Testing & Commissioning Protective Schemes

Generally protective equipment testing may be divided into three stages:

• Factory tests.
• Commissioning tests.
• Periodic maintenance tests.

Factory and commissioning tests confirm the performance of equipment during its development and fabrication, and its operational environment. Periodical maintenance ensures that this performance is maintained.

For example, the relay manufacturer must provide sufficient testing of protective gear before it is accepted and commissioned. The tests performed include:

• Tests in which the operating parameters of the relays, etc. are simulated.
• Conditions such as temperature range, vibration, mechanical shock, electrical impulse withstand, etc. which may affect the operation in service.

In some instances both of the tests aforementioned are conducted simultaneously to check performance.

Commissioning Tests

The purpose of the commissioning tests is to ensure that connections are correct, that the performance of current transformers and relays agrees with the expected results and that no components have been damaged by transport or installation.  This performance test includes correct current transformer ratio, correct calibration of relays and tests confirming that the tripping, inter-tripping and indication of the scheme are in order.

Generally the key points to be checked on a protective scheme are:

• Stability of the system under all conditions of ‘through’ current.
• That the sensitivity of all relays with reference to the primary current is correct.

Prior to performing these final checks on site a very careful preliminary check on the protective gear scheme should be made. Such tests would include:

• The examination of all small wiring connections and insulation resistance tests on all circuits.
• Polarity and ratio check (if possible) on all current transformers.
• The identification of all pilot cables, that is, insulation resistance tests and loop resistance tests on balanced pilot-wire schemes.
• Tests on DC tripping, inter-tripping and operation of all breakers connected with the unit, including alarm circuits and relay signals.
• Checking of calibration of all relay by secondary injection.
• Any other tests depending on special information of a specific scheme.

Saturation Curve

Current transformer saturation curve using a LV local supply that compares closely with results obtained by primary current may be used on site. An LV AC supply is fed to the secondary winding through a control rheostat, and a curve of voltage-current for the secondary is plotted. The equivalent primary current is inferred from the turns ratio. This test is useful for comparing the performance of current transformers needed to have matched characteristics.

In other words, preliminary tests ensure that components are correct. The commissioning process thereafter must depend upon site facilities as discussed in the following sections.

Primary Current Tests

A generator is isolated with the unit under test and by means of primary short circuits and earth faults stability figures, on balanced systems, under full load conditions, operation tests with internal faults may be performed. It is imperative to consult the generator’s manufacturer before it is used for steady unbalanced conditions for instance, testing with one phased earthed, as the distortion of flux combined with armature reaction, if prolonged, may result in excessive heating.

Earthing resistors and transformers should be short-circuited or bypassed during current testing, as they are normally short time rated.

An inverted transformer is a means of stepping up testing current where a transformer is available between the unit under test and the source of test supply e.g. if the transformer has a normal step up ratio of 6.6 kV to 66 kV, then by connecting the testing supply (isolated generator or low-voltage source) to the 66 kV windings and supplying the test current from output of the 6.6 kV windings, a 10/1 step-up of current is obtained. This ‘inversion’ can readily be done by flexible cabling when the connections to the transformer are made through open-type bushings, and is applicable to any power transformer.

A heavy current testing transformer may be designed with an input appropriate for a general range of low-voltage supply. The types available include three-phase and single-phase units with resistance or induction regulator control. For the objective of producing primary current, the output terminals are connected across the primary of the circuit under test, to test windings or test bar primaries embodied in the current transformers. On metal clad switchgear, connection to the primary can be through the circuit spouts with testing plugs; voltage/potential transformer spouts may also be used, but the current-carrying capacity should be examined.

When using externally mounted current transformers around cables such as in core balance types of protection, flexible cables may be employed for threading through the transformers with current supplied from a heavy current testing transformer. You must ensure that the cable is as central as possible, as small errors are introduced if the cable is asymmetrically positioned. Note that, for balancing purposes, if the relative position of each cable in each transformer is the same, the same error will be introduced and the effect neutralized.

Secondary Injection Tests

The techniques aforementioned use primary current; however secondary injection can also be used. The effect of primary current in a current transformer is to develop a secondary voltage; in secondary injection, voltage is applied to the secondary terminals, usually from an injection transformer, hence applying the “output voltage”.

This technique is used for checking relays, calibration, commissioning and maintenance. This method is not in itself adequate for balanced schemes, and must in such cases be supplemented by tests on load; nonetheless, it is possible in most cases to simulate through-faults to earth, and phase-phase conditions, by rearrangement of the secondary current transformer connections. A key limitation is that the ‘load’ current is dependent on the load on the unit, normally determined by network conditions, and on balanced systems, the stability check may be at a low primary current.

The different types of injection transformers typically have a tapped primary winding, with a secondary resistance controlled output covering a range of voltage. This technique gives fine control for calibrating and enables a low power input to be utilized.

Fault Location

Quick location of a fault and being able to have an idea of its nature is a basic prelude to its quick repair. For overhead lines visual inspection from the ground or from a helicopter may be possible. In case of underground cables, inspection of the flag indicators on protective relays and simple Megger or continuity tests will typically offer helpful evidence and enable an appropriate technique of more detailed examinations to be chosen.

Loop Tests

If the fault is of low resistance, a low-voltage battery e.g. car battery, may be enough for the test supply.

For higher-resistance faults about 500 V, a Megger may be needed, whilst for very high resistance, where it is necessary to break down the fault, an HV rectifier set may be employed.

Additional Tests

Fall-of potential, capacitance and pulse reflection tests are more complex techniques. Such tests with loop tests may be accurate to a location within say 20 m of the fault point. For underground cables a more precise location is desirable before excavation.  Induction and discharge methods are generally used in such cases.

Related articles:

• Common Busbar Protection Schemes
• Overvoltage Protection Techniques & Equipment
• The Principles of Power System Protection
• SF6 Circuit Breakers: Features, Operation & Applications
• Testing Power and Distribution Transformers