| Grounding is clearly one of the most important | | | | How to Specify a Grounding Transformer |
| aspects of electrical design, but it steadfastly | | | | The basic parameters required for quoting a |
| continues to be misinterpreted and misunderstood. | | | | grounding transformer are: |
| Millions of dollars in liability and loss can be | | | | - Primary Voltage - This is the system voltage to |
| attributed to ground-fault arcing; thus, | | | | which the grounded winding is to be connected. |
| grounding-related issues should top the checklists | | | | Don't forget to specify the BIL also. In some |
| of any electrical contractor. | | | | cases the BIL will be dictated by equipment |
| Grounding Transformers: | | | | considerations, such as 150 kV BIL ratings on |
| Simply put, a grounding transformer is used to | | | | 34500 volt wind farms because of the limitation |
| provide a ground path to either an ungrounded | | | | on dead front connectors. |
| "Y" or a delta connected system. Grounding | | | | - Rated KVA - Because the grounding |
| transformers are typically used to: | | | | transformer is normally a short time device, its |
| | | | size and cost are less when compared with a |
| 1. Provide a relatively low impedance path to | | | | continuous duty transformer of equal kVA rating. |
| ground, thereby maintaining the system neutral at | | | | For this reason, grounding transformers are often |
| or near ground potential | | | | not sized by "kVA" but by their continuous and |
| 2. Limit the magnitude of transient over voltages | | | | short time current ratings. Regardless of how you |
| when re-striking ground faults occur | | | | rate it, the grounding transformer must be sized |
| 3. Provide a source of ground fault current during | | | | to carry the rated continuous primary phase |
| line-to-ground faults | | | | current without exceeding its temperature limit. |
| 4. Permit the connection of phase to neutral loads | | | | This load includes the magnetizing current of the |
| when desired | | | | core, the capacitive charging current for the |
| If a single line-to-ground fault occurs on an | | | | cables, and any auxiliary load if applicable. The |
| ungrounded or isolated system, no return path | | | | higher this value, the larger and more costly the |
| exists for the fault current, thus no current flows. | | | | transformer will be. Typical continuous current |
| The system will continue to operate but the other | | | | values can be as low as 5 amps to as high as a |
| two un-faulted lines will rise in voltage by the | | | | few hundred. Be sure to include any auxiliary |
| square root of 3, resulting in overstressing of the | | | | loading requirements. |
| transformer insulation and other associated | | | | - Continuous Neutral Current - The continuous |
| components on the system by 173%. MOV | | | | neutral current is defined as three times the |
| lightning arresters are particularly susceptible to | | | | phase to current, or in other words, the zero |
| damage from heating by leakage across the | | | | sequence current. This is usually considered to be |
| blocks even if the voltage increase is not | | | | zero if the system is balanced. However, for the |
| sufficient to flash over. A grounding transformer | | | | purposes of designing a grounding transformer, it |
| provides a ground path to prevent this. | | | | is a value that is expected to flow in the neutral |
| Large multi-turbine wind farms provide an | | | | circuit without tripping protective circuits (which |
| example of the use of grounding transformers | | | | would force the current to be zero) or the |
| for fault protection on ungrounded lines. In many | | | | leakage current to ground that is not a |
| wind farms the substation transformer provides | | | | symmetrical function. Again this value is needed to |
| the sole ground source for the distribution | | | | design for thermal capacity of the grounding |
| system. When a ground fault on a collector cable | | | | transformer. |
| causes the substation circuit breaker for that | | | | - Fault current and duration - This value is needed |
| cable to open, the wind turbine string becomes | | | | to calculate the short time heating that results |
| isolated from the ground source. | | | | from a fault on the system and should be |
| The turbines do not always detect this fault or | | | | determined from an engineered system study. |
| the fact that the string is isolated and ungrounded; | | | | Typical values for this range from a few hundred |
| thus the generators continue to energize the | | | | amps to a few thousand amps with duration |
| collector cable, and the voltages between the | | | | times expressed in seconds and not cycles. For |
| un-faulted cables and the ground rise far above | | | | instance, a value of 400 amps for 10 seconds is |
| the normal voltage magnitude as described above. | | | | typical. The fault duration is a critical parameter |
