Regardless of over voltage protection (ANSI CODE 59) I don’t know G.59 protection but I have heard about G.59 TECHNICAL RECOMMENDATION which consist some protection consideration in diesel generator paralleling with other power supply sources. Please refer to following descriptions.
Engineering Recommendation G.59/1
The full title of this document is “Recommendations for the connection of embedded generating plant to the public electricity suppliers’ distribution systems.” It originated in 1985 as Engineering Recommendation G.59, and following the privatization of the electricity supply industry, was revised in 1991 by the Electricity Association of the United Kingdom who now publish it as Engineering Recommendation G.59/1 (ER G.59/1).
It is intended for the use of the U.K. public electricity suppliers and their consumers and applies to generating plants not exceeding 5 MW rating which is connected to systems operating at 20 kV or below. The term “embedded generating plant” is used to describe any generating plant connected to a public electricity supply, whether it is intended for parallel operation or not. Persons who operate such a plant are defined as embedded generators. Three types of connection are recognized:
- The alternative connection in which the embedded generator operates as an alternative to the public electricity supply. The arrangement must be such that the two supplies cannot in any circumstances be paralleled.
- The parallel connection in which the embedded generator may run in parallel with the public electricity supply for unlimited periods. For this mode of operation an important consideration is the safety of the public electricity supplier’s personnel who may find themselves working on a distribution system which unexpectedly becomes connected to the embedded generating plant.
- Occasional paralleling in which the embedded generator may run in parallel with the public electricity supply for a limited period, typically 5 min, and only for the purposes of maintaining continuity of supply while changing over from one source to the other.
For standby power installations interest is limited to the alternative connection and to occasional paralleling. The document describes the technical requirements for earthing the standby supply, interconnecting the neutrals of the two supplies and the changeover devices, and the electrical protection required during parallel running as followings:
1- Earthing the Neutral of the Standby Supply
It is important that the standby supply has a reliable earthing system; it is not safe to rely on any earthing arrangements that are not under the control of the site operating personnel. If the earthing system for the normal supply is local to the generating set and is under the control of the site operating personnel it may be permissible to use it as the earth for the standby supply. Below figure illustrates typical connections that can be used, the agreement of the network operator having been obtained. If the normal supply earthing conductor uses the sheath or armor of a supply cable that has a remote origin it cannot be relied upon. Such a cable may be disconnected by the network operator during maintenance or may be severed accidentally by a contractor (on or off site).
Earthing Low-Voltage Supplies
A low-voltage standby supply that runs independently of the normal supply should have its neutral solidly connected to a local earth electrode.
If there are separate earthing systems associated with the normal supply and the standby supply they should be bonded together.
When devising the interconnections between the two neutrals it must be kept in mind that the network operator will not allow its neutral to be connected to earth at a second point unless the system incorporates protective multiple earthing. If the system does not incorporate protective multiple earthing a four-pole changeover device will be required as described in the next section. Within the United Kingdom protective multiple earthing is normal and in such cases the standby supply earth electrode should have a resistance to earth not exceeding 20 ohms. This is the resistance allowed for protective multiple earthing in the Electricity Supply Regulations.
Earthing High-Voltage Supplies
A high-voltage standby supply that runs independently of the normal supply usually has its neutral earthed through an earthing resistor which limits any earth fault currents to rated current. The earthing resistor provides two benefits, in the event of an internal fault the damage to the machine is limited, and in the event of an external fault the rise of potential of exposed conductive parts is limited. It should be noted that the earthing conductor between the star point and the earthing resistor will, under fault conditions, experience a rise of potential, the magnitude will depend upon the position of the fault; the earthing conductor should therefore be insulated for the phase voltage.
Within an installation, the Electricity Supply Regulations allow the high-voltage earth electrode system to be interconnected with the low voltage earth electrode system provided that the combined resistance to earth does not exceed 1 ohm. If the resistance exceeds 1 ohm the Regulations forbid any interconnection and require the two electrode systems to be separated sufficiently to ensure that the overlap between the two resistance areas does not cause danger. In most industrial installations it is not practicable to ensure that the two earthing systems are not interconnected, or that the electrode resistance areas do not overlap, in such cases it follows that a combined resistance not exceeding 1 ohm is required for high-voltage neutral earthing.
