ELECTRICAL POWER SYSTEMS

PROTECTION
ENEE-454
(summer term)

Marks allocation
1- Two major exams (25 marks each)
2- Attendance and reports(10 marks)
3- Final exam (40 Marks)
The Course will cover the following main topics.
1- Types of relay (one week)
2- Types of circuit breakers (two weeks)
3- Current and voltage transformers (one week)
4- Protection of power transformer (two weeks)
5- Protection of transmission lines and relay grading (two weeks)
6- Protection of generation and motors ( two weeks)
7- Protection of and bus bars and substations (one week)
8- Earthing in electrical power systems (one week)
 Lecture 1
PROTECTIVE RELAYS

Basic connections of Relay systems

 Steps of the trip Circuits Operation

1- When a short circuit occurs at point F on the transmission line, the current flowing
in the line increases to an enormous value.
2- This results in a heavy current flow through the relay coil, causing the relay to operate
by closing its contacts.
3- This in turn closes the trip circuit of the breaker, making the circuit breaker open
and isolating the faulty section from the rest of the system. In this way, the relay
ensures the safety of the circuit equipment from damage and normal working of the
healthy portion of the system. The operation steps is shown in the Figure below
Different shapes of protective relay

Simple schematic of Electromagnetic relays and operation

Relays in power system zone protection

To limit the extent of the power system that is disconnected when a fault occurs,
protection is arranged in zones. The principle is shown in the Figure below. Ideally, the
zones of protection should overlap, so that no part of the power system is left
unprotected and the circuit breaker should be included in both zones.

Schematic of Power System Zone Protection

What are the main Components Of Protection Systems?

The main components of protection systems are discussed briefly below.

1- Current & Voltage Transformers: they are called instrument transformers. Their
purpose is to step down the current or voltage of a device to measurable values, within
the instrumentation measurement range 5A or 1A in the case of a current transformers
(CTs), and 110V or 100V in the case of a voltage (or potential) transformers (VTs/
PTs). Hence, protective equipment inputs are standardized within the ranges above.

2- Protective Relays : they are electromagnetic or intelligent electronic devices (IEDs)

which receive measured signals from the secondary side of CTs and VTs and detect
whether the protected unit is in a stressed condition (based on their type and
configuration) or not. A trip signal is sent by protective relays to the circuit breakers
to disconnect the faulty components from power system if necessary.
 3- Circuit Breakers: Circuit Breakers act upon open commands sent by protective
relays when faults are detected and close commands when faults are cleared.
They can also be manually opened, for example, to isolate a component for
maintenance.

4- Communication Channels: are the paths that deliver information and

measurements from an initiating relay at one location to a receiving relay (or
substation) at another location.

Types of Relays
Protective relays can be classified in accordance with the function which they carry out,
their construction, the incoming signal and the type of functioning.

1- General function:
• Auxiliary. 3- Incoming signal:
• Protection. • Current.
• Monitoring. • Voltage.
• Control. • Frequency.
• Temperature.
• Pressure.
• Velocity.
• Others.

2- Construction: 4- Type of protection

• Electromagnetic. • Over current.
• Solid state. • Directional over current.
• Microprocessor. • Distance.
• Computerized. • Over voltage.
• Nonelectric (thermal, pressure • Differential.
......etc.). • Reverse power.
• Other.

The electromagnetic relays will be more investigated below.

 Types of Electromagnetic Relays

The Attraction type will be investigated here below.

The Figure shows the:

What is meant by the pickup-current?
Another arrangement of Balanced beam electromagnetic relay
What is meant by Relay time and fault time clearing?

What is meant by relay sensitivity?

Types of overcurrent relays
Problem
EARTH FAULT RELAY
Problem 1:
For the radial system shown in the Figure below, the operating voltage is 11 kV.
Assume the relays used are IDMT-OC and the same power factor for all loads. Select
the relay setting to protect the system. Assume the circuit breaker time of 0.5 Sec,
overshot error of 10 % and current setting multiplier of 125 %.

Solution:
The load currents are
4𝑋106 2.5𝑋106 6.75𝑋106
𝐼1 = 3 ) = 209.95 𝐴 𝐼2 = 3 ) = 131.22 𝐴 𝐼3 = = 354.28 𝐴
√3(11𝑋10 √3(11𝑋10 √3(11𝑋103 )

The normal currents through the sections are given as below

𝐼21 = 𝐼1 = 209.95 𝐴 𝐼32 = 𝐼21 + 𝐼2 = 341.16 𝐴 𝐼𝑠 = 𝐼31 + 𝐼3 = 695.44 𝐴
With the given CTs ratios and with the current setting is 125%
Then the pick-up current for each relay:
209.95 341.16 695.44
𝐼21 = 𝑋1.25 =6.56 A 𝐼32 = 𝑋1.25 =10.66 A 𝐼𝑠 = 𝑋1.25 =10.86 A
(200/5) (200/5) (400/5)

The fault current in the secondary winding of each CT:

2500 3000 3200
𝐼𝑓21 = =62.5 A 𝐼𝑓32 = = 75 𝐴 𝐼𝑠 = =40 A
(200/5) (200/5) (400/5)

From maximum fault currents and the pick-up current, Plug setting multiplier for each relay
can be calculated as:
62.5 75 40
𝑃𝑆𝑀21 = =9.53 𝑃𝑆𝑀32 = =7.04 𝑃𝑆𝑀𝑠 = =3.7
6.56 10.66 10.86

The relay at section 21 is the fastest relay so assume the time setting multiplier (TMS) to be 0.1
For relay at section 21
 0.14(𝑇𝑀𝑆)
𝑡𝑜𝑝 = 𝑆𝑒𝑐.
𝑃𝑆𝑀0.02 − 1
Then
0.14𝑋(0.1)
𝑡21 = = 0.304 𝑆𝑒𝑐.
(9.53)0.02 − 1
For Relay at section 32
As the circuit breaker time of 0.5 Sec and overshoot error of 10%
The relay 32 operation time is T32= (T21+0.5)X1.1= (0.304+0.5)X1.1=0.884 Sec.
0.14𝑋(𝑇𝑀𝑆)
0.884 = .
(3.7)0.02 − 1
TMS (of relay 32)= 0.251
For Relay at section S
The relay 32 operation time is Ts= (T32+0.5)X1.1= (0.884+0.5)X1.1=1.522 Sec.
0.14𝑋(𝑇𝑀𝑆)
1.522 = .
(7.04)0.02 − 1
TMS (of relay S)= 0.288

Relay CT ratio PSM TMS

R21 200/5 9.53 0.1
R32 200/5 7.04 0.251
R3 400/5 3.7 0.288
Problem 2:
Draw the protective zones for the power system shown in the Figure below. State
which circuit breaker should open for the following faults:
1- At P1
2- At P2
3- At P3