PRESENTATION ON SUBSTATION DESIGN

FRANCIS ARTHUR

SubstationThere are four major types of electric substations:

Switchyard at a generating station:

These facilities connect the generators to the utility grid and also provide off-site power to the plant.

Customer Substation:

Functions as the main source of electric power supply for one (or more) business customers. The technical requirements and the business case for this type of facility depend highly on the

customer’s requirements.

SubstationSwitching Substation:

Facilitate the transfer of bulk power across the network. Their feeders typically originate from generating switchyards. They enable the transmission of large blocks of energy from the generators to the load centers.

Distribution Substation:

Provide the distribution circuits that directly supply most electric customers.

Substation Project Triggers Load Growth

System Stability

System Reliability

System Capacity

Substation Design Challenge

Optimal technical performance at least cost.

Design Considerations

Low life cycle cost

Safety

Standardization (Equipment and Station Configurations)

Substation ConfigurationsSingle Bus

All elements (transformers and transmission lines) are directly connected to one bus

Advantages

Cost of construction is relatively low

Disadvantages

Reliability is low

Low Operational Flexibility (E.g Outage required on associated element for maintenance of switchgear)

Suitable Where:

Load & Availability requirements are low

Substation Configurations Typical Single Line Diagram Typical Layout

Substation ConfigurationsMain and Transfer Bus

All elements (transformers and transmission lines) are directly connected to Main bus

Advantages

Cost of construction is relatively low

Operational Flexibility Higher than single bus scheme due to Transfer Bus and Tie Breaker (Outage is not required on associated element for maintenance of switchgear)

Disadvantages

Reliability is low

Suitable Where:

Load & Availability requirements are low

Substation Configurations Typical Single Line Diagram Typical Layout

Substation Configurations Double Bus Single Breaker

Connects each circuit to two buses, and there is a tie breaker between the buses.

Advantages

Cost of construction is relatively low

Reliability Higher than Main & Transfer bus scheme (Bus fault limited to affected bus due to availability of tie breaker)

Disadvantages

Low Operational Flexibility (E.g Outage required on associated element for maintenance of switchgear)

Suitable Where: Load Transfer & Improved Operating Reliability are Important

Substation Configurations Typical Layout Typical Layout

Substation Configurations Double Bus Double Breaker Scheme

Involves two breakers and two buses for each element

Advantages

Increased reliability (Bus fault does not affect any element)

Increased Operational Flexibility (E.g No outage required for maintenance of circuit breakers)

Disadvantages

Cost of construction is relatively high

Suitable Where: reliability and availability of the circuit is a high priority.

Substation Configurations Typical Single Line Diagram

Substation Configurations Ring Bus

All breakers are arranged in a ring with elements connected between two breakers.

Advantages

Increased reliability (Bus fault limited to affected section & faults to individual elements do not affect others)

Increased Operational Flexibility (E.g No outage required on associated element for maintenance of switchgear)

Disadvantages

Cost of construction is relatively high

Suitable Where: reliability and availability of the circuit is a high priority.

Substation Configurations Typical Single Line Diagram Typical Layout

Substation Configurations Breaker and a Half Bus Scheme

Configured with a circuit between two breakers in a three-breaker line-up with two buses

Advantages

Increased reliability (Bus fault does not affect any element)

Increased Operational Flexibility (E.g No outage required for maintenance of circuit breakers)

Disadvantages

Cost of construction is relatively high but justifiable due to above advantages

Suitable Where: reliability and availability of the circuit is a high priority.

Substation Configurations Typical Single Line Diagram Typical Layout

Substation Equipment Categories of Equipment:

Switchgear

Power Transformers

Capacitor Banks & SVC

Instrument Transformers

Protection and Control Equipment

Auxiliaries

SCADA and Communication

Switchgear Disconnector Switch with Earthing Blade Disconnector Switch Without Earthing Blade

Disconnect Switch Mechanical device that conducts electrical current and provides an open point in a circuit for isolation.

Disconnect switches are also installed to by pass breakers or other equipment for maintenance.

They are designed for no-load switching.

Key Requirements

Open and Close reliably whenever necessary

Carry current continuously without overheating

To remain in the closed position under fault current conditions

Circuit Breakers Live Tank Dead Tank

Circuit Breaker

A mechanical switching device capable of making , carrying , and breaking currents under normal circuit conditions and also breaking currents under specified abnormal conditions such as a short circuit.

Surge Arresters

Devices deployed to protect power system equipment from being subjected to lightning or switching surges.

Power Transformer

A transformer is an electrical device for converting ac power at a certain voltage level into ac power at a different voltage, but at the same frequency.

Inductively couples load to the power system at different voltages.

Capacitor Banks

Deployed for local reactive power compensation at the load.

Required for voltage support and reduction of transmission losses.

Static Var Compensators (SVC)

Deployed for dynamic local reactive power compensation at the load.

Required for voltage support and reduction of transmission losses.

Relies on power electronic and other static controllers to enhance control and increase power transfer capability.

Instrument Transformers Current Transformer Voltage Transformer

Instrument Transformers

A high precision transformer designed to:

Provide input into measurement and/or control equipment.