2-Neutral Connections for Single Sets Not Intended to Run in Parallel with the Normal Supply
Where the standby and the normal supplies are not arranged to run in parallel Engineering Recommendation G59/1 describes this as the alternative connection.
The neutral of a low-voltage standby supply should be solidly earthed.
In a single set low-voltage installation the generator will usually be connected, as a four-wire machine, to a distribution board with the star point connected to the neutral busbar which in turn is connected to the earth bar and on to the earth electrode.
Within the United Kingdom four systems of public supply may be encountered, they are described in BS 7671 and are:
- TN-C-S systems in which the neutral and the earthed protective conductors are combined into a single conductor in part of the system. This is the most common system and uses protective multiple earthing (PME). Where a standby supply is installed in such a system a triple pole changeover device will be required to select one supply or the other as illustrated by Figure below.
- TN-C systems in which the neutral and the earthed protective conductors are combined throughout the system. Provided that the system uses multiple earthing the same considerations apply as to the preceding TN-C-S systems.
- TN-S systems in which the neutral and the earthed protective conductors remain separate throughout the system. Where a standby supply is installed in such a system a four-pole changeover device will be required to select one supply or the other as illustrated by Figure Below. The fourth pole is required to avoid earthing the neutral at a second point.
- TT systems in which the neutral is earthed at the power source but the network operator does not provide an earthed protective conductor. Where a standby supply is installed in such a system a four-pole changeover device will be required (above figure). The fourth pole is required to avoid earthing the neutral at a second point.
It will be noted that a parallel path for the neutral current, which may have undesirable consequences. One path uses the neutral conductor but there is another path which uses the protective conductors connecting to the local earth. The current in the protective conductors will be indeterminate, it may exceed the rating of the protective conductor and may upset the current balance in any protective residual current devices. The difficulty can be overcome by using four-pole changeover devices.
In a single set high-voltage installation the generator will usually be connected, as a three-wire machine, to a distribution board. The starpoint will be connected to the earthing resistor which in turn will be connected to the earth bar and on to the earth electrode (figure below).
High-voltage power is usually supplied from a three-wire system, there being no neutral, an earth connection may not always be provided. For such installations triple pole electrically interlocked circuit breakers will be required as illustrated by figure below.
3- Neutral Connections for Multiple Sets Not Intended to Run in Parallel with the Normal Supply
Running Generators in Parallel- and Triplen-Current Flow
Triplen currents have not been previously mentioned and it is appropriate to introduce them here. The harmonic orders 3, 6, 9, 12, etc. are known as triplen harmonics or triplens. They have zero-phase sequence and triplen currents flowing in the phase conductors become additive in the neutral, hence the concern expressed in the following text.
The voltage generated by a loaded salient pole generator will include a third harmonic and other triplen components the magnitude of which will be dependent upon the machine design, its excitation, and its loading. If the machines are identical and are equally loaded and excited the triplen harmonic voltages in each of the machines will be equal, in phase and balanced and there will be no resulting current flow. If the machines are not identical or are not equally loaded or excited, triplen harmonic currents will circulate between the machines as shown in Figure below.
Any triplen load currents taken from them will similarly be additive in the neutral busbar return path as shown in Figure below. It is likely that both the above conditions will exist at the same time and for the triplen currents in the neutral connections to be unexpectedly large. When salient pole generators are to be run in parallel, consideration should be given to the possibility of undesirable third harmonic currents circulating in the neutral connections of the machines. If the neutrals of paralleled generators are not interconnected there can be no triplen harmonic circulating currents and the problem will not arise. If the machines are identical and are running under identical excitation and loading conditions their neutral terminals may be safely connected together. However machines which are thought to be identical may differ and the excitation and loading conditions will be subject to the normal tolerances of the voltage regulators and load sharing systems.
