Protection and coordination of electrical systems form the core of reliability and safety in the distribution network. As a powerline technician, you must understand how protection devices interact to isolate a fault while minimizing the impact on customers. This chapter covers fundamental principles, devices, coordination calculations, and Canadian regulatory requirements.
Fundamental Principles of Protection
Objectives of Protection
Protection of an electrical network aims at three main objectives:
Time-current relationship (inverse, very inverse, extremely inverse)
Standardized Curves (IEEE/ANSI)
Inverse (moderately inverse): T = 0.14 / (I/I_p)^0.02 - 1
Very inverse: T = 13.5 / (I/I_p)^1 - 1
Extremely inverse: T = 80 / (I/I_p)^2 - 1
Where T = time (seconds), I = fault current, I_p = pickup current.
Circuit Breakers
The circuit breaker is a mechanical device capable of making, carrying, and interrupting currents under normal and abnormal conditions.
Types of Circuit Breakers
Type
Voltage
Application
Oil circuit breaker
Medium voltage
Distribution substations
Vacuum circuit breaker
Medium voltage
Modern substations
SF6 circuit breaker
High voltage
Transmission substations
Air blast circuit breaker
High voltage
Older substations
Interrupting Rating
The interrupting rating is the maximum current the circuit breaker can interrupt without damage. It must be greater than the maximum fault current at the point of installation.
Disconnect Switches and Switches
Disconnect switch: device without load-break capability, used to isolate de-energized equipment
Switch: capable of interrupting rated current, but not fault currents
Load-break switch: combines both functions, with load-break capability
Protection Coordination
Coordination Principles
Coordination consists of selecting and setting protection devices so that only the device closest to the fault operates, leaving the rest of the network in service.
Basic Rule
The upstream protection device must have an operating time greater than the downstream device for the same fault current.
Coordination Margins
Between fuses: minimum margin of 0.25 seconds between the total clearing curve of the downstream fuse and the minimum melting curve of the upstream fuse
Between relays: margin of 0.3 to 0.4 seconds (including circuit breaker opening time)
Between relay and fuse: margin of 0.2 to 0.3 seconds
Coordination Steps
74.Identify possible fault points (beginning of line, end of line, substations)
75.Calculate fault currents at each point
76.Plot TCC curves of all devices on the same graph
77.Verify margins between curves
78.Adjust settings (pickup, time dial) if necessary
For proper coordination between two fuses in series:
The upstream fuse must have a higher rated current than the downstream fuse
The curves must be spaced by at least 0.25 seconds
The ratio of rated currents is generally 1.5 to 2
Relay-to-Relay Coordination
Coordination between relays uses the time dial to create a time offset:
The relay closest to the source has a higher time dial
Each upstream relay adds an additional delay
Transformer Protection
Overload Protection
Distribution transformers are protected against overloads by fuses or relays. Overload capacity depends on ambient temperature and prior loading.
Internal Fault Protection
Buchholz relay (oil-filled transformers): detects gas accumulation
Pressure relay: detects rapid pressure variations
Differential relay: compares primary and secondary currents
Transformer Protection Rules
According to the Canadian Electrical Code, Part I (CE Code) (Rule 26-250), each transformer must be protected against overcurrents. The primary fuse must be calibrated to:
Not melt at the transformer's rated current
Melt in the event of a secondary fault
Protect the transformer against internal faults
Transformer Rated Current Calculation
I_rated = S / (√3 × V)
Where:
S = apparent power (kVA)
V = phase-to-phase voltage (V)
Example: 300 kVA transformer, 25 kV:
I_rated = 300,000 / (√3 × 25,000) = 6.93 A
Recommended fuse: 10 A (approximately 1.5 × I_rated)
Distribution Line Protection
Conductor Protection
Conductors must be protected against fault currents that could damage them. The conductor damage curve (I²t) must be above the clearing curve of the protection device.
Overhead Line Protection
Branch fuses: protect lateral taps
Reclosers: automatically interrupt and reclose the circuit
Sectionalizers: isolate faulted sections
Reclosers
The recloser is an automatic circuit breaker with a reclosing sequence. It distinguishes temporary faults (tree branches on lines, lightning) from permanent faults.
Typical Reclosing Sequence
Operation
Time
Description
1st trip
0.1 s
Fault detection
1st reclose
2 s
Fault test
2nd trip
0.1 s
If fault persists
2nd reclose
15 s
New test
3rd trip
0.1 s
If fault persists
Lockout
—
Permanent fault
Coordination with Branch Fuses
The recloser must be coordinated with branch fuses:
Coordination mode: the fuse melts before the recloser (for permanent faults)
Non-coordination mode: the recloser operates before the fuse (for temporary faults)
Grounding and Lightning Protection
Grounding
Grounding is essential for:
Limiting overvoltages
Ensuring protection operation
Protecting personnel
Types of Grounding
Type
Description
Application
System grounding
Connects the neutral to ground
Substations, transformers
Equipment grounding
Connects metal enclosures
Equipment
Work grounding
Protects workers
Live-line work
Surge Arresters
The surge arrester protects equipment against lightning and switching overvoltages.
