Electrical Systems Protection

 

1- Introduction:

After studying and analyzing the power systems under different fault conditions, which vary from three-sided symmetrical faults to asymmetric faults, the results of this study and analysis are used as a basis to determine the conditions that occur in the system during the faults, so that the necessary action can be taken to prevent such malfunctions, and in the event of malfunctions, the necessary action is taken to reduce the effects of destructive malfunctions and to reduce the interruption of electrical supply.

In general, protection systems can be defined as systems that continuously monitor the electrical power system until it is ensured that the maximum continuity of the transmission of electrical power occurs with the least risk to human life or equipment.

The circuits of the protection systems are isolated from the circuits of the electrical forces, so they are fed from the measurement transformers of the type of current transformer (C.T) and voltage transformer (P.T.), and the measurement transformers in turn give the measured variables to the protection system, which in turn detects the occurrence of the fault and sends a signal to the circuit cutting device (C.B) so that the part affected by the malfunction is disconnected from the system only if possible.

The electrical power system is divided into protected zones, which are as follows:

- Generator Area

- Transformer Area

- Rail area

- Transmission and distribution circuit area

- Engine Area

Figure (1) gives a single-phase drawing of a part of the electrical power system while dividing it into protection zones, noting that each zone interferes with the adjacent area to avoid the presence of unprotected areas, which are called (blind zones) and the connections of the current-meter transformers achieve this interference, and Figure (2) gives two of the systems that are likely to be used, noting that a failure occurs in the combined current measurement transformers will operate the circuit breakers in both areas.

Figure 1: Conventional protection zones for part of a strong electrical system

 

Figure (2): Interference around the circuit breaker

 

 

 

2-    Instrument Transformers                    

These transformers are used to measure voltage and current in high power and high voltage networks and to transmit the resulting signals to the circuits of low-current relays. In addition:

1.                  Isolation of relay coils from high voltage circuit

2.                  Feed standard current and voltage values for relays e.g. one and five amps for current, 110 and 120 volts for voltage, thus allowing standard relays for any powerful system.

 

2-1 Current Transformer

Transformer Installation Construction  

There are two main types of these converters:

1.                  Single term adapter for primary file

2.                  Multi turn wound primary converter

Voltage to earth is connected to the circuit and the coils must be isolated to resist the voltage to the ground, and Figure 1 represents a diagram of the current transformer part (a) and part (b) represents a symbolic diagram of how the transformer is connected in the circuit.

 

 Core Switch Core

It is often ring-cut and made of cold-soled grain-oriented steel or a nickel-iron alloy. Or the heart section may be rectangular in shape, and the transformer may be of  a shell type.

 

Coils & Insulation  Winding & Insulation

A- Primary coils: They are in the form  of edge wound copper ship in order to resist the electromagnetic forces that are produced by high-value currents and insulate the rings of the coil by covering with resinous materials  .

B. Secondary coils: They consist of copper wires with a round or rectangular section and are insulated by strips of pressed paper, cloth or other insulating materials.

 

Divisions of the current transformer according to the electrical connections:

Figure (3) shows a single-ratio transformer  that has a primary file and a secondary file only.

 

Figure (3)

1.                  A three-wire transformer, where the primary coil is divided into two equal parts that are completely isolated, and this transformer is often used to measure the electrical power of a three-wire unifacial system and in low-voltage power systems Figure (4).

 

 

Figure (4)

 

1.                    A series parallel primary transformer  , in which the primary coil is divided into two or more symmetrical sections that are connected in series or parallel (Fig. 5) so as to produce the same value  as the camper turn current  in the secondary circuit.

 c)  A transformer with a tapped secondary transformer (Fig. 6) is used when two current signals are required in the secondary circuit, taking into account that only one signal is used at the given time and the other ends are kept open, and it is noted that this will affect the accuracy of the entire secondary coil use case.

 

Figure (5)

 

 

Figure (6)

d)  A transformer with multiple secondary transformers, (Fig. 7) and is used in the case of a tight space where the measuring devices and relays are fed, and for this relay the secondary coil that is not used must be connected to each other, unlike the fragmented coil.

 

 

Figure (7)

 

Devices (relays or measurements) connected to the secondary circuit of transformers are called  burdens  and are not called loads to distinguish between them and loads in the primary circuit.  Secondary loads are expressed in ohms or volt-amperes resistance and power factor. Table (1) summarizes the standard values of these loads for a transformer with a standard current of 5 A in the secondary coil.

