The major electrical assets in an Industrial Plant are Generators, Transformers, Motors, HT & LT switchgears, HT cables, Capacitor banks. The Generators and large power Transformers are usually provided with unit protection but rest of the equipment are usually protected with common Phase Over-Current and Earth Fault protective relays only. The Phase and Earth Over-current protections are pre-set with either Inverse Definite Time Delay (IDMT) or Definite Time Delay (DT). Normally these protective relays are coordinated with the downstream protective relays with desired time grading margin. This time graded over-current system being a conventional practice makes fault clearance at upstream locations prolonged and consequently equipment is subjected to possibilities of severe damages and also the same may cause loss of synchronism of Generators running in parallel with other Generators or Utility systems.
Problems Associated with the Conventional Protection Philosophy
In complex electrical network, delayed action of the relays is resulted due to time grading between upstream and downstream feeders and hence the fault gets cleared after a long time. The clearance of fault after a long time shall cause damage to the equipment and having to expose to fault for longer duration reduces operational life of apparatus drastically.
The operating time of upstream relays might be so high that it becomes nearly impossible to avoid loss of synchronism of Generator in event of severe faults. In industrial plants the numbers of generators run in parallel with each other and/or Utility. In that case it is more than desired that they all remain in synchronism and for instance if the fault clearing time is greater than Critical clearing time then it would severely affect the stability of machines running synchronously and would result in loss of synchronism and blackout of plant. Any three-phase fault that takes place in the system should be cleared before the critical, which can be determined from simulation studies carried out on transient analyzer programs. Owing to large electrical paths of a typical industrial network, it is almost inevitable to co-ordinate each relay to operate below critical clearing time.
For any internal fault taking place in the equipment, the phase and earth over-current relay would operate after a certain time lag in co-ordination with downstream relay, which will cause the fault current to flow in the system for longer duration. Equipment subjected to fault current for too long get affected adversely. The question arises how to safeguard the equipment from having to withstand fault current for a longer duration? Also, the difficulty of satisfactory co-ordination and the possibility of plant black out due to time delayed action of normal protection co- ordination ask for more reliable alternative.
Arcing energy is directly proportional to the fault clearing time. With conventional protection system, because of delayed operation of relays, fault clearing time shall be higher and as a result, arc energy level shall be high. Arc flash incident energy level shall be high and PPE requirements shall be comparatively high.
Furthermore, major drawback of the aforesaid protection philosophy is that for the fault nearest to the power source will result in most prolonged relay operation, which is even worse since the fault MVA is highest at that point.
Perception of Unit Protection Scheme
Entire power system is divided into the sections, which are treated and protected individually as a unit. No reference of the other sections is taken into account while carrying out the settings for these relays. Moreover, the unit protection relays operate almost instantaneously within 100ms considering the breaker ON & OFF time.
Owing to immediate fault clearance, protection of each of the electrical equipment in a plant is assured thereby minimizing the likelihood of damages that they would have been subjected to when fault clearance was prolonged. Moreover, transient stability of synchronous generators shall not suffer since the fault is cleared within
100ms. Though there are lot many factors which affect the dynamic stability of synchronous machines but one of the crucial factors being fault clearance time if taken care of, then probability of loss of synchronism reduces. Unit protection is not a new concept but its application in different aspect is pioneering.
Unit Protection is a form of a Differential Protection whose main principle is to sense the difference in currents between the incoming and outgoing terminals of the equipment being protected. Differential protection operates on the principle that current entering and leaving a zone of protection will be equal. Any difference between these currents is indicative of a fault being present in the zone. If CTs are connected as shown in Figure, it can be seen that current flowing through the zone of protection will cause current to circulate around the secondary wiring. If the CTs are of the same ratio and have identical magnetizing characteristics they will produce identical secondary currents and hence zero current will flow through the relay. If a fault exists within the zone of protection there will be a difference between the outputs from each CT; this difference flowing through the relay causing it to operate.
