Additional Features Of Digital Relays

Published Date: 02 Nov 2017

Protective Relay: For the protective relay to operate, it calculates the electrical quantities of in the electrical circuit by means of the voltage transformer and the current transformer. The circuit breakers will trip when there is a fault detected or abnormal condition occurred. In an electrical power system, the protective relays play a key role in protection and operation. The applications of electromechanical protective relays are now being substituted by microprocessor-based digital protective relays. The digital type possesses a lot of benefits compared with conventional relays. Nearly all microprocessor-based relays possess the feature of fault locating.

Difference between digital relay and conventional protective relay:

A digital relay mainly composes of processor, analog input, digital output system and independent power supply.

To detect electrical or process faults, a digital protective relays with software-based protection algorithms are used in an industrial processes or applied in a control system. Voltage and current are converted to digital form in a digital protective system and the resulting measurements are processed by means of a microprocessor. The digital relay can imitate functions of many conventional protective relays in one device. Hence, the design of protection and maintenance can be simplified. Every digital relay can carry out self-test routines in order to ensure its readiness and alarm if there is a fault occurs. In case of equipment maintenance, changes of protective relays are allowed. Self-testing and communication to supervisory control systems are the benefits of digital relays

Comparison of digital relay and conventional protective relay:

In terms of the construction, protective relays can be classified into three types. They are electromechanical, static and digital protective relay. For electromechanical relays, electromagnetic interaction will create the actuating force. The operating principle of static relays is based on analogue electronic components e.g. transistors, diodes and capacitors etc. The logic in software and micro-processor technology has been implemented in digital protective relays.

In new installations, these protective devices are nearly completely replaced by microprocessor-based digital protective relays. And they copy the characteristics and functions of electromechanical relay with significant in accuracy in precision and convenience in the application. By associating various functions into one, digital relays can save the cost in capital and maintenance compared with electromechanical relays. However, as life span of conventional protective relays is very long, tens of thousands of these are still used to protect electrical apparatus and transmission lines all over the world.

In comparing digital relays and conventional relays, they differ mainly in signal processing input. The input signals of from the current transformers and voltage transformers are always analog signals. In conventional relays, the input signals are directly put to the electromagnetic winding or electronic circuits. For digital relays, there is a conversion of input signals to digital signals before operations. In the analog input system, large inrush in the input signals is suppressed by a surge filter because of the safety of the digital relay.

Additional features of digital relays:

Digital relays have various characteristics. They possess advanced programmable functions which can offer high level of performance. Besides, they are flexible and possess another function of monitoring. Nowadays, they are mainly used in generation unit and transmission system protection.

Digital technology gives various supplementary features which could not be found in earlier traditional technology. These characteristics are measuring of currents, voltages, power and other electrical quantities; event and log recording and oscillography; setting of system remotely and supervision by way of communications; configuring ability of tripping schemes by users; decreased wiring and panel space; low burden on the current transformers and voltage transformers and ongoing self-checking.

Digital protective relays are an ideal selection for a cost-effective system to complete an upgrade program in generator protection. There are several significant reasons for the old generator protection systems to upgrade to use the digital protective relays. These are also the additional features of digital relays over the conventional relays.

1. Reduction in relay testing and maintenance workload

With the shrinkage of manpower in the electric utilities, less people are available in the market of checking and maintaining protection systems. Electromechanical and static relays lack of self checking functions in which a periodic testing of these relays is required. If generator protection is upgraded to digital protective relays with bult-in ongoing self-checking function will reduce the workload in maintenance and testing .

2. Improved protection

In conventional protective relays, they are not sensitive enough to offer proper protection for faults and abnormal situations on the generator. Some of these functions are as follows:

Negative Sequence Overcurrent Protection

Flowing in the stator winding, double frequency current will be induced by negative sequence current in the surface of the rotor. Due to the skin effect, most of this induced double frequency alternating current will flow on the surface of the rotor. Hence, it will cause heating of the surface of the rotor. If the increase of negative sequence current is beyond the generator’s ongoing unbalance current capability, the temperature of the rotor increases. A failure of the rotor will be resulted if the generator is not tripped.

The range for the generator capability of the continuous negative sequence current is between 5% and 10% of the generator rating. In the case of the condition of open generator breaker pole or open conductor, the range for the negative sequence current can be between 10% and 30% of the generator rating. This condition will not be usually detected by other protective relays on the system or on the generator in this circumstance, there is only the negative sequence overcurrent relay to act as protection. When electromechanical relays are used to protect the older generator, only a minimum pickup of 60% of the generator rating can be set by these relays. For the sake of preventing the damage of generator, digital relays must be used as its pickup can be adjusted to 3%. Therefore, older generators should upgrade to digital multifunction relays which will provide a more sensitive protection of the generator from unbalance currents.