Characteristics
Maximum continuous operating voltage (MCOV): maximum voltage in continuous service
Residual voltage: terminal voltage during lightning current discharge
Nominal discharge current: 5 kA, 10 kA, 20 kA depending on application
Insulation Coordination
Insulation coordination consists of selecting insulation levels compatible with surge arresters. The arrester protection level must be lower than the equipment withstand level.
Canadian Regulatory Requirements
Canadian Electrical Code, Part I (CE Code)
The Canadian Electrical Code, Part I (CE Code) (current edition) governs electrical installations of distribution systems. The main rules concerning protection:
Rule
Requirement
Rule 14-100
Protection against overcurrents
Rule 26-250
Transformer protection
Rule 14-010
Conductor protection
Rule 28-100
Motor protection
Rule 10-200
Grounding
Rule 10-300
Lightning protection
CSA B149.1
CSA B149.1 (Natural Gas and Propane Installation Code) is relevant for gas installations near power lines but does not apply directly to electrical protection.
Relevant CSA Standards
CSA C22.2: standards for electrical equipment
CSA C411: surge arresters
CSA C571: distribution transformers
Work Procedures
Verification of Protection Devices
Before any intervention, verify:
153.The physical condition of fuses and cutouts
154.Relay settings (pickup, time dial)
155.Circuit breaker operation indicators
156.The condition of surge arresters
Fuse Replacement
158.Identify the faulty fuse (visual indicator, testing)
159.Verify absence of voltage (voltage detector)
160.Ground the conductors (grounding sticks)
161.Remove the fuse with the universal hot stick
162.Install the new fuse of the same rating
163.Remove the grounds and verify operation
Relay Setting
165.Calculate fault currents at critical points
166.Select the appropriate curve (inverse, very inverse)
167.Set the pickup at 1.25 × the maximum rated current
168.Set the time dial to ensure coordination
169.Document the settings and verify with an operational test
For a phase-to-ground fault with zero fault impedance:
I_fault = V_phase / (Z_source + Z_line)
Voltage Drop Calculation
ΔV = √3 × I × L × (R × cos φ + X × sin φ)
Where:
I = current (A)
L = line length (km)
R = resistance per km (Ω/km)
X = reactance per km (Ω/km)
cos φ = power factor
Complete Coordination Example
Situation: 25 kV distribution line with:
Branch fuse: 15 K (fast curve)
Line head fuse: 40 T (slow curve)
Fault current at the end of the branch: 800 A
Fault current at the beginning of the line: 3,500 A
Verification:
190.For an 800 A fault in the branch:
Melting time of 15 K fuse: 0.05 s
Melting time of 40 T fuse: 0.5 s
Margin: 0.45 s > 0.25 s ✓
194.For a 3,500 A fault at the beginning of the line:
Melting time of 40 T fuse: 0.02 s
The branch fuse is not affected (fault is upstream)
Pitfalls to Avoid
198.Confusing rated current and melting current: the rated current is the continuous current capacity; the melting current is generally 1.5 to 2 times higher.
199.Neglecting the coordination margin: an insufficient margin between two devices can cause simultaneous operation.
200.Forgetting the minimum fault current: coordination must be verified for the minimum fault current (end of line), not just the maximum current.
201.Ignoring ambient temperature: fuses have different characteristics depending on temperature. A fuse operates faster at high temperatures.
202.Using an incorrect fuse rating: always replace a fuse with a fuse of the same rating and type.
203.Confusing minimum and maximum curves: the minimum melting curve and the total clearing curve are not identical.
204.Neglecting ground protection: ground faults are the most frequent and require specific protection.
205.Forgetting to verify the interrupting rating: the device must be able to interrupt the maximum fault current at the point of installation.
206.Not documenting settings: relay settings must be documented and verified periodically.
207.Confusing relay curve types: inverse, very inverse, and extremely inverse curves have different slopes.
Summary
Protection of electrical networks aims to protect equipment, personnel, and ensure continuity of service.
Fuses are simple and economical but offer less flexibility than relays.
Time-current curves (TCC) are the fundamental tool for coordination.
Coordination requires that the upstream device have an operating time greater than the downstream device.
Coordination margins are 0.25 s between fuses and 0.3-0.4 s between relays.
Reclosers distinguish temporary faults from permanent faults.
Grounding and surge arresters protect against overvoltages.
The Canadian Electrical Code, Part I (CE Code) (Rules 14-100 to 14-010, 26-250, 10-200, 10-300) governs distribution installations.
Fault current and coordination calculations must be mastered for the exam.
Personnel safety is paramount: always verify absence of voltage and ground before intervening.
Review Questions
220.What are the three main objectives of electrical network protection?
221.What is the difference between an expulsion fuse and a current-limiting fuse?
222.How do you calculate the rated current of a three-phase transformer?
223.What is the minimum coordination margin between two fuses in series?
224.What is a recloser and how does it work?
225.What are the main rules of the Canadian Electrical Code, Part I (CE Code) concerning protection?
226.Why must coordination be verified for the minimum fault current?
227.What is the difference between a disconnect switch and a switch?
228.How do you choose the rating of a transformer protection fuse?
229.What are the three types of standardized relay curves and their equations?
