Ensuring Reliable Electrical Power Supply: Best Practices and Strategies

Electric power systems are intricate networks linking power plants to loads, spanning large distances and often interconnecting several regions or countries. These entire power systems are self-sufficient, providing electricity from the power plant to the end-users through a grid.

Electrical power systems are among the most extensive and intricate systems that engineers design today, consisting of various components. From low-power devices to massive turbogenerators, power systems are a complicated web of equipment that must work in unison to provide a consistent power supply.

Structure of Electrical Power Systems

An electrical power system comprises various components that generate, transmit, and distribute electricity. Some of the most common components used in power systems include:

• Electric Power Generators

Knowing its basic components is essential to understand how an electrical power system works. The heart of any power system is the three-phase AC synchronous generator, an alternator. A prime mover, a reciprocating engine, a waterwheel, or a steam turbine mechanically drives this generator.

In hydro stations, waterwheels are the first prime movers, and they remain in use today due to their reliability and low operating cost. As water power sources became less available, steam turbines fueled by coal, gas, and oil became the dominant source of power generation. Even nuclear reactors use steam turbines to convert heat into electricity, albeit with more giant turbines due to their fixed costs.

• Transformers

Practical constraints dictate that the voltage level at the alternator's terminals remain relatively low. To transmit power over long distances while minimizing conductor size and losses, transformers are employed to step up the voltage (and reduce the current proportionately). 

A power transformer bank consists of three single-phase transformers with external electrical connections or a more economical three-phase unit housed in a single tank for more extensive power ratings. These transformers typically have oil-immersed windings for insulation and cooling purposes.

• Transmission Lines

The transmission line is a critical component of the electrical power system, typically comprising three conductors, either as individual wires or bundles. One or more neutral conductors may also be present, although sometimes they can be omitted as they only carry the unbalanced return portion of the line current. In a perfectly balanced three-phase circuit, there is no neutral return current. However, the neutral conductor can sometimes be much smaller as the current is accurately balanced among the phases at transmission voltages. An effective neutral return current through the earth can be achieved in certain soil conditions.

• Disconnect Switches and Circuit Breakers

Circuit breakers serve as large three-pole switches at each end of every transmission-line section and on either side of large transformers. Their primary function is to open in the event of a fault or short circuit under the control of automatic protective relays. These relays detect the severity and location of the fault and can even initiate the safe re-energization of the equipment's circuit breakers. A small digital computer may analyze fault conditions and perform logical control functions in critical extra-high-voltage switchyards or substations.

Circuit breakers also act as a switch that a local or remote operator can manually operate to de-energize an element of the network for maintenance. Various methods are used to suppress arcing when a circuit breaker's contacts open under load, such as submerging the contact mechanism in oil or gas like sulfur hexafluoride (SF6). Higher voltage classes use an air blast to quench the arc, using several interrupters or contact sets in series for each phase.

• Voltage Regulators

After electric power is transmitted into the local area, it needs to be transformed back to a distribution-level voltage that consumers can use. It is done through a step-down transformer bank, similar to the step-up bank at the generating station, but sized for local needs. To ensure a constant voltage for customers, a voltage regulator is connected to the output side of the transformer.

The voltage regulator is a particular type of transformer with several discrete taps over a voltage range of ±10%, each with a fractional percent variation. An automatic control circuit senses the voltage and positions the taps to compensate for variations in transmission voltage. Alternatively, the regulator and control circuitry may be incorporated into the transformer, creating an LTC (load tap changer). This process, known as TCUL (tap changing under load), allows for automatic voltage regulation without interrupting power to customers.

• Subtransmission

Certain voltage classes are considered sub-transmission in some power systems, despite being initially designed for transmission purposes. These lines were later tapped to serve additional load centers and became local feeders due to rapid system growth and overlay of higher voltage transmission circuits. 

Sub-transmission types are typically 23, 34.5, and 69 kV; 138 kV may also be included in some more extensive systems. However, this depends on the specific application. Older classes may be relegated to sub-transmission services as higher voltage classes emerge.

• Distribution Systems

A low-voltage distribution system is essential for practical power distribution to multiple customers in a local area. This system is similar to a transmission system, with circuit breakers, lines, and transformers, but with lower power and voltage levels. Despite this, electrical theory and analytical methods are identical for both. Distribution voltages typically range from 2.3 to 35 kV, with 12.5 and 14.14 kV being the most common primary voltages, which are then stepped down to the 240/120 V levels most customers use.

Single-phase distribution circuits use transformers to balance the total load on each phase as closely as possible. Three-phase distribution circuits are only necessary for large industrial or motor loads. To step down the voltage from distribution to customer service level, transformers can be mounted on poles for overhead distribution systems, pads, or vaults for underground distribution. Fuses or fused cutouts typically protect these transformers.

• Loads

Any device that uses electric power can be said to impose a load on the system. From the perspective of the power source, loads can be classified as inductive, resistive, capacitive, or a combination thereof. Furthermore, loads can be time-variant, ranging from slow random swings to rapid cyclic pulses that can cause flickering in nearby lights.

The composite load on a system typically has a predominant resistive component and a small net inductive component. Inductive loads like induction motors are more common than capacitive loads. To minimize the resultant current, capacitors are usually installed in sufficient quantities to balance most of the inductive current. It has been found that the power consumed by the composite load on a power system varies with system frequency. Although this effect is not noticeable to customers within the normal operating frequency range (±0.02 Hz), it can significantly contribute to control systems operating synchronously.

• Capacitors

Capacitors can be classified as transmission or distribution classes when used for power factor correction in a power system. Capacitors should be installed as close to the load as possible to maximize their effectiveness. By reducing the inductive component of current to the load, capacitors can balance the load to the unity power factor, decreasing conductor size and I2R losses. 

While static capacitors can be used at any voltage, high-voltage banks require multiple capacitors connected in series and parallel. High-side circuit breakers and protective relays should protect these banks. For industrial loads that require large amounts of power factor correction, synchronous condensers can be used. These condensers can rapidly control their contribution of capacitive or inductive current over a wide range, and their excitation current can be adjusted using automatic controls.

Conclusion

The electrical power system plays a vital role in our daily lives, providing us with the energy needed to power our homes, industries, and businesses. This large and complex system comprises various components that ensure efficient operation. As highlighted, one component's safety is linked to another's effectiveness and functionality, emphasizing the importance of proper maintenance of the system.

The demand for electrical engineers to design, maintain and troubleshoot electrical power systems has never been higher. With the ever-increasing demand for a robust system, the industry needs skilled professionals. In fact, the U.S. Bureau of Labor Statistics projects that employment of electrical and electronics engineers will grow by 3% from 2020 to 2030, adding about 10,900 new jobs. 

Skill-Lync offers a course on Introduction to Electric Power Systems that can teach you the technicalities involved in designing electric power systems. Talk to our experts to get your free demo.