The failure of a voltage regulator, a speed governor, a load sharing system, or a kVAr sharing system may cause unexpectedly heavy circulating currents to flow in the neutral connections. For these reasons, the generator manufacturer should be consulted before the neutral terminals of paralleled generators are connected together.
The supply should be solidly earthed as described for the single set installation but, for the reasons described in the preceding paragraphs, it may not be advisable to connect together the machine neutrals of a multiple set installation. There are several methods of overcoming the difficulty, the matter should be agreed with the generator manufacturer:
- Each machine is provided with a neutral contactor but only one contactor is closed at any time. A logic system ensures that the neutral of one machine is connected and decides which machine it is; it will usually be the first generator on line. With this method the connected machine necessarily accepts all the neutral or out of balance current; if the neutral current is likely to exceed the neutral current rating of a single machine the method cannot be used. It should be noted that the neutral current to be considered is the rms sum of the fundamental neutral current due to unbalanced loading between the phases and the triplen neutral current generated by the load. Figure below illustrates the electrical connections.
- The supply is earthed through a static balancer having an interstar winding. The generator neutral terminals are not used and the star point of the static balancer becomes the neutral of the supply, and is solidly earthed. The static balancer divides any neutral load current into three equal zero sequence components which return to the load as part of the phase currents and do not pass through the generator windings. The generators share the positive and negative sequence components of the load current, the static balancer affecting only the zero sequence components. Figure below indicates the electrical connections.
The static balancer is a vital component connected to the main busbars and thought should be given to the electrical protection that should be provided. If it is tripped from the busbars due to a fault the standby supply has to be shut down due to the lack of a neutral connection.
It follows that any protection such as over current or earth fault should have current settings and time delays which avoid the probability of spurious tripping.
- Each machine is provided with a reactor connected between its star point and the neutral busbar. These will attenuate the third and higher harmonic currents without offering significant impedance at fundamental frequency. The reactors have the effect of increasing the generator’s zero sequence impedance and data will normally be required from the generator manufacturer before any calculations can be undertaken. If there is fundamental current flow in the neutral due to load imbalance between the phases the reactors will increase the voltage unbalance—a compromise has to be reached between unbalanced phase voltages and triplen current flow.
- With the generator maker’s approval the generator neutrals are paralleled.
Unless the generators are identical and are working under identical conditions of excitation and loading there will be some third harmonic currents circulating in the neutrals.
The supply should be earthed through an earthing resistor as described for the single set installation but, for the reasons described in a foregoing paragraph, it may not be advisable to connect together the star points of a multiple set installation. There are several methods of overcoming the difficulty; the matter should be agreed with the generator manufacturer:
- Each machine is provided with an earthing contactor connected to a common earthing resistor but only one contactor is closed at any time. A logic system ensures that one machine is earthed and decides which machine it is; it will usually be the first generator on line. The resistor should be rated to limit any earth current to the current rating of one machine. Figure below illustrates the electrical connections.
- Each generator is earthed through its own earthing resistor. Circulating currents will be limited to an acceptable level by the resistors and the complications of contactors and a logic system are avoided. The circulating currents are unlikely to be high but the rating of the resistors should be continuously rated to dissipate the resulting losses. Figure below illustrates the electrical connections.
- The supply is earthed through an earthing transformer, which is similar to the low-voltage static balancer but is not rated to carry any neutral current. The generator star point terminals are not used and the star point of the earthing transformer is connected to the earthing resistor.
4- Paralleling the Standby and Normal Supplies Introduction
A facility that allows the standby supply to be occasionally run in parallel with the normal supply is extremely useful. Although paralleling is applicable to installations of any rating, in practice the facility is used with larger installations having ratings of say 500 kVA and above. It will require the agreement of the network operator and there will be requirements regarding the earthing arrangements, the protection relays fitted to the interconnecting circuit breaker and the operating procedures. The network operator must be satisfied that the operating personnel will be competent and will be available as and when necessary.
