High Voltage Switchgear
5.2 HV Switchgear Panel
5.2.1 The Bus Bar Compartment
The main bus bars are made of copper or aluminium and can be rectangular or D-shaped depending on the current rating. The main bus bars and the T-off bus bars are protected by heat shrinkable tubes that serve as an as insulating material. A specially designed insulating cover on the bus bar connecting points ensures that no live parts are exposed; also, any ingress of dust and moisture will not cause any flash over.
Figure 5.4 The Bus Bar Compartment
All bus joints are torque tightened with the standard torque value, and covered with removable insulation boots for easy inspection. The buses are supported and braced to withstand a related arc supported by the T-off bus bars and no supporting insulators are required for the main bus.
The switchgear is equipped with bushings to prevent the arc fault from migrating from one bus bar compartment to other bars to reduce the probability of an insulation breakdown.
5.2.2 The Circuit Breaker Compartment
The insulating spouts in the circuit-breaker compartment hold the fixed contacts and facilitate the connecting of the apparatus to the bus bar compartment and the feeder compartment respectively. They are made of epoxy resin.
Main bus bar
Primary bushing
High Voltage Switchgear
The shutters are metallic and are activated automatically during the movement of the apparatus from the racked-out position to the service position and vice versa. The circuit- breaker compartment can be equipped with a circuit breaker, or other units such as a fused contactor, bus bar earthing truck, etc.
Figure 5.5 Parts of a Circuit Breaker Compartment
In case of an emergency, the door can be opened even if the VCB is not in the test / disconnected position by turning the emergency door opening bolt with an L wrench.
Door interlock actuator
Screw
mechanism
Metal shutter Temp. Sensor
Earthed Contact
Chapter 5
5.2.3 The Cable Compartment
The cable compartment contains the branch system for the connection of power cables to the lower contacts of the circuit-breaker. The cable compartment of each outgoing feeder panel can be fitted with an earthing switch for cable earthing. An indicator on the front of the i chgea h he i i f he ea hi g i ch. Acc di g he e e i e e , the cable compartment can be equipped with a surge arrester, an inline PT, CT and zero sequence current transformers, etc.
5.2.4 Core Balance Current Transformer (CBCT) / ZCT
(CBCT) or Zero CT (ZCT) is a ring type current transformer through the centre of which a three-core cable or three single-core cables of a three-phase system passes. This type of current transformer is normally used for unbalanced current protection (46) for medium voltage systems.
Figure 5.7 Side View of the HV Switchgear Panel
Earthing Switch
Surge Arrestor Low Voltage
Compartment
Circuit Breaker
PT Cable
Compartment CT
Bus bar
Compartment
High Voltage Switchgear
During normal operating conditions, as the vector sum of the three-phase current is always zero, no residual current in the primary will be present and this means a zero-sequence current.
Therefore, there will not be any flux developed in the CBCT core and hence no current in the secondary circuit of the CBCT.
When an earth fault / single-phasing occurs in one of the phases, the zero- sequence fault current will be unbalanced and hence induces a current in the secondary.
Figure 5.8
A ZCT in the Cable Compartment
5.2.5 Surge Arrestor
A surge in voltage is a transient over voltage that steeply rises, followed by a slowly decaying voltage wave. This is caused due to the switching action of certain types of circuit breakers that are connected to inductive loads especially SF6 and vacuum circuit breakers.
Voltage surges of this nature are known as internal or switching surges which can damage the insulation.
The type of surge protection that is installed depends upon the voltage and insulation of the system. Surge arresters or suppressors are connected on the load side between all the three phases and the ground.
The heart of surge arrestors are Metal Oxide Varistors (MOVs), which is highly sensitive to voltage. At normal voltages, the MOV works as an insulator and does not allow current to pass
Power Line ZCT / CBCT
Chapter 5
5.2.6 Metal Oxide Surge Arrestor In Figure 5.10(a) of the surge arrestor, the power cable, cable clamp and ZCT are visible.
Zinc Oxide (90-95%) is used with 5-10% of additives such as Alumina, Antimony Trioxide and Bismuth Oxide.
Figure 5.10(a) Metal Oxide Surge Arrestors
Possible voltages without arrestors
Withstand voltage of equipment
Arrestor power frequency
withstand voltage Voltage limited
by arrestors
Resulting voltage stress in the system
Duration of over voltage Magnitude of over voltage / p.u.
Fast-front (lightning) over voltages sec
Slow-front (switching) over voltages m sec
Temporary
over voltages sec
Highest system voltage Continuously
Figure 5.10(b) Characteristic Curves of a Surge Arrestor
High Voltage Switchgear
5.2.7 The Low Voltage Compartment
All the secondary instruments such as voltage and current, protection relays and control related cabling are housed in the low voltage compartment. Manual operation of the breaker is also done from the low voltage compartment.
Figure 5.11 A Low Voltage Compartment 5.3 Ship s High Voltage Switch Board
5.3.1 Access and Lighting
Switchboards should have an unobstructed passageway not less than one metre wide in front of the switchboard if the switchboard contains withdrawable equipment e.g. a circuit breaker and starter chassis; the unobstructed passage should not be less than 0.4 m wide.
Anti-slip coverings, which in addition are insulated, should also be used in marine applications.
Access at the rear of a switchboard should be not less than 0.6 m to allow adequate room for maintenance; an account should be taken of any stiffeners or frames.
Switch rooms should not be used as workshop or storage areas, the lighting should be sufficient for personnel to work safely and avoid danger as stated in the Electricity at Work Regulations.
Chapter 5
On-board ships the switchboard is usually subdivided. The subdivision may be effected by the bus tie circuit breakers or other suitable means, with the generator inputs and duplicated services split equally.