Current transformers, relays, grading power system, busbar, short circuit fault, load, directional relay overcurrent protection.

Electrical power systems are designed to be as faulty free as possible through careful system design and equipment design, proper equipment installation and periodic equipment maintenance. To achieve a free fault system protection systems or equipment’s should be installed according to the designed electrical power system i.e. grading of relays, transformers and any other protection system. However, this document covers the different methods of protecting a power system as shown in figure 1, using time current relays, current transformers and this document also covers the calculations of faults on each busbar. This is achieved by employing Dig Silent as the simulation software. 

Keywords: current transformers, relays, grading power system, busbar, short circuit fault, load, directional relay overcurrent protection.

1. Introduction & Background 

The short-circuit current available in a distribution system is usually supplied from many sources, which can be grouped into three main categories. The first is the utility transmission system supplying the facility, which acts like a large, remote generator. The second include ‘’local’’ generators either in the plant or nearby in the utility. The third source category is synchronous and induction motors, which are in many plants and facilities. All these are rotating machines; those of the second and third categories have machines currents that decay significantly with time due to reduction of flux in the machine during a short circuit. For a short circuit at its terminal, the induction motor symmetrical current disappears entirely after one to ten cycles while the current of a synchronous motor is maintained at a lower initial value by its energized field. Networks having a greater proportion of induction motors to synchronous motors will have quicker decays of ac-short circuit current components. [1]

Description of short circuit current

Symmetrical and asymmetrical currents. The terms symmetrical current and asymmetrical describe the shape of the ac current waveforms about the zero axis. If the envelopes of the positive and negative peaks of the current waveform are symmetrical around the zero axis, they are called symmetrical currents. If the envelopes of positive and negative peaks are not symmetrical around the zero axis they are called asymmetrical current. The amount of offset that will occur in a fault occur in a fault current waveform depends on the time at which the fault occurs on the ac voltage waveform and the network resistances and reactances. [1, 2] Faults can be a result of two live wires touching, live and ground line wires touching this could be a result of the following:
  • Deterioration of insulation due to age
  • Voltage surges
  • Voltage or mechanical stresses applied to the equipment
  • The intrusion of metallic or conducting objects into the equipment such as grounding clamps, fish tape, tools or pay loaders

What could happen when a fault occurred on an electrical power distribution

At the fault location, arcing and burning can occur damaging adjacent equipment and possibly resulting in an arc-flash burn hazard to personnel working on the equipment. The short circuit currents may be very high introducing a significant amount of energy into the fault and all components carrying the short-circuit currents will be subjected to thermal and mechanical stresses due to the current flowing. This stress varies as a function of the magnitude of the current squared and the duration of the current flow and may damage these components and the system voltage levels drop in proportion to the magnitude of the short circuit currents flowing through the system elements. Maximum voltage drop occurs at the fault location (down to zero for bolted fault, but all parts of power system will be subject to a voltage drop to some degree.

Overcurrent protection

Overcurrent relays should be coordinated for different operation conditions in interconnected networks. In addition, the time interval should not reach under the predefined threshold for each pair of primary and backup relays for the faults occurring in the protection zone of the primary relay. An analytical approach is presented in this paper to calculate the impedance matrix of the network in fault condition to determine the critical fault point accurately. Overcurrent relays are coordinated by using the presented critical fault point instead of the close-in fault used in previous studies. Simulation results show the accuracy of the proposed method for overcurrent relays coordination in comparison with other methods. [2] Generally, overcurrent relays trip or restrain circuit breakers when the magnitude of their input current exceeds the set value known as pick up current.  Can be made to operate in sequence such that the circuit breaker closest to the fault location operates first. 

Graded or Co-ordinated o/c protection.

Satisfactory co-ordination of o/c relays depends on the method of discrimination. The IDMT
These relays show inverse time-current curve at lower values of fault current and definite time-current curve at higher values of fault current. It is therefore a combination of the inverse time and definite time curves. Hence the name IDMT. The time-current relationship for a given time curve could be plotted in the log/log graph paper with the y-axis scaled in seconds and the x-axis scaled in multiples of the pick up or setting current.

Directional relays

Directional overcurrent relaying refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault. There are a variety of concepts by which this task is done. Directional relay uses the phase relationship of sequence components such as positive sequence, negative sequence, and zero sequence to sense fault direction, but other concepts such as using quadrature voltage are included. [4,5]

 2. Objective/ Aim
  • To analyse and design a protection system of the given interconnected power distribution system shown in (figure 1) using DIGSILENT
  • Design and build the interconnected system using DIGSILENT thus proof the functionality of the system (load flow proof)
  • To choose or select protection components for the electrical distribution system.
  • To successfully grade the relays as per my selection and assumptions 

3.Methodology/ project procedure

Electrical power network diagram
Figure 1: Electrical power distribution network (source: power system analysis & design: prob 9 pg577)
Figure 2: Equivalent network diagram designed from dig silent

The designed network has 4 extra busbars as shown in figure 2, I added the busses to connect the synchronous sources to the transformers, an alternative to this could have been (nodes), and I chose using busses as I found this convenient to use and to set up. 

