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# Fundamental overcurrent, distance and differential protection principles

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## Essential protection principles

The aim of this technical article is to cover the most important principles of four fundamental relay protections: overcurrent, directional overcurrent, distance and differential for transmission lines, power transformers and busbars.

Contents:

For simplicity in explaining the key ideas, we consider three phase bolted faults.

### 1. Overcurrent protection

This scheme is based on the intuition that, faults typically short circuits, lead to currents much above the load current. We can call them as overcurrents. Over current relaying and fuse protection uses the principle that when the current exceeds a predetermined value, it indicates presence of a fault (short circuit).

This protection scheme finds usage in radial distribution systems with a single source. It is quite simple to implement.

Figure 1 shows a radial distribution system with a single source. The fault current is fed from only one end of the feeder.

For this system it can be observed that:

• To relay R1, both downstream faults F1 and F2 are visible i.e. IF1 as well as IF2 pass through CT of R1.
• To relay R2, fault F1, an upstream fault is not seen, only F2 is seen. This is because no component of IF1 passes through CT of R2. Thus, selectivity is achieved naturally.
Relaying decision is based solely on the magnitude of fault current. Such a protection scheme is said to be non-directional.

Go back to protection principles ↑

### 2. Directional overcurrent protection

In contrast, there can be situations where for the purpose of selectivity, phase angle information (always relative to a reference phasor) may be required. Figure 2 shows such a case for a radial system with source at both ends. Consequently, fault is fed from both the ends of the feeder.

To interrupt the fault current, relays at both ends of the feeder are required.

In this case, from the magnitude of the current seen by the relay R2, it is not possible to distinguish whether the fault is in the section AB or BC. Since faults in section AB are not in its jurisdiction, it should not trip.

To obtain selectivity, a directional overcurrent relay is required. It uses both magnitude of current and phase angle information for decision making. It is commonly used in subtransmission networks where ring mains are used.

Go back to protection principles ↑

### 3. Distance protection

Consider a simple radial system, which is fed from a single source. Let us measure the apparent impedance (V/I) at the sending end.

For the unloaded system, I = 0, and the apparent impedance seen by the relay is infinite. As the system is loaded, the apparent impedance reduces to some finite value (ZL+Zline) where ZL is the load impedance and Zline is the line impedance. In presence of a fault at a per-unit distance ‘m’, the impedance seen by the relay drops to a mZline as shown in figure 3 below.

The basic principle of distance relay is that the apparent impedance seen by the relay, which is defined as the ratio of phase voltage to line current of a transmission line (Zapp), reduces drastically in the presence of a line fault. A distance relay compares this ratio with the positive sequence impedance (Z1) of the transmission line. If the fraction Zapp/Z1 is less than unity, it indicates a fault. This ratio also indicates the distance of the fault from the relay.

Because, impedance is a complex number, the distance protection is inherently directional. The first quadrant is the forward direction i.e. impedance of the transmission line to be protected lies in this quadrant.

However, if only magnitude information is used, non-directional impedance relay results. Figures 4 and 5 shows a characteristic of an impedance relay and ‘mho relay’ both belonging to this class.

The impedance relay trips if the magnitude of the impedance is within the circular region. Since, the circle spans all the quadrants, it leads to non-directional protection scheme. In contrast, the mho relay which covers primarily the first quadrant is directional in nature.

Thus, the trip law for the impedance relay can be written as follows:

|Zapp| = |VR| / |IR| < |Zset|

then trip; else restrain.

While impedance relay has only one design parameter, Zset; ‘mho relay’ has two design parameters Zn, λ. The trip law for mho relay is given by if:

|Zapp| < |Zn| cos (θ – λ)

then trip; else restrain.

As shown in the Figure 5, θ is the angle of transmission line. Based upon legacy of electromechanical relays λ is also called ‘torque angle’.

Go back to protection principles ↑

### 4. Principle of differential protection

Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from the current leaving it.

Thus by comparing the two currents either in magnitude or in phase or both we can determine a fault and issue a trip decision if the difference exceeds a predetermined set value.

Go back to protection principles ↑

#### 4.1 Differential protection for transmission line

Figure 6 shows a short transmission line in which shunt charging can be neglected. Then under no fault condition, phasor sum of currents entering the device is zero i.e.

IS + IR = 0

Thus, we can say that differential current under no fault condition is zero. However in case of fault in the line segment AB, we get:

IS + IR = IF ≠ 0

i.e. differential current in presence of fault is non-zero.

This principle of checking the differential current is known as a differential protection scheme.

In case of transmission line, implementation of differential protection requires a communication channel to transmit current values to the other end. It can be used for short feeders and a specific implementation is known as pilot wire protection. Differential protection tends to be extremely accurate. Its zone is clearly demarcated by the CTs which provide the boundary.

Differential protection can be used for tapped lines (multiterminal lines) where boundary conditions are defined as follows:

Under no fault condition:

I1 + I2 + I3 = 0

Faulted condition:

I1 + I2 + I3 ≠ 0

Go back to protection principles ↑

#### 4.2 Differential protection for transformer

Differential protection for detecting faults is an attractive option when both ends of the apparatus are physically located near each other. e.g. on a transformer, a generator or a bus bar.