Adaptive Schemes of UPS
- Generator Unit Protection
- Transformer Unit Protection
- Pilot Wire Cable Differential Protection
- Motor Unit Protection
- Capacitor Bank Unit Protection
- Grid Islanding Protection
- Busbar Differential Protection
Brief Descriptions
The protections enlisted above are briefly described in the discussion that follows as under:
Generator Unit Protection
Generator being crucial part of any power system must always be provided with adequate protections. Any fault that takes place inside or in the vicinity of the generator up to the connected busbar shall result in severe damage to the windings and the stator core, the extent of the damage will depend upon the fault current level and the duration of the fault.
For a generating plant or plants running in parallel with Grid, high-speed disconnection of the plant from the rest of the power system may also be necessary to maintain system stability. The generator differential protection is best suited in this case as any internal fault within its zone gets isolated from the rest of the power system almost instantaneously.
3-Phase segregated generator differential protection is provided to detect stator phase faults. This can be set as either a percentage bias scheme with a dual slope characteristic or as a high impedance scheme. This form of unit protection allows discriminative detection of winding faults, with no intentional time delay, where a significant fault current arises. The zone of protection, defined by the location of the CTs, should be arranged to overlap protection for other items of plant, such as busbar or a step-up transformer.
Transformer Unit Protection
In applying the principles of differential protection to transformers, a variety of considerations have to be taken into account. These include compensation for any phase shift across the transformer, possible unbalance of signals from current transformers from either side of windings and the effects of the variety of earthing and winding arrangements. In addition to these factors, which can be compensated for by correct application of the relay, the effects of normal system conditions on relay operation must also be considered.
The differential element must be blocked for system conditions which could result in mal- operation of the relay, such as high levels of magnetizing current during inrush conditions or during transient over-fluxing. However, nowadays most of the numerical relays comes with blocking features based on the second and fifth harmonic in which case can be useful in preventing the relay mal-operation.
The considerations for a transformer protection package vary with the application and importance of the transformer. Small distribution transformers can be protected satisfactorily, from both technical and economic considerations, by the use of restricted earth fault relays. Any internal earth fault in small distribution transformers shall be detected by restricted earth fault relay which is also a kind of unit protection.
Transformer Restricted Earth Fault (REF) Protection Scheme
Depending on location of Neutral CT, 4-CTs or 5- CTs REF protection scheme is used. 5-CTs REF protection scheme is desirable. In 5-CTs REF scheme, 4 CTs (3-phase and neutral) in panel and 1 CT at transformer neutral is required to be installed. In 5-CTs scheme, transformer neutral CT is installed after bifurcation point. In 4-CTs scheme, transformer neutral CT is installed before bifurcation point. 3 CTs (3-phase) in panel and 1 CT at transformer neutral is required to be installed to incorporate 4-CTs REF protection scheme.
High/ Low Impedance Restricted Earth Fault (REF) Protection Scheme
Depending on the neutral earthing of transformer, high or low impedance REF scheme is selected. Low impedance REF scheme is adopted, if neutral point of transformer is NGR earthed (when Line CT and Neutral CT are not same) and model of relay is selected accordingly. Slope characteristic is used in Low Impedance REF scheme. High impedance REF scheme is adopted for transformer having solidly earthed neutral (Line CT and Neutral CT have identical ratio and magnetizing characteristics). As per requirement, stabilizing resistors and metrosils are required in high impedance REF scheme.
Pilot Wire Cable Differential Protection
In industrial plants the distribution of power is usually done with the help of cables and in a normal over-current graded system; faults on cables usually end up with clearing times of around 600-700ms. This causes the generators to unnecessarily feed the fault for that long duration and having the cables undergo short circuit stresses. Further in case of ties between the stations in an industry, normal over-current grading results in even delayed tripping times due to added step. This difficulty can be easily overcome by implementing cable differential protection.
This is a well-established type of protection for feeders. It is based on the Merz-Price circulating current system and suitable for operation over privately owned two core pilots with a relatively high core resistance and low inter-core insulation level. It clears cable fault instantly.
Motor Protection
The major load of any industry comprises of motors, these motors depending upon their ratings as well as their importance should be provided with adequate protections. Possible faults against which motor should be protected are: inter- winding stator faults, faults outside the motor but within protected zone such as faults on motor terminals or on external connections and stator earth faults.
Possible faults against which motor should be protected are: inter-winding stator faults, faults outside the motor but within protected zone such as faults on motor terminals or on external connections and stator earth faults.
