Learn how to use power system analysis tools, diagrams, and curves to select, set, test, and maintain your protective devices in a coordinated manner.

Once you have completed the protection and grading study, it has been confirmed and verified, at regular intervals or following changes the system shall be checked and verified (at least every two years) to check no new large loads, renewable generation or new overlaid cables with different parameters haven’t been added: Plan, Do, Check, Act, Repeat!

How can you best protect and coordinate your power system?

Protecting and coordinating your power system is crucial for ensuring its reliability, safety, and efficiency. Power system protection involves detecting and isolating faults, preventing damage to equipment and personnel, and minimizing service interruptions. Power system coordination involves selecting and setting the protective devices, such as relays, circuit breakers, and fuses, so that they operate in a coordinated manner, clearing the faults as fast as possible and isolating the minimum amount of the system. In this article, you will learn how to best protect and coordinate your power system by following these steps:

1 Understand your power system

The first step is to understand your power system's configuration, load characteristics, fault levels, and equipment ratings. You can use power system analysis tools, such as load flow, short circuit, and stability studies, to model and simulate your power system under normal and abnormal conditions. You can also use power system diagrams, such as one-line diagrams, to visualize and document your power system's topology, components, and connections.

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2 Identify the protection zones

The second step is to identify the protection zones, which are the sections of the power system that are protected by one or more protective devices. The protection zones should cover all the critical elements of the power system, such as generators, transformers, buses, lines, and loads. The protection zones should also be arranged in a way that minimizes the overlap between them, to avoid unnecessary tripping of healthy parts of the system.

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3 Select the appropriate protective devices

The third step is to select the appropriate protective devices for each protection zone, based on the type, location, and function of the element to be protected. The protective devices should have the following characteristics: sensitivity, selectivity, speed, reliability, and simplicity. Sensitivity means that the protective device can detect the smallest possible fault current. Selectivity means that the protective device can discriminate between faults within and outside its protection zone. Speed means that the protective device can operate as fast as possible to clear the fault. Reliability means that the protective device can perform its function correctly and consistently. Simplicity means that the protective device can be easily installed, maintained, and operated.

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4 Set the protective device parameters

The fourth step is to set the protective device parameters, such as current, voltage, time, and angle settings, according to the protection zone requirements and the power system characteristics. The protective device settings should ensure that the protective devices operate in a coordinated manner, meaning that the protective device closest to the fault clears it first, and the backup protective devices operate only if the primary ones fail or are unavailable. You can use coordination curves, such as time-current curves, to graphically compare and adjust the protective device settings.

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Richard Emery CEng MIET MIHEEM

Electrical and Building Services Authorising Engineer at STFC
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When producing the protection and grading study, the verification of selectivity and coordination can be visually checked using the time current curves. However if you use the right software you can virtually inject a fault at a given node or bus and confirm the correct sequence of operation.

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Brad Panike

Retired Senior System Operator
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This is known in the industry as a coordination study. The study documents the setting of each protective device and its expected performance at various fault levels. Settings should not be changed unless the coordination study is performed with the new proposed Settings, and the documentation updated.

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5 Test and verify the protective device operation

The fifth step is to test and verify the protective device operation, using methods such as primary injection, secondary injection, or simulation testing. Primary injection testing involves applying a high current to the power system element to be protected, and observing the response of the protective device. Secondary injection testing involves applying a low current to the protective device terminals, and observing its operation. Simulation testing involves using a software or hardware tool to simulate the power system conditions and faults, and observing the protective device operation. Testing and verification should ensure that the protective devices operate correctly, accurately, and safely.

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Brad Panike

Retired Senior System Operator
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The reason for testing is to verify the device performs to the settings provided in the coordination study. If a device does not perform as expected at the identified settings, coordination will be sacrificed.

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Richard Emery CEng MIET MIHEEM

Electrical and Building Services Authorising Engineer at STFC
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When completing manual verification of the correct operation a relay, you are confirming it operate as expected and remains doing so for its life. However, the only way to verify the correct operation of the scheme is through software, which can inject a calculated current at a point and time the reactions through the circuit.

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6 Monitor and maintain the protective system

The sixth step is to monitor and maintain the protective system, using methods such as periodic inspection, performance analysis, and troubleshooting. Periodic inspection involves checking the physical condition, connections, and settings of the protective devices, and correcting any defects or deviations. Performance analysis involves reviewing the records, logs, and reports of the protective device operation, and identifying any trends, anomalies, or improvements. Troubleshooting involves diagnosing and resolving any problems or malfunctions of the protective devices, using tools such as fault recorders, oscilloscopes, and multimeters. Monitoring and maintenance should ensure that the protective system remains in optimal condition and performance.

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Richard Emery CEng MIET MIHEEM

Electrical and Building Services Authorising Engineer at STFC
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Carrying out condition based maintenance on protection relays is vital for confirming the correct operation during its life. The frequency should be dictated by the manufacturer, local approved standards or maintenance standards like SFG20. Equally important is maintaining power supplies and battery systems providing power to the relays.

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Brad Panike

Retired Senior System Operator
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This includes testing on a routine schedule to assure no degradation of protective equipment has occurred. Agencies like NETA can provide maintenance procedures where none exist.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Richard Emery CEng MIET MIHEEM

Electrical and Building Services Authorising Engineer at STFC
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Once you have completed the protection and grading study, it has been confirmed and verified, at regular intervals or following changes the system shall be checked and verified (at least every two years) to check no new large loads, renewable generation or new overlaid cables with different parameters haven’t been added: Plan, Do, Check, Act, Repeat!

Like

How can you best protect and coordinate your power system?

1

2

3

4

5

6

7

1 Understand your power system

2 Identify the protection zones

3 Select the appropriate protective devices

4 Set the protective device parameters

5 Test and verify the protective device operation

6 Monitor and maintain the protective system

7 Here’s what else to consider

Power Engineering

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How can you best protect and coordinate your power system?

1

2

3

4

5

6

7

1 Understand your power system

2 Identify the protection zones

3 Select the appropriate protective devices

4 Set the protective device parameters

5 Test and verify the protective device operation

6 Monitor and maintain the protective system

7 Here’s what else to consider

Power Engineering

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