Generator Protection Device Testing with the Doble F6150sv Test Set
Generation protection testing differs from transmission and distribution testing primarily due to the greater number of complex elements that require testing on the protection device. Test engineers either test a few elements at a time with one test set or use two test sets simultaneously to provide an effective testing solution. Both methods have drawbacks. Using two test sets is not only costly but inefficient and cumbersome. These two situations can be averted by using Doble’s F6150sv for protection devices that have 120V and 1A nominal secondary inputs.
Generator protection devices and elements
Generator protection devices used by electric utilities such as Schneider Electric Easergy MiCOM P343, Siemens 7UM622, ABB REG 670, Beckwith M-3425A, and SEL-700G all have multiple protection elements integrated into a single unit. The protection elements typically used are:
For a multi-function protection device of this type to protect a generator according to these elements, it requires instrument transformer secondary outputs. Figure 1 shows a single-line diagram of a typical section of a generator network found in a utility.
Figure 1 shows the generator, circuit breaker, multi-function protection device, earthing transformer, potential and current transformers. Regarding the current transformers, Figure 1 reveals that the following secondary current outputs are sent to the protection device:
- 3 phase currents on the output of the generator i.e. IA2, IB2 and IC2
- 3 phase currents on the neutral side of the generator i.e. IA1, IB1 and IC1
- 1 current on the neutral of the generator i.e. IN
In total, seven currents are sent to the protection device.
Concerning the potential transformers, Figure 1 reveals that the following secondary voltage outputs are sent to the protection device:
- 3 phase voltages on the output of the generator i.e. VA2, VB2 and VC2
- 1 voltage on the transformer side of the generator’s circuit breaker i.e. VS
- 1 voltage from the secondary side of the generator’s earthing transformer i.e. VN
In total, five voltages are sent to the protection device.
In summary, seven currents and five voltages are sent to the protection device.
Testing the Generator Protection Device
To test the generator protection device, a secondary injection test set which has 7 current and 5 voltage outputs will be required. Other industry test sets typically have 6 current and 4 voltage outputs on a single unit and these test sets may not have the ability to convert their voltage outputs into current outputs. Users of these test sets can use either of the following methods of testing:
- Method One: Fixed Connection Setup with Two Test Sets
- Method Two: Changing Connection Setup with One Test Set
Both these methods have shortcomings in terms of cost, efficiency and effectiveness.
Method one: fixed connection setup with two test sets
Figure 2 shows a schematic diagram using two test sets to test a multi-function generator protection device.
In Figure 2, Test Set 1 is required to generate the six currents on either side of the generator, and four voltages on either side of the circuit breaker. Test Set 2 is required to generate the remaining neutral current and voltage at the earthing transformer.
Method two: changing connection setup with one test set
Figure 3 shows a schematic diagram using one test set to test a multi-function generator protection device. This is achieved only if inputs to the protection device that are not under test are short circuited. In Figure 3, VN and IN are short circuited to the test set’s ground which is typically done when performing a three-phase current differential element test.
The problem arises when an element to be tested requires non-zero VN and IN inputs such as stator earth fault, neutral overcurrent and neutral overvoltage elements. Prior to testing these elements, the test procedure has to be disrupted and the protection device powered down to make the following connection changes:
- Removal of the shorts on the device’s VN and IN inputs
- Application of a bridge between the device’s VA2 and VS inputs in order to free up a voltage output on the test set
- Application of a bridge between the device’s IA2 and IA1 inputs in order to free up a current output on the test set
- Reconfiguration of the test set to output VN and IN on the freed outputs
- Connection of the test set leads to the device’s VN and IN inputs
The major disadvantage of this test setup is that it requires more time to test the protection device as one has to change the connections between the test set and protection device several times to test all elements. Another disadvantage is that by needing to change these connections several times, the risk of an incorrect connection being made increases.
Testing with Doble F6150sv test set
To make testing generator protection devices more cost-effective, practical, efficient, and error-free, one could use a single Doble F6150sv secondary injection test set as shown in Figure 4.
The F6150sv enables testing of a generator protection device without the need for an additional test set. An F6150sv with the F6810 option has a total of 12 outputs, where six are dedicated current and 6 are convertible outputs.
Each convertible output acts as a voltage output by default, but it can be configured in software to output a current. To achieve the required seven current and five voltage outputs, only one convertible output needs to be configured to output a current.
This configuration of outputs is required to test multi-function generator protection devices. Using a single Doble F6150sv, these protection devices can be tested as shown in Figure 5.
In Figure 5, Doble F6150sv has been configured with seven current and five voltage outputs. These outputs are six currents on either side of the generator, four voltages on either side of the circuit breaker, one neutral current and one neutral voltage at the earthing transformer.
Doble provides the generation industry with a cost-effective, practical, time-saving and efficient testing solution to perform generator protection device testing.
All protective elements typically used on multi-function generator protection devices can be tested with a single Doble F6150sv without the need to change connections between tests, which reduces the testing time, risk of incorrect connections and risk of damage to the protection device.
Doble’s solution is cost-effective and practical as one test set is required to test all elements.