Faults in Power System

In this section we will discuss regarding faults in power system networks. This is the more important fundamental in power system protection since this will be the basis on the study of protection. As part of the study of faults, we need to understand fault calculation. This is the analysis of the behavior of the electrical network under fault conditions. The current and voltages at different part of the network for different types of faults, location of the faults and the various configurations of the power system network.

There are different types of faults that can occur in a network, it can be categorize into three major groups:

1. Short circuit faults
2. Open circuit faults
3. Simultaneous faults

 We shall define each of them to clearly understand what these faults are and how it affects the system.

1. Short circuit faults - a short circuit is an accidental connection between conductors by zero or non-zero impedance. It is referred to as internal if it is located within equipment or external if it occurs on links. The causes of a short circuit can be mechanical, electrical or human. 

           a. Three phase faults 

           b. Two phase faults 

           c.Single phase to earth faults

Effects of short circuit currents:

·         Under short circuit conditions, equipment and connections are subjected to high mechanical stress that can cause breaks and thermal stress that can melt conductors and destroy insulation.

·         Equipments and connections require prompt disconnection or isolation from the fault so as to avoid human casualty and equipment damage.

2. Open circuit faults – this type of fault is not normally considered in fault studies but yet this is still very important. Open circuit faults will not cause overcurrent or overvoltages and therefore normally not dangerous to the network. This type of fault cause heating in rotating machines due to the negative sequence current that will flow in the system. Therefore, the rotating machines are equipped with negative sequence current protection. 

               a. Single phase open circuit 

               b.Two phase open circuit. 

               c.Three phase open circuit.

3. Simultaneous faults are combinations are the combination of short circuit faults and open circuit faults. Say, if one conductor of an overhead line is broken and one end of the line falls down. In this scenario there is both one single phase to earth fault and one single phase open circuited fault in the system.

Generally, the system is protected against phase and earth faults by protection relays. The severity or magnitude of the fault current is dependent on what type of fault occurs in the network. 3 Phase faults normally generate the highest short circuit current that is why short circuit for three phase faults is normally used.

At earth fault, the magnitude of the fault current is depending on the earthing resistance or reactance in the fault. The fault resistance for a phase fault is much smaller than that for an earth fault. For two phase fault, it normally generates low fault currents than three phase faults. However, two phase fault calculation can be necessary to check the minimum fault current level to verify the sensitivity for the back-up protection.

Now we already have the basic information regarding faults, there is still a lot of reading and studying to do to fully understand Faults and Short circuit calculations. You can find few free books online regarding fault study. We will try to discuss in detail if we have still time. Just put your comments and we’ll try to go into details.

 

Requirements on Protection System

The basic requirement on the protection equipment  is that it will clear the fault with sufficient speed to limit the consequential damages in the transmission network or a plant. the fault clearance must be fast enough to avoid a total, or partial power network collapse. A number of different criteria for operation may be established, the most common criteria for power system protection are speed, sensitivity, dependability/reliability and selectivity. We shall define each criteria and more details.

1. Speed - this criteria is the most critical protective relay operating criterion. The relays have to be fast enough to allow clearing of a fault in the minimum time needed to ensure reliable and safe power system operation. In simpler term, this criteria is the ability of the protection to isolate the faults within a short possible time.

2. Sensitivity - this criteria means the capability to detect all type of fault. It is important to detect all faults even if the fault currents is smaller that the load current. Damage to equipment due to induction in low voltage equipment, or person injuries due to rise in earth potential, can occur also for low magnitude faults.  

3. Reliability - it is the ability of the relay or protection scheme to operate correctly when required and to avoid unnecessary operation. Reliability consist of dependability and security of protection. 


4. Selectivity - it is the capability of the protection to determine the fault location and only isolate the faulty region of the network. The consequences of a fault must be limited and the supply of electricity to the consumers secured. The protection system must therefor be capable of distinguishing between an external and an internal faults. 

The protection of transmission lines varies in the principle used and the implementation approach depending on the voltage level. The main principles are associated with the implementation of transmission line protective relaying such as overcurrent protection of distribution feeders, distance and differential protection of transmission lines. 

Common terms used in power system protection

In our earlier blogs we have tackled power system components. Now we need to understand the common terms used in power system protection. We will define each terms simply to better understand each term.

1. Pick-up value(relay) - it is defined as the change of status. Let say when a relay changes it state from un-energized status to energized condition. When a relay is triggered means the status of the output contact will change from normally open (NO) to normally close (NC) and vice versa status.

2. Drop of value(relay) - it is a value of a relay is to change from energized status to de-energized status. The drop-off relay will be determined from the change.

