The ground is clearly one of the most important aspects of electrical design, but remains firm be misinterpreted and misunderstood. Millions of dollars of liability and loss of earth fault arc can be attributed, and so, grounding issues should top the checklists of Electrical Contractors.
Grounding Transformers
In short, a transformer grounding is used to specify a base path, switched to a non-ground "Y" or a triangular system. GroundingTransformers are typically used for:
Enter a relatively low impedance path to ground, maintaining the system at or near ground potential neutral Limit the size of the transient voltages that occur when re-striking earth faults Enter a source of fault current during line-to-ground fault Enable the connection of phase-neutral loads, if desired
When a single line-ground fault on an ungrounded system, or isolated, there is no return channel forFault current, so that no current flow. The system continues to function, but the other two non-perturbed lines of increased tension to the square root of 3, leading to an overload of the isolation transformer and other associated components in the system by 173%. MOV arresters are particularly susceptible to damage by heat losses in the blocks, even if the voltage is not always enough to flashover. A transformer grounding provides a ground path to preventthis.
Large multi-wind turbines provide an example of using transformers grounded to protect against interference from land lines. In many of the wind farm substation transformer provides the sole reason for the source distribution. In the case of ground fault caused to open it in a collector substation circuit breaker cable for this cable, the wind turbine is isolated from the soil the source string.
The turbines do not always recognize the disease or the fact that the stringisolated and not grounded, so that the generators continue to animate the cable collector, and increased tensions between the non-perturbed ground wire and power far beyond the normal range as described above. A transformer is grounded on the string turbine placed on the ground provides a path where the string is isolated from the soil.
Construction:
Grounding transformers are usually built with both
A (Zn) with or without winding connected ZigZagAuxiliary winding or How Wye (YND) of the secondary connected to a Delta or may not be used to provide additional power
The geometry of the zig-zag connection is useful to restrict the movement of third-harmonic and delta connected winding can not be used or 4 - or 5-foot-cores used, usually used for this purpose distribution and power transformers. By eliminating the need for a secondary winding, this option is cheaper and smaller than acomparable with two transformers grounded winding. Furthermore, in a zig-zag grounding transformer provides a unit smaller than a two-winding converter Delta-Wye, the same impedance zero.
Wye connected transformers, on the other hand, is a triangle or the application of the secondary circuit of 4 or 5-leg core design in order to provide a return path for unbalanced load with this primary connection services. Since it is often desirableprovide auxiliary power by the grounding of the transformer secondary, which can provide energy for the benefit of the end user to a two-winding transformer connected to the ground instead of a zig-zag. The current trend in the design of the wind farm is connected to the Wye primary with a delta secondary.
It 'important to understand that both can zig-zag transformers to the ground and two windings are provided with the opportunity to provide more power, and this can be either a star or delta connectedto load.
A transformer with a solid grounding system offers numerous improvements to the security of a system ground. However, it lacks the land alone, the current transformer that limits the ability of a system of grounding resistance. For this reason, the neutral ground resistors are often used in combination with the grounding transformer neutral ground current magnitude limit. Ohm their values must be used in high current ground fault enough to allow allow reliable operationrelay protection equipment, but low enough to limit thermal damage.
How to specify a transformer grounding
The basic parameters for quoting a transformer grounding is necessary:
Primary voltage - This is the voltage at which the winding is connected to the ground. Do not forget to specify BIL. In some cases, the BIL will be considerations of equipment, such as 150 kV BIL dictated reviews of 34 500 volts wind farms because of limitations on the deadFront connectors. Rated KVA - Since the transformer grounding device is usually a short period of time, its size and cost less than a continuous power output kVA transformer. For this reason, transformers are often not grounded in size from "kVA", but from their continuous-time rating and short-circuit current. Regardless of how you evaluate, the grounding transformer can be sized to carry the rated primary phase-continuous, without exceeding the temperature limit. This loadincludes the magnetizing current of the core, the capacitive charging current for the cable and any additional charge, if applicable. The higher the value, the bigger and more expensive transformer. Typical values of the constant current can be as low as 5 amps to a maximum of a few hundred. Be sure to include all the auxiliary load requirements. Continue Neutral Current - The continuous flow defined neutral three times the current stage, or in other words, the zero-sequenceCurrent. This is usually deemed to be zero when the system is balanced. But for the design of a transformer grounding, there is an expected value of being in the neutral circuit without tripping the protection circuits (the power of electricity are set to zero) or the leakage current to ground that is not a symmetric function of the flow. This value is also needed to design the thermal capacity of transformer grounding. Fault current and duration - This value is requiredThe calculation of the heating system a short time to be determined by an error in the system and the study of a technical system. Typical values for this range of several hundred amperes to several thousand amperes with a duration of time in seconds, not cycles. For example, a typical value of 400 A for 10 seconds. The duration of the fault is a critical parameter for the designer transformer. In cases where protection mechanisms use the transformer grounded to activate the feature, a relatively shortSpecified period of time (5 -10 seconds). On the other hand, a period of extended or continuous failure, it would be necessary if the neutral earthing transformer is used in an alarm system ground fault. Impedance - The impedance can be expressed as a percentage or as ohms per phase. But in any case so that the non-phase voltages are disturbed during a ground fault in the temporary surge capacity of the transformer and associated equipment, such asStevedores and terminal. Based on this description, the values range from a minimum of 8% to almost 100%. This value must come from the system designer. The primary winding connection - Specify the type of primary connection, and zig-zag or grounded Wye. Secondary Connection - Enter the secondary voltage and connection, if applicable. Enter the size of the auxiliary load for connecting to one or Zn Wye connected primary windings. If the option is to have twoTransformer with no load secondary winding to know whether the delta can be "buried" (not to be taken out) or when a single female to be placed on the ground or in the tank to check.
Material Safety Data base overall design - note the following features that apply to each transformer
Compartment of poles with transformer assembly tamper-proof tray or wall panel
· External or internal
· Fluid-type of mineral oil,Silicon, Envirotemp FR3
· Connectivity died first, against life, flat plug, lights - cover or side wall, suspended or closed, etc.
· Temperature increase is assumed to be 65 ° C.
· Level site or environmental concerns
Special Paint, if necessary
· Resistance neutral ground - The rated voltage of the NGR should be equal to the line of ground voltage of the transformer grounded. The current rating and the duration shouldGame rating of transformer grounding. Remember, the current rating high enough to put on the cable charging current and grounding transformer magnetizing current.
