Pumps & Systems , November 2007
The second part of this three-part series explores the types of protection applied to specific equipment installed at water and wastewater facilities and some typical criteria used to develop protective settings.
Application of Protective Relaying to Specific Equipment
This section shows what types of protection are applied to specific equipment that might be installed at a water or wastewater facility and some typical criteria that would be used to develop protective settings. The information is intended to give general understanding and guidance but is not a complete discussion of all the equipment characteristics and relay capabilities that must be considered to develop a protective relaying system. The choice of actual protection and relay settings to be applied depends on the specific conditions for each installation and may differ from those discussed in this section.
Transformer Protection
In addition to the damage at the immediate location of an internal fault, a transformer is subject to damage from the fault current flowing in its windings. Damage is caused by heating and by magnetic forces associated with the current. [3] Magnetic forces can cause deformation or destruction of the windings, while thermal and mechanical damage also can occur when the transformer supplies current to an external fault. High speed protection is essential to avoid thermal and mechanical damage for external faults and to limit the damage for internal faults.
For a smaller transformer, perhaps less than 10-MVA capacity, protection may simply be power fuses applied at the high side terminals. Protective relays are typically applied to larger transformers or if there is a design preference to use circuit breakers instead of fuses. Phase and ground overcurrent protection may be the only relaying applied.
For more critical transformers, the main protection is often differential relaying. Overcurrent relays are then used as backup protection. A typical configuration for overcurrent relaying is time and instantaneous overcurrent relays for phase and ground faults on the transformer high side, and separate time overcurrent relaying for phase and ground faults on the low side. If the transformer provides resistance grounding for the system, ground fault current will be limited to values that might not be reliably detected by differential relays. In these cases, separate ground differential relaying can be applied.
Differential relays may require general settings such as the transformer rated voltage, kVA, winding connections, and CT ratios. Protective settings may include minimum pickup, slope of the differential characteristic, and settings to inhibit relay operation when the transformer is energized after being out of service and magnetizing inrush current flows. A typical minimum pickup setting might be 15 percent of the transformer rated current. Differential relays may also provide an instantaneous overcurrent element that measures differential current but is not restrained. The unrestrained instantaneous pickup setting must be high enough so that it can only operate for transformer internal faults.
For example, in a radial system, the unrestrained instantaneous pickup setting would be lower than the current for a fault at the transformer high side terminals but higher than the current for a fault at the low side terminals. If the instantaneous element is not restrained for magnetizing inrush, its setting must also be higher than the inrush current.
Some general guidelines for transformer overcurrent relay settings are as follows:
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High side phase instantaneous overcurrent : Set similar to the unrestrained differential relay instantaneous element. Set higher than the current for a fault at the low side terminals, higher than magnetizing inrush current, and ower than current for a fault at the high side terminals.
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High side phase time overcurrent : Set pickup to allow for normal and emergency loading and to comply with requirements in NEC Table 450.3(A). [11] Set time delay to coordinate with industry-standard transformer damage limits [3] and to coordinate with time overcurrent devices upstream and downstream from the transformer.
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High side ground overcurrent : Transformer winding connections determine the criteria for setting ground overcurrent relays. Usually, high side winding is connected in delta and low side winding is connected in wye. There is no ground current on the delta side for a ground fault on the wye side, and ground relays can be set very sensitively.
There is a possibility that magnetizing inrush current could cause current transformers to be inaccurate, resulting in a current measured by the ground overcurrent relay when no ground fault exists. For security, it may be desirable to set the high side ground instantaneous relay at perhaps 25 percent of the setting for the phase instantaneous overcurrent relay. With a delta-wye connection, high side ground time overcurrent relaying is not required, but may be applied for redundancy. A pickup setting of 25 percent of transformer full load current and time delay of 0.1 second at the maximum ground fault current is suggested.
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Low side phase overcurrent : Instantaneous overcurrent is not recommended because it cannot be coordinated with downstream overcurrent devices. Set the time overcurrent pickup to allow for normal and emergency loading. Set the time delay to coordinate with upstream and downstream overcurrent devices and to coordinate with the transformer damage limits.
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Low side ground overcurrent : As with the low side phase relays, instantaneous overcurrent is not recommended because it cannot be coordinated with downstream overcurrent devices. Set the time overcurrent pickup at 25 percent or more of the transformer full load current to allow for load unbalance and to facilitate coordination with downstream overcurrent devices. Usually, the transformer winding connection eliminates the need to coordinate the low side ground overcurrent relay with devices on the high side, and the time delay can be set to coordinate with downstream devices.
Other protection using devices other than protective relays might be applied for larger transformers. For example, liquid-filled transformers can be equipped with a specialized relay to detect sudden pressure changes caused by faults inside the transformer tank. Arcing vaporizes the insulating fluid in the immediate vicinity of the fault and the resulting pressure wave propagates through the tank volume. The
