Reverse Power Protection of Genset

The reverse trip settings would be determined by code and by the manufacturer of numerous pieces of equipment as far as load capacity/durations, etc.

Turn in the same direction? Are you serious? I would love to see a generator change direction in order to phase sync with the mains feeder.

I sure hope you don't change your supply voltage to your load just based on where you get your power from.

As much as i know, there will be only one setting, time delay to operate, for Reverse Power relay.

Harmful effects on DG set - None, except that you will be spending energy to rotate a DG set instead of getting power from it.

Alternators used in DG Sets are also called Synchronous Generators and similar in construction to a Synchronous Motor - with a main winding & field winding. Field winding is always supplied with power to excite/create magnetic field. When running as Generator (connected to a prime mover to rotate it) voltage is induced in the main winding. When used as motor, we give voltage to main winding, and excitation of field winding is used to control the power factor of the motor.

Practically, you will never know when DG Sets starts motoring and reverse power protection fails.

Supply voltage may drop marginally as you will be adding DG set as additional load on your system. But a definite change in powerfactor depending on its field excitation.

-- sitaram

I am afraid that you give a very wrong and dangerous advice here that during motoring no harmful effects on DG Set. I like to quote from IEEE C 37-102 (Clause 4.5.5):

Anti-motoring protection: Motoring of a generator occurs when the energy supply to the prime mover is cut off while the generator is still on line. When this event occurs, the generator acts as a synchronous motor and drives the prime mover. While this condition is defined as generator motoring, the primary concern is the protection of the prime mover, which can be damaged during a motoring condition.

Motoring causes many undesirable conditions. A steam turbine could overheat due to the loss of the cooling effect provided by the steam. A diesel or gas engine could either catch fire or explode. In a steam turbine, the rotation of the turbine rotor and blades in a steam environment causes idling or windage losses.

Because windage loss is a function of the diameter of rotor disc and blade length, this loss is usually greatest in the exhaust end of the turbine. Windage loss is also directly proportional to the density of enclosing steam. Thus, any situation in which the steam density is high causes dangerous windage losses.

For example, if vacuum is lost on the unit, the density of the exhaust steam increases and causes the windage losses to be many times greater than normal. Also, when high-density steam is entrapped between the throttle valve and the interceptor valve in reheat units, the windage losses in the high-pressure turbine are high.

Windage loss energy is dissipated as heat. The steam flow through a turbine has a two-fold purpose: to give up energy to cause rotation of the rotor and to carry away the heat of the turbine parts. Because no steam flows through the turbine during motoring, the heat of the windage losses is not carried away and the turbine is heated.

Even when the unit has been synchronized, but no load has been applied and enough steam is flowing through the unit to supply the losses, the ventilating steam flow may not be sufficient to carry away all of the heat generated by the windage losses. Although the generator is not motoring under this condition, the problems caused in the turbine are the same, and protection should be provided.

Because the temp. of the turbine parts is controlled by the steam flow, various parts cool or heat at abnormal, uncontrolled rates during motoring. This irregularity can cause severe thermal stresses in the turbine parts. Another problem resulting from this temp. change can be unequal contraction or expansion of the turbine parts. This irregularity could cause a rub between rotating and stationary parts.

Because a rub generates heat, the problem is made more severe (as with windage losses) by the lack of ventilation steam flow to carry the heat away. A maximum permissible time is established for how long a steam turbine can be operated in a motoring condition and this time is generally a function of the rated speed of the unit. These data can readily be obtained from the mfr. for a particular steam-turbine unit.

Windage loss is not a particular problem in other types of prime movers, but they exhibit additional motoring difficulties. Gas turbines, for example, may have gear problems when being driven from the generator end. With hydro turbines, motoring can cause cavitation of the blades on low water flow. If hydro units are to operate as synchronous condensers, the unit will be motoring. This fact should be recognized in any motoring protection.

