Generator protection -- part 1 [Industrial Electrical Power Systems]

Introduction

A generator is the heart of an electrical power system, as it converts mechanical energy into its electrical equivalent, which is further distributed at various voltages. It therefore requires a 'prime mover' to develop this mechanical power and this can take the form of steam, gas or water turbines or diesel engines.

Steam turbines are used virtually exclusively by the main power utilities, whereas in industry three main types of prime movers are in use:

1. Steam turbines: Normally found where waste steam is available and used for base load or standby.

2. Gas turbines: Generally used for peak-lopping or mobile applications.

3. Diesel engines: Most popular as standby plant.

Small- and medium-sized generators are normally connected direct to the distribution system, whilst larger units are connected to the EHV grid via a transformer.

It will be appreciated that a modern large generating unit is a complex system, comprising of number of components:

• Stator winding with associated main and unit transformers

• Rotor with its field winding and exciters

• Turbine with its boiler, condenser, auxiliary fans and pumps.

Many different faults can occur on this system, for which diverse protection means are required. These can be grouped into two categories: Electrical Mechanical Stator insulation failure Overload Overvoltage Unbalanced load Rotor faults Loss of excitation Loss of synchronism Failure of prime mover Low condenser vacuum Lubrication oil failure Loss of boiler firing Over speeding Rotor distortion Excessive vibration We will look briefly at the electrical side in this section.

LV or MV busbars

++++ Small- and medium-sized generators --Main trfr HV busbars --Unit trfr Aux. supply

++++ Larger generating units.

Stator grounding and ground faults

The neutral point of the generator stator winding is normally grounded so that it can be protected, and impedance is generally used to limit ground fault current.

The stator insulation failure can lead to ground fault in the system. Severe arcing to the machine core could burn the iron at the point of fault and weld laminations together. In the worst case, it could be necessary to rebuild the core down to the fault necessitating a major strip-down. Practice, as to the degree of limitation of the ground fault current varies from rated load current to low values such as 5A. Generators connected direct to the distribution network are usually grounded through a resistor. However, the larger generator-transformer unit (which can be regarded as isolated from the EHV transmission system) is normally grounded through the primary winding of a voltage transformer, the secondary winding being loaded with a low ohmic value resistor. Its reflected resistance is very high (proportional to the turns ratio squared) and it prevents high transient overvoltages being produced as a result of an arcing ground fault.

When connected directly through impedance, overcurrent relays of both instantaneous and time-delayed type are used. A setting of 10% of the maximum ground fault current is considered the safest setting, which normally is enough to avoid spurious operations due to the transient surge currents transmitted through the system capacitance. The time delay relay is applied a value of 5%.

Ground fault protection can be applied by using a transformer and adopting a relay to measure the grounding transformer secondary current or by connecting a voltage-operated relay in parallel with the loading resistor.

Loading resistance--Overcurrent relay with time delay

++++ Ground fault protection using a relay to measure secondary current

The current operated relay should incorporate third harmonic filter and is normally set for about 5% of the maximum ground fault current. The third harmonic filter is required because of the low current of the grounding system, which may not be much different from the possible third harmonic current under normal conditions. The time delay is essential to avoid trips due to surges.

Overvoltage relay with time delay-- Loading resistance R

++++ Ground fault protection using a relay in parallel with loading resistor

In the voltage-operated type, a standard induction disk type overvoltage relay is used. It’s also to be noted that the relay is connected across the secondary winding of the transformer and the relay shall be suitably rated for the higher continuous operating voltage. Further, the relay is to be insensitive for third harmonic current.

Phase-to-phase faults clear of ground are less common. They may occur on the end coils or on adjacent conductors in the same slot. In the latter case, the fault would involve ground in a very short time.

Overload protection

Generators are very rarely troubled by overload, as the amount of power they can deliver is a function of the prime mover, which is being continuously monitored by its governors and regulator. Where overload protection is provided, it usually takes the form of a thermocouple or thermistor embedded in the stator winding. The rotor winding is checked by measuring the resistance of the field winding.

Overcurrent protection

It’s normal practice to apply IDMTL relays for overcurrent protection, not for thermal protection of the machine but as a 'back-up' feature to operate only under fault conditions. In the case of a single machine feeding an isolated system, this relay could be connected to a single CT in the neutral end in order to cover a winding fault. With multiple generators in parallel, there is difficulty in arriving at a suitable setting so the relays are then connected to line side CTs.

Overvoltage protection

Overvoltage can occur as either a high-speed transient or a sustained condition at system frequency.

The former are normally covered by surge arrestors at strategic points on the system or alternatively at the generator terminals depending on the relative capacitance coupling of the generator/transformer, and connections, etc. Power frequency overvoltages are normally the result of:

1. Defective voltage regulator
2. Manual control error (sudden variation of load)
3. Sudden loss of load due to other circuit tripping.

Overvoltage protection is therefore only applied to unattended automatic machines, at say a hydroelectric station. The normal setting adopted are quite high almost equal to 150% but with instantaneous operation.

Unbalanced loading

A three-phase balanced load produces a reaction field, which is approximately constant, rotating synchronously with the rotor field system. Any unbalanced condition can be broken down into positive, negative and zero sequence components. The positive component behaves similar to the balanced load. The zero components produce no main armature reaction. However, the negative component creates a reaction field, which rotates counter to the DC field, and hence produces a flux, which cuts the rotor at twice the rotational velocity. This induces double frequency currents in the field system and rotor body.

The resulting eddy currents are very large, so severe that excessive heating occurs, quickly heating the brass rotor slot wedges to the softening point where they are susceptible to being extruded under centrifugal force until they stand above the rotor surface, in danger of striking the stator iron. It’s therefore very important that negative phase sequence protection be installed, to protect against unbalanced loading and its consequences.