| Grounding is clearly one of the most important aspects | | | | transformer offers many safety improvements over |
| of electrical design, but it steadfastly continues to be | | | | an ungrounded system. However, the ground |
| misinterpreted and misunderstood. Millions of dollars in | | | | transformer alone lacks the current limiting ability of a |
| liability and loss can be attributed to ground-fault arcing; | | | | resistive grounding system. For this reason, neutral |
| thus, grounding-related issues should top the checklists | | | | ground resistors are often used in conjunction with the |
| of any electrical contractor. | | | | grounding transformer to limit neutral ground fault |
| Grounding Transformers: | | | | current magnitude. Their ohm values should be |
| Simply put, a grounding transformer is used to provide | | | | specified to allow high enough ground fault current flow |
| a ground path to either an ungrounded "Y" or a delta | | | | to permit reliable operation of the protective relaying |
| connected system. Grounding transformers are | | | | equipment, but low enough to limit thermal damage. |
| typically used to: | | | | How to Specify a Grounding Transformer |
| 1. Provide a relatively low impedance path to ground, | | | | The basic parameters required for quoting a grounding |
| thereby maintaining the system neutral at or near | | | | transformer are: |
| ground potential | | | | - Primary Voltage - This is the system voltage to |
| 2. Limit the magnitude of transient over voltages when | | | | which the grounded winding is to be connected. Don't |
| re-striking ground faults occur | | | | forget to specify the BIL also. In some cases the BIL |
| 3. Provide a source of ground fault current during | | | | will be dictated by equipment considerations, such as |
| line-to-ground faults | | | | 150 kV BIL ratings on 34500 volt wind farms because |
| 4. Permit the connection of phase to neutral loads | | | | of the limitation on dead front connectors. |
| when desired | | | | - Rated KVA - Because the grounding transformer is |
| If a single line-to-ground fault occurs on an ungrounded | | | | normally a short time device, its size and cost are less |
| or isolated system, no return path exists for the fault | | | | when compared with a continuous duty transformer of |
| current, thus no current flows. The system will continue | | | | equal kVA rating. For this reason, grounding |
| to operate but the other two un-faulted lines will rise in | | | | transformers are often not sized by "kVA" but by their |
| voltage by the square root of 3, resulting in | | | | continuous and short time current ratings. Regardless |
| overstressing of the transformer insulation and other | | | | of how you rate it, the grounding transformer must be |
| associated components on the system by 173%. MOV | | | | sized to carry the rated continuous primary phase |
| lightning arresters are particularly susceptible to | | | | current without exceeding its temperature limit. This |
| damage from heating by leakage across the blocks | | | | load includes the magnetizing current of the core, the |
| even if the voltage increase is not sufficient to flash | | | | capacitive charging current for the cables, and any |
| over. A grounding transformer provides a ground path | | | | auxiliary load if applicable. The higher this value, the |
| to prevent this. | | | | larger and more costly the transformer will be. |
| Large multi-turbine wind farms provide an example of | | | | Typical continuous current values can be as low as 5 |
| the use of grounding transformers for fault protection | | | | amps to as high as a few hundred. Be sure to include |
| on ungrounded lines. In many wind farms the substation | | | | any auxiliary loading requirements. |
| transformer provides the sole ground source for the | | | | - Continuous Neutral Current - The continuous neutral |
| distribution system. When a ground fault on a collector | | | | current is defined as three times the phase to current, |
| cable causes the substation circuit breaker for that | | | | or in other words, the zero sequence current. This is |
| cable to open, the wind turbine string becomes isolated | | | | usually considered to be zero if the system is |
| from the ground source. The turbines do not always | | | | balanced. However, for the purposes of designing a |
| detect this fault or the fact that the string is isolated | | | | grounding transformer, it is a value that is expected to |
| and ungrounded; thus the generators continue to | | | | flow in the neutral circuit without tripping protective |
| energize the collector cable, and the voltages between | | | | circuits (which would force the current to be zero) or |
| the un-faulted cables and the ground rise far above | | | | the leakage current to ground that is not a symmetrical |
| the normal voltage magnitude as described above. A | | | | function. Again this value is needed to design for |
| grounding transformer placed on the turbine string | | | | thermal capacity of the grounding transformer. |
| provides a ground path in the event the string | | | | - Fault current and duration - This value is needed to |
| becomes isolated from the system ground. | | | | calculate the short time heating that results from a |
| Construction: | | | | fault on the system and should be determined from an |
| Grounding transformers are normally constructed | | | | engineered system study. Typical values for this range |
| either with | | | | from a few hundred amps to a few thousand amps |
| 1. A ZigZag (Zn) connected winding with or without an | | | | with duration times expressed in seconds and not |
| auxiliary winding or | | | | cycles. For instance, a value of 400 amps for 10 |
| 2. As a Wye (Ynd) connected winding with a delta | | | | seconds is typical. The fault duration is a critical |
| connected secondary that may or may not be used | | | | parameter for the transformer designer. Where |
| to supply auxiliary power | | | | protection schemes use the grounding transformer for |
| The geometry of the Zig-Zag connection is useful to | | | | tripping functions, a relatively short time duration is |
| limit circulation of third harmonics and can be used | | | | specified (5 -10 seconds). On the other hand, a |
| without a Delta connected winding or the 4- or 5-leg | | | | continuous or extended neutral fault current duration |
| core design normally used for this purpose in | | | | would be required when the grounding transformer is |
| distribution and power transformers. Eliminating the | | | | used in a ground fault alarm scheme. |
| need for a secondary winding can make this option | | | | - Impedance - The impedance can be expressed as a |
| both less expensive and smaller than a comparable | | | | percentage or as an ohm value per phase. In either |
| two-winding grounding transformer. Furthermore, use | | | | case it should be chosen so that the un-faulted phase |
| of a Zig-Zag transformer provides grounding with a | | | | voltages during a ground fault are within the temporary |
| smaller unit than a two-winding Wye-Delta transformer | | | | over-voltage capability of the transformer and |
| providing the same zero sequence impedance. | | | | associated equipment, such as arresters and terminal |
| Wye connected grounding transformers, on the other | | | | connectors. Because of this description, the values can |
| hand, require either a delta connected secondary or | | | | vary from as low as 8% to almost 100%. This value |
| the application of 4 or 5 leg core construction to | | | | must come from the system designer. |
| provide a return flux path for unbalanced loading | | | | - Primary winding connection - Specify the type of |
| associated with this primary connection. Since it is | | | | primary connection, either Zig-Zag or grounded Wye. |
| often desirable to provide auxiliary power from the | | | | - Secondary connection - specify the secondary |
| grounding transformer secondary winding, this benefit | | | | voltage and connection when applicable. |
| can sway the end user to specify a two-winding | | | | Specify the size of auxiliary loading to be connected |
| grounding transformer in lieu of a Zig-Zag connection. | | | | for either Zn or Wye connected primary windings. |
| The current trend in wind farm designs is toward the | | | | If the option is to have a two winding transformer with |
| Wye connected primary with a delta secondary. | | | | no secondary load, advise if the delta winding can be |
| It is important to understand that both Zig-Zag and | | | | "buried" (that is not brought out) or if only one bushing is |
| two-winding grounding transformers can be provided | | | | to be brought out for grounding to the tank or testing. |
| with the ability to provide auxiliary power, and this can | | | | - Basic overall construction features - note the |
| be either a Wye or Delta connected load. | | | | following features as they apply to each transformer... |
| A solidly grounded system using a grounding | | | | |