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AM18.5 Medium voltage distribution: Grading (2023)
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AM18.5 Medium voltage distribution: Grading (2023)

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This Applications Manual on medium voltage distribution has been produced to help fill the gap in electrical knowledge in respect of how to safely employ medium voltage for the distribution of electrical power.

Our aim is to provide an understanding of a number of key aspects at different design and construction stages of a medium voltage (11 kV) power distribution system to buildings. Consideration is given to system design and to the selection and erection of equipment, including the associated practical aspects.

It is intended that this Applications Manual will be used by practitioners in conjunction with established international wiring standards and relevant codes of practice. It will also be of interest to designers and authorities who, while not directly concerned with the design or installation of electrical systems, must understand the advice offered to them by specialists.

Further, the manual should be of value to those who wish to enhance their knowledge of electrical power and building services engineering.

Medium voltage distribution is a specialised area of power engineering. This manual is in five parts:

This final part, Grading, provides typical calculations and solutions for the protection of transformers, distribution cables, busbars, generators and motors, and includes examples so that the design of such and the calculations required to achieve protection and coordination with other items of switchgear are clear.

The parts of this Applications Manual will not cover high voltage systems (that is, those using power supply voltages greater than 33 kV). This also means that we will not go into any detail regarding transmission and distribution at these voltages nor specialised power applications within the industry (such as the protection of power factor correction capacitors, harmonic snubber circuits, high voltage direct current, high power static conversion, battery storage or smart grids). For information regarding possible solutions for these applications, it is best to refer to manufacturers’ literature.

1 Introduction

2 The design process

2.1 Time-graded and logic-graded systems

2.1.1 Graphical approach to grading

2.1.2 Overcurrent protection

2.2 Design Example 1: Simple radial distribution

2.2.1 Inverse-time grading

2.3 Design Example 2: Open ring main distribution

2.3.1 Setting the primary MV protection

2.3.2 A workable solution

2.3.3 Transformer overload protection

2.3.4 Design solution, alternatives, and limitations

2.3.5 CT requirements

2.3.6 Low-voltage air circuit breaker?

2.3.7 Ring main faults

2.4 Design Example 3: Closed ring main

2.4.1 Ring-main relays: the starting point

2.4.2 Relay settings for CW1/ACW1

3 Further time-graded application examples

3.1 Transformer earthing protection

3.2 Earth-fault protection

3.2.1 Neutral earthing resistor (NER)

3.3 Balanced earth-fault (BEF) protection

3.3.1 Principle of operation

3.3.2 BEF relay connections

3.4 Restricted earth-fault (REF) protection

3.5 Standby earth-fault (SBEF) protection

3.6 High-set, instantaneous overcurrent (HSOC) protection

3.6.1 High-set versus very inverse (VI) relays

3.7 Buchholz relay

4 Unit or differential protection

4.1 General note

4.2 Zone of protection

4.2.1 Merz-Price principle

4.2.2 High impedance relays

4.3 Design Example 4: Individual winding protection for a power transformer using high-impedance relays

4.3.1 Balanced earth-fault protection (BEF)

4.4 Differential protection applied to a generator (stator faults)

4.5 Biased differential protection applied to a delta-star transformer

4.5.1 Theory of operation

4.6 Design Example 5: Biased differential protection

4.6.1 CT connections

4.6.2 CT recommendations

4.7 Design Example 6: Unit protection for a feeder cable

4.7.1 Introduction

4.8 Closed ring main with unit protection

4.8.1 Check for line-charging current

4.8.2 CT requirements

4.8.3 Speed of operation

4.9 Time-graded backup protection

5 Protection relay configurations for transformers

6 Generator protection

6.1 Stator protection

6.2 Rotor faults

6.2.1 Potentiometer method

6.2.2 AC injection method

6.2.3 DC injection method

6.3 Abnormal operating conditions

6.3.1 Unbalanced loading

6.3.2 Negative sequence protection

6.3.3 Reverse power protection

6.4 Overcurrent backup protection

6.4.1 Generators operating under short-circuit conditions

6.4.2 Design Example 7: Generator overcurrent backup protection

6.4.3 Time-graded relay settings

6.5 Typical protection relay configurations for generators

6.6 Engineering Recommendation G99

Annex A: IEC loading guide for oil-filled transformers

A.1 Transformer loading

A.2 Emergency loading

Annex B: How to read from and plot to a logarithmic scale

B.1 Plotting data to a logarithmic scale

B.2 Reading data from a logarithmic scale

B.3 Using a spreadsheet to plot relay protection curves

Annex C: Overload protection

C.1 Overloads and short-circuits

C.1.1 Sensitivity

C.1.2 Degrees of freedom

C.2 Supply capacity versus protection settings

Annex D: DNO primary substation protection relay and its effect on Design Example 2 (open ring main)

Annex E: Alternative DNO supply arrangement

Annex F: ANSI protection relay codes

Authors: Les Norman (Brunel University London), Adam Rawlinson (PCS Consulting Services Ltd.), Phil Reed (RPS Group PLC)

Peer reviewers: Derek Elliott (Insight PFM Ltd/CIBSE Electrical Services Group), Neil Hitchman (Vinci Construction UK/CIBSE Electrical Services Group), Tony Sung (Energy Reduction Management Ltd/CIBSE Electrical Services Group) (chair)

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