The advantage to be gained from the paralleling facility is the ability to transfer the load from one supply to the other without the load experiencing a break in supply. After running on the standby supply it is possible to return the essential load to the normal supply without a break, and for test runs it is similarly possible to transfer the essential load to the standby supply without a break, while leaving the nonessential load connected to the normal supply. It should be noted that this advantage can only be gained if the normal supply circuit breaker connects to the section of busbar that supplies the nonessential load. These procedures require paralleling for short periods only and the agreement with the network operator usually limits the duration to 5 min.
The technical requirements are set out in Engineering Recommendation G59/1 and its supporting document Engineering Technical Report 113, both published by the Electricity Association.
Features Required on the Generating Set
The generating set should have been constructed with parallel running in mind. The coupling between the engine and alternator should be sufficiently robust to withstand an attempt to synchronize in phase opposition without suffering damage; a shear pin is sometimes provided between the alternator and the coupling. The alternator poles should be provided with pole face damper windings to prevent phase swinging. As the set will be feeding into an infinite system the engine governor will have no control over the speed and the voltage regulator will have no control over the voltage. Instead the governor is used to control power (kWe) and voltage regulator to control power factor (cosø). For synchronizing purposes the engine governor and the voltage regulator will require remote adjusting facilities.
The Supply Authority’s Requirements
For reasons connected with the safety of their operating and maintenance personnel the network operator will require electrical protection to ensure that their distribution network is not energized from the standby supply. For long-term continuous paralleling the protection required is complex but for the short-term occasional paralleling applicable to standby installations the protection required will usually consist of under- and overfrequency, under- and overvoltage, overcurrent, earth leakage, and a 5-min timer, in addition the network operator may require protection against neutral voltage displacement. The settings of the relays required for paralleling will be determined by the network operator.
The purpose of the under- and overfrequency and voltage protection is to detect a loss of normal mains and to prevent the standby generator back-feeding a part of the supply authority’s distribution system (islanding). The purpose of the neutral voltage displacement protection is to prevent the standby generator back-feeding an earth fault on the supply authority’s system. For some installations neutral voltage displacement protection may be expensive or impracticable, and in such cases it is likely that agreement could be reached on the use of some other parameter such as rate of change of frequency, rapid phase angle change, or unbalanced voltages.
Full tests on the protective equipment must be undertaken and recorded by the installer to the satisfaction of the network operator. Where the standby supply is connected to a high-voltage system the network operator has a duty to witness the tests; where it is connected to a low-voltage supply the operator may, at its discretion, wish to witness the tests. The loaded generator is unlikely to produce a pure sine wave of voltage and when it is connected to the normal mains it will cause harmonic currents to flow. As with any distorting load connected to the U.K. system, the harmonic currents must be within the limits set by Engineering Recommendation G.5/4 (ER G.5/4) published by the Electricity Association.
The Low-Voltage Neutral Connections
Where a low-voltage standby supply is intended to run in parallel with the normal supply, the two neutrals may be solidly connected. For TNC-S (PME) systems this is the simplest arrangement as the standby supply neutral may be solidly earthed and connected to the normal supply neutral; the changeover from one supply to the other is then effected by means of triple-pole contactors or circuit breakers. However this will probably not be practicable, it is likely that the resulting triplen harmonic current flow in the neutral will exceed the limits set by ER G.5/4 and some other procedure must be adopted. One procedure is to provide a single-pole neutral contactor in the connection to the generator star point, and to arrange for this to be open when running in parallel. With this arrangement the standby supply star point is not used and the system relies upon the normal supply for single-phase loads, for earthing, and for triplen harmonic currents; all the zero-sequence currents flow from the normal supply phase conductors and return additively in the neutral conductor. Harmonic current flow in the phase conductors of the normal supply must not exceed the limits set by ER G.5/4. The generator manufacturer must provide sufficient information for the network operator to be able to calculate the harmonic current flow before permission for paralleling is given.
There must be electrical interlocks to ensure that:
- It is not possible to connect the two supplies in parallel unless the neutral contactor is open.
- It is not possible to use the standby supply independently unless the neutral contactor is closed.
