How do you receive electrical power at your home or office?
Electricity is one of the most convenient forms of energy available today for our day to day needs. Electrical power is used for domestic lighting, heating and motive power for driving various types of loads in industries etc. Now the question is from where do we receive this power & energy? As you know, electrical power & energy is produced (or converted from one form of energy to electrical energy) at power stations. These stations are located at various places in our country and most of them are far and remote from consumer loads. The sets of equipments installed, from sources up to the consumer loads, performing the processes of generation, transmission/transformation and distribution of electrical energy, is know as an electrical power system.
This means that the generated energy has to be transferred from the sources up to the consumer loads passing a long way. The next question that comes to our mind is that, from which power station do we receive this energy?. This is rather a difficult question to answer, as all of the generating stations are interconnected to each other through transmission lines.
You also know that most generating plants in Sri Lanka (especially if they are hydroelectric) and even in other part of the world are located in remote places (eg. Laxapana, Victoria, Samanalawewa etc.) with respect to the load centres (Colombo, Galle, Kandy, Jaffna). To deliver this generated energy to the load centers, a transmission system is required. The transmission system should be able to carry this energy reliably, and with a minimum loss, at a virtually stable voltage and frequency.
A transmission system can be broken down into three sub systems.
They are,
· Primary Transmission System (generating points to bulk power receiving points)
· Sub Transmission System (bulk power receiving points to area substations)
· Distribution System (area substations to distribution substations)
We know the generated electrical energy is transmitted over long distances to reach the load centres. Generator voltages are in the range of 11 to 30 kV; higher generator voltages are difficult to achieve owing to insulation problems in the narrow confines of the generator stator.
Long distance transmission cannot be done at generator voltage levels (11-30 kV) because of the huge material requirement and the associated high Copper Loss (sometimes we call it I2R loss). Therefore, the voltage is first stepped up at the generating point using transformers, depending upon the power system and the amount of power that has to be transmitted through transmission lines. Then this power flows through the high voltage transmission lines to the load centres.
Transmission voltages worldwide range from 110 to 765 kV. One reason for using higher transmission voltages is to improve transmission efficiency. Basically, transmission of a given amount of power (at a specified power factor) requires a fixed product of voltage and line current. Thus, the higher the voltage, the lower is the current required. Lower line currents cause lower resistive losses (I2R) in the line.
For example the present Sri Lanka Primary Transmission System consists of an island wide network of 220 kV and 132 kV transmission lines feeding several 220/33 kV and 132/33 kV bulk power receiving stations. These receiving stations are also known as grid substations. You should always remember that when we state the voltage of a 3-phase line, we refer to the voltage between any two wires.
At these receiving points, the voltage is stepped down to 33 kV (or 11 kV in a few cases) and fed to the Sub Transmission System for shorter transmission runs. For example, The Sri Lanka sub transmission system comprises a 33 kV network, but there are a few 11 kV sub transmission lines, mainly in urban and suburban areas. Thereafter, the voltage is further reduced to 400 V by means of distribution transformers at distribution substations located in the residential and commercial areas for distribution purposes. Elements of a typical electrical power system are show in figure 4.1.
In this session, we will study about distribution networks, i.e. the network emanating from distribution substations up to the consumers.
Distribution substation
The necessary electrical power for the distribution network is transformed at the distribution substations. A distribution substation consists of transformers, high voltage and low voltage bus bars, feeders, circuit breakers, instrument transformers, different types of relays (such as over current, differential & earth fault relays) etc.
As we mentioned earlier, the main functions of a distribution substation are:
· Stepping down the transmission voltage up to the distribution voltage level.
· Distribution of power in multiple directions.
· Disconnect and re-connect from the H/V transmission grid or L/V distribution feeders using circuit breakers.
Figure 4.2 shows a schematic diagram of a distribution substation. The voltages of lines, which leave the low voltage bus bars, are further stepped down. At the normal operating condition the low voltage bus bars are not connected to each other (i.e. bus-tie circuit breaker is opened). It reduces the short circuit current contribution during faulty conditions to the components installed in the distribution system.
Types of distribution systems:
Let us imagine that several consumer loads (fed through distribution substations) are connected to a single source (main grid substation) located in one area. What kind of a configuration can think of, for the distribution of electrical power among these consumer loads? The simplest method is to connect each consumer load to the grid substation through dedicated feeder lines. Such a network will need a large number of feeder lines to be installed between the grid substation and the consumer loads and therefore is not recommend. Instead of connecting a single consumer to a dedicated feeder, it is recommended to connect a group of consumers to each of these feeder lines thus minimizing the overall distribution cost. However, care should be taken to avoid violation of any technical constraints such as over loading of distribution lines, voltage drops etc. Since all the lines are radialy emanating from the source in this case, this type of distribution system is known as a radial main system (see fig.4.3 (a)). The radial main distribution system is the cheapest because it requires the least amount of conductors and simple line protection methods compared to the other systems available for power distribution which we will discuss later.