Three-phase machine differential protection is provided to detect faults within zone. This can be set either as percentage bias scheme with a dual slope characteristic or as a high impedance scheme. When high impedance is used, additional stabilizing resistance and metrosil will be required.
Further care must be taken to ensure the stability of differential scheme in the sense that it does not operate for through fault.
Capacitor Bank Unit Protection
The capacitor bank protection is an integrated protection, control, and monitoring device for shunt capacitor banks. The protection provides both, bank and system protection schemes for shunt capacitor bank protection.
The current and voltage-based protection functions are designed to provide sensitive protection for grounded, ungrounded single and parallel capacitor banks and banks with taps, for a variety of capacitor bank configurations.
Inherent unbalance in the large shunt capacitor banks may be so large that it may dominate the unbalance that results from loss of 1 or 2 capacitor units. This poses problem in designing the capacitor bank protection. However, voltage differential scheme can be useful in protecting the banks from abovesaid unbalances; the voltage inputs of which are derived from potential transformers employed two per phase. Relay mal- operation that may occur due to switching inrush must be prevented while considering the capacitor protection since the same might be as high as 20 times normal current.
The voltage differential is applicable for both grounded and ungrounded banks. In the ungrounded case, the algorithm uses the neutral point voltage to provide sensitive protection.
Busbar Differential Protection
Busbars are very critical element in a power system, since they are the points of coupling of many circuits, transmission, generation, or loads. A single bus fault can cause damage equivalent to many simultaneous faults and such faults usually draw large currents. Also, these faults severely affect the system stability if not isolated immediately, this calls for busbar differential protection since fault isolation is as fast as 40ms much before the typical critical clearing time of the alternators. It also helps in maintaining the service to as much load as possible. Busbar differential scheme is depicted in below figure.
Bus bar differential schemes are not mandatory for all industrial applications. However, it is must for HV and MV systems of Oil and Gas industries.
Grid Islanding Protection
The process of disconnecting the Plant from the utility in case of Grid disturbance and vice-versa is called the Grid Islanding Scheme. Grid Islanding scheme is a set of protective relays, connected at the incomer bus – these relays will sense a disturbance in the grid or the plant and give a trip command to the incomer breaker whenever the grid/plant disturbance exceeds a set limit following a set logic. By opening a particular breaker, the plant is isolated from the Grid.
It is strongly advisable to disconnect the plant generator from the grid, when the grid is disturbed. The main reason is that the generator may get damaged due to grid disturbances resulting in heavy repair costs and shut downs.
It is also better to disconnect the plant generator from utility whenever the utility fails, by opening the incomer. If not, the generator which is running will experience a severe jolt when the utility comes back as the restored supply may not be synchronous with the generator. After successful islanding the essential loads of the process industry plant should be supplied by in-house generators until the grid supply is restored. If successful and timely isolation of CPP plant does not take place during prolonged disturbances in the grid, there are chances of complete blackout in the process industry plant leading to significant loss in production. Equally important is the fact that after isolation from the grid, there should be a generation –load balance within the industrial plant. And this can be achieved by decrease of load demand to match with available generation. The term used for describing intentional switching off of electrical supply to parts of load is ‘Load Shedding’.
Power system as such is never in a complete steady state. The kinds of changes that continuously take place are load variations, generation variations, occurrences of faults or large switching operations. These disturbances may lead to frequency or voltage collapse of the system, loss of synchronism of generating units or in worst case cascade tripping leading to complete blackout. If Industrial plant generators are not isolated in such a situation of large grid disturbances than they will also be tripped and plant shall have to suffer production loss until restoration process is completed.
However, to achieve proper Grid islanding by deciding settings of protective relays is little tricky; because too sensitive settings shall make frequent separations from grid which will lead to lost opportunities for revenue generation either from selling power to utility in case of plants exporting power or from production loss in case of plants importing power. And on the other hand, waiting too long and disconnecting at only extreme grid disturbances can lead to voltage and frequency sag with the consequences of possible equipment damage. The type of grid disturbances can be categorized as: Grid collapse which leads to loss of supply, faults in the grid due to which relay at the connection point trips the breaker resulting in loss of utility supply, wide fluctuations in frequency or voltage or both.
