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Protection Schemes for Electrical Power System

Electrical Power System is a highly invested area. The more reliable electricity we want, the more is need to protect it. Protection is essential to keep equipment and personnel safe from any kind of damage caused by an electrical unbalance or fault condition. Read more as we cover the objectives of power system protection, different protection devices and schemes to provide complete safety to an electrical power system.

Protection devices perform their purpose by keeping a faulty section isolated from the remaining healthy system to make it work without any disturbances. The function of a protection system is not to prevent faults as its name suggests, rather it minimizes repair costs as it senses fault because it only acts after a fault occurs. Different protection schemes are explained in this article. But first, we need to look at why protecting the power system is crucial?

Objectives of Power System Protection

It takes a lot of effort and money to design a power system. Of course, every investor wants to get a maximum return on investment. But what if some equipment fails?

The whole system may confront the risk of severe damage and deterioration, consequently leading to putting life, property and other equipment in danger. To minimize the probability of damage caused by failure, protection devices come in.

Protection devices reduce the chances of discontinuity of electricity and restrict failure to the failed equipment or area. This way, the system owners keep their customers satisfied with continuous service and the whole system keeps operating without major breakdowns and power outages.

To continuously monitor the system and keep it secure by detaching only the components that are under fault and to retain as much of the grid as possible still in operation, different protection schemes were developed.

Protection Zones

Each protection scheme safeguards a defined area known as a protection zone. The protection zone surrounds each power equipment. When a fault occurs in any of the zone, then only the circuit breaker in that zone trips. Therefore, only a faulty element is disconnected without affecting the rest of the system.

Following six categories of protection zones are possible in a system, we apply here a concept of selective coordination.

  • Generators and generator–transformer units
  • Transformers
  • Buses
  • Lines (transmission, sub-transmission, and distribution)
  • Utilization equipment (motors, static loads, or other)
  • Capacitor or reactor banks (when separately protected).

Protection Devices and Components

Before we mention more about protection schemes, let us go through some elemental components in the power protection system.

Instrument Transformer

Accurate protection cannot be achieved without properly measuring the normal and abnormal conditions of a system. Instrument transformers work as a transducer in electrical systems.

Voltage and Current measurements give feedback on whether a system is healthy or not. Voltage transformers and current transformers measure these basic parameters.

The current transformer has two jobs to do. Firstly, it steps down the current to such levels that it can be easily handled by the relay current coil. Secondly, it isolates the relay circuitry from the high voltage of the High Voltage system. A CT primary is in series with the line in which current is to be measured.

The voltage transformer steps down the high voltage of the line to a level safe enough for the relaying system (pressure coil of the relay) and personnel to handle. A PT primary is connected in parallel at the point where a measurement is desired.

We previously wrote an article on Current Transformer. Have a look at it to understand the basics of current transformer including construction, applications, working principle, grounding and connections.

Relay

RelayRelays are sensing devices. Due to the ability to make decisions about fault recognition, these relays are regarded as the brain of power systems.

Relays are operated by measuring the voltage and current values and converting them into digital and/or analog signals, which in turn isolate the circuits by opening the faulty circuits. Most often, the relays serve two objectives, alarm and trip, once the abnormality is noticed.

In the previous years, the relays had very limited functions and were quite bulky. However, with the advancement in digital technology, relays monitor various parameters, which give the complete history of a system.

Circuit Breaker

circuit breaker

The circuit breaker is an electrically operated switch, which is capable of safely opening and closing circuits. The circuit breaker functions by the output of the linked relay.

When the circuit breaker is in the closed condition, its contacts are held closed by the tension of the closing spring. When the trip coil is energized, it releases a latch, causing the stored energy in the closing spring to bring about a quick opening operation.

The opening of faulty circuits requires some time. However, the circuit breakers, which are used to isolate the faulty circuits, can carry these fault currents until the fault currents are cleared.

Circuit breakers can be classified according to different design considerations like arc quenching media, operation mechanism, voltage levels, etc.

We have written another blog on Selecting the Right Circuit Breaker and Its Type. Read it to understand how circuit breakers work, what are their different types are and how to choose a suitable breaker according to your requirements.

