Introduction
The type and extent of protection for transformers used in industrial and commercial power systems is a compromise between such factors as the importance of the unit, sensitivity, speed and coordination problems versus the cost of the protection scheme. The sketches presented in this guide represent general practice for the most common transformer applications.
Thermal Protection
Thermal protection, Device 49, is supplied by the transformer manufacturer when used. It is generally a thermally actuated device which indicates the maximum temperature of the cooling medium or winding "hottest spot," temperature. It has switches for automatic control of auxiliary cooling equipment and for high temperature alarm. The high temperature switch will normally be set at the temperature above which some accelerated loss in insulating life will begin to occur. It may be used to alarm or trip and the user may adjust its setting based on system conditions and guided by the ANSI C57.96 guides for loading, oil and air cooled transformers respectively.
Thermal protection for heavy overload conditions normally is obtained by the transformer secondary time overcurrent relays (Devices 51 in Figure 1) which will operate before the indicating thermal overload relay in the transformer itself. This is discussed in the "Fault Protection" section which follows.
Fault Protection
The preferred arrangement for transformer protection is shown in Figure 1. This will apply to the vast majority of transformer applications. Figure 2 shows the modifications to the basic scheme when paralleled transformers are used. If the transformer is too small or not sufficiently important in its application to warrant the basic scheme, the protection shown in Figures 3 and 4 is suggested. In Figures 3 and 4, solid neutral grounding is used to provide sufficient primary phase fault current to operate fuses for most secondary ground faults.
The scheme in Figure 1 uses differential relays (Device 87T) as the basic protection. Differential protection schemes using time-overcurrent relays are not recommended. Sensitive ground fault protection is provided by Devices 50G, 51G and 151G. Feeder fault backup, bus fault and transformer overload protection is provided by Device 51 on the secondary. Primary fault backup protection is provided by Device 50/51. The transformer-mounted sudden-pressure relay (Device 63) provides sensitive detection of winding faults in liquid filled transformers.
When a normally-closed secondary bus tie is used as in Figure 2, Device 87TG provides selective and sensitive protection for a ground fault within the secondary circuit of the differential zone. Backup for feeder ground faults is by Devices 151G and 251G and thermal protection for the resistor is by Device 51G. The sudden-pressure relay (Device 63) provides sensitive detection of winding faults. In Figure 2 reverse overcurrent protections provided by Device 67. Bus protection and feeder fault overcurrent backup is provided by Device 51. When a normally open bus tie is used as in Figures 2 and 4, Devices 67 and 67N are not required.
In Figures 3 and 4 the fuses are the primary fault protection. Operation of a single primary fuse for primary ground faults will result in single phasing. This may not provide positive and negative sequence quantities to permit detection. Additional protection should be considered, such as phase unbalance and undervoltage detection, as described in PRSC-2. Devices 51 and 51N of Figures 3 and 4 provide bus and backup feeder protection. In Figure 4 reverse overcurrent protection is provided by Devices 67 and 67N.
If the primary source is grounded and there is a power source on the secondary side, a ground fault on the incoming line will be interrupted by the source breaker, but the secondary breaker will not be relayed open because of the delta primary transformer connection. For various reasons, such as personnel safety, possible damaging transient overvoltages produced by an arcing type fault, and automatic reclosing of the primary breaker, some form of protection is required to insure opening of the secondary breaker. This may be accomplished by pilot protection of the incoming line, transfer tip, or potential ground detection relaying schemes on the transformer primary. Automatic reclosing is a special problem which requires that the secondary breaker be opened before the primary source breaker is reclosed. Publication PRSC-5 provides information for detecting ground faults on primary circuits. ET