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Machine Protection

Introduction

Overheating of a motor due to an abnormal use shortens considerably the life time of its components and causes short-cuts.

Overloads are tolerated only for a limited period of time. Concerning short-cuts, their effects are destructive and they must be quickly cut off.

Fault types

Internal Faults

That type of internal faults generally appears when insulating components are
ageing.

It is then preferable to release the protection as fast as possible.

Those faults can be detected by comparing the ongoing and outgoing currents of the same phase. The protection is then provided by an percentage differential overcurrent relay connected to two sets of three current transformers placed on each input and output phases of the stator.

Because of the imprecision linked to the TC, the fault current must represent 5 to 10 % of TC nominal current. This protection will not be sensitive enough to detect fault currents between the phase and the earth, which are generally limited to 20 A.

The deterioration of the insulating material by exaggerated and repetitive overheating, and the electrodynamic efforts on conductors can cause insulation failures between a phase and the exposed conductive part. As I will show you later, the fault current is limited by a resistance between the earth and the neutral terminal of the main transformer. If a fault current appears, it will not be important enough to unlatch the protections. A tore enclosing the 3 phases is then used.

If the receiver is balanced, the sum of the magnetic fields induced by the currents is null, and there is no secondary current in the tore. When a fault occurs, the tore delivers a current detected by the relay 64.

A synchronous machine is expensive, thereforee it must be well protected.
A failure in the excitation circuit or a break of excitation causes quick dissynchronism and can lead the motor to be pulled out of synchronism. Those failures can be caused by a disfunctioning in the control panel of the excitation regulation, or by a disfonctioning in the rotor.

It is thus necessary to check that a important enough current flows in the rotor, which is also protected against a continuous overcurrent.

The relay number 59 protects against stator overvoltage which can occur when it is fast unloaded.

As the motor is abruptly unloaded, its voltage increases to sometimes important values because its internal voltage drop does not exist anymore.

In both cases, overvoltage can be fatal to dielectric components.

The excitation circuit being isolated from exposed-conductive-parts, the first fault at the rotor will have no consequence. However it must be taken under account, the second fault being in condition to create a short-cut harmful to the rotor. Insulation control is done by injecting a low frequency alternative current. The letter « R » in the protection identification means that the latter is applied to the rotor.

If the load is too heavy, the synchronous motor will need more and more power from the network, until it is unable to convert the power at constant speed. At that point there is a break of synchronism, and the motor is pulled out of synchronism.

External Faults

Faults between phases usually come from ageing of insulating material of the wires. Short-cut faults are instantly cut.

Imbalances come from an unequal repartition of the load between the network phases, or from a bad phase succession in the feeding line, or from an asymetrical fault on the network.

Those faults cause an increase of current value in the stator, and a rise of temperature due to additional losses.

The motor starting under reduced voltage noticeably increases its starting time.If the voltage drop is too high, it is possible that the driving torque will not be sufficient enough to carry (drive) the load (the driving torque of a synchronous motor is proportional to voltage square). In the conception of the installation, it must be checked that the overvoltage at motor starting remains limited, so that the starting doesn’t take too long. The protection used against too-long startings is an undervoltage relay number 27.

When a tension break occurs at the beginning of power supply, synchronous motors are then driven by their load and become alternators : they work as self-driven rotative generators supplying the network with magnetising energy.
The overpower protection number 32 is used to prevent the energy to be sent back to the network, and to prevent a fault on the line to be supplied with energy by a motor. Those protection are use mainly on main machines.

Mechanical defects

The advantage of protection using temperature probes is to take the previous state of the machine into account. They are installed at selected places into the windings. It is possible to detect local rises of temperature, undetectable with other protections.

Nevertheless, they still do not allow to detect fast overheating because of the high thermal constant term of the insulating material in which they are placed. They also are fragile, therefore they are always used alongside other protections.

Temperature image protections take the value of the current as well as the time into account. They consider both direct and negative components of the current so as to take into account strong thermal effects in the rotor area due to unbalanced energy supply of the machine.

That type of protection is particularly useful when the motor is decelerating, because in that case the cooling system is less efficient.