Batteries

The other component which is crucial in a protective system is batteries that are used to ensure uninterrupted power to relays and breaker coils. The operation of relays and breakers require power sources, which shall not be affected by faults.esd protection device

ESD Protection Device

An ESD protection device protects the electronic components from electrostatic discharge. Electrostatic discharge is the buildup of charges which can damage a protective circuitry and can cause malfunction.

Surge Protection Device

surge protection deviceA surge protector is a device used to protect electrical equipment by suppressing voltage spikes. This device attempts to limit the supplied voltage to an electrical device by maintaining it below a safe threshold.

The above items are extensively used in any protective system and their design requires careful study and selection for proper operation.

We also wrote an article on Surge Protection Devices. Read more as we cover various applications and benefits of installing a surge protective device.

Protection Schemes

There are several protection schemes invented along the line as protection engineers face new challenges with the advancement in power systems. Here, we will discuss the most basic ones.

Overcurrent Protection Scheme 

A sudden build-up of current can be considered as an effect of fault. Therefore, over-current protection can be regarded as the most obvious principle of protection because the magnitude of the current can be utilized as an indication of the existence of a fault. But, the magnitude of the fault current is related to the type of fault and the source impedance.

The source impedance depends upon the number of generating units that are in service at a given time and keeps changing from time to time. So, the setpoint for the distinction of fault current magnitude from the normal current as well as the operating time of over-current protection keep changing from fault to fault, and time to time. This has led protection engineers to think of other principles.

Instantaneous OC Relay

Instantaneous means no intentional time delay. The operating time of an instantaneous relay is in milliseconds. Such a relay has only the pick-up setting and does not have any time setting.

Definite Time OC Relay

A definite time overcurrent relay can be adjusted to issue a trip output at an adjustable definite amount of time after it picks up. Thus, it has a pick-up adjustment and a time-setting adjustment.

Inverse Time OC Relay

Inverse time characteristic corresponds with the requirement that the more severe a fault is, the faster it should be cleared to avoid damage to the apparatus. Following inverse time characteristics have been standardized.

1. Inverse definite minimum time (IDMT) OC relay.

2. Very inverse time OC relay.

3. Extremely inverse time OC relay.

 overcurrent relays 

Differential Protection Scheme

Another very expected and appealing principle is differential protection. It is based on the assertion that the current leaving a protected section must be equal to that entering it. Any difference between the two endpoints of a single section indicates a fault. Thus, we can compare the two currents either their phase or magnitude or both.

This method of detecting faults is very popular if both ends of an apparatus are physically very close to each other. It should remain steady in case of an external fault or through-fault which will be outside of its protective zone and should only trip if the fault is internal. The ability of this protection to discriminate between internal and external faults define its stability. Though, it is impracticable to apply this scheme to a transmission line because the ends are at a great distance, and it is not feasible to equate information.

Balanced Circulating Current System, External Fault (stable)
Balanced Circulating Current System, Internal Fault (operate)

Distance Protection Scheme

A distance protection scheme relates the voltage with the current at the same end. This scheme computes the impedance between the protection location and the fault point. Then it compares it with a pre-set value to make the trip decision.

Since the impedance of a transmission line can be directly proportional to the distance of the fault in a line due to the simple series model, it helps in identifying fault location. This type of protection is known as distance protection or under-impedance protection. In practice, the word 'under' is dropped and now it is simply called impedance protection.

Directional Protection Scheme 

In the case of a double-end feed system or parallel lines or a ring main system, a fault gets fed from both sides. To be selective, the protection must be sensitive to the direction of the fault power flow. The protection scheme which exhibits such property is termed as a directional protection scheme.

There are other situations where it becomes necessary to use a directional scheme to supervise overcurrent schemes. Since directional protection units are much more costly and need the provision of power transformers, they should be used only when necessarily needed.

Comparison Summary for all Protection Schemes: 

protection scheme table

There are many protection devices or components available, which are installed along with the equipment so as to ensure safety of not only the equipment but the working personnel as well. Also, we have various protection schemes and each of them is suitable for a particular application and every type has some advantages over the other. Altogether, the different devices and schemes are to make the electrical power system completely reliable and secure.

References:

Book on Differential and Distance Protection Diagram from Power System Relaying

by Horowitz

Book on Differential Protection Diagram and Overcurrent curves from Electrical Power System

by C. L. Wadhwa
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