An alternative procedure is to install a reactor connected between the generator star point and the supply neutral. The reactor is to have sufficient reactance to limit the triplen harmonic current flow to within the limits set by ER G.5/4. This arrangement is discussed in ER G.59/1 and in ETR 113 but it is more complex and expensive than the preceding procedure and would seem to have limited application for single sets.
Earthing Low-Voltage Supplies
When the normal and standby supplies are running in parallel the neutral connection will usually be taken from the normal supply and, depending on the earthing arrangements and the agreement with the network operator, there may be a requirement for an earthing contactor or for an additional pole on the neutral contactor mentioned in an earlier paragraph. For multiple earthed systems, including PME, the neutral and earth connections are combined and a single pole neutral contactor is used. Figure below illustrates the electrical connections.
For single-point earthed systems such as TN-S or TT an additional pole is required on the neutral contactor to avoid a second earth connection to the normal supply neutral. Figure below illustrates the electrical connections. If a separate earthing contactor is used, electrical interlocks, as required for the neutral contactor, must be provided.
Earthing High-Voltage Supplies
For high-voltage systems the network operator may use solid, resistor, reactor, or arc suppression coil earthing methods and a second earthing point will not normally be allowed. When a high-voltage standby supply is operating independently its star point must be connected to earth, usually through an earthing resistor, but when it is operating in parallel with the normal supply, the earth connection must be opened, for which purpose an earthing contactor is used. Figure below illustrates the electrical connections. With this arrangement the standby supply star point is not used and the system relies upon the normal supply for
Overcurrent Protection of the Standby Supply
Behavior of Generators under Fault
When a generator experiences a fault, a large current, dependent upon the subtransient reactance (X"), flows for a short time, this will rapidly decay to a value dependent upon the transient reactance (X') and at the same time the voltage regulator will start to increase the excitation. After a short time the current will have stabilized to a steady-state value determined by the synchronous reactance; this is the steady-state short circuit current which is available for relay operation (the prospective fault current). The time constants of these decrements depend on the generator size and design, the subtransient time constant is measured in milliseconds, the transient time constant in tens of milliseconds and the synchronous time constant in tenths of a second.
For satisfactory fault clearance it is important that each generator is provided with an effective excitation system. The simplest excitations systems use a shaft-mounted rectifier, an ac exciter, and a pilot exciter with permanent magnet excitation.
Alternatively, instead of a pilot exciter, power may be derived from the generator output terminals; with this arrangement, under short circuit conditions, there will be a loss of exciter field unless a system to maintain it is included. One system uses current transformers to maintain excitation during a fault. Another system derives power from a dedicated third harmonic winding in the stator.
The steady-state short circuit current available from an ac generator is normally of the order of three times the rated current.
Protection of the Generator
Small sets up to say 75 kW may be protected against overload and fault conditions by fuses or by molded-case circuit breakers, and this may well be the only form of protection used.
For sets above 75 kW the basic form of protection against overloads is to provide, for each generator, a three-phase overcurrent relay with an inverse time characteristic and having sufficient delay to allow downstream protection to operate. To allow positive discrimination between two relays it is usual to ensure that the two characteristics provide a time difference between 0.3 and 0.4 s. The shorter time would be appropriate for installations using modern electronic relays (for greater accuracy) and quick operating circuit breakers such as those using vacuum tubes.
For sets above 75 kW the form of protection against faults in the distribution system is to provide three instantaneous high set overcurrent elements arranged to operate a timer adjustable up to 5 s. The time delay is to allow any downstream protection to operate before taking the somewhat drastic step of shutting down the entire standby supply. Typical connections are indicated by Figure below.
It is worth noting that if the current transformers measuring overcurrents are mounted at the neutral ends of the stator winding, they will react to any internal phase-to-phase or phase-to-earth faults.
However, except for large sets this is not usually possible and internal faults are monitored by other systems. The following additional protection systems may be considered:
- Restricted Earth Fault Protection.