Other faults

The startings limitation unit counts and controls the motor startings on a definite lap of time : a certain number of startings is allowed during a reference period of time. If the maximum starting number authorised is reached, no other starting will be allowed during a predefinate lap of time.

The too-long startings and rotor-blocking control unit (51 LR) control each starting individually and check if they respect the heating characteristics defined by Id and Td. After the starting, the protection against the blocking of the rotor is provided by a time-independent overcurrent unit.

Protection Module

Constructors offer protection units that provide all protections specific to an installation. It comes as a compact box with a protection settings panel, a screen display, and sending of data to control systems.

Measures and high voltage controls

Before each feeder bay or to an installation downstream, measure devices are placed in HV or LV substations.

For simplicity purposes, the diagrams only show one line where there can be three or four conductor. The diagram will be then filled in in the next slides.

The connections are always represented opened.

The disconnector is used to insulate the energy supply line from the installation. It can not be driven loaded.

When people have to work on HV lines, the earthing switch is closed once the line is insulated to be sure that the lines and the earth are at the same potential. Any risk of electrocution due to capacitive effects in the lines (residual charge) is thus prevented.

The diagram represents only a part of power supply of an installation. The equipment showed is located in the HV substation. As we said, this substation has measure instruments. Here, a triple voltmeter measures the voltage of each three phases. The Potential Transformers (PT) are necessary, because at that point the voltage is to high for the devices. The triple ammeters measure the current in the three phases through the Current Transformers (CT) which transform the current into a measurable current.

An active-energy meter (kilo watt hour meter) gives the power effectively consumed, and the var-hour meter measures the consummation of reactive energy. The indicator PF is a measure of the power factor which has to be maintained up a certain value.
The compensation can be provided by a capacitor bank or by the adjustment of the excitation of the synchronous motor.

On this diagram, the protections of the transformer have been added.

They are the protections 50 and 51. There are only represented on the primary circuit, but they can be found on the secondary as well. They are overcurrent relays.

Short-cuts : the protections release level depends on the short-cut current, and the circuit-breaker should be able to cut the short-cut current. Relay n°50 is operating as soon as the programmed current value is reached.

Overcurrent : through the secondary circuit, the maximum tolerated is 2 to 6 times the rated current of the transformer, during 0,7 to 1 sec., compatible with selectivity and the devices (panels, cables, CT, etc…). Through the primary circuit, the protection threshold is 115 to 125% of the secondary protection, with a time delay of 0,25 to 0,30 longer than of the secondary. 51 is not instantly release, but the higher the overcurrent is, the faster it is released.

The differential protection relay 87 protects the circuit against faults between phases. It is a differential overcurrent percentage relay. Even an unbalanced augmentation of the current through a machine does not systematically release the protection, and even if it is the case, the delay can be very long. On powerful machines (from 5MW), one tries to avoid destructive effects from an long internal fault between phases (for example, a defective insulating material that would lead to an abnormal rise of temperature). In the case of a transformer, 6 identical CTs are placed on the secondary and primary circuits and are connected to relay n°87, which analyses, for each phase, the percentage of the difference between the primary and secondary currents. Because of the CT imprecision, it impossible to detect a fault less than 5 to 8% of nominal current.

Relay n°86 is used to lock the circuit-breaker open until a reset order. Reset can be done manually or thanks to a small electric motor. Here, the reset is manual, no symbol of motor is represented on the diagram.

Main Transformer Protection

WARNING : Metallic exposed-conductive parts being accessible by anybody, they must be equipotential to the earth (when there is no insulating fault). Therefore, they are connected to the earth with a protection conductor (PE).

On the incomplete diagram, there is a resistance between the neutral terminal and the earth to limit the fault current and avoid the devices (motor…) downstream to be deteriorated by thermal effects. In case of insulating fault on a MV or HV device downstream, the fault current would go through the earth circuit (low impedance) to the transformer ’s neutral terminal. An intense current (because of the high voltage) would go through the device enclosure and damage it (hole). This fault current is often limited to 20 A. It won ’t release protections settled on a much higher current threshold. It is thus necessary to put a differential device at the feeder bay of each machine, so that this « low » fault current is detected, and the circuit-breaker is opened. It is the principle of a TT operating phase.
The exposed-conductive parts of the transformer and some devices are connected to a same earthing system (metallic grid for instance). If a fault occurs on a device, the fault current will go up to the transformer neutral terminal and will be detected by the relay 51G (G stands for Ground) through the CT, and open the circuit-breaker and the circuit.

