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  Autor/Fonte: Dr Gareth P. Harrison and Dr A. Robin Wallace
  Data: 16/12/2004

    Network Integration of CHP - how to maximize access


The environmental benefits of Combined Heat and Power are significant and are a necessity for reducing the carbon burden of modern society. The European Union CHP Directive requires EU member states to have at least 18% CHP by 2012 and the UK target is 10 GW by 2010. Current UK installation is some 4.7 GW although a recent study projects that this may be missed by some 20%.

In common with other environmentally beneficial energy sources, such as renewables, CHP is mainly connected to medium or low voltage electrical distribution networks as distributed generation (DG). A significant justification for investing in CHP is the reduction in electricity imports from the network which attract a benefit equal to the purchase price of the electricity. Once site demand is satisfied then there may be potential to export and gain further benefit through electricity sales albeit at a lower rate. Clearly, such financial benefits must be set off against the cost of implementing and maintaining the CHP installation.

Connection of all grid connected plant fundamentally alters the operation of networks. There will be observable impacts on network power flows and voltage regulation particularly where generator capacity is comparable to local demand and specifically where export occurs. There is a risk that new connections will impact, adversely, on the security and quality of local electricity supplies and accordingly they must be evaluated carefully by Distribution Network Operators (DNOs). Where there are negative impacts, a range of options exist to mitigate them, however, under current commercial arrangements the CHP developer will largely bear the costs of implementation. The cost implications can make potential schemes less attractive and have played a part in restricting CHP deployment. As adverse impacts tend to be greater when plant exports to the network this explains to some extent the common requirement for anti-export equipment for CHP. This article reviews the electrical impacts of connecting CHP to distribution networks and examines existing means of mitigating adverse impacts. Further, a new technique is briefly outlined that may assist the network integration of a greater capacity of CHP by allowing DNOs to better guide the siting and sizing of generation to minimise the sterilisation of network access as well as costly upgrades.

Traditional Distribution Networks

Historically, distribution networks were designed to convey power uni-directionally from the high voltage National Grid to consumers at lower voltages (Figure 1). Operated passively with limited control activity, the voltage control is provided by on-load auto-tap transformers down to the 11 kV primary substations which must then accommodate all voltage drops in the network below. While most urban networks are set up as ring circuits (called meshed systems) they are normally operated like rural, radial systems in order to keep fault levels low and simplify protection schemes.



Within the distribution network, the load carried tends to fall with distance from the substation with conductor size and rating reducing in response. This, together with the fact that conductors at lower voltages have relatively higher impedance means that low voltage networks possess significant impedance with consequent implications for voltage control.

It is a licence requirement for DNOs that they ensure that electricity received by consumers is within a defined statutory range in voltage and frequency. For example, the UK Electricity Safety, Quality and Continuity Regulations specify steady-state voltages within +/- 10/6% of nominal for systems up to 132 kV and within +10/ƒ(6% at 400 V. To achieve this, planning procedures specify more conservative limits, while assuming modest load growth from customers who possess a load profile that varies over time in a well defined manner. Until recently, distributed generation was rarely a consideration.

CHP Impacts on the Distribution Network

The connection of all forms of DG, including CHP, have network impacts given the alteration of network power flows (Figure 2). Generation within the distribution network lowers the load seen by the DNO and, particularly where the operation of generation is determined by heating demand or weather patterns, the load profile changes. Its presence can impact significantly through:

1. Bi-directional power flow and the potential to exceed equipment thermal ratings

2. Reduced voltage regulation and violation of statutory limits on supply quality

3. Increased short circuit contribution and fault levels

4. Altered transient stability

5. Degraded protection operation and coordination



Power Flows

Figure 3 shows four scenarios (i)-(iv) for embedding CHP within a simple, but representative, radial network supplying a local load via a substation transformer (1 MVA). The peak value of the local load is 400 kW at 0.98 power factor. A series of CHP capacities ranging from zero to 900 kW (at 0.9 lagging power factor) are connected to the remote end of the feeder.

(i) With zero production the local load is supplied entirely from the network with all equipment operating within thermal limits and the losses in the feeder at 19 kW.
(ii) With production at 300 kW, the power import from the network reduces, along with the losses. This may benefit the DNO by allowing deferral of network upgrades necessitated by load growth.
(iii) With production at 600 kW and exceeding local demand, power will be exported back up the feeder and losses increase again, although the feeder and transformer loadings are within ratings.
(iv) With production at 900 kW the export to the network raises losses beyond their original values, although it would take an even larger generator and low demand to exceed the transformer rating.

Thermal limitations brought about by increasing DG capacity are usually encountered first in substation transformers and switchgear, or at the edges of heavily tapered radial networks where plant capacity is several multiples of the local demand.



Voltage Regulation

Power flows along a distribution feeder will create a voltage gradient in the direction of power flow. Critically, where power flow reverses (as would be the case with CHP export) there will be a local voltage rise at the generator and load. Once again, cases (i)-(iv) are used to demonstrate the impact on the voltage profile (Figure 4). Clearly, when local demand is high and supplied by CHP the voltage rise is reduced, but where local demand is low, say overnight, more power is exported and the voltage rise increases. This effect could cause over-voltage protection to disconnect the plant and, consequently, voltage rise places a major limit on DG capacity, particularly in more rural locations.


Fault Levels

In the event of a short circuit fault on the network all generators will contribute to the fault currents flowing. As such, the switchgear in the DNO network and that of the DG must be rated to withstand the effects of the combined fault currents. As the point of connection becomes more remote from the transmission network the intervening impedance increases lowering the network fault contribution falls. However, connection of DG will tend to raise them, at least locally. If fault levels increase beyond the rating of existing DNO switchgear, the switchgear must be replaced. This effect is most likely to affect urban or meshed networks.

Transient Stability
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