In this system the currents entering and leaving the stator are summed and the sum is applied to a relay. If the sum is not zero an internal winding fault is indicated and the generator should be deexcited and the engine shut down; since no discrimination with downstream protection is involved, no time delay is necessary. Downstream faults do not have any effect on the relay, hence the use of the word restricted in the name. The system requires four current transformers, one in the neutral which can be mounted in the terminal box or in the switchgear, and three current transformers in the phases which are usually mounted in the switchgear. The area protected lies between the three-phase current transformers and the neutral transformer. The system is applicable to sets of above say 100 kW. It is sometimes known as a balanced current system. Typical connections are indicated by Figure below.
- Differential Current Protection
In this system the currents at the start and finish of each phase of the stator windings are compared. If any of the phases is not balanced, this indicates an internal fault and the generator should be deexcited and the engine shut down; since no discrimination with downstream protection is involved, no time delay is necessary. The system requires the three ends of the stator winding to be brought out of the stator and into the neutral terminal box and the installation of three current transformers in the box, the other three current transformers and the relays are usually mounted within the associated switchgear. This system is applicable to large sets, say above 1.5 MW. The area protected lies between the two sets of current transformers and for this reason the term unit protection is sometimes used. The term circulating current system is also sometimes used. Typical connections are indicated by Figure below.
- Standby Earth Fault Protection
This system is applicable to high voltage generators which are resistance earthed. In this system the current flowing to earth from the star point is monitored and is applied to a relay. Any current flow indicates a fault and should operate an inverse time-delayed trip. The system is unrestricted and the time delay is required to allow any downstream protection to operate. The generator should be deexcited and the engine shut down if the fault persists after the expiry of the time delay. This protection is a useful supplement to the restricted earth fault or differential current protection, it also protects the earthing resistor against continuous loading which would have disastrous consequences. Typical connections are indicated by Figure below.
- Reverse Power Protection
This system is needed for any generator which is required to run in parallel with another supply or another generator. A reverse power situation indicates that the generator is acting as a motor and is driving the engine, an undesirable situation. Under reverse power conditions the generator circuit breaker should be opened, it is not important to shut down the engine immediately but it must not be left running unloaded for longer than say 15 min, it depends on whether the generator is attended or not. It is usual for the relay to include a short time delay to prevent any transient effects, particularly during synchronizing operations, from tripping the circuit breaker.
- Unbalanced Load Protection
This system is applicable where there is concern that the generator may be running for long periods on a load which is unbalanced between phases. It is not normally required and is mentioned here because ISO 8528 includes a reference to it.
- Voltage Restrained Overcurrent Relays
The opening paragraphs of this section explain the behavior of a generator under fault conditions.
When running from a standby generator the low value of short circuit current can lead to problems with fault clearance. The voltage restrained relay has two time/current characteristics and may help to resolve the problems. Under conditions of normal voltage the relay has a long time characteristic which is intended to ensure that the generator remains on line for as long as possible. If conditions of low voltage arise during fault clearance, the relay operates to a standard inverse characteristic and, hence, trips in a predictably shorter time.
A Note on Current Transformers and Relays
Current transformers used for protection purposes must be of the correct class for the duty. It is important that their cores do not saturate at the maximum currents expected, it is also important, for differential and earth fault systems, that the ratio and secondary phase angle are sufficiently accurate to ensure proper operation. Until the 1980s most protection relays were electromechanical and based on the induction disc or cup, the principle having remained in use for many years. Current adjustment was by a “plug setting” and a tapped autotransformer, and time adjustment was by a “time multiplier” which determined the angle through which the disc had to rotate to close its contacts. The parts required precision manufacture and skilled assembly and, over a long period of time, as electronic techniques improved, the electromechanical relays became obsolescent and were superseded by electronic versions. For this reason all relays are now electronic in operation and have a very different appearance. Similar comments have already been made in this book in connection with voltage regulators and speed governors.
The modern relays perform the same functions as their predecessors and the same terminology is used, adjustments are made in a different way to achieve the same result. Electronic relays are more accurate than the electromechanical devices, overshoot is less because there are no moving parts, and the current transformer burdens are less. The relays are more versatile and provide, within a single unit, a choice of characteristics such as standard inverse, very inverse, extremely inverse, or definite time. The final output circuits may use electromagnetic relays with metallic contacts, which are better suited to the inductive loads likely to be encountered than are electronic components.
