GROUNDING METHODS IN MEDIUM VOLTAGE NETWORKS Didem ERGUN SEZER Ergun Elektrik Ltd Şti, İzmir didem@ergunelektrik.com ABSTRACT This study briefly discusses the grounding methods of medium voltage networks and attempts to explain which type of network each method is suitable for. Furthermore, the reasons and benefits of grounding the neutral point through a resistor will be examined, and some information will be provided as an example of a medium voltage network and a generator grounding method applied in our country. INTRODUCTION The majority of short circuits occurring in medium voltage networks are phase-to-ground short circuits. The current that will flow in a phase-to-ground short circuit depends on the condition of the fault point. Therefore, the grounding method of the star point is of great importance. The main grounding methods applied in MV networks are: 1. MV networks with directly grounded neutral point 2. MV networks with grounded neutral point via low resistance 3. MV networks with grounded neutral point via high resistance 4. Isolated networks 5. MV networks with neutral point grounded via inductor (Petersen inductor) 6. MV networks with neutral point grounded via resistance through a zigzag transformer. 1. MV Networks with Directly Grounded Neutral Point: Direct grounding is an undesirable grounding method in MV networks. This is because a very high ground fault current will flow in a phase-to-ground short circuit. It is likely to cause significant material damage to the installation. The protective devices must open the circuit very quickly. (Figure-1) Figure-1 2. MV Networks with Neutral Point Grounded via Low Resistance: The star point is grounded via an ohmic resistance. Thus, the ground current is limited to the permitted value. Since resistive ground leaks generally occur in the network, the current is reduced to lower levels, and the star point shift is proportional to this current that will flow (Figure-2). Figure-2 The method of grounding the star point through a low ohmic resistance is applied in transformer neutral grounding in MV networks. It is connected to the star point of the transformer feeding the line. If several transformers feed the line in parallel, some of the resistors should be disconnected with a disconnect switch to prevent the resistance value from decreasing. This method provides protection coordination and saves time for the protection devices. It also prevents the protection devices from opening the circuit and cutting off the power to the installation in transient short circuits. In American networks, the neutral point grounding method with low resistance is generally preferred. In our country, 36kV distribution networks use 36/√3 kV, 1000 A, 20 Ω, 5 s resistors. 3. MV Networks with Neutral Point Grounded by High-Value Resistor: The neutral point is grounded through an ohmic resistor with a value sufficient to limit fault levels at 10A. The star point is isolated by utilizing the current. Ground leakage is indicated by a residual voltage protection device. In 34.5/6.3kV transformers supplying factories, the 6.3kV transformer star is isolated. Thus, the 6.3kV point will be at the ground level in milliamperes, preventing damage to the expensive motor coil pack. At the same time, voltage leakage is not a concern. For example, cement factories with 20 MVA 34.5/6.3 kV or textile factories with 15 MVA 34.5/6.3 kV transformers have isolated star points in their secondary windings and are equipped with appropriate relays for isolated network installations. This system, which is also applied in Europe in factories' 400 V – 525 V – 690 V networks, is used to detect faults and isolation relays. The benefit of leaving the star points isolated in factories is that when a short circuit occurs, the circuit is not opened by the protection devices, allowing the fault to be rectified in a timely manner according to the manufacturing schedule. At 6.3 kV, the circuit is opened in case of a fault, as there is a life-threatening danger at that voltage level. This method is applied in the neutral grounding of the generator. Thus, damage that may occur in the generator windings is reduced. 4. Isolated MV Networks: This method of leaving the star point isolated is applied in networks with small cable distribution networks. Because capacitive levels are low in these networks, and the current level is determined by the capacitance value between the network conductors and the ground. In this situation, the currents are sent to the ground (Figure 4). However, this method is not recommended in long-distance cable networks. The fault current continues to flow, and during this time, there is a possibility of high transient voltages occurring. There is also a risk that the high voltage generated may cause a new fault by puncturing the ground insulation at another point in the network. In this case, very high fault currents flow between the two faulty points, causing material damage at both points. A solution where fault currents do not flow in the network is needed. Also, due to the shift of the star point, the phase-to-ground voltage level may rise during a fault. For this reason, the insulation levels must be selected taking this situation into consideration. Figure 6 shows a phase-to-phase connection in medium voltage networks with a neutral point grounded by a resistor via a zigzag transformer. If the transformer's secondary winding is delta-connected and grounding of the star point is to be done using the ground short-circuit method, a zigzag transformer is mounted at the desired star point, and a resistor is connected to the star point of the zigzag transformer, thus establishing the star point with a resistor. Figure 7 Example 1 (Figure 8) In our country, 36kV distribution networks also use 36/√3 kV, 1000 A, 20 Ω, 5 s resistors. Figure 5 5. MV Networks Grounded with Neutral Point Suppression Coil If the star point of the transformer supplying the network is isolated due to a ground short circuit, the conductors and current level are determined by the capacitance value between the network and ground. Utilizing this principle, the transformer neutral point is grounded with an inductive resistor equivalent to the network-ground capacitive resistance value. The fault current flowing to ground is suppressed and zeroed. This coil is named after its inventor, the Petersen coil. The disadvantage of compensation current is that the Petersen coil is generally preferred in German networks for this grounding method. However, some drawbacks of this method have been observed over time. With this method, the network size and the network capacitance value change with switching operations, and complete ground suppression cannot be achieved. As a precaution, resonance relay controlled, servo motor driven coils are manufactured and installed. This is a high-cost solution. Installation is expensive, but when it works properly, it adjusts the Petersen current. RESULT: In our report, we briefly touched upon the grounding methods applied in MV networks and tried to specify which grounding method should be applied. As can be seen, each method has its own undesirable benefits, but the advantages of resistance grounding of the neutral point, which is used in our country as a grounding method, can be summarized as follows: • Limiting the ground fault current with a resistor in transformers and generators, thus reducing damage to the installation. • Reducing wear and tear on equipment through which the current flows. • Reducing the occurrence of step voltage. • Reducing the severity of voltage sag in case of a short circuit. • A 1000 A resistor is manufactured, but for transformers, the current is selected at 200/5 A to ensure the sensitive operation of the protection devices. Example 2: (Figure-9) In applications, the fault current of 6.3 kV generators is reduced to 10 A by grounding them via a star connection. (Figure-9 shows the resistance point)
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