Examples: Voltmeters Ammeters Watthour Meters, Relays.

Transform currents or voltages from a usually high value to a value easy to handle by protective relays and instruments.

Protection & Control Equipment

Protection & Control Equipment

Dedicated Protection panels for transmission lines, power transformers or capacitor banks.

Protection Equipment ensure speedy isolation of equipment in the event of fault by initiating commands to circuit breakers based on set current or voltage limits.

Control Equipment facilitate the control of switchgear either from the control room or at the switchyard.

Substation Auxiliaries Substation auxiliaries encompass all systems that make the operating voltages of 415Vac, 125Vdc and 48Vdc available at the substation. It includes the following:

Auxiliary Transformers (usually 34.5/0.415kV, 100 (or 250kVA)

Diesel generator sets

Solar Power Systems

125Vdc Rectifiers & 125Vdc Battery Banks

48Vdc Rectifiers & 48Vdc Battery Banks

Automatic Change over switches

AC & DC Distribution Boards

Substation Auxiliaries Auxiliary Transformer Generator

Substation Auxiliaries 125Vdc Rectifier Battery Banks

Substation Auxiliaries 48Vdc Rectifier Change Over Panel

Rectifier

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. (Source: Wikipedia)

Substation Auxiliaries AC Distribution Panel DC Distribution Panel

SCADA & Communication Power Line Carrier (PLC)

SCADA & CommunicationPLC is the superimposition of various signals (ie. data, voice, Fax etc.) on the power line at different frequencies thus relying on the power line as the carrier of the signals.

Functions:

Provides means of high speed fault clearing through use of communication schemes to confirm faulted line section.

Aids in implementation of breaker failure schemes

Aids in remote control requirements

Goal:

To transmit a signal of high quality to the receiving end such that the receiver can interpret the signal.

SCADA & Communication Transmitters & Receivers

The carrier transmitters and receivers are usually mounted in a rack or cabinet in the communication room. (Egs. ETL & FOX panels).

SCADA & Communication

Line Tuners

Works in conjunction with the coupling capacitor to provide a low impedance path for the carrier energy to the transmission line and a high impedance path to the power frequency energy.

SCADA & Communication Line Traps

Directs the carrier signal off the substation bus towards the remote line terminal.

Presents a high impedance path for the carrier energy towards the bus thus directing onto the remote line terminal.

The coil of the line trap presents a low impedance path to the power frequency.

SCADA & Communication

RTU

A remote terminal unit (RTU) is a microprocessor-controlled electronic device that interfaces objects in the physical world to a distributed control system or SCADA (supervisory control and data acquisition) system by transmitting telemetry data to a master system, and by using messages from the master supervisory system to control connected objects. (Source: Wikipedia)

SCADA & Communication

Substation GroundingSubstation grounding is a means of channeling excessive fault currents to ground with the view to preventing damage to equipment and protecting personnel from being subjected to unsafe potentials.

It is achieved with a mat made up of horizontal interconnected bare conductors and ground rods to which all equipment structures are bonded.

The grounding system includes all of the interconnected grounding facilities in the substation area, including the ground grid, overhead ground wires, neutral conductors, underground cables, iron rods in foundations etc.

Substation Grounding

Reasons for Grounding

It provides a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.

It provides a safe environment to protect personnel in the vicinity of grounded facilities from the dangers of electric shock under fault conditions.

Substation Grounding Circumstances for Human Electric Shock

Relatively high fault current to ground in relation to the area of the grounding system and its resistance to remote earth.

Soil resistivity and distribution of ground currents such that high potential gradients may occur at points at the earth surface.

Presence of a person at such a point, time, and position that the body is bridging two points of high potential difference.

Absence of sufficient contact resistance or other series resistance to limit current through the body to a safe value under the above circumstances.

Duration of the fault and body contact and, hence, of the flow of current through a human body for a sufficient time to cause harm at the given current intensity.

Substation Grounding The Design Problem?

To provide a safe condition for personnel within and around the substation area by limiting voltages to safe values.

To provide an adequate earthing system at minimum cost.

Substation Grounding Design

Substation Grounding Earth Grid Operating Platform

Insulation Coordination Over voltages introduce stress to the power system insulation.

Insulation coordination is a combination of measures to prevent the breakdown of insulation.

It is a means of achieving technically acceptable level of insulation that is matched to the level of surge protection.

It comprises the following systems:

Electrical Clearances

Insulation Levels

Insulation Coordination

Insulation Coordination

Insulation Coordination

Insulation Coordination Advantages

Reliable operation of high voltage systems

Minimized equipment damage

Reduced outages

Reduced Operational Losses

Lightning Protection Effects of Lightning in Electrical Installations

Introduces over voltages and damage to non-restoring (solid) insulation

Extreme local heating (vaporization of material, explosion)

Direct strikes generate fields and large mechanical forces

Generates electromagnetic (EMI) and radio frequency interference (RFI) which disturb telecommunication equipment and low voltage digital (and analogue) signal equipment.

Lightning Protection – Shield wires

Lightning Protection – Spikes