In the previous paragraph the characteristics described as standard inverse, very inverse, and extremely inverse are the operating characteristics described in BS EN 60255. The characteristics are inverse with a definite minimum time and are known in full as inverse definite minimum time characteristics, sometimes abbreviated to IDMT.
Protection of the Distribution System
In a normal situation the overload relay characteristic of the supply would appear on the time grading graph to the right of and above the characteristics of the distribution system. Any fault on the distribution system would be cleared, enabling the healthy parts of the system to continue in operation. However, the prospective fault current obtainable from the standby supply will be much less than that obtainable from the normal supply, and will probably be of the order of three times the rated current. On the protection time grading graph the effect is to move to the left, sometimes drastically, the supply characteristic. This characteristic may intersect some of the existing operating characteristics, thus lengthening their operating times or, worse, may leave them isolated in a high current section of the graph where the standby supply cannot operate.
If the standby supply feeds a number of loads through dedicated changeover contactors or circuit breakers there should be no problem in providing protection because the distribution system is in effect duplicated. Where a small standby supply feeds a large distribution system and uses the same switchboard as the normal supply, it will be unlikely to be able to clear faults in the main distribution system and a cable fault near the power source may not be cleared, thus rendering the standby supply inoperative, until it can be restored by manual switching or repair. Such an event is unlikely and the risk may be acceptable; provided that the generator rating is large in relation to the final circuit protective devices, disconnection times complying with BS 7671 should be achievable for the final circuits. The alternative is to connect the standby supply to a downstream point where the protection will be set at a lower level.
For the smaller and simpler generating sets, the switchgear may comprise a single molded-case circuit breaker mounted on the generating set base frame. For larger sets the switchgear is installed separately, and for multiple-set installations the switchgear will comprise several cubicles with busbars and interconnecting wiring. Sometimes, such switchgear is installed in the engine room, but this is an extremely noisy location in which to operate switchgear under the stressful conditions applying after a power or equipment failure. There are advantages to installing the switchgear in a separate room away from the noise and high ambient temperature of the engine room; if there are problems to be resolved it will certainly be easier to discuss the matter or to exchange technical information.
The Cable Connecting the Switchgear to the Generator
The cable entering the generator terminal box should be a flexible cable and should be so arranged and run that it does not come under stress due to vibration or movement of the set. If the cable is not flexible or is not properly installed, it may restrict the freedom of movement of the set, affect the performance of the antivibration mounts, and cause vibration problems.
If the length of the cable run between the switchgear and the generator is short, say not exceeding 10 m, the entire run can be in flexible cable. If the run is longer a cable change box may be installed so that the majority of the cable is of standard distribution type and only a short length of flexible cable is necessary.
For some small sets the main circuit breaker is mounted on a framework fixed directly to the generating set base frame, the manufacturer providing the interconnections between the circuit breaker and the generator. For such an arrangement the interconnections should be in flexible conductors and the cable taking power from the circuit breaker may be of standard distribution type.
For fault protection purposes, this cable may be regarded as an extension of the stator windings, and if the switchgear is remote from the generator some additional protection may be desirable. The probability of a fault developing in this cable is small but the condition can be detected by installing a restricted earth fault protection system as described in the previous section of this chapter. In addition to providing cable fault protection, this system provides protection against internal winding faults and limits the resulting damage to the machine. For installations of more than one set, the restricted earth fault protection should be provided on each set.
For low-voltage installations, the switchgear should be a type tested assembly (TTA) or a partially type tested assembly (PTTA) complying with BS EN 60439-1. The switchgear and controlgear components within the switchboard should comply with BS EN 60947. The enclosures should protect the equipment against the ingress of solid foreign objects, and persons against touching hazardous parts, to one of the degrees of protection in BS EN 60529 which is invoked by BS EN 60439-1.