However, what happens if one of these radial lines goes out of service, some times we call it a “forced outage” in technical terms? Of course, the consumer (or group of consumers), connected to the feeder line will not get any electrical power. This is the main disadvantage of radial systems. Mostly, these systems are used in rural areas. How can we avoid or at least minimize feeder outages when only one source is available? If we can find a way to connect the consumers and the source in a ring, then the consumers will receive supply from both sides and even if a portion of the line is on forced outage, the system still receives supply from either side. This type of distribution system is called a ring main system and a conceptual single line diagram is as shown in fig 4.3 (b). Normally, ring main systems receive supply from multiple sources. Usually this type of system is used or recommended in areas where the higher reliability for the consumers is a requirement. As of today, most of the distribution systems are interconnected to each other, in which the ring main systems have additional interconnections between nodes (fig4.3(c)).
Overhead lines and cables
Length of an overhead transmission line changes over a wide range, from several kilometres up to hundreds of kilometres, for an underground cable it may vary from several meters up to a few kilometres. The equivalent circuit of an overhead line or an under ground cable for purposes of analysis depends on its operating voltage and the line length. The operating voltages of the distribution lines are low and the length is always short (Approximately 8~10 km. for 11~13.8 kV lines etc.) Since the operating voltage is low, the line spacing required to maintain insulation level between the conductors on overhead line at distribution voltage levels will also become less. This leads to the characteristic of line resistance to be equal or greater than to its inductive reactance. Figure 4.4 shows a typical equivalent circuit that can be assumed for a low voltage overhead line in distribution networks.
Z = R + j X
Voltage drops in distribution networks.
Consider a case where an electrical power is delivered from a distribution substation to a consumer load. Depending on the amount of load at the consumer and the line impedance (Z=R+jX), a voltage drop (∆v = I Z) and a transmission line loss (I2R) will appear along the line. So in order to have a VR voltage at the receiving end of a transmission line, you would require a sending end voltage of VR +∆v. Therefore, it is very important for us to know the voltage drop of a transmission line against the power demand and the electrical parameters of the network connected to it. Let us first consider a simple situation where a consumer receives S=P+jQ of apparent power at a power factor of Φ from a distribution substation and demands a receiving end voltage of VR . Now, the question is what voltage we should have at the sending end (substation) in order to maintain VRvoltage at the receiving end. The simplified single line diagram showing the above case is as shown in fig.4.5. Let the sending end voltage be VS.
Usually resistance and the inductive reactance of a distribution line are of the same order or in some cases the resistance is greater than the inductive reactance. As, such the lateral part of the voltage drop component drawn in the phasor diagram can be neglected. For this reason, the voltage drop of a distribution line can be estimated by its longitudinal component as follows:
Example 1:
A radial main distribution network fed from a distribution sub station is as shown in figure 4.7. The length, resistance & inductive reactance per kilometre of lines, power and power factor of the loads drawn at each load location, are given. If the observed voltage at E is 11 kV, what should be the voltage at substation?
Voltage regulation in distribution networks
To understand how voltage regulation methods work, let us look at equation 4.4 again. The resistance (R) and the reactance (X) of distribution lines (sometimes we call them line parameters) are almost constant (however, these parameters can change depending on the type & length of the lines). Active power (P) in the equation (demands by the consumer) has to be delivered through the line. The voltages at the sending end (VS)and the receiving end (VR) can be regulated by changing the reactive power (±Q) at the receiving end. This can be done by installing capacitor banks at the places of consumption. In such a situation, the reactive power flow from the source will be reduced and the capacitor banks installed at the receiving end compensates part of reactive power required by the consumer. The amount of reactive power needed at light load and peak load periods can be calculated using equation 4.4.
Figure 4.8 (a) shows a distribution network in which several loads are connect to a radial line through transformers. The voltage profile during the period of maximum and minimum loads without capacitor banks and the voltage profile at the period of maximum load when the capacitor bank supplies the reactive power are shown in fig.4.8 (b) line 1 and 2 and fig.4.8 (b) line 3 respectively. As you can see from the diagram, the voltage drop at far end is more than its maximum permissible limit. However, when the compensation is present, reactive power flow through the line reduces and voltage at the consumer end is improved. To understand the above, let us look at an example.