The transformer has a second earthing conductor which is connected to the tank. The transformer is insulated from the ground, but its tank is connected to the earth through that conductor. In the same time, one controls the currents which goes from the tank , passes through the CT and then into the ground. In case of an insulation failure in the tank, a high current would go through that conductor and the relay n°51T (T for Transformer) will release the protections. The relay n°63 is the Buchholz relay (2 thresholds) which releases the protection for a certain gas or liquid pressure. The relay n°26 is the temperature probe, placed on the hottest areas.

Differential protection using a tore

This diagram is an example of a smaller transformer using a tore as a protection against internal insulation fault. As before, the protections will be released in case of insulation failures to prevent damages due to fault currents (local overheating). The tore detects any imbalance in the currents.When there is one, a voltage appears in the secondary tore circuit. It is detected by the relay n° 64, specially used to insulation failure detection. To use the tore, the device must be connected by cables and not by bars.

Once again, we find protection n°51 against overload, n°50 against short-cuts, the buchholtz relay, the temperature probe and the measure devices. The protection is reset by a small direct current motor. For safety, there is a mechanical lock (represented by the small triangle on the diagram) between the earthing disconnector and the circuit-breaker : when the circuit-breaker is closed (normal operating phase), the disconnector can not be closed, otherwise it would create an earth-phase dead short. On the contrary, when the disconnector is closed, the circuit-breaker can not be reset. Just before the disconnector, there is a voltage divider used to energise a voltage indicator. The symbol next to the relay n°64 represents some indicator to which measures can be added.

CT ’s specifications :
3×350/5A means that there are 3 nominal currents (=350 A corresponding to a 5 A secondary current) CT (3 phases).
5 VA is the precision power of the CT. It has to be high enough when there are a lot of devices. This is a 0.5 class, and it is used for measure. The 5P20 is the CT protection.

Motor protection

MV motors are wye connected. The neutral line is then accessible, which allows to put a CT in that place. Another CT, upstream from the motor, is used to make a differential protection with relay n° 87. For safety, another differential protection has been added to each tore.
Of course protections n° 51 and 50 are found to be against overcurrents and short-cuts.

Relay n° 50 protects against overloads.
The protection against internal short-cuts is ensured by the differential protection n°87.

Relay n°51r protects against too-long startings, and the effects of long blocking of the rotor. It is an overcurrent relay which is activated only after a so called starting delay. This delay should allow the longest starting under the lowest network voltage to prevent any unwanted trips.

Relay n° 66 protects against too many sartups..

Relay n° 64 associated to the tore detects insulation failures.

Relay n° 46 is a over-negative-component relay. An unbalanced tri-phased network is composed of the three homopolar, direct, and negative components. Concerning a motor, the direct component gives the driving torque, the homopolar component is dissipated into thermal effects, and the negative component becomes the resistive torque. The negative component appears when there is an unequal repartition of the load between the three phase of the network, or when there is a bad succession of the phases, or when there is an asymmetric fault in the network, or a fault between the windings of a machine, or a bad contact in a switching device, or a fuse-break.

The consequences are:

1. an increase of the current in the stator
2. overheating in the rotor, due to additional losses relay n°49 is the thermal image relay

The diagram represents the case of a synchronous motor. We can find the previous protections and relay n°32 : it is a over-reactive-power protection which prevents the energy to be sent back to the network, and thus preventing to energise the fault using the inertia of the load that would be transformed into electric energy.

It is a synchronous brushless motor. The panel supply the fix stimulator with direct power. The magnetic flow is transmitted to a rotating winding bound up with the rotor. An alternative current is then induced, and it is straightened by thyristors and diodes. It can then supply the rotor with direct energy.

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