s) shall be initiated via theCircuit Breaker Fail Protection

Additional Outputs:-

The protection system shall be capable of providing outputs for initiation of the following:-(i) Disconnector sequential isolation.(ii) Trip relay reset.(iii) Lockout of auto reclosure.

Alarm Outputs:-

Protection operation or failure shall be annunciated through the substation alarm and event logging system.The following protection alarm outputs shall be provided:-(i) Busbar protection zone operation (refer to Figure 4).(ii) Trip relay operated indication (where required).(iii) Protection supply supervision alarm for each fused supply (iv) CT Supervision.

(v) Disconnector Auxiliary Switch repeat relay discrepancy alarm (where required).If the protection equipment is of an electronic type, an alarm output of equipment inoperative shall beprovided for the loss of D.C. auxiliary energising supply and internal relay failures.

Fault Recorder:-

Operation of the busbar protection trip relays shall trigger, and be recorded on, a fault recorder.

Operator Interface:-

The following facilities shall be provided for the protection system, where appropriate,

(i) Setting Facilities.(ii) Indications.(iii) Isolation and Testing Facilities. Facilities shall be provided to enable each relay element current to be measured where applicable. Each of the three phase connections from every set of current transformers shall pass through an isolating link or equivalent before being connected to any relay, common bus-wire or busbar selector auxiliary switch.In one position the link shall give normal connection and in the other position it shall short out the current transformer and it shall isolate the current transformer from its associated phase buswire. The three phase set of links, shall have suitable test terminals. The test links shall normally be physically located at the associated circuit-breaker marshalling kiosk, or, in the case of 400 kV busbar stations where circuit-breaker marshalling kiosks may not be provided, at the individual relay room busbar protection rack, panel or primary marshalling cubicle. Where two or more check zones are specified, the test links associated with the current transformers installed at the reserve busbar sectioning point shall be physically located in a marshalling kiosk or boxwhich shall be provided adjacent to the sectioning point. Where the circuit-breaker fail protection utilises the busbar protection trip system for back tripping, sufficientisolation facilities shall be provided to enable complete per circuit trip testing on an individual circuit-breaker fail system without the risk of backtripping to other circuits.Trip circuits in the busbar protection panel or rack shall be fully insulated or shrouded on the trip relay and trip coil(s) sides of fuses or links.