Loss of Field Protection

Losing excitation of a synchronous generator, the synchronous speed will be run at a higher speed and real power will be delivered to the system operate as an induction generator. However, its excitation is obtained from the system meanwhile. Hence, a large reactive drain will be resulted on the system. There will be a problem caused by this large reactive drain for the generator, the power system and the adjacent machines. In addition to, eddy current is induced by the slips in the field winding, rotor body, wedges, and retaining rings which will cause heating effect. Overloaded the stator winding which is caused by the generator’s high reactive current can cause an increase in the temperature of the stator winding. If the state of field loss which is not recongnised rapidly, there will be a devastating consequence on the power system. In such case, this can lead to wide collapse of voltage in an area and trip transmission lines when the reactive power source is not sufficiently available.

The protection for loss of excitation should be reliable to detect this fault condition without reacting to system faults, load swings, and other transients, which should maintain the stability of the machine. The application of offset mho relay is the most common measure to detect the loss of field in generator. The operating principle is to detect the variation of impedance which is seen from the generator terminals. In the old days, offset mho static or electromechanical relay of single zone is used for protection. One half of the transient reactance is set for the offset in the direct axis, and the diameter is positioned at direct axis synchronous reactance. When impedance of the machine is higher than 1.0 p.u. (or may be up to 2.0 p.u.in some conditions), so diameter of the circle is changed to very large. As a result, mal -operations will be occurred during stable power swings.

Multifunctional digital relays combine the characteristic of two-zone offset mho. The diameter of first zone can be set at 1.0 p.u .And a short time delay is also be set so that fast protection will be provided when the loss of field condition happens due to heavy load. A longer time delay is set for the second zone to offer protection in the light load conditions and also to avoid mal-operation during power swings as well. There is also a voltage control function in digital relay to give security additionally. The relay can be blocked when the voltage of generator is above a specific value. The relay characteristics do not change with the operating frequency is the advantage of digital relays over conventional protective relay. In the conditions of power swing, the frequency of generators will change. There will be relay mal-function when there is change with frequency in features

Overexcitation Protection

When the V/Hz greater than 1.05 pu on the generator base, there will be an overexcitation of a generator. When there is an overexcitation condition, saturation of the magnetic core of the connected transformer or the generator can happen. Besides, there is induced stray flux with the non-laminated components which cannot bring flux. There will be damage in a short time It is common practice to have V/Hz protection for transformers and generators from these extravagant levels of magnetic flux density. Over-excitation damage happens most frequently if the unit is off-line before to synchronization. In the older days, inverse time or definite time relays are applied to give protection when this condition occurs.

In modern digital protective relays, both definite time and inverse time components are used to give protection to fulfill features of the short-time over-excitation in a generator. Its associated auxiliary transformer and unit transformer should be matched as well. The time integration characteristic of the volts/hertz protection is gained easily and the simulations of heating and cooling features of the generator and transformer allowed. The design digital relays can give precise V/Hz measurement over a broad range of frequency between 2 Hz and 120 Hz. It is particularly needed for gas turbine generators with convertor start technology where the frequency goes very low.

Additional features of digital relay:

As the older generators were not considered as a problem, some of the protections were not applied. Details of these protection functions are:

Inadvertent (Accidental) Generator Energizing

Recently, the problem of accidental energizing of synchronous generators is aroused in the industry. Some off-line machines were damaged if they were energized accidentally. Using various special protection strategy, protection is provided for inadvertent energizing. When a generator is accidentally activated when it is on a turning gear, it will act as an induction motor. Hence, it will draw high current from the system. It can destroy the machine within a very short period of time because of the operator error of the generator. The protection for inadvertent energizing in conventional relays is limited. The additional inadvertent protection function of digital relays provides an economic upgrade to the conventional generators. Over-current and under-voltage elements are used to carry out inadvertent energizing protection as shown in Figure 1. An instantaneous overcurrent relay is supervised by an under-voltage element with adjustable pickup and dropout time delays. When the generator is taken offline, the under-voltage detectors protect the over-current tripping automatically. The over-current relay will be disabled by the under-voltage detector when the machine is online.

Figure 1 Inadvertent energizing protection logic

100% Stator Ground Fault Protection

Many medium to large generators are earthed with the installation of distribution transformer and a secondary resistor. By means of this method of grounding, a single line-to-ground fault is generally limited to 3 to 25 primary amperes. Over-voltage relay (59N) is applied to protect against ground fault of these generators. The connection is done at the grounding transformer secondary side. Nevertheless, only 90%-95% of the stator winding is given by this relay for protection as shown in figure 2. The ground fault protection in many utilities has upgraded to offer 100% complete coverage stator winding. One of the features in digital relay is to use third harmonic under-voltage (27TN). A third harmonic voltage is given at the neutral of the generator when the generator operates in normal condition. When there is a stator grounded fault near the neutral end of the stator winding which is experienced by a generator, this will lead to a decrease in voltage of the third harmonic. Hence, this method can be applied for ground fault detection near the neutral in the below figure.

Figure 2 100% Stator Ground Fault Protection

Faster fault diagnosis and reduced down time

With the provision of oscillographic and the sequence of events recordings, faults and other system related problems can be detected promptly. If the problem is not identified within the generator, it can be quickly brought back on-line saving the utility from purchasing the lost power from a neighboring utility. Without the help of these recordings it may take a lengthy investigation to determine the problem.