The HT substation is composed of the following equipment and devices.
- Disconnecting Switch
- Circuit Breaker
- Power Fuses
- Potential Transformer (PT)
- Current Transformer (CT)
- Protective Relay
Disconnecting switches are devices to open/close a high voltage circuit when high-voltage equipment are inspected, tested or cleaned. The devices are capable of safely breaking no-load current but not load current.
For safe O&M work, be sure to open/close the disconnecting switch only after opening a circuit breaker, which is located on the secondary side, just downstream of the disconnecting switch.
Circuit breakers are switches that open/close electric-circuits in normal and abnormal conditions (especially in short circuit).
Therefore, the circuit breakers must be capable of tripping the circuits in conjunction with protective relays and by cutting off the short-circuit current definitely and safely, avoiding accidents due to high current.
Circuit breakers for high voltage are categorised into the following types according to their techniques of eliminating arcs:
A. Air Circuit Breakers (ACB)
B. Vacuum Circuit Breaker (VCB)
C. Inert Gas Method (SF6)
The function of a power fuse is to sense and prevent flow of excess current in electrical devices and electrical wire by melting the fuse element and thereby breaking the electric circuit when subjected to a short-circuit.
Power fuses are typically used for smaller electrical systems because they have the capability and speed for breaking the circuit as compared with circuit breakers.
A proper O&M, practice is that even if only one fuse melts due to an accident such as a short-circuit in a three phase switch, all power fuses including the melted one should be replaced.
Voltage Transformer (VT) or Potential Transformer (PT)
Voltage transformers are used mainly in high-voltage distribution equipment to step-down voltage in measurement circuit for safe measurement; single-phase and three-phase types are manufactured.
The typical secondary voltage of the voltage transformer is 110 volt (Phase-to-Phase). They are also applied to protective relays.
O&M issues to be observed in case of a PT are as follows:
• If once short circuit occurs on the secondary side of a VT, excess current flows into the primary side and that may cause the fuse on the primary side to blow. The primary fuse has also to be checked when there is a fault trip or metering mismatch.
Current Transformer (CT)
Current transformers are used for stepping down current to be measured safely. It is also applied to protective relays. The typical secondary current of the current transformer is 5 Amp or 1 Amp.
O&M issues to be checked are as follows:
• If the secondary side of CT is open-circuited, all the current flowing to primary side is excited by magnetic saturation and causes damages to the CT by over-heating. Therefore, the secondary side should never be left open-circuited. Even when the downstream instrument is removed for any repair, the secondary should be shorted.
Protective relays should detect electrical faults promptly, isolate the faults from system and activate alarms when there is a faulty condition sensed in the electrical supply to the circuits or electrical equipment (short circuit, earth fault, single-phase, reverse power flow etc.)
The protective relays should have the following three characteristics:
A. Certainty: The relay should always be sensing the parameters for action when there is a fault or specified abnormality.
B. Selectivity: The relay should obey a selection of the limits beyond which a fault will be judged.
C. Promptness: The relay should sense and operate within the shortest possible time.
Categories according to protective functions are as follows:
A. Over current relay (OCR): Monitor and protect against over load and short-circuit.
B. Under voltage relay (UVR) and over voltage relay (OVR): Detect and protect under voltage (power failure) or over voltage.
C. Earth fault relay: Protect by detecting current leakage to earth.
Protective relay is shown in Figure overleaf.
Categories according to design as follows. An electrochemical protective relay converts the voltages and currents to magnetic and electric forces and torques that press against spring tensions in the relay. The tension of the spring and taps on the electromagnetic coils in the relay are the main processes by which a user sets a relay.
In a Solid State relay, the incoming voltage and current waveforms are monitored by analog circuits, not recorded or digitized. The analog values are compared to settings made by the user by a potentiometer in the relay.
A Digital Relay converts all measured analog quantities into digital signals. Compared to static relays, digital relays introduce Analogue to Digital Converter (A/D conversion) of all measured analogue quantities and use a microprocessor to implement the protection algorithm.
The microprocessor may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to implement the algorithm. Since late 1990s most of the protective relays are of digital type.
Advantages of Digital Relays
• High level of functionality integration
• Additional monitoring functions
• Functional flexibility
• Capable of working under a wide range of temperatures. Internal power requirement is very low.
• They can implement more complex functions and are generally more accurate
• Self-checking features and self-adaptability
• Able to communicate with other digital equipment of contemporary design
• Less sensitive to temperature-related aging
• Economical because can be produced in required numbers and can be set at site
• More accurate
• Signal storage is possible
Limitations of Digital Relay
The devices have short lifetime due to the continuous development of new technologies.
• Needs to be protected against power system transients
• As digital systems become increasingly more complex they require specially trained staff for operation
• Proper maintenance of the settings and monitoring data
These limitations are overcome by progressive improvements in design, ruggedness, cost, low power and heat generation factors, standardized modular design, scalability and simpler training to operating staff.
A transformer is the most important component in substations. Transformers receive electrical power at high voltage and transform it to lower service voltage. They also provide isolation between high voltage and low voltage supply. Cooling system for oil-immersed transformer:
Oil serves as direct cooling medium to disperse the heat that is generated from windings and core.
The oil is in turn cooled by indirect cooling medium such as air at the oil radiator.
Cooling system for dry transformer: Utilize surrounding air or SF6 as cooling medium.
Transformer Efficiency: The efficiency of a transformer varies between 96% and 99%. It not only depends on design, but also on operating load.
The transformer losses are mainly attributed to:
• Constant Loss: This is also called iron loss or core loss, which mainly depends upon the material of the core and magnetic circuit of the flux path. Hysteresis and eddy current loss are two components of constant loss.
• Variable Loss: This is also called load loss or copper loss, which varies with the square of the load current.
The best efficiency of a transformer occurs at a load when constant loss and variable loss are equal.
For distribution transformers, installed in an STP, the best efficiency would occur around 50% load. O&M checks to be made are as follows:
• Check connections of cables for looseness and overheating
• Check the transformer for abnormal vibration and noise
• Check oil and winding temperature regularly with respect to manufacturer’s manual
• Check for moisture ingress by observing the colour of the silica gel
• Check for level of oil in the conservator
A typical transformer is shown in Figure.