Choice of protective device used for grid islanding purpose plays vital role in proper realization of grid islanding. Relay selected should include protection functions as follows:
- Directional phase overcurrent protection
- Directional earth fault protection
- Under/Over voltage protection
- Under/Over frequency protection
- Reverse power protection
- Low forward power protection
- Over power protection
- Rate of change of frequency protection
- Neutral voltage displacement / Residual over voltage protection
- Voltage vector shift protection
- Voltage transformer supervision
- Circuit breaker failure protection
- Programmable scheme logic
Individual protection functions without programmable scheme logics cannot form islanding scheme as operation/non-operation of combination of above cited protection functions is required to make appropriate judgment regarding whether to isolate the plant generators from grid or not. Other factors which contribute to proper grid islanding are: choice of location of protection transformers (CTs, PTs) which provides input to the protective relay, proper configuration of protective relay etc.
Actual Grid islanding protection settings vary according to source configurations, operating philosophy, load sensitivity and grid behavioural pattern but however the idea can be described on what should be the judging criteria which more or less remains same in all cases. Settings of individual protection functions described above should be done in at least two stages, one stage with comparatively lower threshold value should be used for alarm purpose and another stage with little higher threshold settings than previous stage should be used for tripping purpose. Besides these settings of individual protection functions certain logics are also required to be set. These logics shall help to judge grid islanding correctly and further avoid any chances of mal-operation.
Adaptive Generator Neutral Grounding methods and approach to install Transformer immediately after Generator
Primarily, this transformer avoids damages due to plant electric system disturbances as well as Grid disturbances when synchronized for import and export of power.
This Transformer shall restrict the reflection of the secondary side earth fault on the primary side and will thus isolate the Generator from the effect of secondary side earth faults due to Delta-Star configuration of transformer.
Safer value of generator E/F current can be selected by adopted NGT earthing of Generator neutral.
This transformer Limits the value of Phase faults current of the system. It increases the mechanical life of power plant equipment like turbine, generator and its auxiliaries due to avoidance of unnecessary frequent tripping of generator.
Sensitivity of all over current and Earth fault protections can be achieved for time graded protection and even the machine can be set to high set to get isolated from system faster.
It also provides the protection against plant electric system earth faults which are almost 85% of the electric faults.
Generator with step-up Transformer
(Generator NGT earthed and Generator transformer solidly earthed)
This case explains typical industrial network
comprising in-house generation, grid power support and certain plant load demand. It is illustrated in below figure. The distribution to the plant can be done via more than one feeder according to the plant load and process requirement.
Some key aspects of the shown system:
- Generator grounding is done by Neutral Grounding Transformer and Ground Fault current can be limited to low value of about 2-5A.
- Overall plant distribution is done at 33kV.
- From example of above system, it is depicted that for the system having large distribution network, generated power can be stepped up at higher voltage level and equipment cost can be reduced comparatively due to lesser full load current than distribution at lower voltage level.
- Ideally power generation should be at 11kV voltage level. In the system, if generation and utility are getting paralleled then generation shall be stepped up and utility power supply shall be stepped down to distribution voltage level.
- Distribution of power should not be at lower voltage levels than 11kV. HT motors can be connected at 11kV, 6.6kV or 3.3kV voltage levels. Plant auxiliary loads can be connected at 415V by stepping down distribution power from 11kV or 6.6kV to 415V.
Generator with unit Transformer
(Generator NGT earthed and Generator transformer solidly earthed)
In industries where plant load is less, higher distribution voltage cannot be selected. So, generator should be provided with generator transformer of unity ratio. The case under discussion is similar to above case with respect to kind of plant load, in-house generation and grid power support. The distribution to the plant can be done via more than one feeder according to the plant load and process requirement.
Some key aspects of the shown system:
- Generator transformer is provided with generator transformer of unit ratio. It provides electrical isolation to generator and hence earth fault in system does not get reflected on generator. Also, we can have limiting value of earth fault on generator to lower value of about 2-5A by providing neutral grounding transformer.