With diesel engine generating units, explosion and fire from unburned fuel are additional dangers. Motoring protection should, therefore, be provided for all generating units except units designed to operate as synchronous condensers, such as hydro units.

Reverse-power relay: From a system standpoint, the primary indication of motoring is the flow of real power into the generator acting as a synchronous motor. The reverse-power relay detects the reverse flow of power (i.e. watts) that would occur should the prime mover lose its input energy. The magnitude of motoring power varies considerably depending on the type of prime mover, as shown in Table.

The sensitivity and setting of the relay are dependent upon the type of prime mover involved because the power required to motor is a function of the load and losses of the idling prime mover.

The reverse-power relay should have sufficient sensitivity so that the motoring power provides 5 to 10 times the minimum pickup power of the relay.

In gas turbines, for example, the large compressor load represents a substantial power requirement from the system, up to 50% of the nameplate rating of the unit; therefore, the sensitivity of the reverse-power relay is not critical. A diesel engine with no cylinders firing represents a load of up to 25% of rating; therefore, again, no particular sensitivity problem exists.

With hydro turbines, when the blades are under the tail-race water level, the percent motoring kilowatts is high. When the blades are above the tail-race level, however, the reverse power is low, between 0.2% to 2.0% of rated, and a sensitive reverse-power relay may be needed.

Steam turbines operating under full vacuum and zero steam input require about 0.5% to 3.0% of rating to motor. This condition may be detected by a sensitive reverse-power relay. If the turbine were operated with its valves only partially closed to, for example, slightly less than the no-load value, the electrical input from the system could be essentially zero, and the reverse-power relay could not detect the condition.

Because overheating of the turbine could still occur, some additional means of protection is required.

A directional power relay with either definite- or inverse-time characteristics is frequently used to introduce sufficient time delay necessary to prevent operation during power swings caused by system disturbances or when synchronizing the machine to the system. A time delay of 10 s to 30 s is typical. Either a single-phase or a three-phase relay may be used although a single-phase relay calibrated in three-phase watts is frequently selected.

i was told by a friend of mine that the manufacturer of cummins power generators told him to set the reverse power settings to 20 % of the rated genset power for protection of his cummins genset from motoring i.e. use a reverse power relay that will trip the genset ACB when the reverse power exceeds 20% of the rated power . the delay can be 10 second.

You're friend, does he live in the same fantasy land as you?

Depending on the size of the generator, type of prime mover, windings, etc., the reverse power settings will be somewhere around that range (10-20% of rated, with a time delay of a few seconds). This isn't that unusual, but it cannot be taken to apply universally.

Reverse power protection is primarily used to prevent mechanical damage in the prime mover and the generator. Electrically, the generator doesn't care about the direction of power, as that's only a phase angle. The actual volts and amps don't change.

In order to determine an appropriate setting for reverse power protection you should know the frictional losses of the rotating members (diesel shaft(s) & pistons + generator rotor). How much power is required to spin the combined rotors at synchronous speed? No damage will occur until the generator exerts a positive torque on the shaft. Those losses are your "safety zone".

You can push power back into the generator up until the reversed power overcomes the losses and actually pushes the engine instead of the other way around. On a diesel it could be as high as 20% of generator capacity due to the high compression ratio and heavy mass. A steam turbine is usually around 4%, and a vertical hydro unit can be as little as 0.25%. That information should be available from the genset manufacturer.

The setting should be high enough to avoid nuisance trips when running at low load, but it must remain below the total frictional losses for the combined engine-generator. A time delay of 1-2 seconds should be sufficient for a diesel.

I had a genset close to commercial power due to some confusion on the part of an electrician, it wasn't running at the time, 240 volt single phase, in an outdoor enclosure, it didn't motor the genset but the whole enclosure was drumming and vibrating, smoke was pouring out of the generator end of the set, it took what seemed like hours but was actually minutes to determine what happened and the drop the Main breaker. Scared the crap out of me. Considering the volume of smoke I thought it was all done, but it worked fine, I don't suggest it though.