Table 1:Equipment ratings

3.1.1 Procedure

  • I used built-in commands of the Dig SILENT to populate the power network as shown in figure 1 Make some assumptions to obtain the required results.
  •  Inserted all my data as given in table 1
  • Place current transformers on the circuit breakers next to the busbars.
  • Insert the CT-ratings for all the current transformers.
  •  Place all the relays on the same point as the CTs according to the ratings.
  • Plot the Time overcurrent curves. (plotting from clockwise and anti-clockwise direction)

Figure 3:first step on the DIG-SILENT simulation
Figure 4: inserting the ratings for transmission lines 
Figure 5: inserting the rated voltage and reactance’s

Figure (3-5) show the basic procedure I followed to create the power system diagram on the dig-silent software, this also includes inserting the data given, I followed the same procedure to insert rated values of the other parameters such as buses, transformers and other protection components.

3.1.2 Results, discussion and analysis

Figure 6: Shows the load flow test or calculation.
Figure 7:shows the load flow calculation/ proof of successful functionality of the built system
Figure 8:fault calculation at bus 1 (bus 2)

Figure 9:fault calculation at bus 1 (results)

Note that the fault created at bus 1 is a single phase to ground fault this then implies that the currents across line b, c must be equal to zero 

Ib  = Ic = 0kA   But the fault current at  Ia = 7,78kA 

Figure 10: fault calculation at bus 2
Note that the fault created at bus 1 is a single phase to ground fault this then implies that the currents across line b, c must be equal to zero
Ib  = Ic = 0kA   But the fault current at  Ia = 8,38kA 

3.1.3 Over current protection ( with relays and current transformers) Overcurrent protection procedure

When analysing the power system I notice that there are a few equipment that needs to be protected: Transformers, Synchronous generators(sources) and the internal circuitry, I then researched or referred to the overcurrent protections slide and then I discovered the following
There are two types of relays that can be used to protect this system namely :
  • Directional overcurrent relays
  • Directional power relays.

This 2 relays have some similarities as they are both protection devices but basically have different applications. Directional overcurrent relays respond to excessive current flow in a direction in the power system. The relay typically consists of two elements. One is a directional element, which determines the direction of current flow with respect to a voltage reference. When this current flow is in the predetermined trip direction, this directional element enables ("turns on") the other element, which is a standard overcurrent relay, complete with taps and time dial, as found on a normal non-directional overcurrent relay. Because these relays are designed to operate on fault currents, the directional unit is made so that it operates best on a highly lagging current, which is typical of faults in power systems. 

Directional overcurrent relays are normally used on incoming line circuit breakers on buses which have two or more sources. They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources. In complex distribution or sub transmission networks, these relays may be used to improve coordination of the system. [4] Directional power relays measure real power, so they operate best at a high-power factor. Various degrees of sensitivity and speed of operation are available in various models of directional power relays. [4] it is then noted that this relay can be Connected to measure power flow into a transformer from the secondary side, a very sensitive directional power relay can measure core loss power input to the transformer, detecting loss of the primary source to the transformer. The transformer can then be disconnected from the system.

I personally selected the directional relay as the first paragraph state that the directional relays are normally used to protect over currents and are normally used in power networks of many sources or ring networks. This is then the case with the system I am trying to protect

4 conclusions

The project was a success as I managed to simulate the electrical power system on the dig silent, hence the load flow proof shown above, I also manged to protect the system using relays and CT under Directional protection and overcurrent protection as the Time Overcurrent (clockwise and anti-clockwise) graphs are shown above. Also, the grading of the system using some assumptions has given me the values of the operating time of the relays at bus1,2 and 3.
I recommend that in future power relay (32) be used so we can compare the efficiency of the relays. This project has taught me a lot when it comes to the protection of a power system network and it has also proved the theoretical calculations I have covered in the lecture.

5. References

[1] J.D.Glover,M.S Sarma & T.M Overbye power system “analysis & design” Dawn-Business 2nd June 2008.

[2] IEEE Std 242™-2001, IEEE Recommended Practice for Protection and
Coordination of Industrial and Commercial Power Systems (IEEE Buff Book).

[3] H.Arbab.B.Jazi and M.Rezagholizedah’’Symmetrical fault and asymmetrical fault analysis’’ Electrical Distribution and Power systems Volume34,Issue 4, April 2009, Pages 1114-1118

[4] Hossein Mousazadeh, Alireza Keyhani’, Arzhang Javadi and Ahmad Sharifi’’A review of overcurrent protection using relays’’ Volume 13, Issue 8, October 2009, Pages 1800-1818



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