For high-voltage installations, the switchgear should comply with BS EN 60298. The enclosures should protect the equipment against the ingress of solid foreign objects, and persons against touching hazardous parts, to one of the degrees of protection in BS EN 60529. (BS EN 60298 invokes
BS EN 60694 which in turn invokes BS EN 60529).
Prospective Fault Current Level
For a set providing an alternative supply and not arranged to run in parallel with the normal supply, the prospective fault level will be the higher of the two supplies, which will almost certainly be that of the normal supply. For sets running in parallel with the normal supply, the prospective fault current will be the sum of the two fault currents. The mechanical and thermal stresses imposed on the busbars are proportional to the square of the current, it follows that a modest additional current from a generator can have a significant effect within the switchboards.
Information to Be Displayed at the Switchboard
The generator panel of a single set, not intended to run in parallel with another set or the normal supply, is required by ISO8528 to display the following information:
Voltage, with provision for indication of phase-to-phase and phase to- neutral voltages
Current, with provision for indication of each phase
Other indications which should be considered includes:
The frequency of the ac output Operating hours, that is, a cumulative hours-run counter
For large sets only, indication of kW, kVAr, power factor, and cumulative kWh.
Each generator panel of a multiple-set installation not intended to run in parallel with the normal supply should display the following:
Voltage, with provision for indication of phase-to-phase and phase to- neutral voltages
Current, with provision for indication of each phase kW indication for each set For manual synchronizing, the switchboard should include a synchroscope, capable of being connected to any generator and indicating any difference frequency and the phase relationship between the busbars and the generator before paralleling. It is usual to include synchronizing lamps or a zero voltmeter and a check synchronizer as back-up features.
For automatic synchronizing, the switchboard should include an automatic synchronizer feeding signals to motor operated adjustments of the voltage and speed regulators of individual sets.
Other indications which should be considered include:
The frequency of any set or of the busbars to be available for display when required
Operating hours, that is, a cumulative hours-run counter for each set
Indication of kVAr, power factor, and cumulative kWh for each set
Indication of total kW, total kVAr, and the standby supply power factor
Sets Intended to Run in Parallel with the Normal Supply
The information relating to multiple sets applies, with the addition of the following:
Voltage, provision for indication of the busbar voltage which, when the standby supply is shut down is the normal supply voltage.
The synchronizing features should operate between the busbars and the normal supply, and between the busbars and the standby supply, thus enabling synchronizing in either direction. For multiple-set installations the synchronizing features should also operate between any individual set and the busbars. Other indications which should be considered include:
-The frequency of any set or of the busbars to be available for display when required
-Operating hours, that is, a cumulative hours-run counter for each set Indication of kVAr, power factor, and cumulative kWh for each set
-Indication of total kW, total kVAr, and standby supply power factor if more than one generating set is installed
The Synchronizing Facility
A switchboard which provides a paralleling facility will include one set of synchronizing equipment which the user is able to connect between any incoming supply and the busbars. Selection is by a switch, plug and sockets, or any other means, it must not be possible to select more than one incoming supply.
There must be provision for occasionally connecting to “dead busbars” when, obviously, synchronizing is impossible and the control system must take care of this condition. It arises when a single set or the first of multiple sets is started and connected to “dead busbars.”
For manual synchronizing the synchroscope is arranged to be easily visible from each generator panel, and each panel includes raise/lower controls for speed and excitation so that the operator can make fine adjustments while watching the synchroscope. It is likely that the electromechanical synchroscope will be replaced by fully electronic versions, but clear visibility is essential. It is usual to include a check synchronizer in the control system; the check synchronizer prevents circuit breaker closure unless the voltage, frequency, and phase relationship are within limits.
For automatic synchronizing the control system takes care of the adjustments to voltage, frequency, and phase relationship, and closes the circuit breaker when conditions are within limits.
Test Load Facility
Consideration should be given to including in the switchboard a switched outlet to which a test load can be connected when required. The circuit should be fused or otherwise protected and fully rated for the output of one set. For a multiple-set installation this will usually be adequate, but for a large important installations the rating could be increased. This facility is useful after major work has been undertaken on a set and avoids the need for temporary connections within the switchboard in order to connect a test load.