Consider an ideal transformer with the CT connections, as shown in Figure 8.

To illustrate the principle let us consider that current rating of primary winding is 100A and secondary winding is 1000A. Then if we use 100:5 and 1000:5 CT on the primary and secondary winding, then under normal (no fault) operating conditions the scaled CT currents will match in magnitudes. By connections the primary and secondary CTs with due care to the dots (polarity markings), a circulating current can be set up as shown by dotted line.

No current will flow through the branch having overcurrent current relay because it will result in violation of KCL.

Now if an internal fault occurs within the device like interturn short etc., then the normal mmf balance is upset i.e. N1I≠ N2I2. Under this condition, the CT secondary currents of primary and secondary side CTs will not match. The resulting differential current will flow through overcurrent relay. If the pick up setting of overcurrent relay is close to zero, it will immediately pick up and initiate the trip decision.

In practice, the transformer is not ideal. Consequently, even if I2=0, I1≠0, it is the magnetization current or (no load) current. Thus, a differential current always flows through the overcurrent relay.

Therefore overcurrent relay pick up is adjusted above the no load current value. Consequently, minute faults below no load current value cannot be detected. This compromises sensitivity.

Go back to protection principles ↑

#### 4.3 Differential protection for busbar

Ideally, differential protection is the solution for the busbar protection. Figure 9 illustrates the basic idea. If the fault is external to the bus, it can be seen that algebraic sum of the currents entering the bus is zero.

IA + IB + IC + ID + IE = 0

On the other hand, if fault is on the bus (internal fault), this sum is not zero.

IA + IB + IC + ID + IE = IF

Thus, differential protection can be used to protect a bus.

Go back to protection principles ↑

Reference // Fundamentals of Power System Protection – Extract from IIT Bombay NPTEL

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### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

Aug 14, 2020

Gone through the article and like it

Jun 09, 2020

Good night
I’m interested in the solution

3. Aziz Ur Rehman
Feb 22, 2019

What is best probable threshold or parameter setting calculation of medium voltage over head radial feeder to implement in non direction over current relay of 46 BC (Broken conductor feature).

4. Prodip Golder
Aug 25, 2018

Please send me details Differential relay setting calculation. Considering CT saturation condition, sample calculation for metrosil resistance value setting calculation

5. MAHMOUD
Mar 20, 2018

Dear Mr.Csanyi
In an engineering exam I had a question which addition to CT accuracy class C800 and CT ratio 1200/5, gave differential scheme ratio 900/5, which I never haired about, would you please advice me when we have CT ratio , what it means differential scheme ratio ?

6. KAYODE EMMANUEL JEMISEYE
Feb 05, 2018

Thank you so much for this great articles. So educative
I need better understanding/explanation on the subject matter below,

I work as a plant engineer in a power plant,
One of the 1.5MW gas engine do show this fault alarm “differential current” the p.f goes as low as 0.22 and then shutdown.

I am still reading up some materials that explained this
However,your knowledge as an expatriate can help me a great deal
Thanks

7. Yacine ZEKRAOUI
Feb 03, 2018

Many thanks about every golden information in this Great site
Keep me in touch if any ?

Regards.

8. Ghazanfar Ali Khan
Jul 14, 2017

Good Morning Mr. Edward !
Your posts are always found very useful for understanding.

9. Carlos Hernandez
Mar 30, 2017

Good afternoon Mr Edvard Csanyi

Salutes from Venezuela. About this post, I would like to ask you three things, exclusively for the distance protection. At the end of this principle it shown an equation, what is or what does mean Zset, Vr and Ir?.

Second question is, what is or what does mean Zn?

Finally, my third question is Where do I get the value of the torque angle?

Kind Regards

10. FELIX M. S. JR.
Dec 26, 2016

DATE: 26-DEC.-2016

INDEED, WITH THE ABOVE PRESENTATIONS OF TYPICAL APPLICATION CONFIGURATIONS OF PROTECTION RELAYING FOR ELECTRICAL POWER SYSTEM DISTRIBUTIONS, THE INSTALLED PROTECTION RELAY NEAREST THE FAULT LOCATION MUST SEE THE FAULT AND DETECT IT FIRST AND THE PICK-UP SETTING OF THIS RELAY SHOULD BE AT THE MINIMUM MAGNITUDE OF THE FAULT CURRENT OTHERWISE SETTING IT A HIGHER VALE NEAR THE MAX.FAULT WILL EXPOSE THE ELECTRICAL CIRCUIT TO THE EFFECT OF SHORT CIRCUIT THEREBY LETTING THROUGH THE CURRENT TO PASS THRU INCREASING THE TEMPERATURE DUE TO THERMAL AND MAGNETIC FORCE EFFECT OF THE FAULT WITHOUT INTERRUPTING THE FLOW IMMEDIATELY WHICH THEREFORE IS NOT A GOOD DESIGN AND IMPLEMENTATION OF PROTECTION.