Issues with distributed generation protection (bulk power and distributed generators)

Protection requirements for DG

Besides a number of benefits, there are some technical problems with relay protection of distributed generators. It turned out to be one of the most problematical technical issues since its malfunction could cause serious risk for people and components.

Much more small distributed generation units are nowadays connected to power systems than in the past. Protection requirements should relate to the value of the equipment protected. As such, protection requirements for large units differ from those for smaller units.

Furthermore, the location in the power system at which a generator is connected can create site-specific hazards to the generator as well as to the power system to which it is connected.

Generally, generators can be classified as bulk power generators and distributed generators. Both types share many common hazards. Therefore, protection requirements are similar. Smaller generators, common among distributed generators, warrant less sophisticated protection, however, as their cost is significantly less than large units.

Let’s have a word about each type of generators and their basic protection.

Table of Content:

1. Bulk power generators
2. Distributed generators
3. Protection issues with distributed generators
4. Connections and protection schemes
   1. Direct connected
   2. Unit connected

1. Bulk power generators

Bulk power generators are synchronous machines that interconnect into the bulk power transmission system. Such generators are typically above 20 MVA in size and usually range in the 100 to 1200 MVA size.

These generators are often located in power plants that may house one or more generating units. The geographic locations of bulk power plants are selected on the basis of factors such as proximity to fuel supply and load centers, availability of a suitable cooling source, and restrictions related to environmental concerns and public acceptance.

Typical example are hydro-driven generators. These hydro-units have vertical shafts. Steam turbine driven generators have horizontal shafts.

Bulk power generators are usually connected to the power system through a HV switchyard located at the plant location. Some smaller bulk power units may tap into a bulk power transmission line, thus effectively creating a three-terminal line.

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2. Distributed generators

Distributed generators are made up of induction and synchronous machines. An induction generator is simply an induction motor driven above synchronous speed by a prime mover. Induction generators require a source of excitation, which is typically obtained from the power system to which it is connected.

Loss of the power source to the circuit to which an induction generator is connected, therefore, will normally cause the generator to shut down, as its source of excitation is lost. Continued operation of an induction generator is possible after the source to its connected line is removed, however, only if a source of excitation, such as a capacitor bank, exists on the line to which the induction generator remains connected.

To sustain operation in a self-excitation state, the amount of excitation, and load that remains isolated with the induction generator must fall within a suitable range.

There are many different types of distributed generation systems along with a variety of ways in which they are connected to the power system.

Generators located at industrial plants may be connected to the plant’s electrical system electrically at a distance from the point of electrical delivery to the associated industrial complex.

Other types of distributed generation are powered from a variety of sources such as wind, solar, hydro, biomas, geothermal, urban waste, and conventional fossil fuel. The sizes of such units can also vary from very small single-phase units rated at several kVA to larger units exceeding 100 MVA.

Larger distributed generators are usually connected to a sub-transmission system. Distributed generators connected to distribution systems are usually limited to units of about 10-15 MVA in size.

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3. Protection issues with distributed generators

The overall problem when integrating distributed generators in existing networks is that distribution systems are planned as passive networks, carrying the power unidirectionally from the central generation (HV level) downstream to the loads at MV/LV level.

The protection system design in common MV and LV distribution networks is determined by a passive paradigm, i.e. no generation is expected in the network.

For the design and coordination of protective relays in a network, some overall rules have become widely accepted:

Selectivity

A protection system should disconnect only the faulted part (or the smallest possible part containing the fault) of the system in order to minimize fault consequences.

Redundancy

A protection system has to care for redundant function of relays in order to improve reliability. Redundant functionalities are planed and referred to as backup protection.

Grading

For the purpose of clear selectivity and redundancy, relay characteristics are graded. This measure helps to achieve high redundancy
whereas selectivity is not disabled.

Security

The security of a relay protection system is the ”ability to reject all power system events and transients that are not faults so that healthy parts of the power system are not unnecessarily disconnected”.

Dependability

The dependability of a relay protection system is ”the ability to detect and disconnect all faults within the protected zone”.

Different network topologies require different protection schemes. The simplest network structure to protect are radial systems, therefore simple relays are used.

Normally, time-dependent, graded overcurrent protection is installed regarding redundancy (backup protection). More sophisticated relays are used for the protection of rings and meshed grids. Impedance relays trip due to a low voltage-current quotient. Since these relays allow to determine the position of the fault on the line, they are also called distance relays.

Furthermore, a number of other techniques are used, also device-specific ones.

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4. Generator connections and protection

Some common connections for generators are as follows:

4.1 Direct connected

Direct connected (one or several), each through a circuit breaker to a common bus, as illustrated in Figure 1. Usually they are wye-grounded through impedance, but may be undergrounded, or are delta-connected.

They may be connected to a grounded power system or to the power system through a delta-connected transformer.

Typical protection for distributed generator is shown in Figure 2. For such generators undervoltage and overvoltage as well as underfrequency and overfrequency relays are applied for disconnecting the power sources from the utility.

A separate transfer trip channel from the utility to the distributed unit may be required to assure that the unit is not connected when the utility recloses to restore service.

This is important where the distributed generator may be islanded and able to supply the utility loads in the island.

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4.2 Unit connected

Unit connected, in which the generator is connected directly to an associated power transformer without a circuit breaker in between, as shown in Figure 3.

This is the common connection for the large bulk power generators in the utilities.

Most generators are wye-connected, with a few delta-connected. These can either be a single generator or two separate generators (cross-compound) that are supplied by a common prime-mover system. Cross-compound generators may have the separate units directly connected together to a single transformer, or connected to separate secondary delta windings of a three-winding power transformer.

Generators are also connected to the power system through autotransformers.

Typical illustration of protection for the unit-connected generator is shown in Figure 4. The individual protection units shown in Figure 8.3 and Figure 4 may be separate relays or may be combined in various combinations.

The multifunction digital (microprocessor) relays provide many functions in a single package along with digital fault recording, self-checking, and so on.

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Sources:

1. Protective Relaying – Principles and Applications by H. Lee Willis (KEMA T&D Consulting) and Muhammad H. Rashid (University of West Florida)
2. Protection of Power Systems with Distributed Generation: State of the Art by Martin Geidl (Power Systems Laboratory; Swiss Federal Institute of Technology (ETH) Zurich)