Medium voltage networks – Load flow calculation and network planning

Verifying the capacity of MV network

A load flow calculation study is necessary to verify that the electrical system has the adequate capacity to supply the connected load. In addition, this study will provide information regarding the real and reactive power flow, bus voltages (voltage drop), and power factor in each feeder of the electrical system. The load flow calculation was carried out based on the current data of the network.

In the calculation the future development of the network was included, according to previous studies the, load flow forecast for the year 2015 as well as the year 2020 the project reveals a progressive congestion in the medium voltage network, massive overloaded lines and in some areas some voltages problems.

It shows that the yearly consumption increase of the sample network in the city is about 4,1%. It is possible with the same program to evaluate all the calculations used by changing the scaling factors.

Conductor size also influences cable impedance and hence the voltage drops along a feeder due to the load current being taken. It is therefore necessary to check that the drop of voltage along the cable route does not exceed the design criteria for the network or the operating voltage range of the equipment being fed.

This review needs to take into account both the continuous and non-continuous loads and any emergency overload that the cable will be required to carry. Voltage drop is a secondary factor and usually only occurs in very large installations with long cable runs.

Manufacturers’ data sheets also often include voltage drop tables that can be used for a quick parametric check.

Single Line Diagram

A one-line diagram is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, busbars and conductors are shown by standardized schematic symbols.

Instead of representing each of three phases with a separate line or terminal, only one conductor is represented see Figure 1. The theory of three-phase power systems tells us that as long as the loads on each of the three phases are balanced and the lines, transformers and busbars are symmetrical, we can consider each phase separately.

Single-line diagram is usually used along with other notational simplifications, such as the per-unit system. A secondary advantage to using a one-line diagram is that the simpler diagram leaves more space for non-electrical, such as economic, information to be included.

A presentation of a single line diagram of an 11-kV switchgear as an example in Asian country (Iraq) is shown above in Figure 1, and a typical electricity power system in the same country is shown in Figure 2.

Power Quality

Classification of Power system disturbances

To make the study of Power Quality problems useful, the various types of disturbances need to be classified by magnitude and duration. This is especially important for manufacturers and users of equipment that may be at risk. The principal standards in this field are IEC 61000, EN 50160, and IEEE 1159.

Standards are essential for manufacturers and users alike, to define what is reasonable in terms of disturbances that might occur and what equipment should withstand.

The following parameters are relevant for the power quality corresponding the European standard EN 50160:

• Voltage level, slow voltage deviation
• Voltage dips (short, long)
• Voltage drop
• Rapid voltage deviation, flicker
• Unbalance
• Voltage distortion (harmonics, signal, voltage)
• Transient and mains frequency overvoltage
• Frequency

6.3 Load Flow Calculation Method

The goal of a power flow study is to obtain the complete voltage angle and magnitude information for each bus in a power system for specified load and generator real power and voltage conditions. Once this information is known, real and reactive power flow on each branch as well as generator reactive power output can be analytically determined.

Due to the nonlinear nature of this problem, numerical methods are employed to obtain a solution that is within an acceptable tolerance. The solution to the power flow problem begins with identifying the known and unknown variables in the system.

The known and unknown variables are dependent on the type of bus.

• A bus without any generators connected to it is called a Load Bus.
• With one exception, a bus with at least one generator connected to it is called a Generator Bus.
• The exception is one arbitrarily-selected bus that has a generator. This bus is referred to as the Slack Bus.
• Slack Bus at which: P = ∞,  Q = ∞ and V = constant

Recommended Course – Circuit Breaker Failure Protection Course: Function, CB Scheme/Trip Matrix and Various Busbar Schemes

Circuit Breaker Failure Protection Course: Function, CB Scheme/Trip Matrix and Various Busbar Schemes