| A grounding transformer placed on the turbine | | | | for the transformer designer. Where protection |
| string provides a ground path in the event the | | | | schemes use the grounding transformer for |
| string becomes isolated from the system ground. | | | | tripping functions, a relatively short time duration |
| Construction: | | | | is specified (5 -10 seconds). On the other hand, a |
| Grounding transformers are normally constructed | | | | continuous or extended neutral fault current |
| either with | | | | duration would be required when the grounding |
| | | | transformer is used in a ground fault alarm |
| 1. A ZigZag (Zn) connected winding with or | | | | scheme. |
| without an auxiliary winding or | | | | - Impedance - The impedance can be expressed |
| 2. As a Wye (Ynd) connected winding with a delta | | | | as a percentage or as an ohm value per phase. In |
| connected secondary that may or may not be | | | | either case it should be chosen so that the |
| used to supply auxiliary power | | | | un-faulted phase voltages during a ground fault |
| The geometry of the Zig-Zag connection is useful | | | | are within the temporary over-voltage capability |
| to limit circulation of third harmonics and can be | | | | of the transformer and associated equipment, |
| used without a Delta connected winding or the 4- | | | | such as arresters and terminal connectors. |
| or 5-leg core design normally used for this | | | | Because of this description, the values can vary |
| purpose in distribution and power transformers. | | | | from as low as 8% to almost 100%. This value |
| Eliminating the need for a secondary winding can | | | | must come from the system designer. |
| make this option both less expensive and smaller | | | | - Primary winding connection - Specify the type |
| than a comparable two-winding grounding | | | | of primary connection, either Zig-Zag or grounded |
| transformer. Furthermore, use of a Zig-Zag | | | | Wye. |
| transformer provides grounding with a smaller unit | | | | - Secondary connection - specify the secondary |
| than a two-winding Wye-Delta transformer | | | | voltage and connection when applicable. Specify |
| providing the same zero sequence impedance. | | | | the size of auxiliary loading to be connected for |
| Wye connected grounding transformers, on the | | | | either Zn or Wye connected primary windings. If |
| other hand, require either a delta connected | | | | the option is to have a two winding transformer |
| secondary or the application of 4 or 5 leg core | | | | with no secondary load, advise if the delta winding |
| construction to provide a return flux path for | | | | can be "buried" (that is not brought out) or if only |
| unbalanced loading associated with this primary | | | | one bushing is to be brought out for grounding to |
| connection. Since it is often desirable to provide | | | | the tank or testing. |
| auxiliary power from the grounding transformer | | | | · Basic overall construction features - |
| secondary winding, this benefit can sway the end | | | | note the following features as they apply to each |
| user to specify a two-winding grounding | | | | transformer |
| transformer in lieu of a Zig-Zag connection. The | | | | · Compartmental Padmount transformer |
| current trend in wind farm designs is toward the | | | | with integral tamperproof compartment or |
| Wye connected primary with a delta secondary. | | | | substation design |
| It is important to understand that both Zig-Zag | | | | · Outdoor or indoor |
| and two-winding grounding transformers can be | | | | · Fluid type- mineral oil, silicone, |
| provided with the ability to provide auxiliary | | | | Envirotemp FR3 |
| power, and this can be either a Wye or Delta | | | | · Connectivity -dead front, live front, |
| connected load. | | | | spade terminals, location of terminals - cover or |
| A solidly grounded system using a grounding | | | | sidewall , exposed or enclosed, etc |
| transformer offers many safety improvements | | | | · Temperature rise is assumed to be 65'C |
| over an ungrounded system. However, the | | | | · Site elevation or environmental concerns |
| ground transformer alone lacks the current limiting | | | | · Special paint as required |
| ability of a resistive grounding system. For this | | | | · Neutral Ground Resistors - The rated |
| reason, neutral ground resistors are often used in | | | | voltage of the NGR should be equal to the line to |
| conjunction with the grounding transformer to limit | | | | ground voltage of the grounding transformer. The |
| neutral ground fault current magnitude. Their ohm | | | | current rating and duration should match the |
| values should be specified to allow high enough | | | | grounding transformer ratings. Remember to set |
| ground fault current flow to permit reliable | | | | the current rating high enough to be above the |
| operation of the protective relaying equipment, | | | | cable charging current and grounding transformer |
| but low enough to limit thermal damage. | | | | magnetizing current. |