3. Sensitivity - it is a characteristic of a relay that is the minimum pick-up value of the relay. For overcurrent relay the minimum operating current is termed as sensitivity of the relay.

4. Through fault -  this refers to the fault current flowing through the protective zone for a fault outside the protective zone.

5. Stability of Protection - it is defined as stable when the protection system remains in-operative for all system conditions except for which the protection is designed. 

6. Operating Time - the operating time of a relay is the time elapsed from the initiation of the relay to close the output contact.

7. Protected Zone - section or area of the power system covered by a protection scheme.


8. Blind Zone - area of a power system not covered by the protection scheme.

Power System Protection Components

Let us dig deeper into knowing what are the components involved in power system protection. For now it is not necessary to master them in minute details. We only need to know how it works and what are the limitations of each component. 

Here are the main components in power system protection:

1. Relays - it is a protection device which senses the system currents during normal and abnormal condition of the power system. Protection relays are multifunction devices that permanently compare the electrical variables of networks such as current, voltage, frequency, power and impedances with predetermined values, and then automatically send signals for action usually the opening of a circuit breaker or give off an alarm when the monitored value goes above the threshold. 


2. Voltage Transformers (VT) - a voltage transformer is a transformer connected to a very high impedance. It is designed to give the secondary a voltage proportional to that applied to the primary. VT's are also used in metering and indicators. By using voltage transformer all secondary equipments (relays, metering devices, voltmeters etc) are rated to a standard voltage level of 110V. Usually the voltage transformer are denoted by mainly ration and output VA.


3. Current Transformers (CT) - they provide a current proportional to the current flowing through the cable in order to perform energy metering or to analyze this current through a protection device. Usually a current transformer has a secondary value of 1A or 5A.




Summary of the functions instrument transformers (CT, VT):

* To transform currents, or voltages from a high value to a value easy to handle for relays and instruments.
* To isolate the metering circuit from the primary high voltage.
* To provide possibilities of standardization, concerning instruments and relays of rated currents and voltages.

I'll discuss in detail on protection relays in the next following blogs. 

Power System Network

First we need to understand the power system network. Understanding the basics of power system will give a strong background that will be useful in understanding advanced topics in power system protection. To start with, power system network consists of Generation, Transmission and Distribution. Let's define each of them.

1. Generation -  Generator from power plants (e.g. Geothermal, Hyrdo, Combined Cycle) power plants produces power and connected to a bus bar system through a transformer to step up generated power to high voltage. An example is 10.5kV stepped up to 132kV or 400kV. This is just one example. This varies from different country around the world depending on their standards.

2. Transmission - A transmission system consists of High Voltage substations and Overhead Lines. Power from generating plants are sent to different load centers through overhead lines. This is where our focus of protection will be. The overhead lines are connected to the Busbar system in the substations through circuit breakers or load break switches.Transformers in the substations are converted from high voltage to low voltage (400kV to 132kV or 132kV to 11kV) to the distribution network.


3. Distribution - This is the LV side of the network (11kV) and connected to the distribution system. the LV side of the transformer is the incomer to the distribution network and connected to the distribution bus through circuit breaker.

Based on the above concepts, electricity standards varies from different countries. It will be an advantage knowing them once you continue in protection studies. 

There are many sources that summarizes and compares electricity standards. What are the advantages and disadvantages and so on. 


Check this reference material this will be a great resource. ()

Power System Protection

The task of protection system shall, together with the circuit breakers, disconnect faulty parts of the power system to:

- Protection the primary equipment against unnecessary damages.

- Enable continued service in the undamaged part of the network.

- Save people in the vicinity of the electrical plant from injuries.

During abnormal condition in the electrical circuits, high current will flow and may cause damage to the equipments. To protect the equipments from high fault current, the faulty portion of the power system shall be isolated by means of protection devices (fuses, MCB or relays). Hence the function of the protection is to isolate the faulty section.

The correcting measures can be initiation of regulation interference, disconnection of some part of the load, connection of back-up capacity or, as a last possibility, disconnection of the critical apparatus. Mostly of these precautions are initiated by protection relays which supervises the condition in the electrical equipment.

In order to understand the philosophy of the protection, consider the simple example of domestic electrical wiring. In the house we have one Main MCB (incomer) and routed through the branches to different rooms through MCBs or fuses. When there is a fault in any of the branches, the MCB related to that branch will be switched off keeping other branches healthy. the main MCB shall be of higher rating than the MCB in the branches in order to isolate the branches first. Thus we can achieve the protection of the equipment during abnormal condition.

This is just a snapshot on the importance of power system protection. Power system protection is a specialized field where the learning and search for advanced knowledge is continuous. We hope see you guys in our future blogs. Cheers.