- Solid earthing on 11kV distribution system can be provided and installation of CBCTs for earth fault sensitivity as the value earth fault current would not be required, as value of earth fault current would be in line with phase fault currents only and hence shall be sensed by residual connections.
Generator without generator transformer
Adding a generator transformer calls for more capital to be invested for which management may not give permissions then we can have direct connection of generator to plant distribution system.
Some key aspects of the shown system:
- Owing to lesser plant load demand, one can have distribution of power at 11kV, and since generation is at 11kV generator transformer is not provided.
- For system having neutral of generator and grid transformer is earthed through a neutral grounding resistor, the limiting value of earth fault current shall be selected in consideration with generator core damage curve.
- The limiting value of earth fault current for generator cannot be kept too low as the same shall lead to sensitivity issues in downstream system.
Ideal Earth fault current limiting value of Transformer Star point based on voltage level and classification of system
Distribution at 33kV: 200A/300A NGR Distribution at 11kV: 300A/400A NGR
Motor bus at 11kV/6.6kV/3.3kV: Transformer rated
Amp NGR
Distribution at 415V: Solid
In case of Solidly earthed system in HV/MV network, due to higher value of E/F current, ionization of insulating medium causes Phase fault. It results in comparatively higher damage. And blackouts also.
At the same time while designing earthing system, it is required to achieve enough sensitivity of Earth fault detection. Transformer rate Amp NGR shall facilitate adequate sensitivity for Earth fault detection. So, Earth fault current value should be limited accordingly.
Grid Transformer
Above mentioned Earth fault current limiting value of LV side of Grid Transformer i.e. 33kV/11kV/6.6kV/3.3kV shall prevent the conversion of Earth faults into Phase faults.
Provision of Generator Transformer along with future Generator shall ascertain the safety of core against earth fault. Future Generator Transformer distribution side star point
Generator with Generator Transformer
Above mentioned Earth fault current limiting value of distribution side of Generator Transformer i.e. 33kV/11kV/6.6kV/3.3kV shall prevent the conversion of earth faults into Phase faults.
Provision of Generator Transformer along shall ascertain the safety of core against earth fault while CPP’s parallel operation with Grid Transformer in future. Future Grid Transformer distribution side star point shall be earthed through above mentioned NGR value.
Generator with Generator Transformer and Grid
Transformer
Above mentioned Earth fault current limiting value of distribution side of Generator Transformer and Grid Transformer i.e., 33kV/11kV/6.6kV/3.3kV shall prevent the conversion of Earth faults into Phase faults.
Provision of Generator Transformer along shall ascertain the safety of core against earth fault while CPP’s parallel operation with Grid Transformer.
Conclusion
With the advent of newer technologies, it is essential that newer fundamentals shall be set in line with the problems faced in all the industrial plants. Using “Unit Protection” as a concept shall solve majority of the problems related to the time delayed actions of the relays. Any fault shall be isolated from the network almost instantaneously which means the fault current will not be circulating in the power system for a long time. This shall limit damage to the equipment.
Recent industries are already adopting the concept of Unit Protection System during design stage and perceiving the advantages of such schemes. However, it is not the case with old installations which are devoid of such schemes due to unawareness and later innovation. Increased black outs in the industrial plants causes heavy production loss which is not beneficial to the company. Hence, to avoid the black outs and production loss, it is essential that the electrical power system of any industrial plant is appropriate with adequate protections required.
Using the “UNIT PROTECTION SYSTEM” concept will lead to fast clearance of faults from the power system and hence the number of black outs occurring in the plants can be saved which will in turn lead to no production loss to the plant. It shall compensate the cost of required infrastructure for scheme implementation and add the benefits over a period of time. Also, adoption of suggested distribution network i.e., Generator with Generator Transformer and limited Earth fault current value initially from project stage shall result in reduced blackout, less production downtime and improved power system reliability.
Arvind Mehta, BE (Electrical) from M. S. University of Vadodara, Faculty of Technology and Engineering, holds a personal chair in Electrical Engineering and is founder and CMD of Elcon Engineers Private Limited, Vadodara. He has four decade experience in the field of Power System Study and his Specialization is in the field Design, Analysis & Protective Relays. Earlier, he has served with Jyoti Ltd and Tata Consulting Engineers.