 

جدول (1)

P.F	Voltamp	Impedance in ohm	Burden designation
Metering burdens
0.9 0.9 0.9 0.9 0.9	2.5 5.0 n.5 2.2-5 45	0.1  0.2  0.5  0.9 1.8	B - 0.1 B - 0.2 B - 0.5 B - 0.9 B - 1.8
Relaying burdens
0.5 0.5 0.5 0.50	25. 50. 200. 200.	1.0 2.0 4.0 8.0	B-1 B-2 B-4 B-8

 

  Current transformer open circuit voltage

The general rule of the transformer is not to open the circuit of the secondary coil as long as a current passes through the primary coil, because when a current passes through the primary coil, it will generate a high-value overflow in the core, which causes its saturation, and thus a non-high voltage is formed on both ends of the secondary coil, which causes the insulation to collapse and causes danger to people as well. Another reason is the possibility  of residual magnetism in the iron core even after the voltage is removed, which affects the accuracy of the transformer.

 

    Mechanical and thermal characteristics

In addition to the accuracy of the transformers, the mechanical and thermal properties need to be known when selecting transformers, because when a short circuit occurs, a high-value current passes through the circuit and this is accompanied by a thermal and mechanical effect on the transformer.

The forces generated in the transformer at the moment of short are proportional to the square of the number of turns of the initial coil and as the value of the current ampere turns increases  in order to increase the precision, the mechanical resistance will decrease.

As for the thermal capacity, it depends on the section of the wire, its length, the number of windings of both the primary and secondary coils, and the time of the short current passage, and as the size of the conductor increases, the heat capacity will increase, but with the increase in the number of windings.

 

 Choosing Current Transformers

The error ratio is measured in the current value of the transformer, which is known as the conversion ratio, and this shows that Ammeter is measured in both the primary and secondary circuits, and the values are compared with the conversion ratio, from which the error is determined and even a simple method but does not give high accuracy, as well as the voltage is selected by the Impulse voltage curve.

If the transformer is exposed to such efforts, and if the transformer is located outdoors, the test should be conducted under the influence of artificial rain to ensure the validity of the insulation.

 

2-2 Voltage Transformers

Voltage transformers are designed on the basis of being shown between a line and the ground or between two lines, and are used to feed the voltage or control devices or the voltage coil into the power meters for electricity, and the conversion ratio of the primary to secondary coil is so that it gives a value of the voltage in the secondary coil equal to 120,115,110 volts, and Figure 7 shows a diagram of one of these transformers.

 

Installation Construction

a) Electromagnetic types

It depends mainly on the voltage, transformers less than 3.3 kV can be dry transformers, and for higher voltages, the core and coils are immersed in oil for insulation, and with the development and use of sulfur dioxide gas in insulation for cutting devices, it has also been used for voltage transformers.

 

b) Cascade type

Figure (8) shows a six-phase voltage transformer, and this transformer is often used for voltages of more than 145 kV, and it is noted that the primary coil is divided into several magnetic cores, while the secondary coil is only located on one core, the last stage.

 

Figure (8)

 

Use Capacitor Capacitors

The installation of this type is mainly based on the idea of a voltage splitter using capacitors.

 

 Voltage Transducer for Sulfur Hexaoxide Insulated Cutting Devices

The idea of these transformers is based on a voltage splitter using capacitors.

 

Mechanical and Thermal Properties

Since voltage transformers connect between a line and the ground or between two lines, they will not be exposed to thermal and mechanical effects similar to what happens in current transformers, but there is a possibility that they will be exposed to a short circuit in the secondary coil, and if the fuses are used in the secondary circuit to protect them, this load will increase in them, so the fuses are used in the primary coil and this will have a slight effect on the accuracy.

 

Load Voltage Transformer Burden

The load on the transformer is what is related to the secondary coil and is expressed in volts of amperage, and Table (2) shows the load and power factor for some cases.

 

Table 2: ANSI Standard Burdens for voltage transformer

 

Burden P.F	Voltamp. at 120 V	Burden
0.1 0.7 0.85 0.85 0.85 0.2	12.5 25 75 200 400 35	W X Y Z Z2 M

 

The transformer is also assigned  a thermal load,  which is equal to the volts of amperes that pass through the transformer without exceeding the permissible temperature range.

 

Accuracy  

Due to the resistance of the coils, there is a loss of electrical power in them, which causes a rise in the temperature of the coils, as well as the voltage ratio of the primary and secondary coils is not exactly the same as the ratio of the number of turns of the primary to the secondary coil. This is known as the ratio error, in addition to the fact that a part of the current is used in magnetizing the core, and there is also a loss of power in the iron core, so the angle between the two voltages of the primary coil and the secondary coil is not exactly 180, that is, there is an error in the Phase angle error, which is measured in minutes. This last error does not matter in circuits with a power factor equal to one or that feed measuring or controlling devices.